HomeMy WebLinkAbout03 Montana Post-Construction Storm Water BMP Design Guidance ManualMontana Post-Construction Storm Water
BMP Design Guidance Manual
September 2017
Produced for:
Montana’s MS4 Municipalities
Billings
Bozeman
Butte
Great Falls
Helena
Kalispell
Missoula
Prepared by
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Montana Post-Construction
Storm Water BMP
Design Guidance Manual
Produced for:
Montana’s MS4 Municipalities
Prepared by:
HDR
In cooperation with:
Montana Department of Environmental Quality
First Edition
September 2017
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Montana Post-Construction Storm Water BMP Design Guidance Manual
September 2017
Preface
Acknowledgments
This manual was developed as a collaborative effort that included technical representatives from
Montana’s municipal separate storm sewer system (MS4) municipalities (listed below), storm water
management professionals from HDR, representatives from the Montana Department of Environmental
Quality, and professionals from Montana’s development and engineering community. HDR thanks all
individuals and organizations who helped develop this manual and specifically acknowledges the
following individuals and organizations who contributed to this manual’s publication:
Manual Development Team
Vern Heisler, PE – City of Billings Project Manager
Matt Peterson, PE – HDR Lead Author
Spencer Savage – HDR Contributing Author and AutoCAD Designer
Aimee Navickis-Brasch, PE – HDR Technical Advisor
Shanna Adams, PE – HDR Technical Support
Susanna Schippers – HDR Technical Editor
Melissa Widseth – HDR Graphics and Layout
Scott Marshall, PE – HDR Technical Review
Tom Dupuis, PE – HDR Technical Review
Technical Working Group
Boris Krizek, PE City of Billings
Tyler Westrope, PE City of Billings
Kyle Mehrens City of Bozeman
Matt Moore, PE City of Butte
Mike Upton City of Great Falls
Matt Culpo, PE City of Helena
Patrick Jentz City of Kalispell
Tom Tabler, PE City of Kalispell
Bob Hayes City of Missoula
Ida Sajor City of Missoula
Rainie DeVaney Montana Department of Environmental Quality
Manual Reviewers
Eric Anderson, PE, CFM WGM Group
James Connolly, PhD Hyalite Engineers, PLLC
Jonathan Weaver, PE, CFM Great West Engineering
Karen Helfrich, PE Pioneer Technical Services, Inc.
Kim Snodgrass, PE, CFM Water & Environmental Technologies
Lance Lehigh, PE AE2S
Mace Mangold, PE TREC, Inc.
Melissa Matassa-Stone, PE, CFM WGM Group
Mike Day, PE WGM Group
Patty Hamblock, EI Water & Environmental Technologies
Shane Strong, PE KLJ
Stephanie Reynolds, PE WGM Group
Toby McIntosh, PE Jackola
Funding
City of Billings
City of Bozeman
City and County of Butte-Silver Bow Montana
City of Great Falls
City of Helena
City of Kalispell
City of Missoula
Montana Department of Environmental Quality
Montana Post-Construction Storm Water BMP Design Guidance Manual
September 2017
Preface
Foreword
This Montana MS4 Post-Construction Best Management Practice Design Guidance Manual has
been prepared to provide guidance to assist professionals with selecting, designing, constructing,
inspecting, and maintaining post-construction storm water management controls. The information
presented in this manual is the result of research of up-to-date storm water management practices
and the combined expertise of storm water management professionals in Montana and throughout
the United States.
This manual is a guidance document. The information in this manual is intended to provide
professionals with general information on the subject concerned. It is not intended to be an
exhaustive review of all applicable practices or a comprehensive summary of all regulatory and local
requirements. Additionally, best known storm water practices are evolving, along with regulatory and
local requirements, all of which are subject to change and may involve subjective interpretation. With
the exception of referenced permit language, this manual does not convey requirements for storm
water management practices in the state of Montana. However, it is expected that municipalities or
other governing entities may adopt this manual in full or in part to convey requirements for
conducting development or redevelopment activities in their jurisdictions. The professional is
responsible for identifying all applicable requirements for each specific project.
The professional using this manual is responsible for the proper design of a functioning system that
meets all the applicable requirements and considers all unique conditions of individual sites. It is the
professional who is responsible for proper installation of an approved design. Ultimately, it is the
property owner or operator’s responsibility to ensure that all permanent best management practices
(BMPs) function as designed at all times.
This manual does not cover every aspect of engineering necessary for proper BMP design,
construction, and implementation, nor does it cover every possible design scenario. Where the
designer determines that conformance to this manual would be technically or financially infeasible,
alternative design approaches, materials, and methods should be evaluated while consulting local
jurisdictions.
Note: This Manual is not intended to supersede existing procedures and policies for the review of
site, drainage, or infrastructure plans for local jurisdictions. Local policy and ordinances specific to
MS4 jurisdictions must be consulted to guide or dictate permanent storm water management
planning and design. Examples of these include land use codes, right-of-way easements, roadway
setbacks, impervious surface ratios, and a suite of other policies that affect how development is
distributed. Coordinate with the local jurisdiction for more information on site design and storm
water management policies.
Note: Throughout this manual, the words “should” and “recommended” are used for items that are
suggested for good design practice and optimal performance. The words “shall,” “must,” and
“required” convey guidance and/or criteria that, when adhered to, are expected to meet the Post-
Construction Performance Standard (see Section 1.3) based on resources reviewed during this
manual’s development.
Montana Post-Construction Storm Water BMP Design Guidance Manual
September 2017
Contents
Contents
1 Introduction to the Manual ................................................................................................................ 1-1
1.1 Purpose of the Manual ........................................................................................................... 1-1
1.2 Audience for the Manual ........................................................................................................ 1-1
1.3 Post-Construction Storm Water Criteria in the Montana MS4 General Permit ...................... 1-2
1.4 Regulatory Considerations for Storm Water Management in Montana ................................. 1-4
1.5 Best Practices for Storm Water Management ........................................................................ 1-6
2 Site Development ............................................................................................................................. 2-1
2.1 Recommended Site Development Process and Design Approach ....................................... 2-1
2.2 Preliminary Design ................................................................................................................. 2-4
2.3 BMP Selection and Sizing ...................................................................................................... 2-9
2.4 Final Design ........................................................................................................................... 2-9
3 Hydrologic Analysis Methodology .................................................................................................... 3-1
3.1 Hydrologic Basis of the Post-Construction Performance Standard ....................................... 3-1
3.2 Runoff Reduction and Runoff Treatment Volume .................................................................. 3-1
3.3 Runoff Treatment Flow Rate .................................................................................................. 3-2
3.4 Flood Control .......................................................................................................................... 3-4
4 Selection of Post-Construction BMPs .............................................................................................. 4-1
4.1 BMP Selection Process .......................................................................................................... 4-1
4.2 Types and Functions of BMPs ............................................................................................... 4-2
4.3 Screening Factors .................................................................................................................. 4-3
4.4 Cold Climate Considerations ................................................................................................ 4-11
4.5 Offsite Treatment Planning Guidance .................................................................................. 4-12
5 Design Guidance for Post-Construction BMPs ............................................................................. 5.1-1
5.1 Introduction .......................................................................................................................... 5.1-1
5.2 Infiltration Basin ................................................................................................................... 5.2-1
5.3 Bioretention ......................................................................................................................... 5.3-1
5.4 Permeable Pavement Systems ........................................................................................... 5.4-1
5.5 Dispersion ........................................................................................................................... 5.5-1
5.6 Biofiltration Swale ................................................................................................................ 5.6-1
5.7 Extended Detention Basin ................................................................................................... 5.7-1
5.8 Wet Detention Basin ............................................................................................................ 5.8-1
5.9 Proprietary Treatment Devices ........................................................................................... 5.9-1
Works Cited ............................................................................................................................................ WC-1
Montana Post-Construction Storm Water BMP Design Guidance Manual
September 2017
Contents
Tables
Table 1-1. Suggested Uses of the Manual for Intended Audiences .......................................................... 1-1
Table 1-2. Common Regulatory Programs that Influence Design and Storm Water Management
in Montana .................................................................................................................................... 1-4
Table 1-3. LID Planning Principles ............................................................................................................. 1-6
Table 4-1. Storm Water Pollutant Removal Processes ............................................................................. 4-3
Table 4-2. Minimum Setback Requirements from ARM 17.36.323 ........................................................... 4-7
Table 4-3. BMP Summary Table .............................................................................................................. 4-10
Table 4-4. Cold Climate Design Challenges ............................................................................................ 4-11
Table 5.2-1. Recommended Maintenance Activities for an Infiltration Basin ......................................... 5.2-8
Table 5.3-1. Recommended Maintenance Activities for Bioretention Areas ........................................ 5.3-15
Table 5.4-1. Recommended Maintenance Activities for Permeable Surfaces ..................................... 5.4-13
Table 5.5-1. Sheet Flow Dispersion Sizing Guidance ............................................................................ 5.5-5
Table 5.5-2. Channelized Flow Dispersion Sizing Guidance .................................................................. 5.5-6
Table 5.5-3. Recommended Maintenance Activities for Dispersion Areas ............................................. 5.5-8
Table 5.6-1. Flow Resistance Coefficient in Biofiltration Swales ............................................................ 5.6-5
Table 5.6-2. Recommended Maintenance Activities for a Biofiltration Swale ...................................... 5.6-10
Table 5.7-1. Recommended Maintenance Activities for an EDB .......................................................... 5.7-10
Table 5.8-1. Recommended Maintenance Activities for a WDB ........................................................... 5.8-11
Figures
Figure 2-1. Typical Site Development Pathway for Regulated New and Redevelopment Projects .......... 2-1
Figure 2-2. Recommended Design Approach Steps ................................................................................. 2-2
Figure 2-3. Recommended Design Approach Flowchart ........................................................................... 2-3
Figure 2-4. Composite Site Map ................................................................................................................ 2-6
Figure 2-5. Permeable Interlocking Concrete Pavers Walkway Application .............................................. 2-8
Figure 4-1. BMP Selection and Sizing Flow Chart ..................................................................................... 4-1
Figure 4-2. Vegetation in Urban Bioretention Area .................................................................................... 4-8
Figure 4-3. Urban Bioretention ................................................................................................................... 4-9
Figure 4-4. Snow on Storm Drain Inlet ..................................................................................................... 4-12
Figure 4-5. Onsite Versus Offsite Storm Water Management ................................................................. 4-12
Figure 5.2-1. Infiltration Basin ................................................................................................................. 5.2-1
Figure 5.2-2. Infiltration Basin with Outlet Structure ............................................................................... 5.2-5
Figure 5.2-3. Infiltration Basin Plan View and Typical Section ............................................................. 5.2-10
Figure 5.3-1. Bioretention Area ............................................................................................................... 5.3-1
Figure 5.3-2. Example Bioretention Configuration .................................................................................. 5.3-2
Figure 5.3-3. Bioretention Area within Roadway Median ........................................................................ 5.3-4
Figure 5.3-4. Bioretention Section with Typical Porosity Values ............................................................ 5.3-8
Figure 5.3-5. Online Bioretention Area with Overflow Outlet Structure .................................................. 5.3-8
Figure 5.3-6. Offline Bioretention Area ................................................................................................... 5.3-9
Figure 5.3-7. Bioretention Mulch Surface Cover ................................................................................... 5.3-10
Figure 5.3-8. Bioretention Vegetation ................................................................................................... 5.3-12
Figure 5.3-9. Inlet Protection ................................................................................................................. 5.3-13
Figure 5.3-10. Bioretention Plan View and Typical Section (Offline with Full Infiltration Section) ........ 5.3-16
Figure 5.3-11. Bioretention Partial Infiltration Typical Section .............................................................. 5.3-17
Montana Post-Construction Storm Water BMP Design Guidance Manual
September 2017
Contents
Figure 5.3-12. Bioretention No Infiltration Typical Section .................................................................... 5.3-17
Figure 5.4-1. Permeable Interlocking Concrete Pavers in Bozeman ...................................................... 5.4-1
Figure 5.4-2. PICP Parking Lot Application ............................................................................................ 5.4-4
Figure 5.4-3. Full Infiltration Section Cross Section ................................................................................ 5.4-6
Figure 5.4-4. PICP and Bioretention System Treatment Train ............................................................... 5.4-7
Figure 5.4-5. PICP Facility with Elevated Drain ...................................................................................... 5.4-9
Figure 5.4-6. PICPs with Perimeter Barrier ........................................................................................... 5.4-10
Figure 5.4-7. PICP Installation .............................................................................................................. 5.4-11
Figure 5.4-8. Cold Climate PICP Application ........................................................................................ 5.4-13
Figure 5.4-9. Full, Partial, and No Infiltration Sections ......................................................................... 5.4-14
Figure 5.4-10. Sloped Installation Section ............................................................................................ 5.4-15
Figure 5.4-11. Stepped Installation Section .......................................................................................... 5.4-15
Figure 5.5-1. Dispersion Area ................................................................................................................. 5.5-1
Figure 5.5-2. Dispersion Area Adjacent to a Roadway ........................................................................... 5.5-3
Figure 5.5-3. Dispersion Plan View and Typical Section ........................................................................ 5.5-9
Figure 5.5-4. Channelized Dispersion Plan View.................................................................................. 5.5-10
Figure 5.6-1. Biofiltration Swale .............................................................................................................. 5.6-1
Figure 5.6-2. Meandering Biofiltration Swale .......................................................................................... 5.6-3
Figure 5.6-3. Trapezoid Dimensions ....................................................................................................... 5.6-5
Figure 5.6-4. Level Spreader .................................................................................................................. 5.6-7
Figure 5.6-5. Biofiltration Swale Sign ...................................................................................................... 5.6-8
Figure 5.6-6. Degrading Concentrated Flow Inlet ................................................................................... 5.6-9
Figure 5.6-7. Swale Vegetation Establishment ....................................................................................... 5.6-9
Figure 5.6-8. Biofiltration Swale Plan and Section View ....................................................................... 5.6-11
Figure 5.6-9. Biofiltration Swale Typical Details.................................................................................... 5.6-12
Figure 5.7-1. Extended Detention Basin ................................................................................................. 5.7-1
Figure 5.7-2. Detention Basin in Billings, Montana ................................................................................. 5.7-3
Figure 5.7-3. Pretreatment Forebay and Trickle Channel ...................................................................... 5.7-5
Figure 5.7-4. Micropool and Outlet Structure .......................................................................................... 5.7-6
Figure 5.7-5. Example Orifice Plate Outlet Structure (Runoff Treatment Volume Only) ........................ 5.7-7
Figure 5.7-6. Detention Basin Deferred Maintenance .......................................................................... 5.7-10
Figure 5.7-7. Extended Detention Basin Plan View and Typical Section ............................................. 5.7-11
Figure 5.8-1. Wet Detention Basin .......................................................................................................... 5.8-1
Figure 5.8-2. WDB with Permanent Pool ................................................................................................ 5.8-3
Figure 5.8-3. WDB Pretreatment Forebay .............................................................................................. 5.8-5
Figure 5.8-4. Typical Orifice Plate Outlet Structure ................................................................................ 5.8-6
Figure 5.8-5. Outlet Structure Trash Rack .............................................................................................. 5.8-8
Figure 5.8-6. WDB Maintenance Access ................................................................................................ 5.8-9
Figure 5.8-7. Wet Detention Basin Plan View and Typical Section ...................................................... 5.8-12
Figure 5.9-1. Proprietary Treatment Device Installation ......................................................................... 5.9-1
Montana Post-Construction Storm Water BMP Design Guidance Manual
September 2017
Contents
Appendices
Appendix A. Glossary
Appendix B. Additional Hydrology Information
Appendix C. Evaluating Soil Infiltration Rates
Appendix D. Soil Amendments
Appendix E. Standard Forms
Appendix F. Example Inspection Forms
Montana Post-Construction Storm Water BMP Design Guidance Manual
September 2017
Contents
Abbreviations and Acronyms
AASHTO American Association of State Highway and Transportation Officials
BMP Best management practice
BOD Biological oxygen demand
COD Chemical oxygen demand
Construction
General Permit
General Permit for Storm Water Discharges Associated with Construction Activity
CN Curve Number
DEQ Department of Environmental Quality
DNRC Department of Natural Resources and Conservation
EDB Extended detention basin
EISA Energy Independence and Security Act
General Permit General Permit for Storm Water Discharges Associated with Small Municipal
Separate Storm Sewer Systems (MS4s): Permit Number MTR040000
HDPE High-density polyethylene
HSG Hydrologic soil group
H:V Horizontal:vertical
ICPI Interlocking Concrete Pavement Institute
Ksat Saturated hydraulic conductivity
LID Low-impact development
L:W Length:width
MBOGC Montana Board of Oil and Gas Conservation
MCM Minimum control measure
MS4 Municipal separate storm sewer system
NRCS Natural Resources Conservation Service
PICP Permeable interlocking concrete pavers
RRV Runoff Reduction Volume
RTF Runoff Treatment Flow
RTV Runoff Treatment Volume
TMDL Total maximum daily load
TSS Total suspended solids
UDFCD Urban Drainage and Flood Control District
UIC Underground Injection Control
US EPA U.S. Environmental Protection Agency
WDB Wet detention basin
WSDOT Washington State Department of Transportation
WSE Water surface elevation
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Montana Post-Construction Storm Water BMP Design Guidance Manual
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Chapter 1 – Introduction to the Manual
1 Introduction to the Manual
1.1 Purpose of the Manual
This Montana MS4 Post-Construction Best Management Practice Design Guidance Manual provides
guidance for selecting, designing, constructing, inspecting, and maintaining post-construction storm
water management controls, hereafter referred to as best management practices (BMPs), in
accordance with the provisions of Montana’s General Permit for Storm Water Discharges Associated
with Small Municipal Separate Storm Sewer Systems (MS4s): Permit Number MTR040000 (General
Permit). The General Permit is part of the Montana Pollutant Discharge Elimination System, under
which the Montana Department of Environmental Quality (DEQ) issues and enforces permits to
control point source discharges to protect water quality in the state’s surface waters. Following
federal discharge permit regulations, the General Permit contains the following six minimum control
measures (MCMs) that must be addressed in an MS4’s storm water management program:
1. Public education and outreach
2. Public involvement and participation
3. Illicit discharge detection and elimination
4. Construction site storm water management
5. Post-construction site storm water management in new and redevelopment projects
6. Pollution prevention/good housekeeping for permittee operations
This manual provides guidance for addressing the performance standard for new and
redevelopment projects within MCM 5. This manual also provides tools that may be used to address
other portions of an MS4’s storm water management program, such as impaired waterbodies with
approved total maximum daily load (TMDL) waste load allocations. Specific pollutant considerations
are addressed in Chapter 5 in the summary sheet for each BMP.
1.2 Audience for the Manual
Table 1-1 suggests uses of this manual for specific intended audiences.
Table 1-1. Suggested Uses of the Manual for Intended Audiences
Intended Audience Suggested Use
1
MS4 Program Manager To assist with implementation of a storm water management program:
The manual may be adopted and/or adapted for use with local storm water
management program implementation.
MS4 Plan Reviewer To review plans for compliance with the Post-Construction Performance Standard:
2
Chapter 3 provides guidance for hydrologic analyses.
Chapter 4 describes the BMP selection process.
Chapter 5 provides guidance for BMP design and implementation.
Montana Post-Construction Storm Water BMP Design Guidance Manual
1-2 | September 2017
Chapter Chapter 1 1 – – Introduction Introduction to to the the Manual Manual
Intended Audience Suggested Use
1
MS4 BMP Inspector To effectively inspect BMPs:
Chapter 5 provides maintenance considerations for the BMPs included in this manual.
Appendix F provides example inspection forms specific to each BMP that may be
adapted and/or adopted by local jurisdictions.
Design Professionals/
Consultants
To develop storm water designs that can meet the Post-Construction Performance
Standard:
2
Chapter 1 provides background and an overview of permitting requirements.
Chapter 2 provides site development guidance.
Chapter 3 provides guidance for hydrologic analyses.
Chapter 4 provides BMP selection guidance.
Chapter 5 provides design and implementation guidance for the eight BMPs included in
this manual.
Contractors To be informed of construction considerations:
Chapter 5 discusses construction considerations for the BMPs included in this manual.
Project Owners/
Operators
To effectively operate and maintain BMPs:
Chapter 5 discusses maintenance considerations for the BMPs included in this manual.
Appendix F provides example inspection forms specific to each BMP that may be
adapted and/or adopted by local jurisdictions.
Montana DEQ Staff To provide technical assistance to local programs:
The manual allows Montana DEQ to gauge how site plans translate to achieving
performance standards.
Non-MS4 Local
Government or Other
Entity
To assist with implementation of an effective storm water management program:
The manual may be adopted and/or adapted for use with local storm water
management program to communicate design standards to local stakeholders.
Interested Stakeholders
(Businesses,
Watershed
Groups, Citizens)
To be informed:
The manual is an education, outreach, and technical assistance tool for stakeholder
use as guidance in BMP selection, site development, and public education.
1 These are suggested uses for various parties, but not exhaustive, because many types of users will find various sections of the
manual to be helpful for particular purposes.
2 See Section 1.3 for discussion on the Post-Construction Performance Standard.
1.3 Post-Construction Storm Water Criteria in the Montana
MS4 General Permit
MCM 5 in the General Permit requires MS4s to develop, implement, and enforce a program to
address storm water runoff from new development and redevelopment projects that disturb greater
than or equal to 1 acre, including projects less than 1 acre that are part of a larger common plan of
development or sale and that discharge into a permitted small MS4. This program must ensure that
controls are in place that would prevent or minimize water quality impacts. MCM 5 has multiple
General Permit requirements; however, this manual primarily focuses on the performance standard
for new and redevelopment projects.
1.3.1 Post-Construction Performance Standard
For new and redevelopment projects regulated by the General Permit, the Post-Construction
Performance Standard presented in Part II.A.5.b.iii of the General Permit—which was issued on
November 30, 2016, and became effective on January 1, 2017—is as follows:
Montana Post-Construction Storm Water BMP Design Guidance Manual
September 2017 | 1-3
Chapter 1 – Introduction to the Manual
Require that all regulated projects implement post-construction storm water management
controls that are designed to infiltrate, evapotranspire, and/or capture for reuse the post-
construction runoff generated from the first 0.5 inches of rainfall from a 24-hour storm preceded
by 48 hours of no measurable precipitation. For projects that cannot meet 100% of the runoff
reduction requirement, the remainder of the runoff from the first 0.5 inches of rainfall must be
either:
a. Treated onsite using post-construction storm water management control(s) expected to
remove 80 percent total suspended solids (TSS);
b. Managed offsite within the same sub-watershed using post-construction storm water
management control(s) that are designed to infiltrate, evapotranspire, and/or capture for
reuse; or
c. Treated offsite within the same subwatershed using post-construction storm water
management control(s) expected to remove 80 percent TSS
Permittees allowing offsite treatment shall do the following:
a. Develop and apply criteria for determining the circumstances under which offsite
treatment may be allowed.
The criteria must be based on multiple factors, including but not limited to:
i. Technical or logistic infeasibility (e.g. lack of available space;
ii. High groundwater;
iii. Groundwater contamination;
iv. Poorly infiltrating soils;
v. Shallow bedrock;
vi. Prohibitive costs; and
vii. A land use that is inconsistent with capture and reuse or infiltration of storm
water).
Determinations may not be based solely on the difficulty and/or cost of
implementation.
The permittee must develop a formal review and approval process for determining
projects eligible for offsite treatment.
The offsite treatment option is to be used only after all onsite options have been
evaluated and documented through the permittee’s developed formal review and
approval process.
1.3.2 Montana MS4 Standard Terminology
The following terms and definitions are used in this manual to address components of the Post-
Construction Performance Standard presented above:
Post-Construction Performance Standard
The BMP design requirement presented in Part II.A.5.b.iii of the General Permit.
Runoff Reduction Requirement
The portion of the Post-Construction Performance Standard requiring that all regulated projects
implement BMPs that are designed to infiltrate, evapotranspire, and/or capture for reuse the post-
construction runoff generated from the first 0.5 inch of rainfall from a 24-hour storm preceded by 48
hours of no measurable precipitation.
Montana Post-Construction Storm Water BMP Design Guidance Manual
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Chapter Chapter 1 1 – – Introduction Introduction to to the the Manual Manual
Runoff Treatment Requirement
The portion of the Post-Construction Performance Standard requiring that, for regulated projects that
cannot meet 100 percent of the Runoff Reduction Requirement, the remainder of the runoff from the
first 0.5 inch of rainfall be treated using BMPs expected to remove 80 percent total suspended solids
(TSS).
Offsite Treatment Evaluation
An evaluation that must be conducted to determine whether a project is eligible for offsite treatment.
Note: In accordance with the General Permit, MS4s allowing offsite treatment shall develop and
apply criteria for determining the circumstances under which offsite treatment may be allowed.
These criteria must be part of a formal review and approval process developed by the MS4 for
determining whether projects are eligible for offsite treatment. Refer to the local jurisdiction for
additional guidance because the General Permit allows each MS4 to develop its own offsite
treatment criteria and approval process.
1.4 Regulatory Considerations for Storm Water
Management in Montana
For many sites, there are overlapping regulations at the local, state, and federal levels. In addition to
controlling runoff, new and redevelopment projects may have to comply with other requirements
related to storm water, such as floodplains, wetlands, natural streams, and dam safety, among
others. For instance, any new or redevelopment project that disturbs 1 acre or more of land will also
be required to obtain coverage under the Montana DEQ General Permit for Storm Water Discharges
Associated with Construction Activity (Construction General Permit), which provides the
requirements for controlling storm water runoff associated with construction activities.
Table 1-2 outlines the more common regulatory programs/drivers that may intersect with local storm
water programs. While the table is not exhaustive in this regard, it does highlight the degree of
coordination that may be necessary.
Table 1-2. Common Regulatory Programs that Influence Design and Storm Water Management in Montana
1
Permit or Regulatory
Program
Description
Local Construction/
Development Permits
Local Jurisdiction
In accordance with the MS4 General Permit, an MS4 must have a program to
address storm water runoff from construction sites and post-construction
development. Coordinate with the local jurisdiction to determine applicable
standards, submittals, and permits that may be required for development within the
local jurisdiction’s regulated boundary.
General Permit for Storm
Water Discharges Associated
with Construction Activity
Montana DEQ
This permit applies to all construction activities that result in land disturbance of
greater than or equal to 1 acre and projects disturbing less than 1 acre that are part
of a larger common plan of development or sale that would disturb 1 acre or more.
Coverage under this permit is obtained through Montana DEQ. Additional details
are provided on Montana DEQ’s webpage, entitled “MPDES Permits for Storm
Water Discharges” (http://deq.mt.gov/Water/WPB/mpdes/stormwater).
Note that some local jurisdictions may require submittal of the Construction General
Permit package in order to conduct construction activities within their regulated
boundaries.
Montana Post-Construction Storm Water BMP Design Guidance Manual
September 2017 | 1-5
Chapter 1 – Introduction to the Manual
Permit or Regulatory
Program
Description
Multi-Sector General Permit
for Storm Water Discharges
Associated with Industrial
Activity
Montana DEQ
This permit applies to storm water discharges associated with various categories of
industrial, mining, and oil and gas activities as defined in Administrative Rule of
Montana 17.30.1102 (29 and 30). Coverage under this permit is obtained through
Montana DEQ. Additional details are provided on Montana DEQ’s webpage, entitled
“MPDES Permits for Storm Water Discharges”
(http://deq.mt.gov/Water/WPB/mpdes/stormwater).
Other NPDES Permits –
Non-Storm Water General
Permits and Individual
Permits
Montana DEQ
Montana DEQ requires coverage under other general and/or individual permits for
surface water discharges associated with a variety of activities other than storm
water. Examples of such activities include construction dewatering and disinfected
water. A list of individual and general permits can be found on Montana DEQ’s
webpage, entitled “Montana Pollutant Discharge Elimination System”
(http://deq.mt.gov/Water/WPB/mpdes#GP).
Circular DEQ 8: Montana
Standards for Subdivision
Storm Drainage
Montana DEQ
Circular DEQ 8 defines Montana’s minimum standards for subdivision storm
drainage and provides guidance for designing and submitting storm drainage plans
for subdivision development. Note that some local jurisdictions may have storm
drainage standards that are more stringent than the requirements of Circular DEQ
8.
Surface Water Permits
Multiple Agencies
A Joint Application is required for proposed work in Montana’s streams, wetlands,
floodplains, and other waterbodies to protect water quality and aquatic habitats. The
Joint Application allows applicants to apply for the following local, state, or federal
permits: 310 Permit, SPA 124 Permit, Floodplain Development Permit, Section 404
Permit (Clean Water Act), 318 Authorization, 401 Certification, and Navigable
Rivers Land Use License, Lease or Easement. Additional information can be found
on the Montana Department of Natural Resources and Conservation Stream
Permitting webpage (http://dnrc.mt.gov/licenses-and-permits/stream-permitting).
Underground Injection Control
(UIC) Program
U.S. Environmental
Protection Agency
(US EPA)
Montana Board of Oil and
Gas Conservation (MBOGC)
US EPA has minimum requirements for UIC with respect to Class I to VI injection
wells. In Montana, US EPA has primary enforcement authority for Class I and
Class III to VI injection wells, while the MBOGC has primary enforcement authority
for Class II wells. Generally, BMPs such as infiltration basins will be classified as
Class V injection wells (storm water drainage wells) if the basins are deeper than
their largest surface dimension. Class V storm water drainage wells are authorized
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Chapter Chapter 1 1 – – Introduction Introduction to to the the Manual Manual
1.5 Best Practices for Storm Water Management
Modern-day best practices for land development employ various land planning techniques, design
practices, and technologies to simultaneously conserve and protect natural resource systems. This
multistep storm water management approach uses thoughtful site planning and manages rainfall at
its source by using integrated and distributed small-scale BMPs. This approach is also referred to as
low-impact development (LID). While this may not be feasible or the preferred approach for every
project, nationally, LID is becoming increasingly popular as the standard for storm water
management. It is encouraged in Montana’s General Permit. As such, considerations for LID have
been incorporated into this manual. Table 1-3 identifies some of the more common LID principles,
along with their benefits and suggested application. Additionally, LID principles are incorporated into
the site development and BMP selection techniques presented in Chapters 2 and 4, respectively.
Every site is unique and should be assessed on a case-by-case basis to determine whether the LID
BMP would be feasible or beneficial, given the site’s characteristics. Examples of these
characteristics include soil infiltration rates, frost depth, local precipitation and hydrology, vegetation
suitability, winter maintenance considerations, maintenance responsibilities, and regulatory conflicts.
The BMP screening guidance presented in Chapter 4 of this manual may prove helpful when
considering the use of LID principles for a given site.
Table 1-3. LID Planning Principles
LID Principle Example Application Benefits
Preserve natural site features Implement designs that preserve features
such as wetlands, floodplains, woodlands,
riparian areas, and highly permeable soils.
Improved habitat
Reduced storm water runoff
Improved aesthetics
Minimize and disconnect
impervious areas
Minimize runoff by using techniques such as
permeable pavement systems on sidewalks
and parking areas, routing downspouts away
from impervious surfaces, and using street
layouts that reduce the site’s impervious
area.
Reduced storm water runoff
Improved water quality
Increased groundwater
recharge
Disperse small-scale
integrated BMPs throughout
the site
Design sites with multiple small BMPs
(ponds, bioretention, permeable pavers, etc.)
as opposed to one large pond.
Improved aesthetics
Improved water quality
Protect/restore local
watersheds
Control storm water as close
to its source as possible
Design sites to create many small sub-
watersheds and manage runoff close to
where it is created in small decentralized
structures.
Increased groundwater
recharge
Reduced flooding
Create multifunctional
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Chapter 2 – Site Development
2 Site Development
2.1 Recommended Site Development Process and Design
Approach
As discussed in Section 1.3, the General Permit requires implementation of a program that
addresses storm water runoff from new and redevelopment projects that disturb greater than or
equal to 1 acre, including projects less than 1 acre that are part of a larger common plan of
development or sale and that discharge into a permitted small MS4. The site development process is
a key component of this program, which includes elements such as submittal, review, and approval
of site plans as well as construction, inspection, and maintenance of BMPs.
Figure 2-1 illustrates a typical pathway through the site development process. The left side of the
figure refers to activities or actions undertaken by the local jurisdiction (MS4), and the right side
refers to activities and actions by the project owner/applicant. Since each local jurisdiction may have
other plan review and inspection procedures and policies that take precedence, the owner/applicant
is responsible for identifying and following all applicable local engineering standards that pertain to
each phase of design and project implementation. A thorough understanding of the local
jurisdiction’s engineering standards, submittal requirements, and review process will save significant
time, money, and staff resources during design and permitting.
Figure 2-1. Typical Site Development Pathway for Regulated New and Redevelopment Projects
Source: West Virginia Department of Environmental Protection (2)
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Chapter 2 – Site Development
To effectively address the Post-Construction Performance Standard and other storm water
management objectives, consideration of storm water runoff should be integrated into the site
planning and design process. The remainder of this chapter provides an overview of the
recommended approach to site design, which involves a more comprehensive approach to site
planning and requires a thorough understanding of the site’s physical characteristics and resources.
The three primary phases in this recommended approach are shown in Figure 2-2 and are further
illustrated in Figure 2-3. These figures provide an overview of the site planning and design process.
Iteration between phases will likely be necessary.
Figure 2-2. Recommended Design Approach Steps
This design approach provides the site planner with an extensive tool kit to develop a site plan that
meets the Post-Construction Performance Standard and mitigates negative impacts on receiving
waters by managing volume, discharge frequency, and peak flow rates. The remainder of this
chapter primarily focuses on the preliminary design phase. Information regarding phases 2 and 3 in
this chapter is provided only to the extent necessary for the reader to understand how these steps fit
into the overall design process. More details on these phases are described in the subsequent
chapters.
Note: Figure 2-1 is a typical depiction of the site development process. Local jurisdictions are
encouraged to adapt or modify the components of the process in this figure to develop a local site
development process that reflects their requirements. The local jurisdiction should also consider
how this process will be modified for city-owned developments (e.g., library, city roads).
Note: To support the long-term success of site designs, a multidisciplinary design team is
recommended that includes qualified and experienced professionals in land use planning,
landscape architecture, vegetation ecology, geotechnical engineering, soils science, and water
resources engineering.
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Chapter 2 – Site Development
Figure 2-3. Recommended Design Approach Flowchart
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Chapter 2 – Site Development
2.2 Preliminary Design
The primary objectives of the preliminary design phase are to
gather key information and develop a preliminary site layout
that considers all the site characteristics and constraints.
2.2.1 Site Assessment
The site development process typically begins with a site assessment that provides information
about the site and its surroundings, forming the basis to make decisions about BMP placement and
selection. The site assessment should include an inventory and analysis of on- and offsite natural
and developed conditions that would affect the project design. Information typically collected during
the site assessment includes:
Topography
Hydrologic patterns and features
Soil and geotechnical assessment
Native vegetation and soil protection areas
Environmentally sensitive features
Site access
Land use controls
Utility availability and conflicts
The remainder of this section provides details and recommendations for site assessments and
documentation of the information to be collected. Since every site will have a unique set of
characteristics to be considered, additional information can be added to the list, as necessary (3).
Topography
Understanding the topography of the existing site, surrounding areas, and upgradient areas is
important for delineating drainage basins, siting BMPs, and implementing design principles that
promote runoff reduction, such as minimizing grading and preserving existing flow paths. The data
gathered during this step should be used to create a contributing drainage area map which
delineates the offsite runoff flowing to the site and a detailed topographic map of the site and
immediate surrounding areas. Depending on local jurisdiction requirements, projects may require a
topographic survey prepared by a registered land surveyor.
Hydrologic Patterns and Features
Identifying hydrologic patterns and features such as ponds, wetlands, creeks, and swales allows the
designer to determine drainage patterns, evaluate the condition of various drainage features,
determine whether they can be incorporated into the project, and select storm water management
measures to protect ecologically sensitive areas. It may be necessary to divide the site into multiple
subwatersheds, especially if small-scale BMPs will be used to manage storm water runoff.
Soil and Geotechnical Assessment
Understanding the soil and subsurface hydrology is critical to storm water management planning and
design. Typically, the goals of the soil and geotechnical assessment are to evaluate the site’s
feasibility for infiltration and, where appropriate, to determine long-term native soil design infiltration
rates. Soil characterization is also important to help specify materials to be used in design. For
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Chapter 2 – Site Development
example, geotextile layers for separation may not be needed on the sides or bottom of excavations
for bioretention or permeable pavement if the native site soils are not expected to migrate into the
various BMP layers based on grain size distributions.
During the preliminary design phase, obtaining soil data from sources such as the Natural
Resources Conservation Service (NRCS) Web Soil Survey may be an appropriate approach to
make initial judgments about a site’s suitability for infiltration and the placement of site features and
BMPs. One key piece of information to be obtained during this phase is the hydrologic soil group
(HSG), which provides general information regarding the infiltration rate of the soils, summarized as
follows:
Group A: Low runoff potential, high infiltration rate, well-drained sands and gravels
Group B: Moderate infiltration rate, well-drained sandy loam and fine to coarse gravels
Group C: Slow infiltration rate, silty loam and moderately fine to fine texture types
Group D: High runoff potential, slow infiltration rate, clay and soils with high water table
Detailed onsite geotechnical assessments such as infiltration test pits and soil borings should be
conducted as early as possible to determine infiltration rates and depth to groundwater. These
assessments should be conducted by a qualified professional such as a certified soil scientist,
professional engineer, geologist, hydrogeologist, or engineering geologist. See Appendix C,
Evaluating Soil Infiltration Rates, for detailed discussion of recommended methods for evaluating
native soil infiltration rates.
Native Vegetation and Soil Protection Areas
Protecting onsite native vegetation and soil helps reduce runoff, increase evapotranspiration, and
reduce erosion from the site, which can reduce the size of BMPs necessary to achieve the Post-
Construction Performance Standard. Vegetation surveys are recommended to determine baseline
conditions, establish long-term management strategies, and determine appropriate application of
dispersion techniques if storm water is directed to a protection area.
Environmentally Sensitive Features
It is important to identify environmentally sensitive features early in the design process because
these features typically need to be avoided or mitigated, both for habitat protection and permitting
reasons. Some of the typical features that should be identified and mapped during the site
assessment include wetlands, streams, riparian areas, floodplains, and cultural resources.
Site Access
Vehicular and pedestrian access areas are project elements that should be identified during the site
assessment because access can often represent a controlling element for the site’s design. Consult
the local jurisdiction to determine the street classification and site access requirements, which will
identify constraints such as the number of allowed access points, the width of the access, the
spacing of access points between sites on the same or opposite side of the adjacent street right-of-
way, and pedestrian circulation requirements along and through the site. The designer can use this
information to complete the access assessment, which typically includes mapping the location of
roads, driveways, and other points of ingress and egress within 50 feet of the site.
Land Use Controls
It is important to understand land use regulations to determine the allowable land uses and
development standards for the project site. Coordination with the local jurisdiction’s planning
department—along with a review of the local planning standards, comprehensive plans, and zoning
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Chapter 2 – Site Development
classifications—will reveal whether land use controls will place limitations on development, such as
limits on the amount of impervious surface coverage, minimum landscaping and lot area
requirements, setback requirements, parking requirements, and site design standards associated
with building placement and orientation.
Utility Availability and Conflicts
The location of wet (e.g., water, sewer, storm water) and dry (e.g., power, phone, cable) utilities
should be identified and the adequacy or concurrency of these utilities should be confirmed. Where
utilities already exist on the site, easements or other covenants that may stipulate onsite restrictions
should be identified and mapped. The county auditor or recorder’s office or a title company is often a
good source for finding restrictions and easements that may be recorded against the title of the
property. Also consider directly contacting the utility purveyors for this information.
If new utilities need to be extended to the site, the designer will need to understand where the utility
will come from, and potentially extend to, and the impact that easements and restrictions may have
on the site design. Existing utilities and utility easements, including any applicable setbacks, should
be mapped on the site plan. Existing utilities that may need to be moved and any new utilities to be
extended to the site should also be mapped.
Site Mapping Process
Through the site assessment process, map layers can be produced to delineate important site
features. These map layers may be combined to provide a composite site map that guides the layout
of streets, structures, and other site features (see Figure 2-4). This composite site map may be used
for all development types and will form the basis for the site layout, discussed in Section 2.2.3.
Figure 2-4. Composite Site Map
Source: Courtesy of AHBL, Inc. (3)
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Chapter 2 – Site Development
2.2.2 Identify Design Standards and Requirements
The next step is identifying and reviewing the local jurisdiction’s engineering and development
standards and any applicable state and federal requirements that will influence the design. This step
typically involves meeting with the local jurisdiction to discuss the proposed project and approach to
meeting the standards and requirements. This meeting, often in the form of a pre-application
meeting, usually occurs concurrent with the site assessment. Local standards and requirements that
may influence site design include:
Storm water regulations and design standards, including post-construction water quality and
flow control requirements
Setback requirements for infiltration facilities
Setback requirements for structures
Soil and subsurface hydrology evaluation and reporting requirements
Sizing methodologies to be used to demonstrate compliance with applicable storm water
requirements
Street network design standards
Maintenance agreement requirements for storm drainage systems and BMPs
A list of potential state and federal requirements is provided in Table 1-2 in Chapter 1. During this
review of design standards and requirements, the design team should also confirm local jurisdiction
requirements for design submittal preparation. By understanding all of the requirements and their
relative importance at the start of design process, the team can develop a site plan that efficiently
prioritizes and achieves all applicable objectives (3).
2.2.3 Preliminary Site Layout
Developing the preliminary site layout is an iterative process intended to optimize site development
and ensure that the site requirements and constraints are considered, including water quality
considerations. This process typically takes place after the majority of the site assessment has
occurred. Some of the standard content and general guidance to consider when developing a
preliminary site layout is provided in this section.
Preliminary Site Layout Content
Local jurisdictions usually have specific requirements for the contents of a preliminary site layout;
some of the more standard components are as follows:
Site grading (existing and proposed topography)
Roads
Buildings
Drainage facilities (conveyance, flood control facilities, maintenance access, and BMPs)
Recreational areas (parks, trails, etc.)
Utilities (water, sewer, gas, etc.)
Parcel boundaries
Natural resource protection areas (wetlands, floodplains, etc.)
Storm Water Management Strategies
This section identifies strategies that can be employed during the preliminary design phase to
promote the natural hydrology. Implementation of such recommendations typically reduces the
Runoff Reduction Volume (RRV) (see Section 3.2.1) and results in smaller BMPs (4).
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Chapter 2 – Site Development
Conserve Existing Site Features
During the preliminary site layout process, identify portions of the site that should be protected or
improved. Such areas may include:
Natural wetlands
Floodplains
Steep slopes
Woodlands
Wildlife habitats
Open spaces
Streams and riparian areas
Soils with high infiltration rates
Aquifers and their recharge areas
Some areas are generally legally or logistically unbuildable and therefore must be avoided; consult
the local jurisdiction for additional information.
Minimize Impervious Area
Multiple strategies may be used to reduce the site’s impervious area. A few options include:
Consider using cluster development to conserve open space.
Confine construction traffic to areas where structures, roads, and right-of-ways will exist after
construction, which limits compaction of native soils.
Reduce paved areas and compacted soils.
Use non-impervious drainage conveyances where appropriate.
Figure 2-5. Permeable Interlocking Concrete Pavers Walkway Application
Source: Courtesy of the City of Bozeman
Permeable interlocking concrete pavers in Bozeman create an aesthetically pleasing walkway and provide storm
water management by reducing the effective impervious area.
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Chapter 2 – Site Development
Strategically Locate BMPs
Consider the following recommendations when selecting locations for BMPs:
Select BMP areas that promote greater infiltration.
Where practical, consider combining flood control facilities with water quality BMPs to
achieve multiple storm water management objectives.
BMPs can sometimes be located in areas that promote multiple uses. An example includes
permeable surfaces, which may be ideal in areas where space constraints are a concern.
2.2.4 Hydrologic Analysis
The hydrologic analysis is an iterative process that should be initiated during the preliminary site
layout step. The delineation of subwatersheds and approximation of impervious areas within each
subwatershed provide critical information for this analysis. Once this information has been
determined, calculate the RRV using Equation 3-1 in Section 3.2 and the Runoff Treatment Flow
(RTF) rate using the procedure described in Section 3.3 for each subwatershed delineated. Each of
these values should be recalculated as the site layout is adjusted throughout the design process.
Note: Additional hydrologic analysis will likely be required to address flow control in accordance
with the local jurisdiction’s requirements (see the local jurisdiction’s engineering and development
standards).
2.3 BMP Selection and Sizing
Each project has unique design goals and constraints. As such, there is no one-size-fits-all BMP
which meets the Post-Construction Performance Standard. Some of the many items that should be
considered when selecting a BMP for a given site include land use, target pollutants, performance
capabilities, and physical site capabilities. For sites that are not conducive to runoff reduction,
documentation of the BMP selection process is important, especially if offsite treatment will be used.
For these projects, coordinate with the local jurisdiction to determine whether offsite treatment is
allowed and to work through the offsite treatment evaluation process. See Chapter 4 for additional
guidance on BMP selection and use of offsite treatment.
Preliminary sizing of BMPs will be necessary to determine if the Post-Construction Performance
Standard can be achieved with any given BMP or if several BMPs need to be applied in series, using
a “treatment train” approach. The selection and sizing process will likely be iterative because
multiple options may need to be considered to determine the most efficient and effective BMP(s) for
the site. See Chapter 5 for BMP sizing guidance.
2.4 Final Design
After the preliminary design phase has been completed and the proposed BMPs have been
preliminarily sited and sized, the design team should transition to the final design phase.
Coordination with the local jurisdiction is recommended prior to beginning the final phase to verify
that the preliminary site layout and proposed BMPs adequately address the local jurisdiction’s
design standards and requirements. Development of a final design for BMPs should include
components such as final siting and sizing, landscaping plans, construction considerations, and
operation and maintenance considerations. See Chapter 5 for guidance on BMP sizing and final
design considerations.
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Chapter 3 – Hydrologic Analysis Methodology
3 Hydrologic Analysis Methodology
3.1 Hydrologic Basis of the Post-Construction
Performance Standard
The overall goals of MCM 5 within the General Permit are to have the hydrology associated with new
development reflect the predevelopment hydrology and to improve redeveloped sites’ hydrology.
Pre-development hydrology, in terms of permit compliance, is defined as the natural conditions
where runoff from approximately 90 percent of the annual rainfall is either infiltrated, taken up by
plants, or conveyed by shallow subsurface flow (or interflow) to streams and rivers.
Historical rainfall data support the characterization that, on average, 90 percent of the rainfall events
occurring across Montana’s MS4 areas are 0.5 inches or less; therefore, the General Permit requires
that all regulated projects implement BMPs that are designed to infiltrate, evapotranspire, and/or
capture for reuse the post-construction runoff generated from the first 0.5 inches of rainfall from a
24-hour storm preceded by 48 hours of no measurable precipitation (5).
3.2 Runoff Reduction and Runoff Treatment Volume
3.2.1 Runoff Reduction Volume
In accordance with the Runoff Reduction Requirement (see Section 1.3.2), the RRV is the volume of
storm water runoff generated from the first 0.5 inches of rainfall from a 24-hour storm preceded by
48 hours of no measurable precipitation. Montana DEQ requires that when calculating the RRV, at a
minimum, designers must use impervious areas from new and redevelopment projects, and
including contiguous drainage areas that may contribute storm water (6). Equation 3-1 is
recommended to calculate the RRV (7).
RRV= PRv
A
12
Equation 3-1
Where:
RRV = Runoff Reduction Volume (acre-ft)
P = Water quality rainfall depth (use 0.5 inches)
Rv = Dimensionless runoff coefficient, Rv = 0.05 + 0.9(I)
I = Percent impervious cover draining to the facility, converted to decimal form
A = Site drainage area (acres)
3.2.2 Runoff Treatment Volume
The Runoff Treatment Volume (RTV) is defined as the remainder of the RRV that was not infiltrated,
evapotranspired, or captured for reuse onsite; hence, this volume must be treated onsite or
managed offsite (see Section 1.3). Equation 3-2 is recommended to calculate the RTV.
RTV = RRV - Vi,e,c
Equation 3-2
Where:
RTV = Runoff Treatment Volume (acre-ft)
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Chapter 3 – Hydrologic Analysis Methodology
RRV = Runoff Reduction Volume (acre-ft)
Vi,e,c = Volume of water infiltrated, evapotranspired, or captured for reuse onsite
3.3 Runoff Treatment Flow Rate
The RTF is the peak flow rate associated with the RRV or RTV, which is used to size flow-based
systems such as a biofiltration swale and flow diversion structures for offline storm water
management practices. The following procedure is recommended to calculate the RTF. This
procedure relies on the volume of runoff computed using Equation 3-1 and uses an adaptation of the
NRCS TR-55 Graphical Peak Discharge Method (7). Users are encouraged to refer to TR-55 for
more discussion on procedures and limitations.
Step 1: Determine the Runoff Curve Number
Determine the NRCS Runoff Curve Number (CN) using Equation 3-3, which is derived from the CN
method described in Chapter 2 of TR-55:
CN=
1000
[10+5P+10Q-10(Q
2
+1.25QP)
1/2
]
Equation 3-3
Where:
CN = Runoff Curve Number
P = Rainfall depth (use 0.5 inches)
Q = Runoff depth (watershed inches)
Compute the runoff depth (Q) in watershed inches using Equation 3-4:
Q=
RRV*12
A
Equation 3-4
Where:
A = Total area (acres)
Note: The RTV should be used in place of the RRV in Equation 3-4 in cases where a Runoff
Treatment BMP will be used.
Step 2: Calculate Time of Concentration
The time of concentration (tc) is the time it takes for runoff to travel from the hydraulically most
distant point of the watershed to a point of interest within the watershed. Water generally flows
through a watershed as sheet flow (overland), shallow concentrated flow, open channel flow, or
some combination of these. The minimum tc is 5 minutes, even when the calculated tc is less than
5 minutes.
Time of Concentration
A component of tc is travel time (Tt), which is the time it takes water to travel from one location to
another in a watershed. Tc is computed by summing all the travel times for consecutive components
(that is, sheet flow, shallow concentrated flow, and open channel) of the drainage conveyance
system, as shown in Equation 3-5:
tc
=Tt
1+Tt
2+. . . Tt
m Equation 3-5
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Chapter 3 – Hydrologic Analysis Methodology
Where:
tc = Time of concentration (hrs)
m = Number of flow segments
Travel time for sheet flow is calculated directly using Equation 3-7. For shallow and open channel
flow, travel time is calculated based on the ratio of flow length to flow velocity using Equation 3-6:
Tt
=
L
3600*V
Equation 3-6
Where:
Tt = Travel time (hrs)
L = Flow length (ft)
V = Average velocity (ft/sec)
3600 = conversion factor from seconds to hours
Sheet Flow
Sheet flow is shallow flow over land that usually occurs in the uppermost portion of a watershed and
occurs for very short distances in urbanized conditions. The maximum allowable sheet flow is
300 feet; however, most sheet flow distances will be shorter. Calculate the sheet flow travel time
using Equation 3-7:
Tt
=
0.007(nL)
0.8
(P2
)
0.5
s0.4 Equation 3-7
Where:
Tt = Sheet flow travel time (hrs)
n = Manning’s roughness coefficient (see Table B-1 in Appendix B)
L = Flow length (ft)
P2 = 2-year, 24-hour rainfall (in) (refer to local jurisdiction for rainfall depths)
s = Slope of hydraulic grade line (land slope, ft/ft)
Shallow Concentrated Flow (i.e., Street Gutter Flow)
After a maximum of 300 feet, sheet flow usually becomes shallow concentrated flow. Calculate the
average velocity for this flow using Equation 3-8 or Equation 3-9, depending on the watercourse
slope and type of channel. After determining average velocity, use Equation 3-6 to estimate travel
time for the shallow concentrated flow segment.
Unpaved: V=16.1345*(s)
0.5
Equation 3-8
Paved: V=20.3282*(s)
0.5
Equation 3-9
Where:
V = Average velocity (ft/sec)
s = Slope of hydraulic grade line (watercourse slope, ft/ft)
Open Channel Flow and Pipe Flow
The velocity in open channels and pipes can be determined using Manning’s equation if the shape,
flow depth, slope, and channel type are known. Channels can be in either natural or improved
conditions. Calculate the velocity for open channel flows using Equation 3-10. After determining
average velocity, use Equation 3-6 to estimate travel time for the open channel and/or pipe flow
segments.
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Chapter 3 – Hydrologic Analysis Methodology
V=
1.49
n
R
2
⁄ 3
√S Equation 3-10
Where:
V = Average velocity (ft/sec)
n = Manning’s roughness coefficient (see Table B-2 in Appendix B)
R = Hydraulic radius (ft)
S = Slope of the channel (ft/ft)
Note: The hydraulic radius, R, depends on the depth of flow in the channel or pipe. The depth of
flow associated with the RTF should initially be assumed, and should be confirmed upon final
calculation of the RTF.
Step 3: Calculate RTF
The RTF is computed using the following procedures, based on Chapter 4, “Graphical Peak
Discharge Method,” of TR-55.
Calculate the initial abstraction (Ia) using Equation 3-11.
Ia
=0.2* (1000
CN
-10) Equation 3-11
Compute the ratio Ia/P where P = 0.5 inches.
Use the calculated values for tc and Ia/P to read the unit peak discharge (qu) from TR-55
Exhibit 4-I or Exhibit 4-II as recommended by the local jurisdiction (see Appendix B).
Accuracy of peak discharge estimated by this method will be reduced if Ia/P values are used
that are outside the range given in Exhibit 4-I or Exhibit 4-II. In such cases, the limiting Ia/P
values are recommended for use.
Compute the RTF using Equation 3-12:
RTF=q
u
AQ Equation 3-12
Where:
RTF = Runoff treatment flow rate (cfs)
qu = Unit peak discharge (cfs/mi2/inch)
A = Drainage area (mi2)
Q = Runoff depth (in watershed inches) (see Equation 3-4)
3.4 Flood Control
Flood protection controls are designed based on a design storm with a specific return frequency that
is identified by local regulating jurisdictions. Generally, a 10- or 25-year return design storm is used
to size storm drainage infrastructure and a 100-year return design storm is used to address
upstream and downstream flooding, including restrictions and backwater conditions. Coordinate with
the local jurisdiction for flood control regulations and associated hydrologic analysis procedures (8).
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Chapter 4 – Selection of Post-Construction BMPs
4 Selection of Post-Construction BMPs
4.1 BMP Selection Process
The selection of suitable BMPs for any given project is based
on a review of the available BMPs, their performance
capabilities and design criteria, and screening factors such as
physical site constraints, treatment objectives, aesthetics,
safety, maintenance requirements, and cost. The guidance provided in this chapter builds on the
preliminary design phase discussed in Chapter 2 and is based on the presumption that the designer
has already conducted a site assessment (Section 2.2.1) and identified the design standards and
requirements (Section 2.2.2) for the project.
As discussed in Chapter 1, the General Permit requires that all regulated new and redevelopment
projects use onsite runoff reduction to satisfy the Post-Construction Performance Standard. In cases
where this cannot be achieved, the remainder of runoff must be addressed using one of the following
approaches, listed in the order of priority for selection: (1) treated onsite prior to discharge using
BMPs expected to remove 80 percent of TSS; (2) managed offsite within the same subwatershed
using BMPs that are designed to infiltrate, evapotranspire, and/or capture for reuse; or (3) treated
offsite within the same subwatershed using BMPs expected to remove 80 percent TSS (see
Figure 4-1).
The selection of BMPs, regardless of the option selected, should include a feasibility analysis to
verify that the selected BMP will safely and efficiently meet performance objectives. The remainder
of this chapter discusses the types and functions of BMPs available for use, along with the
recommended screening factors that should be reviewed when selecting a BMP for a given site.
Figure 4-1. BMP Selection and Sizing Flow Chart
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Chapter 4 – Selection of Post-Construction BMPs
4.2 Types and Functions of BMPs
No one-size-fits-all BMP exists. In particular, the types of treatment provided by a BMP and how
BMPs receive runoff can vary. This section introduces the primary functions of BMPs associated
with this manual, differentiates the two ways in which BMPs can receive runoff, and defines the
pollutant removal processes by which BMPs provide treatment.
4.2.1 Primary Function
The BMPs addressed in this manual have been placed into one of two categories, based on the
primary function they provide: runoff reduction or runoff treatment. For the purposes of this manual,
these terms are defined as follows:
Runoff Reduction: Implementation of a BMP (or series of BMPs) designed to infiltrate,
evapotranspire, or capture for reuse the RRV.
Runoff Treatment: Implementation of a BMP (or series of BMPs) expected to remove
80 percent TSS from the RTV.
4.2.2 Online Versus Offline BMPs
Storm water BMPs can be designed as either online or offline facilities. Online BMPs are designed to
intercept and manage all runoff generated from the contributing watershed. They provide treatment
for the RRV or RTV, and any additional runoff from larger storms is conveyed through an outlet
structure and/or overflow spillway. Offline BMPs are designed to receive only a portion of the runoff
generated from the contributing watershed. A flow regulator (diversion structure, flow splitter, etc.),
located upstream of the BMP, intercepts all the contributing runoff and then diverts a specified
volume or flow rate, such as the RRV or RTV, to the BMP, while the remaining volume of runoff
bypasses the BMP (9).
Note: Use of online BMPs in a natural drainage way may be discouraged or prohibited by the local
jurisdiction or applicable regulatory entities. Early coordination with applicable regulatory
personnel is highly recommended when considering the use of an online BMP in a natural
drainage way.
4.2.3 Pollutant Removal Process
Some BMPs remove pollutants from runoff through a variety of physical, chemical, and biological
processes. The pollutant removal processes associated with a BMP dictate which pollutants the
BMP will be effective at removing. Primary processes include biological uptake, filtration, infiltration,
density separation, and sorption. For sites subject to the Post-Construction Performance Standard,
the primary objective of BMPs is to remove gross solids and suspended sediment. For meeting
these treatment goals, BMPs that provide the following processes are effective: filtration, infiltration,
and density separation. When other pollutants are targeted, such as dissolved metals, other
processes such as biological uptake and adsorption/absorption may be necessary. Table 4-1
provides a brief overview of the more common physical, chemical, and biological processes by
which the BMPs remove pollutants (10). This table will be useful for MS4 program managers when
considering management strategies in areas that discharge to an impaired waterbody and will aid
designers when selecting BMPs for sites located upstream of impaired waterbodies.
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Table 4-1. Storm Water Pollutant Removal Processes
Removal Process Description and Pollutants Affected BMPs
Biological Uptake Definition: Broadly referred to as the transfer of pollutants from
runoff by plants or microbes; can include evapotranspiration.
Pollutants: hydrocarbons, nutrients, metals, organics, biological
oxygen demand (BOD), particulate chemical oxygen demand
(COD)
Bioretention
Dispersion
Biofiltration swale
Wet detention basin
Chemical
Transformation
Definition: Process by which pollutants react with other
compounds to change structure and are removed from the runoff.
Pollutants: nitrogen (ammonia, nitrate, nitrite), organics,
hydrocarbons
Bioretention
Biofiltration swale
Filtration Definition: Straining of pollutants by passing storm water through
a media (sand, natural soil, vegetation, etc.) finer than the target
pollutants.
Pollutants: solids, pathogens, particulate nutrients, particulate
metals, BOD, particulate COD
Bioretention
Permeable surfaces
Biofiltration swale
Mechanical treatment
1
Infiltration Definition: Pollutant reduction is achieved through runoff volume
reduction, such as infiltrating storm water through existing soils
below the surface grade.
Pollutants: solids, pathogens, nutrients, metals, organics, BOD,
particulate COD
Infiltration basin
Bioretention
Permeable surfaces
2
Dispersion
Density
Separation
Definition: Uses density differences between pollutants and
storm water for removal. This includes sedimentation and flotation.
Sedimentation is the gravitational settling of solids denser than
water. Alternatively, floatation is removal of those lighter than
water.
Pollutants: sediment, solids (particulates associated with other
pollutants such as nutrients and metals), oil (hydrocarbons), BOD,
particulate COD
Biofiltration swale
Extended detention
basin
Wet detention basin
Mechanical treatment
Sorption Definition: Includes adsorption and absorption. Absorption occurs
when a substance of one state is incorporated into another
substance of a difference state (that is, liquids being absorbed by
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Rural
Rural development consists of low-density projects generally occurring on lot sizes of greater
than 1/3 acre. Impervious cover is typically widely dispersed, and a significant portion of the
acreage is usually either forested, turf grass, vegetated open space, and/or used for
agriculture.
Rural lands are typically well-suited for minimization of impervious areas and small-scale
vegetated BMPs such as bioretention, dispersion, and biofiltration swales.
Residential
Residential development consists of medium- to high-density developments generally
occurring on lot sizes of less than 1/3 acre. The land cover includes areas with a mixture of
houses, paved areas, and vegetation.
Limitations on selection and siting of BMPs are typically related to traffic safety, large storm
conveyance, and lack of available space for pretreatment. Also, BMPs may need to be
located close to residences, where public safety, nuisance insects, and maintenance access
are common concerns.
Commercial Development
Commercial developments can consist of a wide variety of lot sizes and land cover
characteristics. They often contain a high percentage of impervious cover.
BMP selection and siting considerations vary depending on the size and type of commercial
development.
Industrial Development
Industrial areas are often dominated by impervious or semi-impervious (gravel) surfaces.
Runoff generated in these areas often has the highest and most variable concentrations of
certain pollutants compared with other land uses.
Limitations on BMPs are generally based on the potential for storm water hotspots. For
instance, infiltration-based BMPs are often prohibited in certain industrial areas because of
the potential for groundwater contamination. Additional discussion on storm water hotspots is
provided in Section 4.3.4.
Local Roads
Local roads are developed within linear corridors that vary in size depending on the extent of
the project. Land cover generally consists of impervious areas and grass right-of-way. Roads
typically generate higher storm water pollutant loads, most of which is generated from
vehicles (oil, grease, brake dust, etc.) and winter sanding and/or deicing activities.
BMPs such as biofiltration swales and bioretention areas are often suitable, given their ability
to have a linear footprint and fit within the right-of-way. In some cases, it may be necessary
to use a conveyance system to route flows to a larger BMP such as an extended detention
basin or wet detention pond.
4.3.2 Storm Water Management Objectives
Often, multiple objectives are identified to manage storm water runoff from a new or redevelopment
project. Three objectives common to storm water management in Montana’s MS4 areas include
meeting the Post-Construction Performance Standard, providing treatment for specific pollutants,
and addressing flood control requirements. When selecting BMPs, it is important to understand the
storm water management objectives specific to the project and consider whether certain BMPs will
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help or hinder the ability to meet those objectives. The following sections summarize the three
primary objectives.
Post-Construction Performance Standard
The ability to meet the Post-Construction Performance Standard—either through runoff
reduction or runoff treatment—is a key consideration when selecting BMP(s) for a given site.
Chapter 5 provides information on BMPs that can be designed, operated, and maintained to
meet the Post-Construction Performance Standard.
Treatment of Specific Pollutants
Areas that drain to a sensitive receiving waterbody may have requirements related to
treatment of specific pollutants within storm water runoff. For instance, wet detention basins
can raise the water temperature of storm water runoff prior to discharge and, therefore,
should be avoided if discharging directly to waterbodies listed as impaired for temperature.
Table 4-3 provides a brief overview of pollutant removal capabilities for the BMPs presented
in this manual.
Flood Control
Flood control is an important consideration when designing a site and selecting BMPs.
Given space constraints or other site-related factors, it may be beneficial to combine water
quality BMPs with flood control facilities.
4.3.3 Physical Site Characteristics
Physical site characteristics, such as contributing drainage area and the native soil’s infiltration
capabilities, are critical screening factors when determining whether a BMP will be able to meet the
desired objectives. For instance, infiltration-based BMPs will have a high probability of failure if sited
on soils that are not conducive to infiltration. Therefore, it is important for the designer to consider,
identify, and document any physical constraints at the project site that may restrict or preclude the
use of a particular BMP. Most physical site characteristics will be identified during the site analysis
step discussed in Chapter 2. A brief summary of site characteristics that should be considered is
discussed below; however, additional details specific to each BMP discussed in this manual are
addressed in Sections 5.2 to 5.9.
Contributing Drainage Area
Contributing area is defined as the total area, including pervious and impervious surfaces,
contributing to a BMP. This screening factor reflects the recommended minimum and/or
maximum drainage area that is considered optimal for a given BMP.
The maximum allowable contributing drainage area for each BMP should be decreased
when higher-than-normal pollutant loads are expected or may be increased when lower-
than-normal pollutant loads are expected.
The contributing drainage area used for final sizing and design calculations must be
determined from the final grading plan.
Soil Characteristics
The primary soil characteristic to be considered during the screening process is the
infiltration rate of the soils on site.
Determining the infiltration rate is a critical factor in determining the feasibility of runoff
reduction BMPs because implementing such BMPs on soils not conducive to infiltration will
most likely result in failure of the BMP.
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Obtaining the HSG information from the NRCS Web Soil Survey is typically sufficient for
initial screening of BMPs; however, confirmation of infiltration rates is required prior to final
design when the use of infiltration-based BMPs is proposed.
Depth to Groundwater and/or Bedrock
Depth to groundwater or bedrock is defined as the distance measured from the bottom or
floor of the BMP to the seasonal high water table or bedrock formation.
Shallow water tables may lead to BMP failure, significant maintenance concerns, and/or
contamination of groundwater. Designers must be careful not to “remove” pollution through a
system with the potential to adversely affect groundwater.
Shallow bedrock may limit the constructability of all BMPs and the effectiveness of infiltration
practices.
Depth to groundwater may be initially estimated using the NRCS Web Soil Survey, local
records, Montana Bureau of Mines and Geology studies, or historic data. When limited
information exists or shallow groundwater is likely, field observations should verify the depth
to groundwater for final design. Consult the local jurisdiction for additional information
regarding groundwater investigation requirements.
Site Topography
Site topography refers to the land slopes within the nearby drainage area of the BMP. This
screening factor reflects the potential effect of topographic influences on the functionality of a
BMP.
Steep slopes increase the potential for erosion, resulting in increased pollutants draining to
the BMP. If not properly accounted for during design and construction, this can result in
increased maintenance or failure of the BMP.
The topography in the immediate vicinity of a BMP affects both the ability of water to flow
through a BMP and the site’s retention capacity.
Topographic information may be obtained using online resources such as U.S. Geological
Survey topographic maps for preliminary design purposes, but should be surveyed on site for
final design.
4.3.4 Special Storm Water Management Areas
Special storm water management areas are those that possess certain on- or offsite (downstream)
characteristics that would limit the use of certain BMPs given their potential negative effect on the
immediate or surrounding areas. This section provides a general description of four special storm
water management areas and considerations for BMP selection in these areas.
Karst Geology
Karst is a dynamic landscape formed by the dissolution of soluble bedrock such as limestone
or dolomite. Karst geology is often associated with sinkholes, springs, caves, and a highly
irregular soil-rock interface (11).
BMPs that store and/or infiltrate runoff that are located within karst geology have the
potential to promote sinkhole formation and to threaten the integrity of the BMP and nearby
structures (11).
Karst geology provides rapid pathways for water to travel from the surface to the
groundwater, creating a risk of groundwater contamination (12).
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If the presence of Karst is suspected, a detailed site investigation, including a subsurface
materials investigation, should be conducted. Specific site and BMP design considerations
must be employed in areas of karst geology.
Sensitive Receiving Waterbodies
Sensitive receiving waterbodies are particularly sensitive to certain pollutants in runoff
because of the harmful effect that the pollutant(s) have on the waterbodies ability to support
certain uses. Examples include impaired waterbodies, cold-water fisheries, and high-quality
wetlands.
Developments that drain to a sensitive receiving waterbody often require additional
measures to protect or restore the waterbodies unique properties.
Consulting the State’s 303(d) list (impaired waterbodies) and the local jurisdiction’s storm
water master plan or watershed plan is often a good starting place to identify BMP design
considerations and limitations for specific watersheds and/or receiving waterbodies.
Storm Water Hotspots
Storm water hotspots are areas that produce higher concentrations of pollutants than is
normally found in urban runoff. Examples include gas stations, vehicle maintenance/repair
areas, and auto recyclers.
When selecting a BMP for an area that receives runoff from a storm water hotspot, it is
important to quantify the BMP’s ability to provide treatment for the expected pollutants. For
instance, the use of infiltration-based BMPs often poses the risk of groundwater
contamination when used to provide treatment for storm water hotspots.
Oftentimes, the entire project area may not be a hotspot; therefore, the designer may choose
to isolate hotspot areas with BMPs designed to handle the expected pollutants (11).
Water Supply Areas
Water supply areas include locations near water supply wells and within source water
protection areas. Designers should be aware of any design limitations or restrictions for
projects located in or near these areas.
Setback requirements can vary based on project location and the local jurisdiction’s
standards; however, minimum setback requirements have been established by the State of
Montana through ARM 17.36.323. Table 4-2 briefly summarizes these requirements.
Table 4-2. Minimum Setback Requirements from ARM 17.36.323
From To
Horizontal Setback Distance
(ft)
1
Storm water ponds
and ditches
Drinking water wells 25
2
Sealed components and other components 10
Drain fields/soil absorption systems 25
1 Local jurisdiction may have more stringent standards.
2 The setback is 100 feet for public wells, unless a deviation is granted under ARM Title 17, Chapter 38,
Subchapter 1.
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4.3.5 Maintenance
Routine and proper maintenance is essential for long-term effectiveness of all BMPs. Even when
BMPs are correctly designed and installed, they will likely become eyesores and cease to function if
not properly maintained. Because maintenance requirements vary for different BMPs, maintenance
must be considered during the BMP design and selection process. A brief summary of maintenance
items that should be considered during the BMP selection process is discussed below; however,
additional details specific to each BMP discussed in this manual are addressed in Sections 5.2 to 5.9
(10).
Compatibility with the Project Owner’s Maintenance Capabilities
Each BMP requires certain equipment and skills to conduct proper maintenance. For
instance, sediment removal will be a common maintenance requirement for BMPs which
have a sediment forebay.
When selecting a BMP, it is important to consider who will conduct the long-term
maintenance on the BMP and assess whether the project owner/operator has the correct
maintenance equipment, necessary skills, and is in agreement with the required
maintenance schedule.
Vegetated BMPs
Vegetated BMPs such as
bioretention and biofiltration swales
require special care to maintain the
functionality of the BMP.
When planning to use vegetated
BMPs, the maintenance frequency
and need for specialized training
often varies depending on the
type(s) of vegetation selected for
use. For example, the designer
should consider whether vegetation
will require supplemental irrigation
throughout the growing season and
verify that the project
owner/operator will irrigate and
maintain the vegetation.
Accessibility
Access must be considered because it is critical that all BMPs be accessible for inspections
and maintenance.
When selecting and siting a BMP, consider what type of equipment will be needed to
conduct the required maintenance activities and the frequency at which the maintenance will
be conducted. Large BMPs such as extended detention basins will require both regular
access for equipment such as lawnmowers and less frequent access for large equipment to
remove accumulated sediment from the main treatment cell.
Difficult access situations, including those with safety concerns, must also be considered.
These include BMPs close to buildings and high traffic areas (13).
Figure 4-2. Vegetation in Urban Bioretention Area
Source: HDR
This urban bioretention area will require considerable
maintenance to sustain healthy vegetation. It is important to
consider maintenance and vegetation management
requirements when selecting and designing BMPs.
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4.3.6 Community Factors
Considering community screening factors such as safety and aesthetics helps the designer consider
whether a BMP is well-suited for the general project area. This section describes two community
factors that should be considered during the BMP selection process (13).
Safety
Consideration of safety will help to determine whether certain BMPs pose a safety risk to the
surrounding community.
Safety considerations often include the potential for a drowning hazard due to the deep water
associated with BMPs such as wet detention basins.
Safety hazards can sometimes be mitigated by the addition of project features such as a
fence around a pond and trash racks on outlet structures.
Aesthetics
For some projects, aesthetics is an important screening factor, particularly if the community
or project owner requests that storm water facilities blend in with the existing or proposed
landscape.
In these cases, the surrounding land use and users should be considered when selecting
and designing a BMP. For example, considering whether the BMP will be visible and
assessing who will see the BMP may help the designer determine which BMP(s) may be
suitable.
When designing and/or selecting a BMP to be aesthetically appealing, it is of utmost
importance that functionality and maintainability are not compromised.
Figure 4-3. Urban Bioretention
Source: HDR
This bioretention facility is an example of a multifunctional facility with several functions including storm water
management, plant habitat, and aesthetics.
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Table 4-3. BMP Summary Table
Site Applicability
Maximum
Site
Slope
5%
5%
6%
Low
to
Moderate
Low
to
Moderate
15%
25%
Varies for different units
1 BMP is expected to infiltrate, evapotranspire, or capture for reuse the RRV when designed, operated, and maintained as described in this manual.
2 BMP is expected to remove 80 percent TSS from the RTV when designed, operated, and maintained as described in this manual.
3 TSS is the only pollutant regulated in the General Permit.
4 Runoff treatment may be the primary function in cases where an underdrain is required.
5 Additional research is necessary in cases when pollutant removal considerations are not provided.
Depth to
Groundwater
and/or Bedrock
3-foot minimum
Infiltration:
3-foot minimum
No infiltration:
1-foot minimum
Infiltration:
3-foot minimum
No infiltration:
1-foot minimum
3-foot minimum
1-foot minimum
2-foot minimum
No restrictions
Soil
Characteristics
HSG A or B
Applicable to
most soil types
Applicable to
most soil types
Applicable to
most soil types
Applicable to
most soil types
Applicable to
most soil types
Low infiltration
rates preferred
Contributing
Drainage
Area
0 to 50 acres
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4.4 Cold Climate Considerations
Storm water management facilities in Montana are subject to cold climate conditions in the winter,
requiring special considerations when selecting, siting, and designing BMPs. This section discusses
some of the BMP selection and design considerations associated with cold climates.
4.4.1 Design Challenges
Cold climates present multiple design challenges that, if not accounted for, can detrimentally affect
the structural integrity, performance, and maintainability of BMPs. Some of the primary issues
encountered when selecting, siting, and designing BMPs for cold climate conditions can include high
pollutant concentrations within snowmelt runoff; operational challenges resulting from ice, cold
water, and decreased biological activity; and challenges related to sand and/or deicing materials.
Table 4-4 summarizes some of the additional design challenges that should be considered and
mitigated when designing BMPs in cold climates.
Table 4-4. Cold Climate Design Challenges
Cold Climate
Characteristics
BMP Design Challenge
Cold temperature Pipe and/or outlet structure freezing
Ice formation on permanent pools
Reduced biological activity
Reduced oxygen levels during ice cover
Reduced settling velocities
Deep frost line Pipe freezing
Reduced soil infiltration
Frost heaving
Short growing season Reduced time period to establish vegetation
Selection of appropriate plant species for cold climates
Significant snowfall High runoff volumes during snowmelt and rain-on-snow
High pollutant loads during spring melt
Impacts from road salt/deicers
Snow management may affect BMP storage capabilities
Source: Adapted from Caraco and Claytor (14)
4.4.2 BMP Siting, Design, and Operations Adaptations
Each of the design challenges presented in Table 4-4 will have varying solutions depending on the
type of BMP being implemented, the expected amount of snowfall, the severity of cold temperatures,
and the surrounding land use that will drain to the BMP. While each designer should consider these
items and apply appropriate mitigating techniques, some general siting, design, and operations
strategies are provided as follows:
Careful Site Selection: When possible, consider placing BMPs in areas where they will not
receive immediate runoff from roads that receive high concentrations of sand and/or gravel.
A common method to address this is placing filter strips or dispersion areas along roadways
to promote settling of sand and gravel prior to runoff entering a BMP (15).
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Careful Plant Selection and
Placement: When vegetated BMPs
will be subject to runoff from roads,
salt-resistant plants should be used.
Plant placement should also be
considered if BMPs are located near
roadways that will be plowed during
the winter because piles of ice and
snow can damage vegetation (15).
Snow Storage Planning: If
dedicated snow storage areas will
be incorporated into a new or
redevelopment project, consider
placing such areas on pervious
surfaces where a portion of the
runoff will be infiltrated or directing
runoff from these areas to
pretreatment BMPs such as vegetated filter strips or swales prior to entering a BMP that is
more difficult to clean (for example, an infiltration basin or wet detention pond).
Perform Timely Maintenance Activities: Perform maintenance activities such as street
sweeping as soon as the spring melt has occurred, which will help to limit the amount of
debris carried into BMPs and the storm drain conveyance system (15).
4.5 Offsite Treatment Planning Guidance
Offsite treatment is a storm water management approach in which regional BMPs strategically
located within a subwatershed are sized to manage runoff from multiple projects that drain to the
facility (see Figure 4-5). For this manual, offsite treatment BMPs are defined as regional facilities
designed to manage storm water runoff from multiple development projects located within the same
subwatershed. This process is typically managed by the local jurisdiction, where individual project
owners may assist in financing the regional BMP. This section discusses the General Permit’s
requirements regarding offsite treatment and considerations for evaluating the offsite treatment
option (9).
Figure 4-5. Onsite Versus Offsite Storm Water Management
Figure 4-4. Snow on Storm Drain Inlet
Source: Courtesy of the City of Missoula
Snow storage planning can help reduce the amount of
pollutants discharged into the storm drain system.
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4.5.1 Offsite Treatment Evaluation
As described in Section 1.3, offsite treatment is an option which may be employed to attain the Post-
Construction Performance Standard only when it meets the offsite treatment eligibility criteria defined
by the local jurisdiction. Subsequently, if offsite treatment is to be used, an evaluation must be
conducted that documents the evaluation of both onsite and offsite storm water management options
(hereby referred to as an Offsite Treatment Evaluation).
According to the General Permit, each MS4 has the latitude to develop its own criteria and
evaluation process for assessing the feasibility of onsite BMPs to determine the circumstances
under which offsite treatment may be allowed. The following tools are provided in this manual to
assist both MS4 program managers and designers in determining the feasibility of BMPs:
Site selection guidance is provided for each BMP in Sections 5.2 to 5.9.
A template for an Offsite Treatment Evaluation Form is provided in Appendix E.
4.5.2 Advantages and Disadvantages of Offsite Treatment
The use of offsite treatment has both advantages and disadvantages that should be understood
when considering this option for storm water management. Some of the advantages and
disadvantages are discussed below (9).
Advantages of Offsite Treatment BMPs
Cost Effective: Regional BMPs are often more cost-effective than onsite BMPs because it is
more efficient and less expensive to build, operate, and maintain one large facility compared
with several small BMPs.
Consolidated Maintenance: It is typically easier to track and accomplish maintenance
requirements for one large regional BMP than it is for multiple small BMPs located
throughout a watershed.
Higher Probability of Maintenance: Regional BMPs are often more likely to be properly
maintained when compared with onsite BMPs because they are large and have a higher
visibility, and are typically the responsibility of the local jurisdiction.
Potential Integration with Flood Control: Pairing water quality BMPs with flood control
facilities can be an effective use of space in situations where a local jurisdiction takes a
regional approach to flood control through the use of large regional detention basins.
More Effective Treatment Capabilities: Some sites are not conducive to onsite BMPs
(because of high groundwater, geotechnical concerns, etc.). In these cases, regional BMPs
are often safer and/or more effective at reducing pollutants in runoff.
Disadvantages of Offsite Treatment BMPs
Disruption of Natural Hydrology: Infiltration or treatment of storm water close to its source
is a way to mimic predevelopment hydrology and to attempt to maintain or restore natural
conditions. The use of regional BMPs disrupts this process.
Potential Location and Siting Difficulties: Regional BMPs usually require a large amount
of contiguous space and, therefore, can be difficult to site, especially for large facilities or in
areas with existing development.
High Capital Costs: It can often be difficult to fund a regional BMP because of the large
amount of capital required for initial construction. The matter is complicated by fact that the
facility must be completed prior to development of the full upstream area.
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Need for Planning: The implementation of regional BMPs requires substantial planning,
financing, and permitting; each of which must be in place ahead of future projected growth.
4.5.3 Important Considerations for the Use of Regional BMPs
The use of regional BMPs requires significant planning. Many items must be considered so that the
system functions properly, treatment objectives are met, and environmental concerns are
addressed. Some of the items that should be considered are discussed below (9).
The conveyances between the individual upstream developments and the regional facility
must be able to handle the design peak flows and volumes without causing adverse
environmental impacts or property damage. In the event that natural waterways are used for
conveyance to the facility (as opposed to facilities such as storm drains), upstream BMPs
may be required to protect the natural drainage way and additional permitting requirements
may apply (see Table 1-2).
An analysis must be conducted to verify that the Post-Construction Performance Standard
will be achieved for the proposed new and/or redevelopment project(s) in the BMP’s
drainage area. Full build-out conditions that account for the projected future land use within
the BMP’s drainage area are recommended for use in the design analysis; however, in cases
where this is not done, a procedure should be set in place to assess the regional facility’s
ability to manage runoff from future development in the drainage area.
A funding analysis should be conducted to determine how the facility will initially be paid for,
how future maintenance will be paid for, and what fees will be applied to projects draining to
the BMP.
Siting and designing regional BMPs should be done within the context of storm water master
planning or watershed planning.
It is highly recommended that online regional BMPs be avoided because of environmental
issues and permitting complexities.
Note: Design of offsite treatment BMPs must account for the current and future conditions of the
entire drainage area—not just the project site. The BMPs presented in Chapter 5 of this manual are
applicable to offsite treatment only when the current and future conditions of the entire drainage
area are accounted for. This manual does not include technical guidance on partial onsite or offsite
treatment or partial use of the Runoff Reduction Requirement and Runoff Treatment Requirement.
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Chapter 5 – Design Guidance for Post-Construction BMPs
5 Design Guidance for Post-Construction
BMPs
5.1 Introduction
This chapter provides guidance for design, construction, and maintenance of a number of BMPs that
can be used at new and redevelopment sites to meet the Post-Construction Performance Standard.
The guidance presented in this chapter is based on information from multiple storm water
management manuals and resources from across the United States, while accounting for items such
as Montana’s MS4 General Permit requirements and cold climate considerations. This chapter
includes the following BMPs:
Section 5.2 – Infiltration Basin
Section 5.3 – Bioretention
Section 5.4 – Permeable Pavement Systems
Section 5.5 – Dispersion
Section 5.6 – Biofiltration Swale
Section 5.7 – Extended Detention Basin
Section 5.8 – Wet Detention Basin
Section 5.9 – Proprietary Treatment Devices
Each BMP section contains the following information:
Description: An overview of the characteristics and function of the BMP.
Performance: The expected performance of the BMP with respect to meeting the Post-
Construction Performance Standard (see Section 1.3 of this manual). This section describes
the BMP’s potential effectiveness in meeting either the Runoff Reduction Requirement or
Runoff Treatment Requirement.
Site Selection: Items to consider when evaluating a potential site for locating the BMP.
Additional site selection guidance is also provided in Chapter 4.
Design and Sizing Procedure: Guidance and procedures to be used in conjunction with the
hydrology calculations in Chapter 3 to properly size and design the BMP.
Vegetation Considerations: Guidelines for considering, evaluating, and selecting
vegetation.
Construction Considerations: Suggestions and considerations for construction.
Maintenance: An outline of recommended protocols for maintaining the BMP.
Plan View and Typical Details: Example plan view drawings and typical details.
Note: The BMP functions presented in this chapter may include runoff reduction, runoff treatment,
and combined water quality/flood control; however, while this manual references flood control
aspects of storm water management, that is not its intended use. Some of the BMPs identified in
this chapter may be designed to provide flood control benefits, but they should be used in
conjunction with appropriate flood control design guidance and floodplain management measures.
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5.2 Infiltration Basin
5.2 Infiltration Basin
Figure 5.2-1. Infiltration Basin
Source: HDR
Description
A constructed basin designed to collect and retain
storm water runoff so that it can infiltrate into
underlying soils. These facilities remain dry between
runoff events and often have permanent vegetation
ranging from grass to small shrubs.
Primary Components Primary Function
Inlet structure
Pretreatment
Infiltration cell
Overflow outlet
structure (optional)
Runoff reduction
Runoff treatment
Benefits Limitations
Effective method for achieving the Runoff
Reduction Requirement within large drainage
basins
Provides groundwater recharge
Mimics pre-development hydrology
Not suitable for sites containing soils with low
permeability
Susceptible to clogging by sediment and organic
debris if proper pretreatment measures are not
employed
May not be suitable for industrial sites or locations
where spills could occur
Design and Site Selection Considerations
Setbacks
Depth to groundwater or bedrock
Soil permeability
Soil preparation/amendments/compost
Pretreatment
Inlet and outlet spacing
Energy dissipater/level spreader
Underdrain
Facility liners
Landscaping/planting
Fencing
Size of contributing drainage area
Area required
Incorporate flood control
TMDL Considerations Maintenance Requirements
Avoid Preferred
Total suspended solids (TSS)
Total phosphorus
Total nitrogen
Temperature
Metals
Fecal coliform
Access roads or pullouts
Sediment removal
Irrigation
Vegetation management
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5.2 Infiltration Basin
5.2.1 Description
An infiltration basin is an above ground earthen impoundment that uses the natural filtering ability of
soils to remove pollutants in storm water runoff. Storm water runoff is retained in the basin with the
only means of emptying being through evapotranspiration and infiltration. Infiltration basins have
high pollutant removal efficiencies and can help recharge groundwater.
The primary characteristics of an infiltration basin are as follows:
An infiltration basin consists of an inlet structure, pretreatment BMP(s), an infiltration cell,
and an optional outlet structure.
The recommended maximum drawdown time for the design volume is 72 hours. Storage in
excess of 72 hours may result in both water quality and mosquito breeding issues.
An infiltration basin can be designed to provide both runoff reduction and flood control.
5.2.2 Performance
Runoff Reduction
An infiltration basin is expected to infiltrate 100 percent of the RRV when designed, operated, and
maintained as described in this manual.
Runoff Treatment
An infiltration basin is designed to infiltrate the entire RRV from a contributing drainage area;
therefore, runoff treatment is not applicable.
5.2.3 Site Selection
Basic guidelines are provided below to aid in evaluating whether infiltration basins are feasible for
use at an individual site.
Contributing Drainage Area
Infiltration basins are best suited for sites with a contributing drainage area of less than 50
acres.
This guidance assumes that, in most cases, impervious surfaces will constitute more than
50 percent of the contributing drainage area and that most of this impervious area is directly
connected. The recommended maximum contributing area to an infiltration basin may be
increased if pervious surfaces constitute the majority of the contributing drainage area and
soils are highly permeable (HSG A or B).
It is recommended that contributing drainage areas have a maximum 5:1 ratio of impervious
area to infiltration area (16).
Soil Characteristics
Native soils should have an infiltration rate such that the facility is capable of infiltrating the
design volume in a maximum of 72 hours (or in accordance with local jurisdiction
requirements).
Soil characteristics can initially be estimated from NRCS soil data, but must be field verified
prior to final design using the onsite soil investigation methods discussed in Chapter 2 and
Appendix C of this manual.
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5.2 Infiltration Basin
A minimum of two soil profile pits are recommended within the infiltration cell area to confirm
its ability to function as designed. Consult the local jurisdiction for soil assessment
requirements.
Infiltration basins should only be sited on natural, uncompacted soils.
Depth to Groundwater and/or Bedrock
A minimum vertical distance of 3 feet is recommended between the bottom of the infiltration
basin and the seasonal high water table or bedrock layer (16).
An evaluation of the depth to groundwater should be conducted, as described in Section
4.3.3.
Site Topography
Infiltration basins should be located on relatively flat areas, and the grade immediately
adjacent to the basin (within 15 to 20 feet) should be less than 5 percent to limit erosion, but
greater than 1 percent to promote flow toward the basin.
The area of the facility intended for siting of the infiltration cell should be as level as possible
to uniformly distribute runoff.
If steep grades are present throughout a project site, the basin can be split into multiple cells
with adequate conveyance between the cells to take advantage of relatively flat areas.
Unless slope stability analyses demonstrate otherwise, basins should be located a minimum
horizontal distance of 200 feet from down-gradient slopes greater than 20 percent (17).
Land Use and Considerations of Surrounding Area
Use caution when placing infiltration basins in drainage areas that produce high sediment or
trash/debris loads because such loads may cause clogging (17).
Infiltration basins located above sloped areas may result in shallow lateral flow (interflow)
that can re-emerge and negatively affect down-gradient structures. For these sites, an
assessment of the impact on down-gradient structures is recommended.
Consider minimum setback requirements, as discussed in Section 4.3.4.
Community and Environmental Considerations
Infiltration basins should be avoided at locations where storm water runoff could pose a risk
of groundwater contamination (i.e., storm water hotspots) (17).
Safety concerns may be associated with standing water as an infiltration basin drains.
Section 5.2.4 provides guidance and recommendations for designing basin depths and side
slopes that may help alleviate safety concerns. Consult the local jurisdiction for fencing
requirements.
Opportunities may be available for an infiltration basin to be located within or near multiuse
facilities such as parks and open space.
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5.2 Infiltration Basin
5.2.4 Design and Sizing Procedure
The following steps outline the design procedure and criteria for an infiltration basin.
1. Infiltration Cell Volume
The infiltration cell is the primary component of
the facility where storm water runoff is captured,
treated, and released through infiltration. Design
the infiltration cell volume to be 100 percent of
the RRV. Determine the RRV using the
guidance provided in Section 3.2. Additional
sizing requirements are as follows:
Calculate the maximum basin depth for
the design infiltration rate using Equation
5.2-1. For highly permeable sites, the
calculated maximum basin depth may be
greater than the maximum depth
permitted by the local jurisdiction.
Coordinate with the local jurisdiction for
additional details.
Dmax
=
i
2
*td
Equation 5.2-1
Where:
Dmax = Maximum depth of the
infiltration cell (in)
i = Field-verified infiltration rate for
the native soils (in/hr)
td = Maximum draw down time (hrs)
Note: For design purposes, the field-tested subgrade soil infiltration rate (i) is divided by 2 within Equation
5.2-1 as a factor of safety to account for potential compaction during construction and to approximate long-
term infiltration rates.
Calculate the infiltration bed area using Equation 5.2-2.
A=
V
D
Equation 5.2-2
Where:
A = Infiltration cell bottom area (ft2)
V = Infiltration cell volume (ft3)
D = Design depth of the infiltration cell (ft)
Note: The entire RTV is assumed to be instantaneously ponded within the infiltration basin.
2. Inlet and Conveyance
Design the infiltration basin so that the inlet discharges into a pretreatment facility. The inlet locations
should be designed to dissipate flow energy to limit erosion and promote particle sedimentation.
Infiltration basins may be constructed as either offline or online systems; see Section 4.2.2 for
additional guidance.
Infiltration Basin Minimum Design Criteria
1
Required Components
Inlet structure
Pretreatment
Infiltration cell
Overflow outlet structure or spillway (only if facility
is designed to accommodate flood control flows)
Design and Sizing
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5.2 Infiltration Basin
3. Pretreatment Forebay
Pretreatment facilities are an important component of an infiltration basin because they protect the
infiltration cell from the build-up of trash, solids, and particulate matter. A pretreatment forebay is
recommended at each major inlet to allow larger particles to settle out prior to discharging flows to
the infiltration cell. Guidance for forebay sizing and design are as follows:
Maximize the length of the flow path through the forebay and minimize the slope to
encourage settling.
Provide a depth between 4 and 6 feet with a volume equal to 10 percent of the RRV.
A barrier separating the pretreatment forebay and infiltration cell should be constructed to
contain the forebay opposite of the inlet. If the barrier is an earthen berm, a minimum top
width of 8 feet and side slopes no steeper than 4:1 are recommended. The barrier should be
armored with material such as gabions, concrete, or riprap.
It is recommended that a level spreader be provided in the transition area between the
pretreatment forebay and infiltration cell to enable even distribution.
A concrete bottom is recommended to facilitate sediment removal during maintenance.
Provide a way to monitor sediment accumulation. Options include a metered rod within the
forebay or concrete lining that defines sediment removal limits.
4. Infiltration Cell Shape
Basin side slopes should be stable and gentle to facilitate maintenance and access. 4:1 (H:V) or
flatter side slopes are preferred to allow for conventional maintenance equipment and for improved
safety and aesthetics. Side slopes should be no steeper than 3:1 (H:V); however, local design
standards should be consulted to confirm the maximum allowable slopes. Using walls is discouraged
because of maintenance constraints.
5. Infiltration Cell Bottom
The bottom of the infiltration cell should be as flat as possible to enable even distribution and
infiltration of storm water. Lateral slopes should have a 0 percent grade and longitudinal slopes
should range from 0 to 1 percent. It is not recommended to use any type of filter fabric on the bottom
of the basin because this could reduce infiltration rates and possibly clog the practice entirely (17).
6. Outlet Structure (Optional)
An infiltration basin may be designed as
part of an online, combination system that
provides both water quality and flood
control for storm water runoff. In such
cases, an outlet structure will be required to
attenuate flood flows. General outlet
structure design guidance is provided in
Section 5.8.4, in the Outlet Structure
subsection, and guidance specific to an
infiltration basin is as follows:
Design the outlet structure to
manage flows greater than the RTF
(or greater than the design
infiltration capacity if infiltration
capacity is larger than the RTF).
Figure 5.2-2. Infiltration Basin with Outlet Structure
Source: HDR
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5.2 Infiltration Basin
The lowest weir or orifice elevation should be above the water surface elevation associated
with the RRV so that the infiltration process is not bypassed.
The structure must be designed so that outflow velocities are non-erosive.
7. Embankment and Overflow Spillway
Infiltration basins are typically constructed with an overflow spillway designed to safely convey
excess flows through the facility. Design guidance for the overflow spillway and embankment is as
follows:
If the embankment falls under the jurisdiction of Montana DNRC, it must be designed to meet
the applicable requirements (Table 1-2).
Embankment soils should be compacted as determined by a licensed engineer.
Slopes that are 4:1 (H:V) or flatter are preferred to allow for conventional maintenance
equipment and for improved safety, maintenance, and aesthetics.
Locate the overflow spillway at a point where waters can best be conveyed downstream.
Design spillway structures and associated freeboard in accordance with applicable state or
local regulations.
In accordance with the local jurisdiction’s design standards, materials such as concrete,
riprap, or articulated concrete block mats may be necessary to mitigate the potential for
erosion and failure of the spillway during less frequent events.
8. Maintenance Access
Consideration of maintenance access during the design phase of an infiltration basin is critical
because it will facilitate long-term performance of the facility. Guidelines for the design of
maintenance access are as follows:
Provide appropriate maintenance access to the pretreatment facility, infiltration cell, basin
bottom, and outlet structure if one is present. For larger basins, this typically means
stabilized access designed to withstand the expected loads from maintenance vehicles.
Stabilized access typically includes materials such as concrete, articulated concrete block,
concrete grid pavement, or reinforced grass pavement.
If stabilized access is not provided, a maintenance plan that provides detail, including
recommended equipment, on how trash and debris will be removed from the basin may be
required by the local jurisdiction.
9. Guidelines for Incorporating Flood Control
Infiltration basins can be designed to provide flood control by increasing the basin volume for flood
detention storage and designing the outlet structure to detain and release flood flows. Appropriate
flood control design guidance and local regulations should be referenced when incorporating flood
control into an infiltration basin.
5.2.5 Vegetation Considerations
Vegetation is crucial because it provides erosion control and enhances site stability. Developing a
landscaping plan for the infiltration basin and surrounding area is required to indicate how the
infiltration basin will be stabilized and established with vegetation. Considerations when developing
the vegetation and landscaping plan are as follows:
Keep adjacent vegetation from forming an overhead canopy above the infiltration basin. This
prevents litter, fruits, and other vegetative material from clogging the facility (16).
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5.2 Infiltration Basin
The use of sod is not recommended over the infiltration cell bottom. For designs that call for
a grass cover over the infiltration cell bottom, seeding and use of biodegradable erosion
control matting are recommended (17).
Stabilize and plant the interior of the infiltration basin as well as the surrounding
embankments, spoil areas, borrow areas, and other disturbed areas using native plant
species. Without healthy vegetation, the surface soil pores will quickly clog.
Salt-resistant vegetation should be used in locations where adjacent salt application is
probable, such as roadsides and parking lots.
Schedule planting and seeding activities during optimal growing seasons.
Determine the location and type of irrigation facilities, if necessary. Where possible, place
irrigation heads outside the infiltration cell bottom because irrigation heads can become
buried with sediment over time.
The infiltration basin should not be operated until vegetation is established.
Note: Given the wide range of native vegetation across Montana, designers should consult local
specialists, landscape architects, and/or agencies for recommendations on appropriate plant
species and landscaping considerations for the site.
5.2.6 Construction Considerations
Basic construction considerations and guidelines are provided below.
Construction Site Management
Acquire all applicable permits prior to construction. See Section 1.4 for more information.
Apply appropriate erosion control measures to minimize erosion during construction.
Siting a construction storm water BMP within the location of an infiltration basin is
discouraged; however, if this approach is used, excavation for the construction storm water
BMP should be at least 2 feet above the final design elevation of the infiltration cell bottom.
Conduct the initial excavation to within 1 foot of the final elevation of the infiltration cell
bottom. Defer the final excavation to the finished grade until all disturbed areas within the
contributing drainage area are stabilized or protected with BMPs. The final phase of
excavation should remove all accumulated sediment.
Contributing drainage areas should be properly stabilized with the appropriate erosion and
sediment controls or permanent seeding before allowing storm water runoff to drain to the
infiltration basin.
To avoid excessive compaction, prevent construction equipment and vehicles from traveling
over the proposed location of the infiltration cell. Excavation and construction of the
infiltration cell should be performed using equipment placed outside the limits of the
infiltration cell.
If compaction occurs during construction, consider tilling the infiltration cell to a depth of at
least 18 inches below subgrade. This technique has been shown to increase infiltration and
reduce compaction from construction activities (16).
Post-construction testing may be required by the local jurisdiction to ensure that the design
drain time has been achieved.
Construction Inspection
Inspections are recommended during the following phases of construction:
o Pre-construction meeting
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5.2 Infiltration Basin
o Initial site preparation
o Excavation/grading
o Implementation of the vegetation and landscaping plan
o Final inspection
Inspectors should be familiar with project plans and specifications to ensure the contractor’s
interpretation of the plans are consistent with the designer’s intent. The inspectors should
take frequent photos and notes of construction activities and features as work progresses
and at all critical points during the construction process. Check dimensions and depths of all
installed materials, and all materials and products should be verified or tested for
conformance with the specifications (16).
Transition to Post-Construction
Impervious area construction should be completed and pervious areas should be established
with vegetation prior to introduction of storm water into an infiltration basin.
Coordinate with the local jurisdiction prior to terminating coverage of the Construction
General Permit.
5.2.7 Maintenance
Maintenance is required on all BMPs. Recommended maintenance activities are provided in Table
5.2-1, which may be used as a guide when developing a maintenance plan. Additionally, an example
inspection form is provided in Appendix F that may be adapted or adopted as part of the
maintenance plan.
Table 5.2-1. Recommended Maintenance Activities for an Infiltration Basin
Activity Frequency
Remove litter/debris from all components of the infiltration basin.
Repair structural components including inlets, diversion structures, and outlet structure (if
applicable).
Inspect the basin for signs of erosion and repair eroded areas accordingly. Perform spot-
reseeding if necessary.
Observe drain time following rainfall events to determine if the facility is clogged. If the
observed drain time is longer than the local jurisdiction’s allowable maximum drain time,
corrective action must be taken to return the infiltration basin to the design drain time.
Regularly manage all vegetation associated with the infiltration basin and remove all clippings.
Repair maintenance access routes, if applicable.
As needed
Trim vegetation for aesthetics and to prevent the establishment of woody vegetation that may
drop leaf litter, fruits, and other vegetative material that may clog the facility.
Remove all green waste and dispose of properly to prevent clogging.
Semiannually
Inspect all components of the infiltration basin in accordance with an approved inspection
form in accordance with local jurisdiction requirements. An example inspection form is
provided in Appendix F.
Remove sediment from inlets, pretreatment facilities, diversion structures, and overflow
structures (if applicable).
Annually
5.2.8 Considerations for Use of Subsurface Infiltration Facilities
The guidance provided within the subsections above is specific to above ground infiltration basins;
however, subsurface infiltration facilities may also be designed and constructed to achieve the Post-
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5.2 Infiltration Basin
Construction Performance Standard. A subsurface infiltration facility stores storm water runoff
beneath the ground surface and slowly releases the runoff through the underlying, uncompacted
soil. Examples of these facilities include a vault, large-diameter perforated pipes, and/or boulder pits.
While subsurface infiltration facilities have similar performance capabilities and site selection
constraints, there are design, construction, and maintenance criteria that differ from an above
ground infiltration basin. Subsurface infiltration facilities are common throughout certain areas of
Montana; however, designers should consult the local jurisdiction to determine whether they are
allowed. Consultation with the local jurisdiction and use of appropriate guidance materials should be
adhered to for the design and implementation of underground infiltration facilities. Basic
considerations for subsurface infiltration facilities are discussed as follows (18).
Pretreatment for Subsurface Infiltration Basins
Pretreatment is recommended for all subsurface infiltration facilities to prevent clogging.
Contact the local jurisdiction to discuss acceptable pretreatment BMPs.
Component Requirements for Subsurface Infiltration Basins
Filter fabric is recommended along the top and sides of a subsurface infiltration facility to
prevent the migration of fine particles from the surrounding soil, unless the basin is enclosed
in an impermeable structural housing. Filter fabric should not be used along the bottom of the
facility because it may result in a reduced infiltration rate.
Any aggregate used in a subsurface infiltration facility must be free from debris, silt, or other
material that could contribute to clogging.
Access Requirements
Where applicable, at least one inspection access point that extends into the facility should be
provided to monitor functionality. The location of the inspection point must be shown in the
maintenance plan. Additionally, the maximum design volume depth should be marked on the
structure and its level included in the design report and maintenance plan.
All points of access must also be covered to prevent sediment or other material from entering
the facility and to prevent the accumulation of standing water, which could lead to mosquito
breeding.
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5.2 Infiltration Basin
5.2.9 Plan View and Typical Details
Figure 5.2-3. Infiltration Basin Plan View and Typical Section
Source: Adapted from Minnesota Stormwater Manual (19)
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5.3 Bioretention
5.3 Bioretention
Figure 5.3-1. Bioretention Area
Source: Courtesy of the City of Bozeman
Description
Bioretention areas are shallow, landscaped
depressions that capture and infiltrate or filter storm
water runoff through plants, an engineered soil media,
and often an underdrain.
Primary Components Primary Function
Inlet
Pretreatment
Surface ponding area
Bioretention soil media
Bioretention plants
Underdrain (optional)
Runoff reduction
Runoff treatment
Benefits Limitations
Siting is generally not limited by native soils;
design accommodations can be made for most
soil types
Dimensions are flexible, allowing this BMP to fit
various site conditions
Good retrofit capability
Not recommended for contributing drainage
basins greater than 2.5 acres
Not recommended in developing or erosive
watersheds given the potential for high sediment
loads that can clog the BMP
Not recommended for sites with steep slopes
Design and Site Selection Considerations
Setbacks
Depth to groundwater or bedrock
Soil permeability
Soil preparation/amendments/compost
Pretreatment forebay
Inlet and outlet spacing
Energy dissipater/level spreader
Underdrain (optional)
Facility liners (optional)
Landscaping/planting
Fencing
Size of contributing drainage area
Area required
Incorporate flood control
TMDL Considerations1 Maintenance Requirements
Avoid Preferred
Total suspended solids (TSS)
Total phosphorus
Total nitrogen
Temperature
Metals
Fecal coliform
Access roads or pullouts
Sediment removal
Irrigation, if applicable
Vegetation management
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5.3 Bioretention
5.3.1 Description
Bioretention facilities are vegetated, shallow landscaped areas that capture and temporarily store
storm water runoff. Runoff is directed into the bioretention area and then treated by the interaction of
plants, engineered soil media, and microorganisms. The treated runoff is either infiltrated or returned
to the down gradient conveyance system via an underdrain.
The primary characteristics of bioretention areas are as follows:
Bioretention areas usually consist of a pretreatment facility, surface ponding area, surface
cover, bioretention soil media, optional underdrain, and overflow outlet.
The recommended maximum drawdown time for the design volume is 48 hours. Storage in
excess of 48 hours may result in both maintenance and mosquito breeding issues.
Depending on site characteristics, bioretention areas can be designed to provide runoff
reduction, runoff treatment, and/or flood control.
Note: Bioretention areas differ slightly from rain gardens in that they typically have a larger
drainage area and may also have an underdrain.
5.3.2 Performance
Runoff Reduction
When located on soils that are conducive to infiltration, bioretention areas are expected to infiltrate
100 percent of the RRV when designed, operated, and maintained as described in this manual.
Runoff Treatment
Bioretention areas are expected to achieve an 80 percent or greater removal rate of TSS from the
RTV when designed, operated, and maintained as described in this manual (20) (21).
Figure 5.3-2. Example Bioretention Configuration
Source: Courtesy of Minnesota Pollution Control Agency
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5.3 Bioretention
5.3.3 Site Selection
Basic guidelines are provided below to aid in evaluating whether bioretention areas are feasible for
use at an individual site.
Contributing Drainage Area
Bioretention areas are best suited for sites with a contributing drainage area of 2.5 acres or
less. Smaller practices such as rain gardens typically have a contributing drainage area of
0.5 to 1.0 acre (22).
Bioretention requires a stable watershed that is not subject to high levels of erosion.
Pretreatment is required when the watershed includes phased construction, sparsely
vegetated areas, or steep slopes in sandy soils (23).
Bioretention areas should not receive continuous dry-weather flow, excessive irrigation
water, or other non-storm water flows (22).
Soil Characteristics
The site’s soil characteristics do not typically limit the use of bioretention; however, soil
characteristics must be used to determine whether an underdrain system is needed. At a
minimum, underdrain systems are required when the bioretention area is used for treating
runoff from storm water hotspots or located above contaminated groundwater and/or soils.
An underdrain system is recommended if the bioretention area is located on soils that cannot
infiltrate the design volume within 48 hours. When calculating drawdown time for the design
volume, divide the field-tested soil infiltration rate by 2 as a factor of safety to account for
potential compaction during construction and to approximate long-term infiltration rates.
In locations where potentially expansive soils or bedrock exist, placement of a bioretention
area adjacent to structures and pavement should be considered only if the BMP includes an
underdrain and an impermeable liner designed to restrict infiltration (23).
Soil characteristics can initially be estimated from NRCS soil data but must be field-verified
prior to final design using the onsite soil investigation methods discussed in Chapter 2 and
Appendix C of this manual.
Depth to Groundwater and/or Bedrock
Soil acts as a filter for pollutants between the bottom of the facility and the underlying
groundwater; therefore, a minimum vertical distance of 3 feet is recommended between the
bottom of the facility and the seasonally high groundwater table or bedrock for full and partial
infiltration sections (24).
For systems with an impermeable liner and underdrain system, a minimum vertical distance
of 1 foot is recommended between the bottom of the facility and the seasonally high
groundwater table or bedrock.
An evaluation of the depth to groundwater should be conducted as described in Section
4.3.3.
Site Topography
Bioretention areas should be located on relatively flat terrain, and the grade immediately
adjacent to the basin (within 15 to 20 feet) should be between 1 and 5 percent to promote
drainage while limiting the potential for erosion (22).
If steep grades are present, bioretention areas should be split into multiple cells throughout a
project site.
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5.3 Bioretention
Bioretention areas with an underdrain are constrained by the invert elevation of the existing
conveyance system to which the practice discharges (i.e., the bioretention area underdrain
must connect to the storm drain or down-gradient natural conveyance system). In general, 4
to 5 feet of elevation change between the top of natural ground and the outlet invert is
typically needed for bioretention areas with an underdrain (22).
Land Use and Characteristics of Surrounding Area
Bioretention can be easily integrated into areas such as parking lot islands, roadway
medians, and right-of-ways along roads because it is not limited to a specific shape.
Runoff from hotspot areas should not be treated with infiltrating bioretention areas. An
impermeable liner with an underdrain is required when treating runoff from hotspot areas.
Interference with underground or overhead utilities should be avoided whenever possible.
Consult applicable utility companies or agencies for site-specific requirements prior to
implementing bioretention areas (25).
When bioretention areas are located adjacent to buildings or pavement areas, protective
measures should be implemented to avoid adverse impacts to these structures.
Oversaturated subgrade soil underlying a structure can cause the structure to settle or result
in moisture-related problems. Wetting of expansive soils or bedrock can cause swelling,
resulting in structural movements. A geotechnical engineer should evaluate the BMP’s
potential impact on adjacent structures based on an evaluation of the subgrade soil,
groundwater, and bedrock conditions at the site (23).
Consider minimum setback requirements, as discussed in Section 4.3.4.
Community and Environmental Considerations
Bioretention areas can be an urban aesthetic feature when installed in locations such as
parking lot islands, street medians, and landscaped areas between roads and sidewalks.
Figure 5.3-3. Bioretention Area within Roadway Median
Source: HDR
Bioretention areas can be designed as an urban aesthetic feature shaped to fit the surrounding area.
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5.3 Bioretention
5.3.4 Design and Sizing Procedure
The following discussion provides design criteria and procedures for the various components of
bioretention areas. Annual precipitation across Montana can vary considerably; therefore, the
character of bioretention areas will vary across the state. Some or all of the components may be
used for a given application, depending on the site characteristics, restrictions, pollutant loading, and
design objectives.
1. Full Infiltration, Partial Infiltration, or
No Infiltration Section
Bioretention areas can be classified into three
different types of sections, depending on
whether an underdrain is used. The section
used will depend on site characteristics such as
land use, proximity to adjacent structures, and
soil characteristics. The three section types are
described as follows:
Full Infiltration Section: This section
does not have an underdrain and,
therefore, infiltrates all water captured by
the facility into the native subsurface
soils. Full infiltration sections should be
used when the native subsurface soils
have the ability to infiltrate the design
volume and infiltration will not adversely
affect the environment or adjacent
structures.
Partial Infiltration Section: This section
uses both infiltration and an underdrain
to discharge treated runoff from the
BMP. A partial infiltration section does
not include an impermeable liner. Any
storm water that does not infiltrate into
the native subsurface soils will be
discharged to the downstream
conveyance system via an underdrain.
This type of section should be used
when native subsurface soils do not
have the ability to infiltrate 100 percent of the design runoff volume and when infiltration will
not adversely affect the environment or adjacent structures.
No Infiltration Section: This section uses an underdrain to discharge 100 percent of the
runoff that has been captured and stored in the BMP. The section also includes an
impermeable liner to prevent infiltration. A no infiltration section should be used when
infiltration has the potential to adversely affect the environment or adjacent structures (for
example, when treating runoff from a storm water hotspot).
Bioretention Minimum Design Criteria
1
Required Components
Inlet structure(s)
Pretreatment
Surface ponding area (filter area)
Surface cover
Bioretention soil media
Overflow outlet structure (for online facilities)
Design and Sizing
General
Basin storage volume sized for 100% of RRV or
RTV (minimum)
Bioretention soil media with minimum depth of
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5.3 Bioretention
2. Basin Storage Volume
The volume of runoff that can be stored and managed within a bioretention facility is a combination
of the surface ponding volume, bioretention soil media storage volume, and underdrain aggregate
layer storage volume (if applicable). Calculate the minimum required basin storage volume using the
following guidance:
Full Infiltration Section: Design the bioretention storage volume to be 100 percent of the
RRV. Calculate the RRV using the guidance provided in Section 3.2.
No Infiltration Section or Partial Infiltration Section: Design the bioretention storage
volume to be 100 percent of the RTV. Calculate the RTV using the guidance provided in
Section 3.2.
Note: The design volume must be greater than or equal to the RRV or RTV if there is only one
BMP in the contributing drainage area. Where multiple BMPs are used as part of a treatment train,
the design volume may be only part of the overall RRV for the drainage area, with the sum of each
BMP’s design volume equaling or exceeding the RRV.
The following discussion provides guidance for sizing a bioretention facility by calculating the
available runoff storage volume of the surface ponding area, bioretention soil media, and underdrain
aggregate layer (if applicable). The sum of the available storage volume of each of these
components of the facility will be equal to the total available storage volume.
Surface Ponding Area
The surface ponding area is the area where runoff is captured and stored before it begins to infiltrate
into the underlying bioretention soil media. When designing the surface ponding area, provide an
area and depth that offers adequate volume to store a portion of the design volume and to allow it to
begin filtering through the bioretention soil media. It is recommended that the surface ponding
volume account for at least 50 percent of the total basin storage volume. Calculate the surface
ponding area volume using Equation 5.3-1.
SPv
=SAp
dp
Equation 5.3-1
Where:
SPv = Surface ponding area volume (ft3)
SAp = Average ponding surface area, calculate using Equation 5.3-2 (ft2)
=
(SA at top of ponding area)(SA at bottom of ponding area)
2
Equation 5.3-2
dp = Design ponding depth (ft)
Bioretention Soil Media
The bioretention soil media provides additional storage volume within the void spaces, referred to as
porosity (η), to manage the design volume. Calculate the bioretention soil media storage volume
using Equation 5.3-3.
Note: The minimum surface ponding volume requirement is based on the need to capture runoff
from the 0.5-inch rainfall event from a full range of expected storm intensities. During high-intensity
storm events, the surface ponding area may fill up faster than the collected storm water runoff is
able to filter through the soil media. In addition, the infiltration rate of the bioretention soil media will
vary over the maintenance life-cycle of the practice. Therefore, an adequate surface ponding
volume is necessary to allow the runoff to begin to filter into the bioretention soil media before the
runoff bypasses the BMP or overflows the surface ponding area (22).
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5.3 Bioretention
BSMv
=SAb
dBSM
η
BSM
Equation 5.3-3
Where:
BSMv = Bioretention soil media storage volume (ft3)
SAb = Bottom surface area of the bioretention soil media and aggregate layer (ft2)
dBSM = Depth of the bioretention soil media (ft)
ηBSM = Effective porosity of the bioretention soil media (typically 0.25)
Underdrain Aggregate Layer Storage
The underdrain aggregate layer is only applicable for partial infiltration and no infiltration sections.
When applicable, this layer provides additional storage to manage the design volume. Calculate the
underdrain aggregate layer storage volume using Equation 5.3-4.
ALv
=SAb
dAL
η
AL
Equation 5.3-4
Where:
ALv = Aggregate layer storage volume (ft3)
SAb = Bottom surface area of the bioretention soil media and aggregate layer (ft2)
dAL = Depth of the aggregate layer (ft)
ηAL = Effective porosity of the aggregate layer (typically 0.40)
Verify Total Design Volume
Verify that the combination of the surface ponding area volume, bioretention soil media storage
volume, and underdrain aggregate layer storage volume (if applicable) is greater than or equal to the
total basin storage design volume. The recommended porosity values for the storage components
are listed below.
Surface ponding area (ηp) = 1.0
Bioretention soil media (ηBSM) = 0.25
Underdrain aggregate (ηAL) = 0.40
Dv
=SAb[(dBSM*η
BSM)+(dAL
*η
AL)]+(SAp
*dp) Equation 5.3-5
Figure 5.3-4. Bioretention Section with Typical Porosity Values
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5.3 Bioretention
3. Surface Ponding Area Geometry
The surface ponding area, located above the bioretention soil media as shown in Figure 5.3-4, is the
portion of the facility that collects and temporarily stores runoff. Guidance for the surface ponding
area geometry and design is as follows:
Select a ponding depth between 6 to 12 inches. Consider the local jurisdiction’s fencing
requirements associated with ponding areas for storm water management.
The bottom of the surface ponding area (filter area) should be flat to enable even distribution
and infiltration of storm water runoff.
The recommended minimum bottom width of the surface ponding area is 2 feet.
Provide freeboard above the top of the design ponding elevation. The recommended
minimum freeboard is 6 inches or as specified by the local jurisdiction.
The side slopes of the surface ponding area should be 3:1 (H:V) or flatter; however, in highly
urbanized or space-constrained areas, concrete vertical sidewalls may be used to conserve
space. When using vertical sidewalls, the vertical distance from the top of the side wall to the
bottom of the ponding area should not exceed 12 inches.
4. Inlet and Conveyance
Inlet and conveyance considerations for
design of bioretention areas include online
vs. offline facilities, available inlet
configurations, and surface overflows.
Guidance for each of these considerations
is discussed as follows:
Online systems operate such that
all runoff from the drainage area
flows into the bioretention area.
Flows that exceed the design
capacity flow through the facility
and exit through an overflow
structure or weir without being
treated.
Offline systems operate such that
flow is split or diverted so that only
the design flow enters the
bioretention area and larger flows
bypass the facility. Offline
bioretention areas are often
designed by establishing a
maximum ponding depth—at which
point higher flows bypass the
facility. Curb cuts along a roadside
or parking lot area are commonly
used as inlets for offline facilities
(see Figure 5.3-6).
Offline facilities are typically
preferred over online facilities,
especially for contributing drainage
Figure 5.3-5. Online Bioretention Area with Overflow Outlet
Structure
Source: HDR
Figure 5.3-6. Offline Bioretention Area
Source: Courtesy of the City of Spokane
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5.3 Bioretention
basins greater than 0.5 acre. Larger drainage basins can generate flows that overwhelm or
damage bioretention areas for online facilities (22).
The inlet of a facility will depend on characteristics of the bioretention area and drainage
basin, such as topography, land use, flow velocities, and design volume. Suitable types of
flow entrances range from sheet flow across paved or landscaped areas to curb cuts to
concentrated piped flow entrances.
Overflow conveyance systems are necessary for all bioretention facilities to safely convey
flows that exceed the facility’s design capacity. Surface overflow systems may include
vertical catch basins or stand pipes connected to underdrain systems, horizontal drainage
pipes, armored overflow channels, or curb cuts at the down-gradient end of the area to direct
overflows back to the street (26).
Bioretention areas must be designed with an internal flow path such that the treatment
mechanisms are not bypassed or short-circuited. Travel time from each inlet to the outlet
should be maximized by locating the inlets and outlets as far apart as possible, and incoming
flow must be distributed as evenly as possible across the entire filter surface area (22).
5. Pretreatment Facilities
Pretreatment facilities are recommended to reduce the maintenance cycle by reducing trash and
sediment accumulation within the filter area. The type of pretreatment facility will vary depending on
the drainage area characteristics and inlet configuration; however, common pretreatment facility
options include a gravel diaphragm, grass filter strip, vegetated swale, pretreatment cell, and
proprietary treatment devices. Guidance for pretreatment facility design options is as follows:
Gravel or stone diaphragm (sheet flow): A gravel diaphragm is a 1- to 2-foot-wide strip of
gravel located at the edge of a road or parking lot to provide pretreatment for sheet flow from
a contributing drainage area. The gravel diaphragm should be oriented perpendicular to the
sheet flow path with a 2- to 4-inch drop from the edge of pavement to the top of the stone.
Size the stone to dissipate flows and prevent erosion based on the expected rate of
discharge.
Grass filter strip (sheet flow): A grass filter strip is a gently sloped vegetated area that
provides pretreatment for sheet flow from a road or parking lot. Grass filter strips should be
oriented perpendicular to the sheet flow path with a 2- to 3-inch drop from the edge of
pavement to the top of the grass. The filter strip should extend from below the edge of
pavement to the bottom of the surface ponding area at a 5:1 slope or flatter.
Vegetated swale (concentrated flow): A vegetated swale is a broad and shallow vegetated
channel which is similar to a biofiltration swale. Vegetated swales can be used to provide
pretreatment for runoff from concentrated flows, such as piped or curb cut inlets.
Pretreatment cell (concentrated flow): Similar to a forebay, a pretreatment cell is located at
piped inlets or curb cuts leading to the bioretention area and consists of an energy dissipater
sized for the design flow rates. A storage volume equivalent to at least 15 percent of the total
design volume with a 2:1 length-to-width ratio is recommended. The cell may be formed by a
wooden or stone check dam or an earthen or rock berm. Pretreatment cells do not need
underlying engineered soil media.
Note: Designers must include provisions for safe conveyance of larger flows either contained
within properly sized pipe or channel systems or as overland flood routing to a receiving
waterbody so as to minimize public safety risks and property damage (22).
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5.3 Bioretention
Proprietary treatment device (concentrated flow): If allowed by the local jurisdiction, a
proprietary treatment device may be used to provide pretreatment (22).
6. Surface Cover
The surface cover for bioretention areas is variable and depends on the landscape context (for
example, highly visible versus less visible, routine mowing versus managed landscape). The choice
of surface cover will also influence the complexity and frequency of long-term maintenance activities.
Surface cover options are provided as follows:
Mulch: A 2- to 3-inch layer of mulch on the surface of the filter area enhances plant survival,
suppresses weed growth, regulates soil temperatures, and provides pretreatment before
runoff reaches the bioretention soil
media. Shredded hardwood bark
mulch, aged for at least 6 months, is
recommended because it retains a
significant amount of pollutants and
typically will not float.
Alternative to Mulch Cover:
Alternative surface covers include
turf grass, native groundcover,
erosion control matting (coir or jute
matting), river stone, or pea gravel.
The surface cover selected for use
must be able to support plant growth
for the type of vegetation that will be
used in the facility. Stone or gravel
are not recommended in parking lot
applications because they increase
soil temperature and have low
water-holding capacity (22).
7. Bioretention Soil Media
The bioretention soil media is an engineered soil mixture that is essential to the performance of a
bioretention area. Located below the ponding area, the bioretention soil media is designed to
maintain long-term permeability while also providing nutrients to support plant growth. More
specifically, soil media mixes should balance the following four primary design objectives:
Provide high enough infiltration rates to meet minimum surface water drawdown time
Provide long-term infiltration rates that are not too high in order to optimize pollutant removal
capability (typically less than 6 inches per hour)
Support long-term plant and soil health
Balance nutrient availability and retention to reduce or eliminate nutrient export (27)
Soil media is typically composed of a mixture of sand and organic matter, and ranges in depth from
18 to 36 inches. The mixture will vary depending on local climate characteristics, material availability,
and performance objectives. Recommendations for design of soil media mixtures, also referred to as
soil amendments, are provided in Appendix D.
Figure 5.3-7. Bioretention Mulch Surface Cover
Source: HDR
Mulch surface cover enhances plant survival, suppresses
weed growth, regulates soil temperatures, and provides
pretreatment.
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5.3 Bioretention
8. Underdrain System (If Applicable)
An underdrain system is required for partial infiltration and no infiltration sections in order to drain the
bioretention area after storm water runoff has been treated. An underdrain system consists of an
aggregate choking layer, a slotted pipe that conveys treated runoff out of the bioretention area, and
an aggregate layer that reduces clogging of the underdrain. Design guidance for the underdrain pipe
and aggregate layer is as follows.
Choking Layer
A choking layer is a layer of choker stone placed between the bioretention soil media and above the
underdrain aggregate layer to prevent the media from migrating into the aggregate layer. The
recommended choking layer thickness is 2 inches, composed of ASTM No. 8 or No. 89 washed
gravel.
Underdrain Pipe
Underdrains should be slotted plastic pipe. The openings should be smaller than the smallest
aggregate gradation for the underdrain layer aggregate to prevent migration of material into the drain
and clogging. This configuration also allows for pressurized water cleaning and root cutting if
necessary. Additional underdrain pipe recommendations are as follows:
Use slotted subsurface drain polyvinyl chloride (PVC) pipe in accordance with ASTM D1785-
12 SCH 40. Pipe diameters may range from 4 to 8 inches, depending on the required
hydraulic capacity.
Slots should be cut perpendicular to the long axis of the pipe and be 0.04 to 0.069 inch by 1
inch long and be spaced ¼ inch apart (spaced longitudinally). Slots should be arranged in
two rows spaced on 45-degree centers and cover one half of the circumference of the pipe.
Perforated PVC or flexible slotted high-density polyethylene (HDPE) pipe are not
recommended because they cannot be cleaned with pressurized water or root cutting
equipment and are less durable.
The underdrain can be installed with slots oriented on the top or bottom of the pipe.
Underdrains should be sloped at a minimum of 0.5 percent and spaced a maximum of 20
feet on center.
Design the underdrain system to drain the design volume within 48 hours.
Provide at least one cleanout to enable maintenance and observation of infiltration rates over
the life of the facility. For pipe lengths greater than 100 feet, two cleanouts are recommended
(one on each end of the pipe).
Wrapping the underdrain pipe in a filter fabric is not recommended because this has been
shown to increase the potential for clogging (26).
Underdrain Aggregate Layer
An aggregate filter layer buffers the underdrain system from sediment input and clogging. The
underdrain system should be placed within a 6-inch-thick section of washed ASTM No. 57 stone or
similar aggregate filter material.
Orifice and Other Flow Control Structures
An orifice flow control structure may be used to regulate flows discharging from the underdrain
system. A minimum orifice diameter of 3/8 inch is recommended to avoid clogging (23).
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5.3 Bioretention
9. Impermeable Liner (if Applicable)
An impermeable liner is required for no infiltration sections to prevent storm water runoff from
infiltrating into the underlying soils. Design guidance for impermeable liners is provided as follows:
Install a 30 mil (minimum) PVC geomembrane liner on the bottom and sides of the basin,
extending up at least to the top of the underdrain layer.
Provide at least 9 inches (12 inches if possible) of cover over the membrane where it is
attached to the wall to protect the membrane from UV deterioration (23).
When high groundwater is anticipated, the design should consider the potential effect that
buoyancy forces may have on the facility.
10. Guidelines for Incorporating Flood Control
Bioretention areas can be designed to provide flood control by increasing the basin storage volume
for flood detention storage and designing the outlet structure (if applicable) to detain and release
flood flows. Basin storage volume may be increased by expanding the surface ponding footprint
and/or by incorporating additional subsurface storage within the underdrain aggregate layer.
Appropriate flood control design guidance and local regulations should be referenced when
incorporating flood control into an infiltration basin.
5.3.5 Vegetation Considerations
Vegetation is crucial because it provides treatment and enhances stability of the bioretention facility.
Developing a landscaping plan for the bioretention area is required in order to indicate how the
facility will be stabilized, established with vegetation, and maintained. The landscaping plan should
include information such as area delineations, plant
lists and quantities, handling instructions, planting
sequence, and plant-specific maintenance
requirements (22). Additional considerations when
developing the vegetation and landscaping plan
are as follows:
Consider the level of maintenance that will
be associated with the vegetation selected
for the bioretention area and whether the
facility’s owner/operator is willing and able
to conduct the required maintenance.
For sites where less technical maintenance
is desired, consider using managed turf
grass for vegetation. Seeded turf grass is
preferred because sod may reduce the
facility’s infiltration capabilities.
In general, plants should tolerate summer
drought, ponding fluctuations, and
saturated soil conditions for lengths of time
anticipated according to the facility’s design
(26).
If the bioretention area will be used for
snow storage or to treat runoff from a
surface where salt is used as a deicer, the
Figure 5.3-8. Bioretention Vegetation
Source: HDR
Consider whether the project owner/operator is able to
conduct the required maintenance when selecting
vegetation.
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5.3 Bioretention
area should be planted with salt-tolerant, non-woody plant species (26).
Consider nearby infrastructure, underground utilities, and whether an impermeable liner will
be used when selecting vegetation. When using an impermeable liner, select plants with
diffuse (or fibrous) root systems, not taproots. Taproots can damage the liner and/or
underdrain pipe (23).
Trees should not be planted above an underdrain, but should be located closer to the
perimeter of the facility (22).
Select plants that will tolerate the expected pollutants and pollutant loadings from the
contributing drainage area (26).
Schedule planting and seeding activities during optimal growing seasons.
Provide a plan to address weed control, especially within the first 2 to 3 years during the
vegetation establishment period.
Irrigation systems will likely be necessary to establish vegetation. These systems can be
temporary or permanent depending on the type of vegetation to be used. Place irrigation
heads outside the filter area because irrigation heads can become buried over time.
When pedestrian traffic through bioretention areas is anticipated, consider incorporating
elevated pathways to prevent vegetation damage. Where necessary, provide pipes through
elevated berms to allow flows from one cell to another (26).
Note: Given the wide range of native vegetation across Montana, designers should consult local
specialists, landscape architects, and/or agencies for recommendations on appropriate plant
species and landscaping considerations for the site.
5.3.6 Construction Considerations
Basic construction considerations and guidelines are provided below.
Construction Site Management
Acquire all applicable permits prior to construction. See Section 1.4 for more information.
Construction on bioretention areas is not recommended until the entire contributing drainage
area is stabilized. If this is not feasible, apply appropriate erosion control measures to
minimize erosion during construction
and protect the bioretention area from
sediment loading during construction.
Using bioretention areas for
construction storm water management
controls is not recommended. However,
if bioretention areas are used for this
purpose, notes and graphical details
should specify that the maximum
excavation depth at the construction
stage should be at least 1 foot above
the post-construction installation and
that the facility must contain an
underdrain (25).
Onsite soil mixing or placement is not
recommended when the bioretention
Figure 5.3-9. Inlet Protection
Source: Courtesy of the City of Missoula
Protect the bioretention area from runoff until the BMP
construction is finalized and the contributing drainage area
is stabilized.
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5.3 Bioretention
soil media or subgrade soil is saturated (26).
It is recommended that the bioretention soil media be compacted to between 80 to
85 percent of modified maximum dry density. Optimal compaction of the bioretention soil
media mix can be achieved by placing the soils in 6-inch lifts and boot packing the soils
between lifts.
When using an impermeable liner, ensure enough slack in the liner to allow for backfill,
compaction, and settling without tearing the liner. Provide quality assurance and quality
control to ensure the liner will perform as designed (e.g., inspect for tears, inspect seams,
conduct tests as specified by the manufacturer) (23).
Care should be taken to not over compact the soils, which can reduce their permeability. To
avoid excessive compaction, prevent construction equipment and vehicles from traveling
over the proposed location of the filter area. Excavation and construction of the filter area
should be performed using equipment placed outside of the limits of the filter area.
If compaction of subgrade soils occurs, it may be necessary to till the bottom soils to a depth
of 6 to 12 inches to promote greater infiltration rates.
Extreme care is required during construction to ensure that the design grades and drainage
patterns are implemented. This is especially important for offline facilities with inlets such as
curb cuts, where incorrect final grading can result in runoff bypassing the bioretention area.
Refer to the local jurisdiction’s construction site storm water management program for
additional guidance and local requirements.
Construction Inspection
Inspections are recommended during the following phases of construction:
o Pre-construction meeting
o Initial site preparation
o Excavation/grading
o Installation of the bioretention soil media
o Implementation of the vegetation and landscaping plan
o Final inspection
Inspectors should be familiar with project plans and specifications to ensure the contractor’s
interpretation of the plans are consistent with the designer’s intent. The inspectors should
take frequent photos and notes of construction activities and features as work progresses
and at all critical points during the construction process (such as immediately prior to
backfilling). Check dimensions and depths of all installed materials, and all materials and
products should be verified or tested for conformance with the specifications (24).
Transition to Post-Construction
Develop a plan prior to construction that will allow for an effective transition from construction
storm water management BMPs to post-construction BMPs without compromising the
integrity of the post-construction BMPs.
Coordinate with the local jurisdiction prior to terminating coverage of the Construction
General Permit.
5.3.7 Maintenance
Bioretention areas require consistent vegetation, soil, and surface cover maintenance to ensure
optimum performance. Where applicable, maintenance of bioretention areas can be integrated into
routine landscape maintenance tasks. If landscaping contractors will be expected to perform
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5.3 Bioretention
maintenance, their contracts should contain specifics on the unique bioretention landscaping needs,
such as maintaining elevation differences needed for ponding, proper mulching, sediment and trash
removal, and limited use of fertilizers and pesticides (22).
Frequent and well-timed maintenance (e.g., weeding prior to seed dispersal) is especially critical
during the first 2 to 3 years while vegetation is being established. A portion of the plant stock may die
off in the first year, so consider including a care and replacement warranty in construction contracts
to ensure that vegetation is properly established and survives during the first growing season
following construction.
Recommended maintenance activities are provided in Table 5.3-1, which may be used as a guide
when developing a maintenance plan. Additionally, an example inspection form is provided in
Appendix F that may be adapted or adopted as part of the maintenance plan.
Table 5.3-1. Recommended Maintenance Activities for Bioretention Areas
Activity Frequency
Inspect the bioretention area and contributing drainage area following rainfall events. Conduct
any needed repairs or stabilization.
One-time, spot fertilization may be needed for initial plantings.
Follow the watering schedule provided by the designer because frequent watering is typically
needed to establish vegetation.
Remove and replace dead plants.
Upon
establishment
Perform spot weeding, trash removal, and mulch raking. Semiannually
during growing
season
Add reinforcement planting to maintain the desired vegetation density.
Manage all vegetation associated with the bioretention area.
Remove sediment from inflow points, pretreatment facilities, diversion structures, and overflow
structures (if applicable).
Remove any dead or diseased plants and invasive plants using recommended control
methods.
Stabilize the contributing drainage area to prevent erosion.
Observe drain time following rainfall events to determine if the facility is clogged. If the
observed drain time is longer than the local jurisdiction’s allowable maximum drain time,
corrective action must be taken to return the facility to the design drain time.
As needed
Inspect all components of the bioretention area in accordance with an approved inspection
form in accordance with local jurisdiction requirements. An example inspection form is
provided in Appendix F.
Supplement mulch where needed to maintain a 2- to 3-inch layer.
Plants can provide nutrient uptake during the growing period. Once a year, prior to the
dormant season, plants should be cut back to maintain the nutrient mass removal. If
vegetation is left to decay, it will release nutrients back into the bioretention soil media (28).
Annually
Remove sediment in pretreatment facility and inflow points.
Remove and replace the mulch layer and the top 2 to 3 inches of the bioretention soil media.
This is necessary because TSS can accumulate in the top layers of the facility and reduce
infiltration rates (21). For designs that include pretreatment upstream of the bioretention area,
the frequency with which the top layer is removed may be reduced. For bioretention soil
media mixes with a design infiltration rate of less than 2 inches per hour, the top layer should
be removed more frequently.
Once every
2 to 3 years
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5.3 Bioretention
5.3.8 Plan View and Typical Details
Figure 5.3-10. Bioretention Plan View and Typical Section (Offline with Full Infiltration Section)
Source: Adapted from Minnesota Stormwater Manual (29)
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5.3 Bioretention
Figure 5.3-11. Bioretention Partial Infiltration Typical Section
Source: Adapted from Minnesota Stormwater Manual (29)
Figure 5.3-12. Bioretention No Infiltration Typical Section
Source: Adapted from Minnesota Stormwater Manual (29)
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5.4 Permeable Pavement Systems
5.4 Permeable Pavement Systems
Figure 5.4-1. Permeable Interlocking Concrete Pavers
in Bozeman
Source: Courtesy of the City of Bozeman
Description
A pavement system with a permeable surface that
allows storm water runoff to move through surface
voids into an underlying aggregate reservoir for
temporary storage and/or infiltration.
Primary Components Primary Function
Permeable pavement
Bedding material
Base reservoir
Subbase reservoir
Soil subgrade
Runoff reduction
Runoff treatment
Benefits Limitations
Accomplishes storm water management in areas
with a different primary purpose (i.e., parking lots)
Decreases effective impervious area
Less likely to form ice on the surface when
compared with conventional pavements
Good retrofit capability
Not recommended in developing or erosive
watersheds due to potential for high sediment loads
that can clog the facility
Not recommended for sites with steep slopes
Limited to pedestrian and low-speed traffic areas
Design and Site Selection Considerations
Setbacks
Depth to groundwater or bedrock
Soil permeability
Soil preparation/amendments/compost
Pretreatment forebay
Inlet and outlet spacing
Energy dissipater/level spreader
Underdrain (optional)
Facility liners (optional)
Landscaping/planting
Fencing
Size of contributing drainage area
Area required
Incorporate flood control
TMDL Considerations1 Maintenance Requirements
Avoid Preferred
Total suspended solids (TSS)
Total phosphorus
Total nitrogen
Temperature
Metals
Fecal coliform
Access roads or pullouts
Sediment removal
Irrigation
Vegetation management
Erosion and embankment stabilization repair
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5.4 Permeable Pavement Systems
5.4.1 Description
The term permeable pavement system, as used in this manual, describes any one of several
surfaces that allow storm water runoff to filter through surface voids into an underlying aggregate
reservoir for temporary storage and/or infiltration (30). Examples of these systems include
permeable interlocking concrete pavers (PICPs), pervious concrete, and pervious asphalt.
Note: The guidance in this section focuses exclusively on PICPs because they are currently the
more common application of permeable pavement systems in Montana. A brief introduction to
additional types of permeable pavement systems is provided in Section 5.4.9. While most
permeable pavement systems have similar performance capabilities and site selection constraints,
the design, construction, and maintenance criteria differ from PICPs. Consult with the local
jurisdiction and follow appropriate guidance for the design and implementation of other types of
permeable pavement systems.
The primary characteristics of PICPs are as follows:
PICP facilities usually consist of a subbase reservoir layer, a base reservoir layer, a bedding
course layer, and concrete pavers with joint areas that are filled with small-sized aggregates
to allow infiltration of runoff.
The recommended maximum drawdown time for the design volume is 48 hours.
PICP facilities can be designed to provide both runoff reduction and flood control. Treatment
of pollutants may also be provided for systems that infiltrate into the underlying subgrade.
5.4.2 Performance
Runoff Reduction
When using a full infiltration section on soils conducive to infiltration, a PICP system is expected to
infiltrate 100 percent of the RRV when designed, operated, and maintained as described in this
manual.
Runoff Treatment
For the purposes of this manual, runoff treatment is applicable to BMPs that discharge treated runoff
to a waterbody or downstream conveyance system. Specifically, runoff treatment BMPs must be
expected to remove 80 percent TSS from storm water runoff. PICPs with an underdrain system
provide some TSS reduction; however, these systems have not consistently demonstrated
80 percent removal of TSS when used as a stand-alone BMP. The Runoff Treatment Requirement
can be met when PICPs are used as part of a treatment train with other BMPs; however, it is the
responsibility of the designer to determine performance capabilities of BMP treatment trains.
5.4.3 Site Selection
Basic guidelines are provided below to aid in evaluating whether PICPs are feasible for use at an
individual site.
Contributing Drainage Area
PICPs are best suited for sites that receive runoff only from impervious areas. Runoff from
non-impervious areas is not recommended because it may increase the potential for
clogging (31).
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If runoff from non-impervious areas drains to PICPs, the drainage area should be stable with
shallow slopes to limit the potential for sediment to drain onto the facility and clog the PICPs.
Pretreatment is recommended for sites where runoff drains from non-impervious areas.
The recommended ratio of contributing drainage area to PICP surface area is 2:1 or less.
This ratio may be increased to no greater than 5:1 if a large portion of the runoff is generated
from rooftops, where runoff tends to have low sediment contents or pretreatment BMPs are
in place to reduce the sediment content within runoff prior to entering the PICP facility (32).
Soil Characteristics
The site’s soil characteristics do not typically limit the use of PICPs; however, soil
characteristics must be used to determine whether an underdrain system is needed. At a
minimum, underdrain systems are required when the PICP facility is used for treating runoff
from storm water hotspots and located above contaminated groundwater and/or soils.
Underdrain systems are recommended when the PICP facility is located on soils that cannot
infiltrate the design volume within 48 hours.
Siting of infiltrating PICPs is not recommended above fill soils because of the potential for
reduced infiltration rates and slope stability issues. Infiltrating PICP installations should only
be placed on fill soils if laboratory tests indicate the compacted fill will be stable when
saturated and the slope stability of deep fills has been verified by a geotechnical engineer.
PICPs sited above fill soils typically require an impermeable liner and underdrain system
(32).
Soil characteristics can initially be estimated from NRCS soil data, but must be field-verified
prior to final design using the onsite soil investigation methods discussed in Chapter 2 and
Appendix C of this manual.
Depth to Groundwater and/or Bedrock
The bottom of the subbase reservoir should be a minimum of 3 feet above the seasonally
high groundwater table or bedrock (hardpan) layer. A high groundwater table may cause
seepage into the bottom of a PICP facility, and both groundwater and bedrock can prevent
complete drainage. Also, soil acts as a filter for pollutants between the bottom of the subbase
reservoir and the underlying groundwater. For systems with an impermeable liner and
underdrain system, a minimum vertical distance of 1 foot is recommended between the
bottom of the subbase reservoir and the seasonally high groundwater table (32).
An evaluation of the depth to groundwater should be conducted, as described in Section
4.3.3.
Site Topography
PICPs should be located on relatively flat areas that can be graded to maintain a finished
surface grade of between 1 and 6 percent.
Pavement slopes of at least 1 percent are recommended to evenly distribute flow and
provide an alternative means for drainage if an area becomes clogged because of lack of
maintenance (32).
Pavement slopes of less than 6 percent are recommended because steep pavement slopes
allow runoff to migrate downslope through the reservoir and pool at the lower end of the
PICP facility.
For pavement surface slopes greater than 3 percent, a terraced surface and/or terraced
subgrade is recommended to provide a more even distribution of ponded runoff within the
aggregate reservoir.
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Land Use and Considerations of Surrounding Area
PICPs are best suited for areas that receive pedestrian use and low-speed (less than 40
mph) vehicle traffic. Examples of such areas include parking lots, street parking lanes,
residential driveways, residential streets, alleyways, recreational trails, sidewalks, and patios.
To avoid adverse effects from seepage, care must be taken when siting infiltrating PICPs
near building foundations, hardscapes, or conventional pavement areas. An impermeable
liner may be necessary to prevent the PICP facility from being hydraulically connected to
nearby infrastructure.
Interference with underground utilities should be avoided whenever possible. Consult
applicable utility companies or agencies for site-specific requirements prior to implementing
PICP areas (31).
Runoff from hotspot areas should not be treated with infiltrating PICPs. An impermeable liner
with an underdrain is required when treating runoff from hotspot areas.
PICPs should not be used in areas that produce high sediment loads because such loads
may cause clogging.
Consider minimum setback requirements, as discussed in Section 4.3.4.
Community and Environmental Considerations
PICPs can be an urban aesthetic feature when installed in locations such as parking lots,
sidewalks, and patio areas.
Figure 5.4-2. PICP Parking Lot Application
Source: HDR
Parking lots can be designed to use PICPs within the parking stall areas while using conventional pavements in other
areas that receive more vehicle traffic.
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5.4.4 Design and Sizing Procedure
This section provides a general outline of the design procedure and criteria for PICP facilities. A
variety of PICP paver products are available, each of which may have unique design constraints.
Manufacturers and/or suppliers should be consulted for materials and guidelines specific to each
product. Additionally, the Interlocking Concrete Pavement Institute (ICPI) is recommended as a
reference for more information because it provides technical information on best practices for PICP
facility design, specifications, construction, and maintenance.
Design of PICP facilities is an iterative process
in which the designer must balance site
constraints, hydrologic design requirements, and
structural design requirements. This section
does not provide a step-by-step process for
PICP design, but rather is separated into three
general sections to aid in the design process:
PICP FACILITY OVERVIEW: This
subsection identifies and describes the
types of facilities and typical components
within a facility. The considerations
provided in this section can be combined
with guidance in the Facility Sizing
section to size the facility.
FACILITY SIZING: The sizing of a PICP
facility depends on a hydrologic analysis
and structural analysis because the
facility’s depth must be designed to
retain or detain the entire design runoff
volume and accommodate expected
traffic loads. This section provides
guidance on calculating the design
volume and sizing the facility for the
design volume, and provides structural
analysis considerations.
ADDITIONAL DESIGN COMPONENTS: After the facility has been sized, additional
components such as a perimeter barrier and observation wells can be designed. In some
cases, the additional components are integral to the facility sizing (i.e., underdrains);
therefore, guidance in this section may need to be considered when sizing the facility.
PICP FACILITY OVERVIEW
Full Infiltration, Partial Infiltration, or No Infiltration Section
PICP facilities can be classified into three different types of sections, depending on whether an
underdrain is used. The section used will depend on site characteristics such as land use, proximity
to adjacent structures, and soil characteristics. The three section types are described as follows:
Full Infiltration Section: This section does not have an underdrain and, therefore, infiltrates
all water captured by the facility into the subgrade below. Full infiltration sections should be
used when the native subgrade soils have the ability to infiltrate the design volume and
infiltration will not adversely affect the environment or adjacent structures.
PICP Minimum Design Criteria
1
Required Components
Soil subgrade
Open-graded subbase reservoir
Open-graded base reservoir
Bedding course
Concrete pavers
Design and Sizing
General
Facility is sized for 100% of RRV or RTV
(minimum)
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Partial Infiltration Section: This section uses both infiltration and an underdrain to
discharge treated runoff from the BMP. A partial infiltration section does not include an
impermeable liner. Any storm water that does not infiltrate into the underlying soils will be
discharged to the downstream conveyance system through an underdrain. This type of
section should be used when native subgrade soils are not able to infiltrate 100 percent of
the design runoff volume and infiltration will not adversely affect the environment or adjacent
structures.
No Infiltration Section: This section uses an underdrain to discharge 100 percent of the
runoff that has been captured and stored in the BMP. This section also includes an
impermeable liner to prevent infiltration. A no infiltration section should be used when
infiltration has the potential to adversely affect the environment or adjacent structures (that
is, when treating runoff from a storm water hotspot).
PICP Facility Layers
PICP facilities typically consist of concrete pavers and three aggregate layers: (1) open-graded
bedding course, (2) open-graded base reservoir, and (3) open-graded subbase reservoir. Design
these layers to meet runoff storage goals and support the anticipated traffic loads. Descriptions and
design recommendations for the facility layers are as follows.
Concrete Pavers
Several brands of PICPs are available, most of which have design and installation specifications
provided by the manufacturer. Common design information is as follows (33):
Concrete pavers are typically a minimum of 3⅛ inches thick for vehicular areas and 2⅜
inches thick for pedestrian areas.
The joint areas between pavers are usually filled with small-sized aggregates, such as ASTM
No. 8, 89, or 9 stone in accordance with ASTM D448. These joint areas allow water to
infiltrate through the pavers to the open-graded bedding course.
Open-Graded Bedding Course
The open-graded bedding course layer is a
highly permeable aggregate layer that
provides a level bed for the pavers. Design
recommendations for this layer are as
follows (33):
Provide a 1½- to 2-inch thick layer of
small-sized, open graded aggregate.
Use an ASTM No. 8 stone or similar
sized material.
Open-Graded Base Reservoir
The open-graded base layer is an aggregate
layer that provides a structural transition
between the bedding and subbase
aggregate layers and has the ability to store
runoff. Design recommendations for this
layer are as follows (33):
Design the layer to be 4 inches thick.
Figure 5.4-3. Full Infiltration Section Cross Section
Source: Courtesy of ICPI
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Use crushed stone from 1 inch
down to ½ inch.
Use ASTM No. 57 or similar sized
material.
Open-Graded Subbase Reservoir
The open-graded subbase layer provides
additional storage for runoff and
structural support for the expected traffic
loads. Design recommendations for this
layer are as follows (33):
A minimum depth of 6 inches is
recommended; however, the
depth of this layer will vary
depending on the expected traffic
loading, infiltration rate of the
underlying soils, and the design
volume. Ultimately, the thickness of this layer must accommodate both hydrological and
structural needs, so the thicker of two sections must be selected for construction.
Use crushed stone from 3 inches down to 2 inches.
Use ASTM No. 2, 3, or 4 stone.
Maintain a subbase slope of less than 1 percent for full or partial infiltration sections. Use a
stepped installation on sites where this is not achievable.
A subbase layer may not be necessary in pedestrian or residential driveway applications;
however, in such instances, the base layer thickness should be increased to provide runoff
storage and structural support.
Subgrade
The subgrade must balance the need for structural support and infiltration, particularly for full and
partial infiltration sections. Consider the native soil characteristics, expected loading, and the design
infiltration rate when specifying compaction, if any, of the subgrade.
Stepped Installation (if applicable)
Sloped and stepped installations contain flow barriers, check dams, or soil berms along the subbase
to enable even distribution of ponded runoff and facilitate infiltration across the entire facility (see
Figure 5.4-10 and Figure 5.4-11). A sloped or stepped installation is recommended for facilities with
a surface slope of greater than 3 percent.
FACILITY SIZING
Hydrological Analysis
Aggregate Reservoir Depth
The aggregate reservoir consists of a combination of the open-graded base reservoir and open-
graded subbase reservoir, where storm water runoff is stored and released through infiltration or an
underdrain. Calculate the minimum depth required for management of the RRV for the aggregate
reservoir using Equation 5.4-1.
DAL
=
(PAi
Rvi)+(PAp)
ηAp
Equation 5.4-1
Figure 5.4-4. PICP and Bioretention System Treatment Train
Source: Courtesy of Altitude Training Associates, LLC
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Where:
DAL = Minimum depth of aggregate reservoir (inches)
P = Water quality rainfall depth (use 0.5 inch)
Ai = Area draining to the PICP facility; this does not include the PICP area (acres)
Rvi = Dimensionless runoff coefficient of Ai, Rvi = 0.05 + 0.9(I)
I = Percent impervious cover draining to the facility converted to decimal form
Ap = PICP area (acres)
η = Porosity of aggregate layer (typically 0.40)
Equation 5.4-1 makes the following assumptions:
The PICP area, Ap, is known. This is often the case for PICP facilities given the nature of the
areas in which they are located (e.g., parking lots, sidewalks, roadways). For designs where
the PICP area is not known, an area can be assumed and final design dimensions can be
reached by iterating aggregate reservoir depth and area options.
The runoff from the contributing drainage area and the rainfall that falls directly onto the
PICPs are calculated separately. A portion of the runoff from the contributing drainage area
drains to the aggregate reservoir, using the methods discussed in Chapter 3 to calculate the
RRV. 100 percent of the rainfall that falls onto the PICP surface area drains to the aggregate
reservoir.
The surface area of the PICPs is equal to that of the aggregate reservoir.
Note: The minimum volume to be managed by a PICP facility must be equal to the RRV or RTV
(depending on how the site runoff will be managed). Most PICP facilities receive runoff from all
storm events that occur within a contributing drainage area; therefore, designers typically need to
account for additional runoff volumes to accommodate flood control requirements. Coordinate with
the local jurisdiction to determine overall design volume requirements.
Maximum Allowable Infiltration Depth (Full and Partial Infiltration Sections Only)
It is recommended that full and partial infiltration sections be able to infiltrate the design volume
within 48 hours. Use Equation 5.4-2 to calculate the maximum allowable depth of runoff to be
infiltrated within the aggregate reservoir. If the maximum allowable depth is less than the design
depth calculated using Equation 5.4-2, increase the PICP surface area, use an overflow system, or
find ways to reduce the volume of water draining to the facility.
DAL-max
=
itd
2η
Equation 5.4-2
Where:
DAL-max = Maximum allowable infiltration-depth of the reservoir, see Figure 5.4-5 (inches)
i = Field-verified infiltration rate for the native soils (in/hr)
td = Maximum allowable draw down time (typically 48-hrs)
η = Porosity of aggregate layer (typically 0.40)
Note: For design purposes, the field-tested subgrade soil infiltration rate (i) is divided by 2 within Equation
5.4-2 as a factor of safety to account for potential compaction during construction and to approximate long-
term infiltration rates.
Structural Analysis
Structural support for PICP facilities is provided by a combination of the concrete pavers and
underlying aggregate layers. The structural design procedure for PICP facilities is the same as for
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flexible pavements because the load distribution and failure modes of PICPs are similar to those for
other flexible pavement systems. Therefore, the 1993 American Association of State Highway and
Transportation Officials (AASHTO) Guide for Design of Pavement Structures can be used. This
design process uses a structural number, given the expected axle loads, soil type, climatic, and
moisture conditions (33).
The structural design considerations are often outside the realm of typical storm water BMP design.
An engineer who is qualified to analyze and design pavement systems should reference applicable
sources such as the 1993 AASHTO Guide for Design of Pavement Structures and Smith’s
Permeable Interlocking Concrete Pavements for detailed structural design guidance for PICP
facilities.
ADDITIONAL DESIGN COMPONENTS
Pretreatment
Pretreatment facilities are recommended for all sites that receive run-on from non-impervious areas
to reduce the potential for clogging of the PICPs. Sheet flow is the preferred flow entrance method
for run-on. Place sheet flow pretreatment practices such as vegetated filter strips adjacent to non-
impervious stabilized areas to trap coarse sediment particles before they reach the PICP surface
(31).
Conveyance and Overflow
PICP areas typically receive all runoff from the contributing drainage area; therefore, the facility
should be designed to accommodate and/or convey runoff events that are greater than the design
volume. Coordinate with the local jurisdiction to determine which runoff events should be considered.
The following types of conveyance designs are generally used:
Underdrain System: An underdrain system consists of perforated pipes at the bottom of the
aggregate storage layer that discharge to a downstream waterbody or conveyance system.
PICP underdrain systems are similar to bioretention underdrain systems (see Section 5.3.4)
with the exception that most bioretention underdrain systems will not be designed to
accommodate traffic loading. Also, the PICP underdrain system can be designed with an
orifice outlet structure so that the facility will act as an underground detention system. When
designing the underdrain system, consult with the pipe manufacturer to verify that the
underdrain pipe is appropriate for locations with traffic loading and verify that the cover
thickness of aggregate over the underdrain pipe will accommodate the expected vehicle
loads.
Elevated Drain: An elevated drain,
also referred to as an overflow
system, consists of slotted or
perforated pipes suspended within
the aggregate reservoir that
discharge to a downstream
waterbody or conveyance system
(see Figure 5.4-5). Runoff that
pools beneath the elevated drains
infiltrates into the underlying soils
and excess runoff is able to exit the
facility through the overflow system.
These systems should be designed
Figure 5.4-5. PICP Facility with Elevated Drain
Source: Adapted from Smith (33)
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to maximize the use of infiltration while limiting the duration when ponded water is stored in
the facility in order to protect the subgrade from oversaturation.
Oversized Subbase Reservoir Layer: Some sites may be able to accommodate an
increased depth within the subbase reservoir layer to store and infiltrate large runoff events.
Sites that may be conducive to this option include soils with high infiltration rates and sites
with limited contributing drainage areas. When considering this option, check the maximum
allowable depth using Equation 5.4-2.
Perimeter Barrier
A structural barrier should be installed along the perimeter of the PICP facility to restrain movement
of the pavers and reduce lateral flow. The type of barrier to be used depends on whether the facility
is for pedestrian, residential, or a parking lot or street use. Perimeter barrier recommendations are
as follows:
For vehicular installations, use a cast-in-place curb (typically 9 inches deep) that rests on the
top of the subbase or that extends the full depth of the base and sub-base (34).
If a PICP facility is adjacent to an existing road or parking lot, provide a curb that is level with
the adjacent surface. The curb should extend to the subbase of the PICP facility, or an
impermeable liner should be used to
protect the adjacent subgrade material
from excessive moisture (33).
For pedestrian areas and residential
driveways, cast-in-place concrete curbs
or dense-graded berms that provide a
base to secure spiked metal or plastic
edge restraints can be used (33).
An additional option for a pedestrian
and light parking application is a
subsurface concrete grade beam with
pavers cemented to the concrete beam
to create a rigid paver border (34).
Observation Wells
An observation well is recommended for PICP facilities that are subject to vehicular traffic to verify
that the facility drains within the maximum allowable drain time. Provide a vertical 4- to 6-inch
perforated pipe that extends 4 to 6 inches into the soil subgrade and is located a minimum of 3 feet
from the edge of the facility at the lowest elevation of the subbase (33).
Geotextiles (Optional)
Geotextiles are optional for use within PICP facilities. Recommendations for use of geotextiles are
as follows:
Geotextile is optional for placement between the soil subgrade and aggregate base. The
purpose of geotextile is to prevent the bottom of the aggregate base from intrusion by
underlying soils, although there is some concern that using geotextiles can lead to clogging
of the facility over time.
To prevent erosion of adjacent soil into the aggregate reservoir, geotextiles are
recommended on the sides of PICP facilities when a full-depth concrete curb is not used.
The geotextile fabric should extend horizontally at least 1 foot onto the subgrade bottom,
Figure 5.4-6. PICPs with Perimeter Barrier
Source: Courtesy of the City of Bozeman
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5.4 Permeable Pavement Systems
resting on the soil subgrade. A minimum overlap of 1 foot is recommended for well-drained
soils and 2 feet for poorly draining soils (33).
Impermeable Liner (If Applicable)
An impermeable liner is required for no infiltration sections to prevent storm water runoff from
infiltrating into the underlying soils. Design considerations for impermeable liners are as follows:
Install a 30 mil (minimum) PVC geomembrane liner on the bottom and sides of the facility,
extending up at least to the top of the underdrain layer.
Provide at least 9 inches (12 inches, if possible) of cover over the membrane where it is
attached to the wall to protect the membrane from UV deterioration (23).
5.4.5 Vegetation Considerations
Vegetation considerations are not applicable to PICP facilities.
5.4.6 Construction Considerations
Installation of PICPs involves numerous steps and typically requires a variety of construction
equipment that ranges from excavators to vibratory plate compactors. Basic construction
considerations and guidelines are provided below. Consultation with PICP manufacturers or
suppliers and the ICPI is necessary for more installation instructions and considerations specific to
the PICP selected for a project.
Construction Site Management
Acquire all applicable permits prior to construction. See Section 1.4 for more information.
Installation of many PICP products requires special construction techniques. Some project
owners and/or local jurisdictions may require the contractor to have previous PICP
installation experience or certification through ICPI.
A pre-construction meeting is recommended to review the design and installation
requirements and discuss items such as plans for sediment management and construction
sequencing.
To reduce the potential for clogging of the facility, keep stockpiled aggregate material,
installed base material, and installed
pavers protected from construction
site runoff and tracking of mud and
sediment from construction
equipment.
If the excavated PICP area will be
used as a construction storm water
management control prior to
installation of the PICPs, then
excavation for the construction storm
water management control should be
at least 6 inches above the final
design elevation of the soil subgrade.
Excavation to the final bottom
elevation should not occur until
immediately before installing the
aggregate subbase and base (33).
Figure 5.4-7. PICP Installation
Source: HDR
Pavers often need to be cut to fit along the perimeter of a
facility.
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Compaction of the subgrade will reduce the infiltration rate of the native soil and should be
avoided unless required in the plans and specifications.
Store aggregate materials on a hard surface or geotextile material (as opposed to natural
ground) so that sediment is not introduced to the aggregates.
Paver installation can be by hand or with mechanical equipment. Mechanical equipment is
faster and may be more cost effective, depending on the size of the installation.
Cut pavers should be no smaller than one-third of a whole paver for facilities that will be
subject to vehicle loading (33).
When using an impermeable liner, ensure enough slack in the liner to allow for backfill,
compaction, and settling without tearing the liner (23).
Project specifications should require the contractor to revisit the site 6 months after project
completion to inspect the joints and top them with aggregate if they have settled to more than
¼ inch below the paver surface (33).
Construction Inspections
Inspections are recommended during the following phases of construction:
o Pre-construction meeting
o Initial site preparation
o Excavation/grading
o Installation of the impermeable liner (if applicable), aggregate base, bedding layer, and
pavers
o Final inspection
Inspectors should be familiar with project plans and specifications to ensure the contractor’s
interpretation of the plans is consistent with the designer’s intent. The inspectors should take
frequent photos and notes of construction activities and features as work progresses and at
all critical points during the construction process (such as immediately prior to backfilling).
The photos will serve as a helpful resource when creating inspection reports. Check
dimensions and depths of all installed materials, and all materials and products should be
verified or tested for conformance with the specifications (16).
Transition to Post-Construction
Develop a plan prior to construction that will allow for an effective transition from construction
storm water management BMPs to post-construction BMPs without compromising the
integrity of the post-construction BMPs.
Coordinate with the local jurisdiction prior to terminating coverage of the Construction
General Permit.
5.4.7 Maintenance
Maintenance is required on all BMPs. Recommended maintenance activities are provided in Table
5.4-1, which may be used as a guide when developing a maintenance plan. An example inspection
form is provided in Appendix F that may be adapted or adopted as part of the maintenance plan.
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Table 5.4-1. Recommended Maintenance Activities for Permeable Surfaces
Activity Frequency
Observe the system during and following rainfall events to determine if the facility is clogged.
If clogging is suspected, test the surface infiltration rate using ASTM C1701. Vacuum, refill
joints with clean aggregate, sweep the surface clean, and retest the infiltration rate again in
accordance with ASTM C1701. The retest should result in a minimum 50 percent increase or
a minimum 10 inches/hour (33).
Stabilize the contributing drainage area to prevent erosion.
Regularly manage all vegetation around the permeable pavers and remove all clippings.
Keep the pavers free of trash, debris, and sediment.
As needed
Vacuum sweep the surface with equipment such as a regenerative air vacuum sweeper.
Adjust the vacuum settings to remove visible sediment without uptake of aggregate from
paver openings. Additional aggregate may be needed between pavers after vacuuming (34).
Semiannually
(typically spring
and fall)
Inspect the PICP facility.
Replenish aggregate in joints if more than ½ inch of space exists between aggregate and
chamfer bottoms on the paver surface (33).
Inspect and repair all paver surface deformations exceeding ½ inch (33).
Repair pavers offset by more than ¼ inch above/below adjacent pavers or curbs, inlets, etc.
(33).
Replace cracked pavers.
Check underdrain system and outfalls for free flow of water and outflow from the observation
well(s) after a major rainfall event.
Flush the underdrain system to check for clogging (if applicable).
Inspect all components of the PICP facility in accordance with an approved inspection form
according to local jurisdiction requirements. An example inspection form is provided in
Appendix F.
Annually
Considerations for Winter Operations and Maintenance
Winter operation and maintenance of PICPs varies from traditional pavement surfaces primarily
because of the potential for clogging. Winter operations and maintenance recommendations are as
follows:
Snow can be plowed from pavers. Most pavers have chamfered edges to reduce chipping
from snowplows; however, skids on the corners of the plow blades are recommended as well
as raising the blade slightly above the paver surface to reduce the potential for damage to
the pavers (34).
Deicing materials and application
of sand is not recommended.
Deicing materials can infiltrate into
the subgrade and sand may clog
the facility. If traction is required,
ASTM No. 8, 89, or 9 stone (or
similar) may be applied (33).
If sand is used, the PICP surface
should be vacuumed in the spring
to reduce the potential for clogging
(33).
Locate large snow piles in adjacent
grassy areas so that sediments
and pollutants in snowmelt are
partially filtered before they reach
the PICP facility.
Figure 5.4-8. Cold Climate PICP Application
Source: Courtesy of the City of Bozeman
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5.4.8 Plan View and Typical Details
Figure 5.4-9. Full, Partial, and No Infiltration Sections
Source: Adapted from Smith (33)
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Figure 5.4-10. Sloped Installation Section
Source: Adapted from Smith (33)
Figure 5.4-11. Stepped Installation Section
Source: Adapted from Smith (33)
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5.4.9 Additional Types of Permeable Pavement Systems
Description Example Photo
Pervious Concrete
Pervious concrete is similar to conventional concrete with reduced or
no fine aggregates (sand). The reduction of fine aggregates creates
interconnected voids that allow runoff to filter through the concrete
into the underlying aggregate base and subgrade. One way that
pervious concrete differs from most other pervious pavement
systems is that it is a ridged system and does not rely as heavily on
the aggregate base for structural support. Pervious concrete can be
used for various light to heavy duty applications supporting low to
moderate speeds (34).
Source: HDR
Pervious Asphalt
Pervious asphalt is very similar to standard hot or warm-mix asphalt
except that the aggregate fines have been removed to create
interconnected void spaces. These void spaces allow runoff to filter
through the asphalt into the underlying aggregate base and
subgrade. Permeable asphalt applications can include facilities such
as parking lots, residential access and collector roads, light arterial
roads, pedestrian and bicycle paths, and utility access (34).
Source: HDR
Concrete Grid Pavers
Concrete grid pavers are similar to PICPs except that they have
larger open areas that are filled with topsoil planted with grass or
small aggregates. Like other permeable pavement systems, the open
areas allow runoff to infiltrate into the underlying aggregate base and
subgrade. These facilities provide a greenspace that can withstand
vehicle loading without compaction and loss of infiltration capabilities.
Common applications include alleys, driveways, patio areas, utility
access, and overflow parking areas.
Source: Public Domain. By Immanuel Giel
Plastic Grid Systems
Plastic grid systems consist of flexible plastic interlocking units that
allow for infiltration through large gaps filled with gravel or topsoil
planted with turf grass (35). These systems provide the largest void
space compared with other permeable pavement systems. Similar to
concrete grid pavers, plastic grid systems provide a greenspace that
can withstand vehicle loading without compaction and loss of
infiltration capabilities. Common applications include parking lots,
overflow parking areas, and emergency access routes.
Source: Courtesy of Emmons & Olivier
Resources, Inc. and Minnesota Pollution
Control Agency
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September 2017 | 5.5-1
5.5 Dispersion
5.5 Dispersion
Figure 5.5-1. Dispersion Area
Source: Courtesy of WSDOT
Description
A BMP that achieves runoff reduction by using
vegetation, soils, and gentle slopes located adjacent
to impervious surfaces to impede the velocity of storm
water runoff and encourage infiltration.
Primary Components Primary Function
Sheet flow conditions
Level spreader
Vegetated sloped area
Infiltrative soils
Runoff reduction
Runoff treatment
Benefits Limitations
Maintains and preserves natural hydrologic
functions
Siting is generally not limited by native soils. Design
accommodations can be made for most soil types.
Reduces directly connected impervious areas that
can result in reduced runoff volumes
Storm water runoff must maintain sheet flow across
a dispersion area
Limited to small contributing drainage areas
Not recommended for sites with steep slopes
Design and Site Selection Considerations
Setbacks
Depth to groundwater or bedrock
Soil permeability
Soil preparation/amendments/compost
Pretreatment forebay
Inlet and outlet spacing
Energy dissipater/level spreader
Underdrain
Facility liners
Landscaping/planting
Fencing
Size of contributing drainage area
Area required
Incorporate flood control
TMDL Considerations Maintenance Requirements
Avoid Preferred
Total suspended solids (TSS)
Total phosphorus
Total nitrogen
Temperature
Metals
Fecal coliform
Access roads or pullouts
Sediment removal
Irrigation
Vegetation management
Erosion and embankment stabilization repair
Specialized equipment and training
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5.5 Dispersion
5.5.1 Description
Dispersion areas are composed of vegetated slopes that receive runoff as sheet flow from
impervious or pervious areas. The vegetation and sloped dispersion area reduce the velocity of
runoff, promoting infiltration and evapotranspiration. Dispersion often occurs naturally; depending on
site characteristics, only minor construction activity may be required to implement this BMP.
The primary characteristics of dispersion areas are as follows:
A dispersion area usually consists of a level spreader, vegetated sloped area, and infiltrative
soils.
Sheet flow must be maintained through dispersion areas to promote infiltration.
Dispersion areas should be designed to provide runoff reduction.
5.5.2 Performance
Runoff Reduction
Dispersion areas are expected to infiltrate 100 percent of the RRV when designed, operated, and
maintained as described in this manual (36).
Runoff Treatment
Dispersion areas are designed to manage the RRV from contributing drainage areas such that no
runoff leaves the dispersion area; therefore, runoff treatment is not applicable.
5.5.3 Site Selection
Basic guidelines are provided below to aid in evaluating whether dispersion is feasible for use at an
individual site.
Contributing Drainage Area
Dispersion areas require sheet flow to operate properly; therefore, impervious contributing
drainage areas that promote sheet flow are preferred (e.g., roads and parking lots). Where
sheet flow cannot be maintained, a flow spreader can be used to promote sheet flow across
the dispersion area. See Section 5.5.4, in the Pretreatment Diaphragms and Flow Spreaders
subsection, for guidance.
For sheet flow dispersion, the sheet flow path leading to the dispersion area should not be
longer than 150 feet (not including pervious shoulders and side slopes of a road or parking
area) (36).
The resultant slope of the contributing drainage area must be less than or equal to
9.4 percent (see Figure 5.5-3). Calculate the resultant slope using Equation 5.5-1.
SCFS≤(G
2
+e
2)0.5
Equation 5.5-1
Where
SCFS = Resultant slope of the lateral and longitudinal slopes (%)
G = Lateral slope (superelevation) (%)
e = Longitudinal slope (grade) (%)
Soil Characteristics
Dispersion areas are suitable for most soil types; however, some sites may have native soils
conditions that limit the ability to establish and maintain vegetation. This decision may be
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September 2017 | 5.5-3
5.5 Dispersion
based on visual observations of the existing site or by testing the organic content of the soils.
Specifically, an organic content of 8 percent is recommended. In some cases, soil
amendments will be necessary.
Soil characteristics can initially be estimated from NRCS soil data, but must be field-verified
prior to final design using the onsite soil investigation methods discussed in Chapter 2 and
Appendix C of this manual.
Depth to Groundwater and/or Bedrock
A vertical distance of 3 feet is recommended between the existing ground elevation and the
seasonal high water table or bedrock layer. Note that this depth recommendation applies to
the entire limits of the dispersion area (36).
Site Topography
Dispersion areas should be located on sites with low to moderate slopes (less than
33 percent).
Infiltration of storm water runoff above steep slopes can create landslide hazards. Dispersion
areas should not be located above slopes greater than 33 percent or above erosion hazard
areas without evaluation by a geotechnical engineer and approval by the local jurisdiction
(36).
Land Use and Considerations of Surrounding Area
Dispersion areas typically look like a natural vegetated slope and are not always
recognizable as a BMP. As such, they should be sited in areas where they are likely to
receive maintenance and protection from future development. Examples of these areas
include public right-of-way, designated open space, and protected conservation easements.
Dispersion areas are best suited for
areas adjacent to linear facilities
such as roadside ditches and
parking lots.
Depending on soil characteristics,
dispersion areas may not infiltrate
runoff from larger rainfall events
(e.g., 10- or 25-year event).
Coordinate with the local jurisdiction
to determine flood flow conveyance
requirements. Ensure that the
dispersion area will not increase
runoff to down-gradient properties or
structures.
Consider minimum setback
requirements as discussed in
Section 4.3.4.
Community and Environmental Considerations
Dispersion should be avoided at locations where storm water runoff could pose a risk of
groundwater contamination (i.e., storm water hotspots).
Figure 5.5-2. Dispersion Area Adjacent to a Roadway
Source: Courtesy of WSDOT
This dispersion area adjacent to a roadway provides storm
water management within the existing right-of-way.
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5.5 Dispersion
5.5.4 Design and Sizing Procedure
The following steps outline the design procedure and criteria for dispersion. The information
provided has been adopted from the Washington State Department of Transportation (WSDOT)
Highway Runoff Manual, with minor revisions to account for local considerations. Guidance and
standards from the local jurisdiction should be considered during the design process.
1. Sheet Flow or Channelized Dispersion
Dispersion areas can be classified into two
different types of facilities depending on how
runoff drains to the BMP:
Sheet Flow Dispersion: Runoff flows
across and leaves the contributing
drainage area as sheet flow. The
dispersion area is immediately adjacent
to the contributing drainage area.
Channelized Dispersion: Runoff from
the contributing drainage area is
conveyed to the dispersion area. A flow
spreader is used to evenly distribute
runoff across the dispersion area and to
reestablish sheet flow.
2. Dispersion Area Geometry
Geometry guidance for both sheet flow and
channelized dispersion areas is as follows:
Storm water runoff must enter the
dispersion area as sheet flow.
If a level spreader is not located
immediately upstream of the dispersion
area, the average lateral slope of the
dispersion area may not exceed
15 percent or 6:1 (H:V).
If a level spreader is located immediately upstream of the dispersion area, the average
lateral slope of the dispersion area may not exceed 33 percent or 3:1 (H:V).
The width of the dispersion area (perpendicular to the direction of sheet flow) must be
greater than or equal to the width of the contributing drainage area (see Figure 5.5-3).
Provide a uniform slope across the dispersion area to promote sheet flow. There should be
no discernible flow paths through the dispersion area.
Dispersion areas are generally considered online facilities (refer to Section 4.2.2 for
discussion of online and offline facilities). The limitations on the contributing drainage area
size for dispersion BMPs minimizes the need for large storm conveyance and overflow
systems; however, in channelized dispersion cases where inflow comes from a pipe or
channel and must be converted to sheet flow, the dispersion area can be implemented as an
offline facility by using a diversion structure in conjunction with the flow spreader. The
purpose is to prevent the conveyance system design discharge (e.g., 10- or 25-year peak
flow) from scouring a channel or rill through the dispersion area. Figure 5.5-4 shows a
diversion structure in conjunction with a level spreader (37).
Dispersion Minimum Design Criteria
1
Required Components
Dispersion area grading to maintain sheet flow
Turf grass established throughout dispersion area
Design and Sizing
General
Contributing drainage area maximum sheet flow
length of 150 feet
Dispersion area width greater than or equal to
width of contributing drainage area
Maximum lateral slope of 3:1 (H:V)
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5.5 Dispersion
3. Dispersion Sizing Guidance
Sheet Flow Dispersion Sizing Guidance
Sheet flow dispersion occurs at sites where a dispersion area is implemented immediately adjacent
to the contributing drainage area. Sizing guidance and requirements that are specific to sheet flow
dispersion are as follows:
Sizing of a dispersion area is based on soil and contributing drainage area characteristics.
Dispersion design length requirements are presented in Table 5.5-1.
Table 5.5-1. Sheet Flow Dispersion Sizing Guidance
Soil Characteristic
Contributing Drainage Area
Characteristics
Dispersion Area Sizing Requirement
All HSG A soils and HSG B
soils with saturated
hydraulic conductivity
(Ksat) of 4 inches/hour or
greater
Impervious surfaces Provide a minimum lateral dispersion
area length of 10 feet
Add 0.25 feet of lateral dispersion
area length for every 1 foot of
contributing area sheet flow length
beyond 20 feet
All HSG A soils and HSG B
soils with Ksat of
4 inches/hour or greater
Pervious surfaces (bare soil
and nonnative landscaping)
Provide 1 foot of lateral dispersion
area length for every 6 feet of
contributing area sheet flow length
All HSG C and D soils and
HSG B soils with Ksat of
less than 4 inches/hour
All surface types Provide a minimum lateral dispersion
area length of 100 feet
Provide 6.5 feet of lateral dispersion
area length for every 1 foot of
contributing area of sheet flow length
Channelized Flow Dispersion Sizing Guidance
For sites where a dispersion area cannot be implemented immediately adjacent to a sheet flow area,
the flows may be channelized and then redispersed over a dispersion area with a flow spreader (see
Figure 5.5-4). Sizing guidance and requirements that are specific to channelized dispersion are as
follows:
Do not allow runoff from adjacent drainage areas to intersect with the channelized flow
conveyance system of dispersion area.
Locate discharge points a minimum of 100 feet upgradient of steep slopes (slopes steeper
than 40 percent within a vertical elevation change of at least 10 feet), wetlands, and streams.
Sizing is based on soil characteristics. Channelized dispersion design length requirements
are presented in Table 5.5-2.
Note: If roadway side slopes will be considered part of the dispersion area, the sizing must
consider the roadway side slope soil type. In particular, the Ksat may be lower on the side slope
because of typical compaction activities associated with construction.
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5.5 Dispersion
Table 5.5-2. Channelized Flow Dispersion Sizing Guidance
Soil Characteristic Dispersion Area Sizing Requirement
All HSG A soils and HSG B soils with
saturated hydraulic conductivity (Ksat) of 4
inches/hour or greater
Provide a minimum lateral dispersion area length of 10
feet
Add 0.25 feet of lateral dispersion area length for every 1
foot of contributing area sheet flow length beyond 20 feet
All HSG C and D soils and HSG B soils
with Ksat of less than 4 inches/hour
Provide a minimum lateral dispersion area length of 100
feet
Provide 6.5 feet of lateral dispersion area length for every
1 foot of contributing area of sheet flow length
4. Pretreatment Diaphragms and Flow Spreaders
Level Spreader Design Guidance
A gravel diaphragm level spreader is a 1- to 2-foot-wide strip of gravel located at the edge of
a road or parking lot to provide pretreatment for sheet flow from a contributing drainage area.
The gravel diaphragm should be oriented perpendicular to the sheet flow path with a 2- to
4-inch drop from the edge of pavement to the top of the stone. Size the stone to dissipate
flows and prevent erosion. A design velocity of 1 foot per second or less is recommended.
Use of a level spreader is recommended for all sheet flow dispersion areas and is required
for all sheet flow dispersion areas with slopes steeper than 6:1 (H:V).
For any existing slope that will lead to a dispersion area, if evidence of channelized flow (rills
or gullies) is present, use a level spreader before those flows are allowed to enter the
dispersion area.
Flow Spreader Design Guidance (Channelized Dispersion)
Flows collected in a pipe or ditch conveyance system require energy dissipation and
dispersal at the end of the conveyance system before entering the dispersion area.
Flow spreaders should be sized to reduce the velocity from the conveyance system to less
than 1 foot per second.
Design the flow spreader to evenly distribute the design flow rate over the dispersion area. In
cases where a diversion structure is used, the diversion structure must divert, at a minimum,
the RTF rate to the flow spreader and dispersion area. Section 3.3 provides guidance for
calculating the RTF.
Flows discharging from a flow spreader must traverse the design length as provided in Table
5.5-2.
5. Signs
Installing signs that identify a dispersion area as a storm water management area is recommended
because dispersion areas are not always recognizable as a BMP. Signs will increase the probability
that the dispersion area will protected from additional landscaping, application of pesticides and
fertilizers, future construction activity, or other disturbances.
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5.5 Dispersion
5.5.5 Vegetation Considerations
Vegetation is crucial because it provides erosion control, promotes filtration of pollutants, and
facilitates infiltration. Development of a landscaping plan for the dispersion area is required to
indicate how the dispersion area will be stabilized and established with vegetation. Considerations
when developing the vegetation and landscaping plan are as follows:
If the site’s existing soils are not conducive to establishment of healthy vegetation, topsoil
should be imported from an offsite location and applied over the entire dispersion area prior
to seeding.
Durable, dense, and drought-tolerant grasses are recommended. Grass selection should
consider both short-term and long-term maintenance requirements because some varieties
have higher maintenance requirements than others (38).
Salt-resistant vegetation should be used in locations where adjacent salt application is
probable, such as roadsides and parking lots.
Use of sod is not recommended for dispersion areas because seeding establishes deeper
roots and sod may contain soil that is not conducive to infiltration (37).
Trees are not recommended dispersion areas because they may affect the level spreading of
flows across the surface (39).
Schedule planting and seeding activities during optimal growing seasons.
Irrigation systems will likely be necessary to establish vegetation. These systems can be
temporary or permanent depending on the type of vegetation to be used. Irrigation
scheduling must be appropriate for the selected vegetation since overwatering can decrease
the permeability of the soil and under watering may hinder vegetation establishment and
reduce the straining capabilities of the vegetation (38).
If possible, divert runoff (other than necessary irrigation) during the period of vegetation
establishment. Where runoff diversion is not possible, protect graded and seeded areas with
suitable erosion control measures.
The entire dispersion area should have mature vegetation coverage by the end of the
establishment period because unplanted areas may decrease infiltration and promote
erosion (39).
Note: Given the wide range of native vegetation across Montana, designers should consult local
specialists, landscape architects, and/or agencies for recommendations on appropriate plant
species and landscaping considerations for the site.
5.5.6 Construction Considerations
Basic construction considerations and guidelines are provided below.
Construction Site Management
Acquire all applicable permits prior to construction. See Section 1.4 for more information.
Apply appropriate erosion control measures to minimize erosion during construction.
If possible, minimize disturbance, excavation, and clearing and grubbing in the location and
vicinity of dispersion areas to maintain existing plant root systems (36).
To the extent practicable, construction equipment should be restricted from the dispersion
area to prevent compaction of the native soils. If construction equipment is used within the
dispersion area, use low-ground-pressure vehicles to minimize compaction of soils.
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5.5-8 | September 2017
5.5 Dispersion
Contributing drainage areas should be properly stabilized with the appropriate erosion and
sediment controls before allowing storm water runoff to drain to the dispersion area.
Perform fine grading, application of topsoil, and seeding only after upgradient areas have
been stabilized.
Construction Inspections
Inspections are recommended during the following phases of construction:
o Pre-construction meeting
o Initial site preparation
o Excavation/grading
o Implementation of the vegetation and landscaping plan
o Final inspection
Transition to Post-Construction
Develop a plan prior to construction that will allow for an effective transition from construction
storm water management BMPs to post-construction BMPs without compromising the
integrity of the post-construction BMPs.
Coordinate with the local jurisdiction prior to terminating coverage of the Construction
General Permit.
5.5.7 Maintenance
Maintenance is required on all BMPs. Recommended maintenance activities are provided in Table
5.5-3, which may be used as a guide when developing a maintenance plan. Additionally, an example
inspection form is provided in Appendix F that may be adapted or adopted as part of the
maintenance plan.
Table 5.5-3. Recommended Maintenance Activities for Dispersion Areas
Activity Frequency
Apply irrigation until vegetation has been established
Inspect the dispersion area for signs of erosion and immediately stabilize eroded areas with
grass cover
Upon
establishment
Remove trash and debris from the dispersion area
Regularly manage all vegetation in accordance with the designer’s recommendations. For
locations where the grass is mowed, remove all clippings.
As needed
Maintain and/or restore the level spreader so that flows are spread evenly over the entire area
Remove sediment deposits and re-level so lateral slopes are even and flows pass evenly
through the dispersion area
Reseed as needed during fall seeding season to maintain 90% turf grass cover
Inspect all components of the dispersion area in accordance with an approved inspection form
according to local jurisdiction requirements. An example inspection form is provided in
Appendix F.
Annually
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5.5 Dispersion
5.5.8 Plan View and Typical Details
Figure 5.5-3. Dispersion Plan View and Typical Section
Source: Adapted from WSDOT (36)
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5.5 Dispersion
Figure 5.5-4. Channelized Dispersion Plan View
Source: Adapted from WSDOT (36)
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September 2017 | 5.6-1
5.6 Biofiltration Swale
5.6 Biofiltration Swale
Figure 5.6-1. Biofiltration Swale
Source: Courtesy of the City of Kalispell
Description
A vegetated channel designed to remove suspended
solids from storm water runoff. Biofiltration swales
have a trapezoidal cross-section and low longitudinal
slopes to promote shallow concentrated flow that
allows for filtration of storm water by plants.
Primary Components Primary Function
Inlet structure
Level spreader
Check dams (optional)
Outlet structure
Runoff reduction
Runoff treatment
Benefits Limitations
Typically provides a less expensive and more
attractive storm drainage conveyance feature
when compared with curb and gutter systems
Reduces directly connected impervious areas that
can result in reduced runoff volumes
Not recommended for basins greater than 5 acres
Requires more land area than storm sewers
Poor design and/or construction can create
erosion, standing water, and mosquito problems
Design and Site Selection Considerations
Setbacks
Depth to groundwater or bedrock
Soil permeability
Soil preparation/amendments/compost
Pretreatment forebay
Inlet and outlet spacing
Energy dissipater/level spreader
Underdrain
Facility liners
Landscaping/planting
Fencing
Size of contributing drainage area
Area required
Incorporate flood control
TMDL Considerations Maintenance Requirements
Avoid Preferred
Total suspended solids (TSS)
Total phosphorus1
Total nitrogen1
Temperature
Metals
Fecal coliform
Access roads or pullouts
Sediment removal
Irrigation
Vegetation management
Erosion repairs
Specialized equipment and training
1 Biofiltration swales that use compost amendments have been shown to release phosphorus and nitrogen. Avoidance of biofiltration
swales with compost-amended topsoil is recommended in areas that drain to waterbodies listed as impaired for phosphorus and
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5.6 Biofiltration Swale
5.6.1 Description
Biofiltration swales are densely vegetated channels designed to provide runoff treatment while
slowly conveying storm water runoff. A trapezoidal cross-section paired with low longitudinal slopes
facilitates decreased velocities and shallow concentrated flows, allowing for filtration of pollutants by
plant stems and leaves.
The primary characteristics of a biofiltration swale are as follows:
Biofiltration swales consist of an inlet structure, level spreader, main treatment channel, and
an outlet structure. Some swales will also use check dams to reduce velocities and increase
the hydraulic residence time (the time for runoff to travel the full length of the channel).
A minimum hydraulic residence time of 9 minutes for the RTF rate facilitates the removal of
TSS.
5.6.2 Performance
Runoff Reduction
Runoff reduction is not considered to be a primary function of biofiltration swales because they
generally discharge a volume equivalent to the entire inflow runoff volume.
Runoff Treatment
Biofiltration swales are expected to achieve an 80 percent or greater removal rate of TSS from the
RTF rate when designed, operated, and maintained as described in this manual (40).
5.6.3 Site Selection
Basic guidelines are provided below to aid in evaluating whether biofiltration swales are feasible for
use at an individual site.
Contributing Drainage Area
Biofiltration swales are generally suited for sites with a contributing drainage area of 5 acres
or less. Contributing drainage area limitations are related to the design flows and maximum
allowable velocity within the swale, because the swale’s required bottom width may become
impractical for larger contributing drainage areas (37). Design flow and velocity requirements
are further discussed in Section 5.6.4.
Some local jurisdictions may have smaller contributing drainage area limitations.
Biofiltration swales should be located to avoid flows from springs or other dry weather flows.
Soil Characteristics
Biofiltration swales are suitable for most soil types; however, some sites may have native soil
conditions that limit the ability to establish thick and healthy vegetation. This decision may be
based on visual observations of the existing site or by testing the organic content of the soils.
Specifically, an organic content of 8 percent is recommended. In some cases, soil
amendments and/or importing topsoil will be necessary.
An impermeable liner may be appropriate for groundwater protection considerations at sites
where there is a sensitive underlying aquifer and the underlying soils allow for infiltration
(40).
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5.6 Biofiltration Swale
Depth to Groundwater and/or Bedrock
A minimum vertical distance of 1 foot is recommended between the bottom of the swale and
the seasonal high water table (37).
An evaluation of the depth to groundwater should be conducted, as described in Section
4.3.3.
Site Topography
Biofiltration swales should be located on sites with low to moderate slopes.
The longitudinal slope along the length of the swale should be less than 5 percent. Swales
with longitudinal slopes greater than 5 percent may have erosion problems and will have
difficulty meeting the velocity constraints discussed in Section 5.6.4.
Swales with longitudinal slopes of less than 1 percent must be carefully monitored during
construction to avoid flat areas that may hold pockets of standing water (41).
For slopes greater than 2.5 percent, check dams are recommended to reduce the effective
slope and increase hydraulic residence time.
Land Use and Characteristics of Surrounding Area
Biofiltration swales are well-suited for areas adjacent to linear facilities such as roadside
ditches, alongside parking lots, and along property boundaries (42).
Biofiltration swales can sometimes be used to replace traditional curb and gutter systems.
Consider minimum setback requirements, as discussed in Section 4.3.4.
Community and Environmental Considerations
Biofiltration swales should be placed in a drainage or maintenance easement to increase the
probability of regular maintenance.
Figure 5.6-2. Meandering Biofiltration Swale
Source: HDR
Swales can often be designed to meander through a site to reduce the longitudinal slope and velocity of runoff.
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5.6 Biofiltration Swale
5.6.4 Design and Sizing Procedure
The following steps outline the design procedure and criteria for a biofiltration swale. The information
provided has been adopted from the WSDOT Highway Runoff Manual, with minor revisions that
account for local considerations. Determining swale dimensions is typically an iterative process to
develop a geometry that balances slope, flow depth, and velocity criteria. Guidance and standards
from the local jurisdiction should be considered during the design process.
1. Runoff Treatment Design Flow Rate
Calculate the RTF rate to determine the runoff
treatment design flow rate. See Section 3.3 for
guidance.
2. Biofiltration Swale Geometry
Swale geometry depends on site constraints
such as natural topography, available area, and
elevations of adjacent drainage structures, and
design requirements such as peak velocity and
minimum hydraulic residence time. Guidance
and constraints related to swale geometry are
as follows:
The recommended longitudinal slope is
between 1.5 and 5 percent. For
longitudinal slopes above 2.5 percent,
consider using drop structures, such as
check dams, to accommodate velocity
constraints. Energy dissipation
techniques should be used downstream
of each drop structure to prevent erosion.
A trapezoidal cross section is required to increase pollutant contact area and maximize
pollutant removal capabilities.
Provide a bottom width between 2 and 10 feet. When the calculated bottom width exceeds
10 feet, two parallel swales can be constructed and divided in half using a non-erodible
weather-resistant material such as plastic lumber. The maximum allowable total width for
parallel swales is 16 feet.
Side slopes should be stable and gentle to facilitate maintenance and access. 4:1 (H:V) or
flatter side slopes are preferred to allow for conventional maintenance equipment and for
improved aesthetics. Side slopes should be no steeper than 3:1 (H:V); however, local design
standards should be consulted to confirm the maximum allowable slopes.
3. Select Soil and Vegetation Cover
The type of vegetation and condition of the underlying soil influence the swale’s flow capacity. Use
Table 5.6-1 to determine the Manning’s n coefficient associated with the type of vegetation and
expected soil condition for the biofiltration swale.
Biofiltration Swale Minimum Design Criteria
1
Required Components
Inlet
Level spreader (at inlet)
Trapezoidal cross section with healthy vegetation
Outlet
Design and Sizing
General
Runoff treatment design flow rate is 100% of RTF
Longitudinal slope less than 5%
Bottom width between 2 and 10 feet (or up to
16 feet if using a swale divider)
Design flow depth between 2 and 4 inches (for
RTF)
Design flow velocity is ≤1 foot/second (for RTF)
Hydraulic residence time is ≥9 minutes (for RTF)
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5.6 Biofiltration Swale
Table 5.6-1. Flow Resistance Coefficient in Biofiltration Swales
Vegetation and Soil Condition
Manning’s n
Coefficient
1
Grass-legume mix on compacted native soil 0.20
Grass-legume mix on lightly compacted topsoil 0.22
Grass-legume mix on lightly compacted topsoil with 3-inch medium compost blanket 0.35
1 The Manning’s n coefficients presented in this table should be used only in conjunction with the RTF because they
represent expected resistance for shallow flows. Separate Manning’s n values should be used when calculating the
swale’s capacity for larger flows (i.e., the 10- or 25-year event).
4. Design Flow Depth
The flow depth associated with the RTF rate must be between 2 and 4 inches so that the vegetation
is able to filter pollutants within runoff. Select a design flow depth based on the condition and type of
vegetation that will be used in the swale. Recommendations are as follows:
2 inches if swale is mowed frequently
3 inches if swale consists of dryland grasses
4 inches if swale is mowed infrequently or inconsistently
5. Bottom Width
Calculate the bottom width of the biofiltration swale using Manning’s equation (Equation 5.6-1).
RTF=
1.49
n
AR
2
⁄ 3
s
1
⁄ 2
Equation 5.6-1
Where:
RTF = Runoff treatment design flow rate (cfs)
A = Wetted area (ft2)
R = Hydraulic radius (ft)
s = Longitudinal slope of swale (ft/ft)
n = Manning’s coefficient (see Table 5.6-1)
Equation 5.6-1 cannot be directly solved for the bottom width of a trapezoid; however, for trapezoidal
channels that are flowing very shallow (4 inches or less), the hydraulic radius is approximately equal
to the depth of flow. Using this assumption, Manning’s equation can be rewritten as follows:
b=
(n*RTF
1.49
)
[(y
5
⁄ 3
)(s
1
⁄ 2
)]
-zy Equation 5.6-2
Where:
RTF = Runoff treatment design flow rate (cfs)
s = Longitudinal slope of swale (ft/ft)
n = Manning’s coefficient (see Table 5.6-1)
b = Bottom width of the swale (ft)
z = Side slope of the swale in the form of z:1 (H:V)
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5.6 Biofiltration Swale
width of 10 feet. If the required bottom width is greater than 10 feet, use two parallel swales with a
combined width of up to 16 feet in conjunction with a device that splits the flow evenly between
swales.
6. Design Velocity
The maximum allowable runoff treatment design flow velocity is 1 ft/sec because a velocity greater
than 1 ft/sec can flatten grasses and reduce filtration capabilities. Compute the design flow velocity
using Equation 5.6-3. If the velocity is greater than 1 ft/sec, increase the bottom width, reduce the
slope, or provide check dams to reduce the design velocity and then repeat the calculation process
described above until the velocity is less than or equal to 1 ft/sec.
V=
RTF
A
Equation 5.6-3
Where:
V = flow velocity at RTF (ft/sec)
RTF = Runoff treatment design flow rate (cfs)
A = Wetted area for RTF (ft2)
7. Swale Length and Hydraulic Residence Time
The hydraulic residence time, t, must be a minimum of 9 minutes, and the swale must have a
minimum length of 100 feet. Compute the swale length using Equation 5.6-4 with an assumed
hydraulic residence time of 9 minutes.
L =Vt(60
sec
min
) Equation 5.6-4
Where:
L = swale length (ft)
V = flow velocity at RTF (ft/sec)
t = hydraulic residence time (set at 9 minutes)
8. Maintenance Access
Provide access to the swale for mowing or other vegetation management equipment and design the
side slopes to safely operate the expected maintenance equipment.
9. Additional Considerations
Insufficient Space
If there is not sufficient space for the biofiltration swale, consider the following solutions:
Divide the site drainage to flow to multiple BMPs.
Use small infiltrating BMPs upstream of the biofiltration swale to provide a lower design flow
rate.
Alter the design depth of flow, if possible.
Reduce the developed surface area to gain space for the biofiltration swale.
Reduce the longitudinal slope by meandering the biofiltration swale.
Conveyance for Larger Flow Rates
If the biofiltration swale is designed as an online BMP, then it will need to also provide conveyance
for flows greater than the RTF (as determined by the local jurisdiction). If applicable, provide a total
swale depth that is designed to accommodate larger runoff events and meet local jurisdiction
conveyance and freeboard requirements. In these situations, it is important to verify that the swale is
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5.6 Biofiltration Swale
designed to provide both runoff treatment for the RTF (with a maximum RTF flow depth of 4 inches)
and adequate conveyance for larger rainfall events. It is also important to verify that drainage is
being handled without flooding critical areas, structures, or adjacent streets.
Note: Consider the expected depth of flow when selecting the Manning’s n coefficient during the
design process. The design Manning’s n coefficient to be used for checking conveyance of larger
runoff events will likely be lower than the value used in conjunction with the RTF.
Level Spreaders
Level spreaders are objects that are aligned
perpendicular to the direction of flow to
evenly distribute and maintain level flow in
the swale. They should be installed at the
head of the biofiltration swale and every
50 feet of swale length if the swale is 6 feet
or greater in bottom width. Level spreaders
and swale dividers may be constructed
using plastic boards, concrete, or other
materials that will not leach pollutants.
Constructed level spreaders should be
staked into the bottom of the swale with
nongalvanzed metal pins at 4 feet on center
minimum. Also consider installing sediment
cleanouts at the head of the swale if high
sediment loads are expected.
Check Dams
Check dams are 6- to 12-inch-tall obstructions that are aligned perpendicular to the direction of flow
to reduce the swale’s effective slope, thereby increasing the hydraulic residence time. Check dams
are recommended for swales on longitudinal slopes exceeding 2.5 percent. Design
recommendations for check dams are as follows:
Design of check dams should consider the full range of design flows.
The swale should have a continuous grade between check dams.
Firmly anchor check dams into the bottom and side slopes of the swale.
Provide a weep hole or similar drainage feature within the check dam to allow ponded water
to drain following runoff events.
Armoring with quarry spalls may be needed at the downstream toe of the check dam to
prevent erosion.
Construct check dams using wood, concrete, stone, or other non-erodible material.
Check dams may take the place of level spreaders if they are designed and installed to
maintain level flow in the swale.
Use Equation 5.6-5 to calculate the effective slope of a swale when using check dams. Use
the effective slope to verify a design velocity of less than 1 ft/sec and hydraulic residence
time of at least 9 minutes for the RTF.
Se
=St
-
h
L
Equation 5.6-5
Figure 5.6-4. Level Spreader
Source: Courtesy of the City of Kalispell
Level spreaders at concentrated flow inlets help to evenly
distribute flow within the swale.
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5.6 Biofiltration Swale
Where
Se = effective slope (%)
St = longitudinal slope of the swale (%)
h = height of check dam (ft)
L = distance between check dams (ft/ft)
5.6.5 Vegetation Considerations
Vegetation is an essential component of biofiltration swales because it provides erosion control,
enhances site stability, and filters pollutants. Developing a landscaping plan for a biofiltration swale
and surrounding area is required to indicate how the swale will be stabilized and established with
vegetation. Considerations when developing the vegetation and landscaping plan are as follows:
Durable, dense, and drought-tolerant grasses are recommended. Grass selection should
consider both short- and long-term maintenance requirements because some varieties have
higher maintenance requirements than others.
Biofiltration swales should be planted with salt-tolerant plant species if roadway salt will be
applied to the contributing drainage area.
Topsoil should be imported from an offsite location if the site’s existing soils are not
conducive to establishing healthy vegetation.
Irrigation systems will likely be
necessary to establish vegetation.
These systems can be temporary or
permanent, depending on the type of
vegetation to be used in the swale.
Irrigation scheduling must be
appropriate for the selected vegetation
because overwatering can decrease the
permeability of the soil and under
watering may hinder vegetation
establishment and reduce the straining
capabilities of the vegetation (38).
If possible, divert runoff (other than
necessary irrigation) during the period of
vegetation establishment. Where runoff
diversion is not possible, protect graded
and seeded areas with suitable erosion
control measures.
Use of sod is not recommended for
biofiltration swales because seeding
establishes deeper roots and sod may
contain soil that is not conducive to
infiltration (37).
Note: Given the wide range of native vegetation across Montana, designers should consult local
specialists, landscape architects, and/or agencies for recommendations on appropriate plant
species and landscaping considerations for the site.
Figure 5.6-5. Biofiltration Swale Sign
Source: HDR
Signs can discourage activities that would cause
compaction and disturbance of vegetation.
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5.6 Biofiltration Swale
5.6.6 Construction Considerations
Basic construction considerations and recommendations are provided below.
Construction Site Management
Acquire all applicable permits prior to construction. See Section 1.4 for more information.
Apply appropriate erosion control measures to minimize erosion during construction.
Topsoil should be stripped, stockpiled, and reapplied just prior to seeding of the biofiltration
swale.
To the extent practicable,
construction equipment should be
restricted from the swale area to
prevent compaction of the native
soils.
Perform fine grading, application of
topsoil, and seeding only after
upgradient areas have been
stabilized and all work crossing the
swale has been completed (43).
Contributing drainage areas should
be properly stabilized with the
appropriate erosion and sediment
controls or permanent seeding
before allowing storm water runoff to
drain to the swale.
Construction Inspections
Inspections are recommended
during the following phases of
construction:
o Pre-construction meeting
o Initial site preparation
o Excavation/grading
o Implementation of the
vegetation and landscaping
plan
o Final inspection
Inspect level spreaders and check
dams (if applicable) to verify they
are at correct elevations and are
properly installed.
Check that outfall protection/energy
dissipation measures at
concentrated inflow and outflow
points are stable (37).
Inspectors should be familiar with project plans and specifications to ensure the contractor’s
interpretation of the plans is consistent with the designer’s intent. The inspectors should take
frequent photos and notes of construction activities and features as work progresses and at
Figure 5.6-6. Degrading Concentrated Flow Inlet
Source: HDR
To prevent erosion, inlet areas must be designed to dissipate
flows and be stabilized prior to allowing runoff to enter the
facility.
Figure 5.6-7. Swale Vegetation Establishment
Source: HDR
Post-construction inspections should be conducted to verify
that thick and healthy vegetation is being established.
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5.6 Biofiltration Swale
all critical points during the construction process. The photos will serve as a helpful resource
when creating inspection reports (16).
Transition to Post-Construction
Develop a plan prior to construction that will allow for an effective transition from construction
storm water management BMPs to post-construction BMPs without compromising the
integrity of the post-construction BMPs.
Coordinate with the local jurisdiction prior to terminating coverage of the Construction
General Permit.
5.6.7 Maintenance
Maintenance is required on all BMPs. Recommended maintenance activities are provided in Table
5.6-2, which may be used as a guide when developing a maintenance plan. Additionally, an example
inspection form is provided in Appendix F that may be adapted or adopted as part of the
maintenance plan.
Table 5.6-2. Recommended Maintenance Activities for a Biofiltration Swale
Activity Frequency
Make sure full coverage of turf grass or erosion control fabric has been achieved following
construction, both on the channel bed and side slopes
Inspect the swale during and after runoff events to ensure that the swale is operating as
designed and inspect for erosion
Upon
establishment
Remove litter/debris from all components of the biofiltration swale
Manage all vegetation during the growing season and maintain grass heights as specified
during the design documents. Remove all clippings.
As needed
Ensure that the contributing drainage area is clear of debris
Ensure that the contributing drainage area is stabilized and perform spot-reseeding if or where
necessary
Repair undercut and eroded areas as needed at swale inflow and outflow structures
If applicable, inspect upstream and downstream of check dams for evidence of undercutting
or erosion, and remove trash or blockages at weepholes
Quarterly
Reseed as needed during fall seeding season to maintain 90% turf grass cover
Remove any accumulated sand or sediment deposits behind check dams
Examine channel bottom for evidence of erosion, braiding, excessive ponding or dead grass
Check inflow points for clogging and remove any sediment
Inspect side slopes and filter strips for evidence of erosion
Inspect all components of the biofiltration swale in accordance with an approved inspection
form according to local jurisdiction requirements. An example inspection form is provided in
Appendix F.
Annually
Source: West Virginia Department of Environmental Protection (37)
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5.6 Biofiltration Swale
5.6.8 Plan View and Typical Details
Figure 5.6-8. Biofiltration Swale Plan and Section View
Source: Adapted from WSDOT (40)
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5.6 Biofiltration Swale
Figure 5.6-9. Biofiltration Swale Typical Details
Source: Adapted from WSDOT (40)
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5.7 Extended Detention Basin
5.7 Extended Detention Basin
Figure 5.7-1. Extended Detention Basin
Source: Courtesy of UDFCD
Description
A constructed basin designed to capture and treat
storm water runoff. Runoff is detained for a minimum
of 48 hours, providing time for pollutants to settle out
prior to discharge. These facilities are sometimes
referred to as a “dry ponds” because they are
designed to remain empty between runoff events.
Primary Components Primary Function
Inlet structure
Pretreatment forebay
Main treatment cell
Micropool
Outlet structure
Runoff reduction
Runoff treatment
Benefits Limitations
Siting is generally not limited by native soils; design
accommodations can be made for most soil types
Maintenance can be achieved using equipment and
skills common to most MS4s
The facility can be designed for multiple uses such
as runoff treatment, flood control, and open space
Not recommended for contributing drainage basins
of less than 5 impervious acres
Typically require a relatively large continuous area
Relatively ineffective at removing dissolved
pollutants
Ponding time and depths may generate safety
concerns and vector issues
Design and Site Selection Considerations
Setbacks
Depth to groundwater or bedrock
Soil permeability
Soil preparation/amendments/compost
Pretreatment forebay
Inlet and outlet spacing
Energy dissipater/level spreader
Underdrain
Facility liners
Landscaping/planting
Fencing
Size of contributing drainage area
Area required
Incorporate flood control
TMDL Considerations Maintenance Requirements
Avoid Preferred
Total suspended solids (TSS)
Total phosphorus
Total nitrogen
Temperature
Metals
Fecal coliform
Access roads or pullouts
Sediment removal
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5.7 Extended Detention Basin
5.7.1 General Description
An extended detention basin (EDB) is a sedimentation basin designed to detain and slowly release
storm water over an extended period of time following a rainfall event. This BMP is similar to a
detention basin used for flood control except it uses a smaller outlet that extends the emptying time
of the more frequently occurring runoff events to improve pollutant removal.
The primary characteristics of an EDB are as follows:
EDBs consist of an inlet, a pretreatment forebay, and a main treatment cell that includes a
trickle channel, a micropool, and an outlet structure.
An EDB has a minimum 48-hour drain time for the RTV, facilitating the removal of TSS (44).
EDBs can be designed to provide both runoff treatment and flood control.
5.7.2 Performance
Runoff Reduction
Runoff reduction is not considered to be a function of EDBs because they generally discharge a
volume equivalent to the entire inflow runoff volume.
Runoff Treatment
An EDB is expected to achieve an 80 percent removal rate of TSS from the RTV when designed,
operated, and maintained as described in this manual (45).
5.7.3 Site Selection
The basic guidelines are provided below to aid in evaluating whether EDBs are feasible for use at an
individual site.
Contributing Drainage Area
EDBs are best suited for sites with contributing basins ranging from 5 impervious acres to
1 square mile. EDBs located at sites with drainage areas of less than 5 impervious acres can
result in small orifice sizes that are prone to clogging (44).
Soil Characteristics
EDBs are suitable for almost all soil types; however, special consideration should be taken
for sites located within karst regions, as discussed in Chapter 4.
Sites containing soils with high infiltration rates may have the potential for seepage through
the embankment. An impermeable liner may be appropriate in these situations.
Soil characteristics can initially be estimated from NRCS soil data, but should be field-verified
prior to final design using the onsite soil investigation methods discussed in Chapter 2 and
Appendix C of this manual.
Depth to Groundwater and/or Bedrock
The seasonal high water table and/or bedrock should be 2 or more feet below the bottom of
the basin unless a licensed engineer with geotechnical expertise (PE), or other licensed
professional acceptable to the local jurisdiction, judges that conditions are acceptable for
keeping the basin dry and maintainable based on site-specific test data or analysis and the
potential for groundwater contamination has been evaluated (44).
An evaluation of the depth to groundwater should be conducted, as described in Section
4.3.3.
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5.7 Extended Detention Basin
Site Topography
The site should be able to accommodate a slope within the EDB that is steep enough to
ensure that flows are able move through the system.
It is recommended that the overall slope of the site should be less than 15 percent (46).
Land Use and Characteristics of Surrounding Area
Locating EDBs in densely developed areas (i.e., downtown areas) is difficult and often cost-
prohibitive because of the amount of land needed to properly size the facility.
EDBs located on soils with high infiltration rates or near steep slopes may result in shallow
lateral flow (interflow) that can reemerge and negatively affect down-gradient structures. For
these sites, an assessment of the impact on down-gradient structures is recommended.
Community and Environmental Considerations
Water temperatures may increase between the inlet and outlet of an EDB during summer
months.
EDBs have the potential to affect naturally sensitive features such as wetlands and trees
located within or directly adjacent to the site. For example, changes in inundation frequency
can affect established trees or other vegetation (47).
Shallow wet areas have the potential to create conditions that lead to mosquito breeding.
Constructing EDBs with consistent slopes can facilitate proper draining between events and
help reduce the potential for shallow wet areas to develop.
Opportunities may be available for an EDB to be located within or near multiuse facilities
such as parks and open space.
Figure 5.7-2. Detention Basin in Billings, Montana
Source: HDR
EDBs can be sized to incorporated both runoff treatment and flood control
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5.7 Extended Detention Basin
5.7.4 Design and Sizing Procedure
The following steps outline the design procedure and criteria for an EDB in a contributing basin
greater than or equal to 5 impervious acres. The information provided has been adopted from the
Urban Drainage and Flood Control District (UDFCD) Urban Storm Drainage Criteria Manual,
Volume 3, with minor revisions that account for local considerations. EDBs contain certain features
for which local standards and preferences may affect the design process (e.g., outlet structures,
trash racks, and embankments). Guidance and standards from the local jurisdiction should be
considered during the design process.
1. Basin Storage Volume
Design the basin storage volume to be at least
100 percent of the RTV. This volume begins at
the invert of the lowest orifice in the outlet
structure, as shown in Figure 5.7-7. Calculate
the RTV using Equation 3-2 in Section 3.2. If the
EDB is designed for flood control, the flood
volume would be stacked on top of the RTV
volume in the basin.
2. Basin Shape
Maximize the distance between the inlet and the
outlet by providing a basin length-to-width ratio
of at least 2:1 to minimize short circuiting and
improve sediment removal. The flow path length
is defined as the distance from the inlet to the
outlet as measured at the surface. The width is
defined as the mean width of the basin (see
Figure 5.7-7).
Design the main treatment cell with a depth
between 2 and 5 feet, depending on local
standards (note that this depth includes the
initial surcharge volume discussed below). The
design depths for the forebay, trickle channel,
and micropool are discussed in their respective
sections below.
3. Basin Side Slopes
Basin side slopes should be stable and gentle to
facilitate maintenance and access. 4:1 (H:V) or
flatter side slopes are preferred to allow for
conventional maintenance equipment and for
improved safety and aesthetics. Side slopes should be no steeper than 3:1 (H:V); however, local
design standards should be consulted to confirm the maximum allowable slopes. Using walls is
discouraged because of maintenance constraints.
Extended Detention Basin Minimum Design Criteria
1
Required Components
Inlet structure
Pretreatment forebay
Main treatment cell
Trickle channel
Micropool
Outlet structure
Design and Sizing
General
Basin storage is 100% of RTV (minimum)
Basin shape of 2:1 (L:W) (minimum)
Main treatment cell depth of 2 feet (minimum)
Provide maintenance access
Provide a landscaping plan
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5.7 Extended Detention Basin
4. Inlet
Inlet locations should be designed to
dissipate flow energy using materials such
as riprap or concrete to limit erosion and
promote particle sedimentation.
5. Pretreatment Forebay
A pretreatment forebay, located at each
major inlet, provides an opportunity for
larger particles to settle out prior to
discharging flows to the main treatment cell.
This feature helps to preserve the capacity
of the main treatment cell. Guidance for
forebay sizing and design are as follows:
Maximize the length of the flow path
through the forebay and minimize
the slope to encourage settling.
Provide a depth between 4 and 6 feet with a volume equal to 10 percent of the RTV.
Size the forebay outlet structure to discharge the RTV at a flow rate equal to 2 percent of the
100-year undetained peak discharge. This structure can be created as an armored earthen
berm with 3:1 (H:V) side slopes (or flatter) using gabion, concrete, or riprap along the
separation embankment preceding the main treatment cell. Protecting the berm from erosion
is important because it will overtop frequently.
A concrete bottom is recommended to facilitate sediment removal during maintenance.
Provide a way to monitor sediment accumulation. Options include a metered rod in the
forebay or concrete lining that defines sediment removal limits.
6. Trickle Channel
Convey low flows from the forebay to the micropool using a trickle channel. The trickle channel may
be either concrete or soft-bottomed, with a flow capacity equal to the maximum discharge from the
forebay outlet (2 percent of the 100-year undetained peak discharge).
Concrete Trickle Channels: A concrete trickle channel will help establish the bottom of the
basin in the long term and may also facilitate regular sediment removal. It can be a “V”-
shaped concrete drain pan or a concrete channel with curbs. A flat-bottom channel facilitates
maintenance. A slope between 0.4 and 1 percent is recommended to encourage settling
while reducing the potential for low points in the pan.
Soft-bottom Trickle Channels: Soft-bottom trickle channels offer an attractive alternative to
concrete and can improve water quality. However, they are not appropriate for all sites
because maintenance requires mechanical removal of sediment and vegetation, and this
option can increase the likelihood of creating a mosquito habitat. Therefore, they should be
considered on a case-by-case basis with the approval of the local jurisdiction. Soft-bottom
trickle channels should be designed with a consistent longitudinal slope from the forebay to
the micropool, and they should not meander. This geometry will allow for reconstruction of
the original design when sediment removal in the trickle channel is necessary. The trickle
channel may also be located along the toe of the slope if a straight channel is not desired.
The recommended minimum depth of a soft-bottom trickle channel is 1.5 feet, which will help
limit potential wetland growth to the trickle channel, preserving the bottom of the basin.
Figure 5.7-3. Pretreatment Forebay and Trickle Channel
Source: Courtesy of UDFCD
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5.7 Extended Detention Basin
Riprap and soil riprap lined trickle channels are not recommended because of increased
maintenance requirements. Specifically, when sediment is removed during routine maintenance,
riprap may be inadvertently removed and need to be replaced.
7. Micropool
A micropool is a small pool located in front
of the outlet structure, designed to prevent
sediment resuspension, protect the low flow
outlet riser orifices or perforated plate from
clogging, and reduce mosquito breeding
areas (44). Design guidance for the
micropool is as follows:
The side slopes may be stabilized
vertical walls or stabilized slopes of
up to 3:1 (H:V).
Provide a depth of at least 2.5 feet
with a minimum surface area of 10
square feet.
A concrete bottom is recommended.
8. Outlet Structure
The purpose of the outlet structure is to detain and slowly release runoff, allowing pollutants to settle
out prior to release and to safely discharge runoff volumes accumulated from larger storm events.
An EDB will typically have a multistage outlet control structure that includes a low-flow water quality
outlet (typically an orifice), a 10- or 25-year design storm outlet depending on local jurisdiction
requirements (typically a drop inlet, pipe, or weir), and may also include an auxiliary or emergency
spillway designed to pass the 100-year runoff event (weir or armored spillway built into the
embankment).
This manual provides guidance for sizing an orifice plate outlet structure (see Figure 5.7-5);
however, a variety of outlet structure configurations could be used to meet the project’s storm water
management objectives. Consult the local jurisdiction prior to selecting an outlet structure
configuration because preferences may vary throughout the state.
General outlet structure design guidance is as follows:
Locate the outlet structure in the embankment of the EDB and provide a permanent
micropool directly in front of the structure.
The outlet may be sized for the RTV only or it may have a multistage control structure,
depending on whether the facility is designed for water quality only or includes flood control.
Outlet structure orifice sizing guidance is provided in Section 5.8.4, in the Outlet Structure
subsection.
A minimum drawdown time for the RTV of 48 hours is required, and 72 hours is the
maximum drawdown time recommended. Refer to local standards because some
jurisdictions have different maximum drawdown time recommendations.
For orifice plate outlets, the trash rack may be submerged to the bottom of the micropool.
This will reduce potential for clogging of the trash rack because it allows water to flow
through the trash rack below the elevation of the lowest orifice even when the area above the
water surface is plugged. This will prevent shallow ponding in front of the structure, which
Figure 5.7-4. Micropool and Outlet Structure
Source: Courtesy of UDFCD
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5.7 Extended Detention Basin
provides a breeding ground for mosquitoes (large, shallow puddles tend to produce more
mosquitoes than a smaller, deeper permanent pond).
The low-flow orifice should be adequately protected from clogging by either an acceptable
external trash rack (recommended minimum orifice of 3 inches) or by internal orifice
protection that may allow for smaller diameters (recommended minimum orifice size of 1
inch). Orifices less than 3 inches in diameter may require extra maintenance because of the
increased potential for clogging.
Perforated riser pipes should be used with caution in cold climates because ice cover can
cause clogging of the orifices.
Ensure that the outlet structure is designed to accommodate the peak flows generated from
each design event. For instance, if the EDB is designed to incorporate flood control, the
outlet structure must be sized to safely pass the design flood flow while the basin maintains
freeboard requirements specified by the local jurisdiction. An overflow outlet and/or weir may
be required by the local jurisdiction to safely pass volumes greater than the RTV.
Figure 5.7-5. Example Orifice Plate Outlet Structure (Runoff Treatment Volume Only)
Source: Adapted from UDFCD (48)
1
9. Initial Surcharge Volume
Providing a surcharge volume above the micropool for frequently occurring runoff events minimizes
standing water and sediment deposition in the remainder of the basin. The initial surcharge volume
is not part of the micropool nor does it include the micropool volume; rather, it is the available
storage volume that begins at the water surface elevation of the micropool and extends upward to a
grade break in the basin (typically the trickle channel’s invert). Design guidance for incorporating the
initial surcharge volume is as follows:
The surface area of the initial surcharge volume, when full, is typically the same as or slightly
larger than that of the micropool.
The recommended initial surcharge volume is at least 0.3 percent of the RTV at a depth of at
least 4 inches.
10. Trash Rack
Most basins will collect a certain amount of trash and debris from incoming flows. Floating debris
such as grass clippings, tree limbs, leaves, trash, construction debris, and sediment bed load from
upstream watersheds are common. A trash rack located at the outlet control structure of the facility
can help reduce the potential for clogging. General trash rack design guidance is as follows:
1
Additional guidance pertaining to the analysis and design of orifice plate outlet structures is available in Fact
Sheet T-12: Outlet Structures, in UDFCD’s Urban Storm Drainage Criteria Manual, Volume 3 (available at
http://udfcd.org/).
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5.7 Extended Detention Basin
The trash rack’s size should provide the necessary hydraulic capacity while the rack is
partially clogged.
Openings should be small enough to limit clogging of the individual orifices.
Where applicable, it is recommended that trash racks be installed at a shallow (~15°) angle
to prevent ice formation (49).
All drop inlet spillways designed for pressure flow should have adequate anti-vortex devices.
An anti-vortex device is not required if weir control is maintained in the riser through all flow
stages, including the maximum design storm. Examples of anti-vortex devices include a
baffle or plate installed on top of the riser, or a headwall set on one side of the riser (50).
11. Embankment and Overflow Spillway
EDBs are typically constructed with an overflow spillway designed to safely convey excess flows
through the facility. Design guidance for the overflow spillway and embankment is as follows:
If the embankment falls under the jurisdiction of Montana DNRC, it must be designed to meet
the applicable requirements.
Embankment soils should be compacted as determined by a licensed engineer.
Slopes that are 4:1 (H:V) or flatter are preferred to allow for conventional maintenance
equipment and for improved safety, maintenance, and aesthetics.
Locate the overflow spillway at a point where waters can best be conveyed downstream.
It is recommended that the overflow spillway be designed to safely convey runoff from the
100-year storm, at a minimum.
Design spillway structures and associated freeboard in accordance with applicable state or
local regulations.
Materials such as concrete, riprap, or articulated concrete block mats may be necessary to
mitigate the potential for erosion and failure of the spillway during less frequent events.
12. Maintenance Access
Considering maintenance access during the design phase of an EDB is critical because it will help to
facilitate the facility’s long-term performance. Guidelines for the design of maintenance access are
as follows:
Provide appropriate maintenance access to the forebay and micropool/outlet works. For
larger basins, this typically means stabilized access designed to withstand the expected
loads from maintenance vehicles.
Stabilized access typically includes materials such as concrete, articulated concrete block,
concrete grid pavement, or reinforced grass pavement.
Grades of less than 10 percent for haul road surfaces and 20 percent for skid-loader and
backhoe access are preferred. A cross slope of 2 percent is recommended for drainage.
If stabilized access is not provided, a maintenance plan that provides details including
recommended equipment and a plan for sediment and trash removal from the outlet
structure and micropool may be required by the local jurisdiction.
13. Guidelines for Incorporating Flood Control
EDBs can be designed to provide flood control by increasing the surcharge volume for flood
detention storage and designing the outlet structure to detain and release flood flows. Reservoir
routing calculations may be used to assist in the outlet structure design for larger runoff events.
Appropriate flood control design guidance and local regulations should be referenced when
incorporating flood control into an EDB.
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5.7 Extended Detention Basin
5.7.5 Vegetation Considerations
Vegetation is an essential component of an EDB because it provides erosion control and enhances
site stability. Developing a landscaping plan for the EDB and surrounding area is required to indicate
how the EDB will be stabilized and established with vegetation. Considerations when developing the
vegetation and landscaping plan are as follows:
Delineate landscaping zones within and surrounding the EDB.
Select suitable plant species, including drought-tolerant plants, where applicable.
Include sources of native plant material in a planting plan.
Determine the location and type of irrigation facilities, if necessary. Where possible, place
irrigation heads outside the basin bottom because irrigation heads in an EDB can be buried
with sediment over time.
Delineate areas that should be avoided during construction to avoid compacting native soils.
Omit woody vegetation within 15 feet of the toe of the embankment or within 25 feet of the
principal spillway structure because of potential impacts of root systems (47).
Note: Given the wide range of native vegetation across Montana, designers should consult local
specialists, landscape architects, and/or agencies for recommendations on appropriate plant
species and landscaping considerations for the site.
5.7.6 Construction Considerations
Basic construction considerations and recommendations are provided below.
Construction Site Management
Acquire all applicable permits prior to construction. See Section 1.4 for more information.
Apply appropriate erosion control measures to minimize erosion during construction. Refer to
the local jurisdiction’s construction site storm water management program for additional
guidance and local requirements.
If used as a construction storm water management BMP, an EDB should be dewatered,
dredged, and re-graded prior to post-construction implementation.
Construction Inspection
Inspections are recommended during the following phases of construction:
o Pre-construction meeting
o Initial site preparation
o Excavation/grading
o Installation of the embankment, spillway(s), and outlet structure
o Implementation of the vegetation and landscaping plan
o Final inspection
Transition to Post-Construction
Develop a plan prior to construction that will allow for an effective transition from construction
storm water management BMPs to post-construction BMPs without compromising the
integrity of the post-construction BMPs.
Coordinate with the local jurisdiction prior to terminating coverage of the Construction
General Permit.
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5.7 Extended Detention Basin
5.7.7 Maintenance
Maintenance is required on all BMPs. Recommended maintenance activities are provided in Table
5.7-1, which may be used as a guide when developing a maintenance plan. Additionally, an example
inspection form is provided in Appendix F that may be adapted or adopted as part of the
maintenance plan.
Table 5.7-1. Recommended Maintenance Activities for an EDB
Activity Frequency
Remove litter/debris from all components of the EDB.
Repair inlet, outlet, trickle channel, and all other structural components required for the basin to
operate as intended.
Repair and revegetate eroded areas. If turf grass requires replacement, use a species with
similar growth requirements.
Regularly manage all vegetation and remove all clippings.
Where applicable, irrigate during dry weather and replace broken sprinkler heads. Completely
drain the irrigation system before the first winter freeze and check for damaged components
upon reactivation in the spring.
Repair maintenance access routes, if applicable.
Inspect the EDB for signs of mosquito larvae during summer months and provide treatment
when breeding is found. If available, a local mosquito control service could be used to carry out
these inspections.
As needed
Trim vegetation for aesthetics and mosquito control. Prevent establishment of woody
vegetation on or near berms or embankments.
Evaluate the health of vegetation and remove and replace any dead or dying plants.
Remove all green waste and dispose of properly.
Semiannually
Inspect all components of the EDB in accordance with an approved inspection form according
to local jurisdiction requirements. An example inspection form is provided in Appendix F.
Annually
Remove sediment from the micropool when the depth has been reduced to approximately
18 inches.
Remove sediment from the forebay before it becomes a significant source of pollutants for the
remainder of the EDB.
Typically 1 to
4 years
Remove accumulated sediment and re-grade when the accumulated sediment volume exceeds
10 percent of the main treatment cell design volume. Dispose of sediment properly.
Typically 10 to
20 years
(or as needed)
Figure 5.7-6. Detention Basin Deferred Maintenance
Source: Courtesy of the City of Bozeman
Conducting routine maintenance is a critical component to the performance of all BMPs. Deferred maintenance
allowed this BMP to be overrun with vegetation.
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5.7 Extended Detention Basin
5.7.8 Plan View and Typical Details
Figure 5.7-7. Extended Detention Basin Plan View and Typical Section
Source: Adapted from UDFCD (44)
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5.8 Wet Detention Basin
5.8 Wet Detention Basin
Figure 5.8-1. Wet Detention Basin
Source: HDR
Description
A constructed basin that maintains a permanent pool
of water and is designed to manage storm water
runoff. Runoff is detained for a minimum of 48 hours,
providing time for pollutants to settle out prior to
discharge.
Primary Components Primary Function
Inlet structure
Pretreatment forebay
Permanent pool
Outlet structure
Runoff reduction
Runoff treatment
Benefits Limitations
Siting is generally not limited by native soils. Design
accommodations can be made for most soil types.
Maintenance can be achieved using equipment and
skills common to most MS4s
The facility can be designed for multiple uses such
as runoff treatment, flood control, and open space
Not recommended for basins less than 10 acres
Typically require a relatively large continuous area
There is a potential for safety concerns associated
with open waters
Attraction of water fowl can increase nutrients and
bacteria leaving the pond
The facility must be able to maintain a permanent
pool of water
Design and Site Selection Considerations
Setbacks
Depth to groundwater or bedrock
Soil permeability
Soil preparation/amendments/compost
Pretreatment forebay
Inlet and outlet spacing
Energy dissipater/level spreader
Underdrain
Facility liners
Landscaping/planting
Fencing
Size of contributing drainage area
Area required
Incorporate flood control
TMDL Considerations Maintenance Requirements
Avoid Preferred
Total suspended solids
Total phosphorus
Total nitrogen
Temperature
Metals
Fecal coliform
Access roads or pullouts
Sediment removal
Irrigation
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5.8 Wet Detention Basin
5.8.1 Description
A wet detention basin (WDB) is a constructed storm water basin designed to capture, detain, and
slowly release runoff to promote pollutant removal through sedimentation and biological uptake. This
BMP maintains a permanent pool of water throughout most of the year. Water in the permanent pool
is partially displaced during each runoff event, allowing storm water runoff to mix with permanent
pool water prior to discharge. An additional function of the permanent pool is to minimize
resuspension of sediments and other pollutants deposited during prior runoff events.
The primary characteristics of a WDB are as follows:
A two-cell pond that is separated by a baffle or berm. An inlet structure conveys runoff into
the first cell, which is referred to as the pretreatment forebay. The second larger cell is
referred to as the wetpool cell, which contains an outlet structure at its downstream end.
A minimum 48-hour drain time for the RTV facilitates the removal of TSS.
WDBs can be designed to provide both runoff treatment and flood control.
5.8.2 Performance
Runoff Reduction
Runoff reduction is not considered to be a function of WDBs because they generally discharge a
volume equivalent to the entire inflow runoff volume.
Runoff Treatment
A WDB is expected to achieve an 80 percent or greater removal rate of TSS from the RTV when
designed, operated, and maintained as described in this manual (51).
5.8.3 Site Selection
Basic guidelines are provided below to aid in evaluating whether WDBs are feasible for use at an
individual site.
Contributing Drainage Area
A site with a consistent inflow (typically via groundwater) is desirable to help maintain a
permanent pool.
WBDs are best suited for sites with contributing basins greater than 10 acres. WDBs with
drainage areas of less 10 acres can result in outlet structures that have small orifice sizes
that are prone to clogging and may have difficulty maintaining a permanent pool.
Contributing basins of less than 10 acres may be acceptable, particularly if the groundwater
table provides a base flow to the pond and a water balance indicates that a permanent pool
can be sustained.
Soil Characteristics
Soils with low infiltration rates are preferred to maintain a permanent pool; however, sites
containing soils with high infiltration rates may be acceptable if an impermeable liner is
provided and a water balance demonstrates that a permanent pool will be maintained.
WDBs are not recommended for Karst terrain.
Soil characteristics can initially be estimated from NRCS soil data, but should be field-verified
prior to final design using the onsite soil investigation methods discussed in Chapter 2 and
Appendix C of this manual.
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5.8 Wet Detention Basin
Depth to Groundwater and/or Bedrock
In general, no minimum separation distance is required for WDBs because intercepting the
groundwater table can help to maintain a permanent pool. However, a separation distance or
impermeable liner may be appropriate for groundwater protection considerations at sites
where there is a sensitive underlying aquifer and the bottom material of the pond allows for
infiltration.
An evaluation of depth to groundwater should be conducted, as described in Section 4.3.3.
Site Topography
The site should be able to accommodate an elevation difference between the inlet and outlet
that is large enough to ensure that flows are able move through the system.
It is recommended that slopes immediately adjacent to the WDB be less than 25 percent to
limit erosion, but greater than 0.5 percent to promote flow toward the pond.
Land Use and Characteristics of Surrounding Area
Large open areas are typically required to site a WDB, which may not be cost-effective in
dense urban areas.
Use caution when placing a WDB in a drainage area where development will not be
completed for an extended period or where the potential for a chemical spill is higher than
typical. When these conditions exist, it is critical to provide adequate containment and/or
pretreatment (44).
WDBs located on soils that facilitate infiltration or near steep slopes may result in shallow
lateral flow (interflow) that can reemerge and negatively affect down-gradient structures. For
these sites, an assessment of the impact on down-gradient structures is recommended.
Community and Environmental Considerations
Water temperatures may increase between the inlet and outlet of a WDB during summer
months (52).
WBDs can be an attractive
landscape and promote habitat for
fish and upland wildlife; however,
attraction of geese and waterfowl is
typically not desirable because their
droppings add to nutrients and
bacterial loading in the WDB and
downstream waterways. Strategies
for discouraging waterfowl use are
discussed in Section 5.8.5.
Safety concerns are often
associated with open waters.
Consult the local jurisdiction for
fencing or sign requirements around
ponds.
Figure 5.8-2. WDB with Permanent Pool
Source: HDR
A WDB should maintain a permanent pool throughout the
growing season.
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5.8 Wet Detention Basin
5.8.4 Design and Sizing Procedure
The following steps outline the design procedure and criteria for a WDB. The information provided
has been adapted from the UDFCD Urban Storm Drainage Criteria Manual, Volume 3, with minor
revisions that account for local considerations. WDBs contain certain features for which local
standards and preferences may affect the design process (e.g., outlet structures, trash racks, and
embankments). Guidance and standards from the local jurisdiction should be considered during the
design process.
1. Permanent Pool Volume
The permanent pool provides storm water
quality treatment between runoff events,
primarily through sedimentation. Design the
permanent pool volume to be 100 percent of the
RTV. Determine the RTV using the guidance
provided in Section 3.2.
2. Surcharge Volume
The surcharge volume is the volume located
directly above the permanent pool water surface
elevation (WSE) and encompasses both the
wetpool cell and pretreatment forebay (see
Figure 5.8-7). Design the WDB to accommodate
a minimum surcharge volume equal to
100 percent of the RTV. The surcharge volume
may be increased if additional storage capacity
is desired for flood control considerations.
3. Base Flow and Water Budget
Maintaining a permanent pool is critical to the
performance of a WDB. Climatic conditions vary
throughout the state, but overall most areas are
relatively arid, which makes maintaining a
permanent pool challenging if base flow is not
provided to the facility. For this reason, a
groundwater base flow is strongly
recommended.
To ensure a permanent pool is maintained,
develop an overall water budget to confirm
inflows will exceed losses attributable to infiltration and evaporation. Some considerations when
performing a water budget are as follows:
Potential inflows include runoff, base flow, and rainfall.
Net inflow calculations should be conservative to account for annual variations in hydrologic
conditions.
Potential outflows include infiltration, surface overflow, and evapotranspiration.
Evaporation can be estimated from existing local studies or from the National Weather
Service Climate Prediction website.
Wet Detention Basin Minimum Design Criteria
1
Required Components
Inlet structure
Pretreatment forebay
Wet pool cell
Outlet structure
Design and Sizing
General
Permanent pool is 100% of RTV (minimum)
Basin shape of 2:1 (L:W) (minimum)
Surcharge volume is 100% of RTV (minimum)
Ability to maintain a permanent pool
Provide maintenance access
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5.8 Wet Detention Basin
4. Basin Shape
Maximize the distance between the inlet and outlet by providing a basin length-to-width ratio of at
least 2:1, which will minimize short circuiting and improve sediment removal. The flow path length is
defined as the distance from the inlet to the outlet as measured at the surface. The width is defined
as the mean width of the basin.
The wetpool cell should have two depth zones:
Safety-Wetland Bench: The safety-wetland bench is a gently sloped bench located along
the perimeter of the wetpool cell that provides a shallow area allowing people or animals that
inadvertently enter the open water to gain footing to get out of the pond. Additionally, the
bench facilitates aquatic plant growth along the perimeter of the permanent pool, which can
help strain surface flow into the pond, protect the banks from erosion by stabilizing the soil at
the edge of the pond, and provide biological uptake. Design the bench to be 6 to 12 inches
deep and a minimum of 4 feet wide.
Open Water Zone: The remaining pond area should be open, providing a volume to promote
sedimentation and nutrient uptake by phytoplankton. Design the permanent pool with a depth
between 4 and 12 feet, depending on local standards. The minimum depth helps to prevent
resuspension of settled pollutants and encourages proper mixing, and the maximum depth
helps to minimize stratification and an imbalance between pool volume and surface area (53)
(54). For safety considerations associated with open waters, a fence or signs may be
required by the local jurisdiction.
5. Inlet
Design the WDB so that inlets discharge into a pretreatment forebay. The inlet locations should be
designed to dissipate flow energy to limit erosion and promote particle sedimentation.
6. Pretreatment Forebay
A pretreatment forebay, located at each major inlet, provides an opportunity for larger particles to
settle out prior to discharging flows to the wetpool cell, helping preserve the capacity of the wetpool
cell. Guidance for forebay sizing and design are as follows:
Maximize the length of the flow path through the forebay and minimize the slope to
encourage settling.
Provide a depth between 4 and 6 feet with a volume equal to 10 percent of the RTV.
A barrier separating the
pretreatment forebay and wetpool
cell should be constructed to contain
the forebay opposite of the inlet. If
the barrier is an earthen berm, a
minimum top width of 8 feet and
side slopes no steeper than 4:1 are
recommended. The barrier should
be armored with using material such
as gabions, concrete, or riprap to
minimize erosion because the berm
may overtop frequently.
A concrete bottom is recommended
to facilitate sediment removal during
maintenance.
Figure 5.8-3. WDB Pretreatment Forebay
Source: HDR
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5.8 Wet Detention Basin
Provide a way to monitor sediment accumulation. Options include a metered rod in the
forebay or concrete lining that defines sediment removal limits.
7. Side Slopes
Basin side slopes should be stable to facilitate maintenance and access. Side slopes above the
safety-wetland bench should be no steeper than 4:1 (H:V), preferably flatter to allow for conventional
maintenance equipment and for improved safety. The side slope below the safety-wetland bench
should be 3:1 (or flatter when access is required or when the surface could be slippery). The steeper
3:1 slope below the safety wetland bench can deter algae growth because it will reduce the shallow
area of the pond, thus reducing the amount of sunlight that penetrates the pond bottom. Local
design standards should be consulted to confirm the maximum allowable slopes.
8. Outlet Structure
The outlet structure detains and slowly releases runoff, allowing pollutants to settle out prior to
release and to safely discharge runoff volumes accumulated from larger storm events. A WDB will
typically have a multistage outlet control structure that includes a low-flow water quality outlet
(typically an orifice), a 10- or 25-year design storm outlet depending on local jurisdiction
requirements (typically a drop inlet, pipe, or weir), and may also include an auxiliary or emergency
spillway designed to pass the 100-year runoff event (weir or armored spillway built into the
embankment).
This manual provides guidance for sizing an orifice plate outlet structure (see Figure 5.8-4);
however, a variety of outlet structure configurations could be used to meet the project’s storm water
management objectives. Consult the local jurisdiction prior to selecting an outlet structure
configuration because preferences may vary throughout the state.
Figure 5.8-4. Typical Orifice Plate Outlet Structure
Source: Adapted from UDFCD (55)
2
General outlet structure design guidance is as follows:
Locate the outlet structure in the embankment of the WDB. This allows access for
maintenance.
A minimum drawdown time for the RTV of 48 hours is required, and 72 hours maximum is
recommended. Refer to local standards because some jurisdictions have different maximum
drawdown time recommendations.
2
Additional guidance pertaining to the analysis and design of orifice plate outlet structures is available in Fact
Sheet T-12: Outlet Structures, in UDFCD’s Urban Storm Drainage Criteria Manual, Volume 3 (available at
http://udfcd.org/).
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5.8 Wet Detention Basin
It is recommended that the low-flow orifice be adequately protected from clogging by either
an acceptable external trash rack (recommended minimum orifice of 3 inches) or by internal
orifice protection that may allow for smaller diameters (recommended minimum orifice size of
1 inch).
Orifices less than 3 inches in diameter may require extra maintenance because of the
increased potential for clogging.
An overflow outlet and/or weir may be required by the local jurisdiction to safely pass
volumes greater than the RTV.
Perforated riser pipes should be used with caution in cold climates because ice cover can
cause clogging of the orifices.
Ensure that the outlet structure is designed to accommodate the peak flows generated from
each design event for the WDB. For instance, if the WDB is designed to incorporate flood
control, the outlet structure must be sized to safely pass the design flood flow while the basin
maintains freeboard requirements specified by the local jurisdiction.
Orifice Sizing for the RTV (Average Hydraulic Head and Average Discharge Method)
Multiple methods could be used for designing the outlet structure configuration and orifice sizing.
This manual proposes using a simplified method whereby the full RTV is assumed to fill the basin
instantaneously, and the drawdown time is the time it takes to drain the RTV. Guidance for the low-
flow orifice sizing is as follows:
Calculate average release rate for the RTV using Equation 5.8-1.
QRTV
=
RTV
t
Equation 5.8-1
Where:
QRTV = Average orifice discharge for the RTV (cfs)
RTV = Runoff treatment volume (ft3)
t = Drawdown time, converted to seconds (48-hrs*3600 sec/hr)
Find the elevation associated with the RTV using the WDB’s stage-storage table and
calculate the approximate average head on the orifice using Equation 5.8-2.
h=
(WSERTV
-WSEPerm Pool)
2
Equation 5.8-2
Where:
h = Average head on orifice (ft)
WSERTV = Surcharge WSE, see Figure 5.8-4 (ft)
WSEPerm Pool = Permanent pool WSE, see Figure 5.8-4 (ft)
The orifice equation (Equation 5.8-3) can be reconfigured to calculate the diameter of the
orifice using Equation 5.8-4.
QRTV
=CAo√2gh Equation 5.8-3
dRTV
= [ 4*QRTV
πC√2gh
]
0.5
Equation 5.8-4
Where:
QRTV = Average orifice discharge for the RTV (cfs)
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5.8 Wet Detention Basin
C = Coefficient of discharge (typically 0.6 or as specified by vendor)
Ao = Cross sectional area of orifice (ft2)
g = Gravitational acceleration (32.2 ft/sec2)
h = Average head on orifice (ft)
dRTV = Diameter of low-flow orifice (ft)
9. Pond Drain
WDBs should be equipped with a method to drain the permanent pool when sediment removal is
required. A gravity drain pipe that can completely or partially drain the permanent pool within
24 hours is preferred; however, in cases where a low-level gravity drain is not feasible (such as in an
excavated pond), pumping may be necessary. The pond drain should be equipped with a valve that
will be opened only for maintenance.
10. Trash Rack
Most basins will collect a certain amount of
trash and debris from incoming flows.
Floating debris such as grass clippings, tree
limbs, leaves, trash, construction debris,
and sediment bed load from upstream
watersheds are common. A trash rack
located at the outlet control structure of the
facility can reduce the potential for clogging.
General trash rack design guidance is as
follows:
Size the trash rack to provide the
necessary hydraulic capacity while
the rack is partially clogged.
Openings should be small enough to
limit clogging of the individual
orifices.
Where applicable, trash racks should be installed at a shallow (~15°) angle to prevent ice
formation (49).
All drop inlet spillways designed for pressure flow should have adequate anti-vortex devices.
An anti-vortex device is not required if weir control is maintained in the riser through all flow
stages, including the maximum design storm. Examples of anti-vortex devices include a
baffle or plate installed on top of the riser, or a headwall set on one side of the riser (50).
11. Embankment and Overflow Spillway
WDBs are typically constructed with an overflow spillway designed to safely convey excess flows
through the facility. Design guidance for the embankment and overflow spillway is as follows:
If the embankment falls under the jurisdiction of Montana DNRC, it must be designed to meet
the applicable requirements (see Table 1-2).
Embankment soils should be compacted as determined by a licensed engineer.
Slopes that are 4:1 (H:V) or flatter are preferred to allow for conventional maintenance
equipment and for improved safety, maintenance, and aesthetics.
Locate the overflow spillway at a point where waters can best be conveyed downstream.
Figure 5.8-5. Outlet Structure Trash Rack
Source: HDR
A trash rack can reduce the potential for clogging of the outlet
structure.
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5.8 Wet Detention Basin
The overflow spillway should be designed to safely convey runoff from the 100-year storm, at
a minimum.
Design spillway structures and associated freeboard in accordance with applicable state or
local regulations.
In accordance with the local jurisdiction’s design standards, materials such as concrete,
riprap, or articulated concrete block mats may be necessary to mitigate the potential for
erosion and failure of the spillway during less frequent events.
12. Maintenance Access
Consideration of maintenance access during the design phase of a WDB is critical because it will
facilitate the WDB’s long-term performance. Guidelines for the design of maintenance access are as
follows:
Provide appropriate maintenance access to the pretreatment forebay, wetpool cell bottom,
and outlet structure. For larger basins, this typically means stabilized access designed to
withstand the expected loads from maintenance vehicles.
Stabilized access typically includes materials such as concrete, articulated concrete block,
concrete grid pavement, or reinforced grass pavement.
Grades of less than 10 percent for maintenance road surfaces and 20 percent for skid-steer
and backhoe access are preferred. A cross slope of 2 percent is recommended.
If stabilized access is not provided, a maintenance plan that provides details including
recommended equipment and a plan for sediment and trash removal from the outlet
structure may be required by the local jurisdiction.
13. Guidelines for Incorporating Flood Control
WDBs can be designed to provide flood control by increasing the surcharge volume for flood
detention storage and designing the outlet structure to detain and release flood flows. Reservoir
routing calculations may be used to assist in the outlet structure design for larger runoff events.
Appropriate flood control design guidance and local regulations should be referenced when
incorporating flood control into a WDB.
Figure 5.8-6. WDB Maintenance Access
Source: HDR
Provide stable maintenance access designed to withstand the expected loads from maintenance vehicles.
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5.8 Wet Detention Basin
5.8.5 Vegetation Considerations
Vegetation is an essential component of a WDB because it provides erosion control and enhances
site stability. Developing a landscaping plan for the WDB and surrounding area is recommended to
indicate how the WDB will be stabilized and established with vegetation. Considerations when
developing the vegetation and landscaping plan are as follows.
Berms and side-sloping areas should be planted with native grasses or irrigated turf,
depending on the local setting and proposed uses for the pond area.
The safety wetland bench should be vegetated with wetland plants that can tolerate a
saturated root zone. This vegetation around the perimeter of an open water body can
discourage frequent use of the pond by geese.
Woody vegetation is not recommended within 15 feet of the toe of the embankment or within
25 feet of the principal spillway, inlet, and outlet structures because of the potential impacts
of root systems.
The soils of the area immediately surrounding a WDB are often severely compacted during
the construction process to ensure stability. The density of these compacted soils can be so
great that it effectively prevents root penetration. Therefore, it is recommended to excavate
large and deep holes around proposed planting sites and backfill them with uncompacted
topsoil or other organic material.
Note: Given the wide range of native vegetation across Montana, designers should consult local
specialists, landscape architects, and/or agencies for recommendations on appropriate plant
species and landscaping considerations for the site.
5.8.6 Construction Considerations
Basic construction considerations and recommendations are provided below.
Construction Site Management
Acquire all applicable permits prior to construction. See Section 1.4 for more information.
Apply appropriate erosion control measures to minimize erosion during construction.
If used as a construction storm water management BMP, a WDB must be dewatered,
dredged, and regraded prior to post-construction implementation.
Consider preserving existing trees in the area surrounding the WDB during construction
because it is often desirable to locate forest conservation areas adjacent to ponds.
To the extent practicable, construction equipment should be restricted from the WDB area to
prevent compaction of the native soils.
Contributing drainage areas should be properly stabilized with the appropriate erosion and
sediment controls or permanent seeding before allowing storm water runoff to drain to the
storm water pond.
Construction Inspections
Inspections are recommended during the following phases of construction
o Pre-construction meeting
o Initial site preparation
o Excavation/grading
o Installation of the embankment, spillway(s), and outlet structure
o Implementation of the vegetation and landscaping plan
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5.8 Wet Detention Basin
o Final inspection
Transition to Post-Construction
Develop a plan prior to construction that will allow for an effective transition from construction
storm water management BMPs to post-construction BMPs without compromising the
integrity of the post-construction BMPs.
Coordinate with the local jurisdiction prior to terminating coverage of the Construction
General Permit.
5.8.7 Maintenance
Maintenance is required on all BMPs. Recommended maintenance activities are provided in Table
5.8-1, which may be used as a guide when developing a maintenance plan. Additionally, an example
inspection form is provided in Appendix F that may be adapted or adopted as part of the
maintenance plan. It is recommended that maintenance responsibilities are clearly defined and/or
maintenance agreements are executed prior to construction.
Table 5.8-1. Recommended Maintenance Activities for a WDB
Activity Frequency
Remove litter/debris from all components of the WDB.
Repair basin inlets, outlets, and all other structural components required for the basin to
operate as intended.
Repair and revegetate eroded areas.
Regularly manage all vegetation along maintenance right-of-ways and the embankment.
Remove all clippings.
Repair maintenance access routes, if applicable.
Inspect the WDB for signs of mosquito larvae during summer months and provide treatment
when breeding is found. If available, a local mosquito control service could be used to carry out
these inspections.
When necessary, drain the WDB during dry periods to prevent the release of untreated water.
Inspect the WDB for damage and excessive sediment deposition following large storm events.
As needed
Trim vegetation for aesthetics and mosquito control. Prevent excessive growth of woody
vegetation on or near berms or embankments.
Evaluate the health of vegetation and remove and replace any dead or dying plants.
Semiannually
Inspect all components of the WDB in accordance with an approved inspection form according
to local jurisdiction requirements. An example inspection form is provided in Appendix F.
Open the riser to access and test the valves (if applicable).
Annually
Remove sediment from the forebay before it becomes a significant source of pollutants for the
remainder of the WDB.
Typically 1 to
4 years
Remove accumulated sediment from the bottom of the wetpool cell to maintain volume and
deter algae growth. This typically requires heavy equipment, designated corridors, and
considerable expense. Harvesting of vegetation may also be desirable for nutrient removal.
When removing vegetation from the pond, take care not to create or leave areas of disturbed
soil susceptible to erosion. If removal of vegetation results in disturbed soils, implement proper
erosion and sediment control BMPs until vegetative cover is reestablished. Dispose of
sediment properly (56).
Typically 10 to
20 years
(or as needed)
Montana Post-Construction Storm Water BMP Design Guidance Manual
5.8-12 | September 2017
5.8 Wet Detention Basin
5.8.8 Plan View and Typical Details
Figure 5.8-7. Wet Detention Basin Plan View and Typical Section
Source: Adapted from Minnesota Stormwater Manual (57)
Montana Post-Construction Storm Water BMP Design Guidance Manual
September 2017 | 5.9-1
5.9 Proprietary Treatment Devices
5.9 Proprietary Treatment Devices
Figure 5.9-1. Proprietary Treatment Device
Installation
Source: City of Kalispell
Photo is used as example only—not an endorsement.
Description
Manufactured devices used to treat storm water runoff
through various processes that may include
sedimentation, filtration, and sorption. These devices
can provide treatment for a variety of different
pollutants.
Primary Components Primary Function
Variable depending on
treatment device
See manufacturer’s
technical specifications
Runoff reduction
Runoff treatment
Benefits Limitations
Can often be easily incorporated into urban sites or
space-constrained areas
Devices are typically underground and do not
consume a large amount of site space
Good retrofit capability
May require frequent maintenance for sites that
discharge a large amount of sediment
Certain devices may have cold-climate limitations,
depending on installation depth
Each type of device has specific design constraints
and limitations for use
Design and Site Selection Considerations
Setbacks
Depth to groundwater or bedrock
Soil permeability
Soil preparation/amendments/compost
Pretreatment forebay
Inlet and outlet spacing
Energy dissipater/level spreader
Underdrain
Facility liners
Landscaping/planting
Fencing
Size of contributing drainage area
Area required
Incorporate flood control
TMDL Considerations1 Maintenance Requirements2
Avoid Preferred
Total suspended solids (TSS)
Total phosphorus
Total nitrogen
Temperature
Metals
Fecal coliform
Access roads or pullouts
Sediment removal
Irrigation
Vegetation management
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5.9-2 | September 2017
5.9 Proprietary Treatment Devices
5.9.1 Description
Proprietary treatment devices are storm water BMPs that are commercially designed and
manufactured by vendors. They may be used individually or with other BMPs as part of a treatment
train, depending on the site constraints and storm water management objectives. Numerous devices
are available, each with different benefits, limitations, performance capabilities, and maintenance
requirements. Most devices can be categorized as either a hydrodynamic separator or filtering
system, as discussed below.
Hydrodynamic Separators
Hydrodynamic separators are manufactured chambers that use sedimentation to remove pollutants
such as sediment from storm water runoff. They are usually round, flow-through devices that induce
a circular motion to promote sedimentation as runoff flows through the chamber. They may also be
designed to remove oil, grease, and other floatables from runoff through the use of baffles.
Maintenance of these devices requires regular removal of accumulated sediment and floatables
(58).
Filtering Systems
Filtering systems are manufactured units that typically consist of an underground chamber or catch
basin that includes a filter media insert. The system is designed to pass storm water runoff through
filter media to remove pollutants. The pollutants to be treated depend on the type of filter media
selected, which may include a screen, fabric, activated carbon, perlite, zeolite, or other materials.
These systems may be designed to provide treatment for nutrients, sediments, floatables, metals,
oils, and/or organic compounds. Maintenance of these devices typically requires replacing the filter
media (59).
Note: This manual does not recommend or endorse any specific proprietary treatment devices.
This section is included to provide general recommendations and considerations to assist with the
selection and implementation of proprietary treatment devices.
5.9.2 Performance
Runoff Reduction
Runoff reduction is not considered to be a function of proprietary treatment devices because they
generally discharge a volume equivalent to the entire inflow runoff volume.
Runoff Treatment
Certain proprietary treatment devices have been shown to achieve an 80 percent or greater removal
rate of TSS from the RTV or RTF. It is the responsibility of the designer and/or project owner to
document that the proposed device has demonstrated 80 percent TSS reduction for the given site
conditions.
5.9.3 Site Selection
Basic guidelines are provided below to aid in evaluating whether proprietary treatment is feasible for
use at an individual site.
Montana Post-Construction Storm Water BMP Design Guidance Manual
September 2017 | 5.9-3
5.9 Proprietary Treatment Devices
Contributing Drainage Area
Many proprietary treatment devices are flow-through systems sized for a design flow rate
(typically the RTF). Contributing drainage area characteristics (e.g., size, percent impervious)
will determine the design flow rate.
Most devices are available in various sizes to accommodate multiple design flow rates or
volumes.
Consult manufacturer recommendations for guidance and limitations regarding contributing
drainage areas and design flow rates or volumes.
Soil Characteristics
Soil characteristics may necessitate design modifications to or limitations on the type of
structural material that can be used on a given site. For example, corrosive soils may require
modifications to treatment devices manufactured with steel.
Depth to Groundwater and/or Bedrock
High groundwater can result in buoyancy and seepage of groundwater into a device. These
considerations should be evaluated and mitigated in areas with consistently or seasonally
high groundwater tables.
Site Topography
Site topography considerations vary for individual devices.
Sites with steep slopes may require the use of energy dissipation features upstream of a
device to reduce runoff velocities that could damage the BMP.
Land Use and Considerations of Surrounding Area
Proprietary treatment devices are typically small—as such, they are best suited for areas
where opportunities to use larger BMPs are limited by the lack of available space. Examples
of these areas include redevelopment sites, downtown areas, and space-constrained
transportation corridors.
5.9.4 Guidelines for Using Proprietary Treatment Devices
This section provides general guidelines to be used when considering implementation of a
proprietary treatment device. Specific design and implementation guidelines vary for different
devices; therefore, manufacturers and/or suppliers should be consulted for guidance and design
specifications specific to a given device.
Performance Verification
The manufacturer and/or project engineer should provide an independent third-party scientific
verification showing that the proprietary device is able to meet storm water management objectives
for a given project. One primary objective will likely be the ability to meet the Runoff Treatment
Requirement, which states that the BMP must be expected to remove 80 percent TSS from the RTV.
Consult with the local jurisdiction to determine whether they have specific product verification
requirements.
Record of Longevity
The manufacturer and/or project engineer should provide data that indicate when maintenance is
required (maintenance cycle). Furthermore, the data should demonstrate that when maintained
correctly, the device is expected to meet storm water management objectives throughout its entire
life cycle.
Montana Post-Construction Storm Water BMP Design Guidance Manual
5.9-4 | September 2017
5.9 Proprietary Treatment Devices
Ability to Function in Local Conditions
The device must be able to function in and withstand project site conditions such as climate, rainfall
patterns, and soil types. Certain conditions may preclude the use of certain devices, while other
conditions may simply require design adaptations. For instance, depending on site conditions,
design adaptation may or may not be feasible for devices subject to freezing in cold climates. The
manufacturer should provide data that indicate any limitations in function or performance of the
device because of weather. The data should indicate that the device can meet the project’s storm
water management objectives at the proposed site.
Flow Control Considerations
The site must be designed to handle the full range of expected flows. Some devices have flow
bypass systems built into the design, while other systems may require a flow diversion upstream of
the structure to divert high flows around the device.
Maintainability
There must be documented procedures for required maintenance, including collection and removal
of pollutants or debris. As with all BMPs, the designer should also consider who will conduct the
maintenance and whether they have access to the proper equipment and have the capability to
perform the required maintenance tasks. Additionally, the designer must ensure there is adequate
maintenance access capable of handling the equipment necessary to conduct maintenance
activities.
Note: The list of considerations provided in this section is not comprehensive. Designers must
consider all of the project’s storm water management objectives and site constraints when
selecting and specifying a device.
5.9.5 Maintenance
Maintenance is required on all BMPs because clogging of devices can hinder pollutant removal
capabilities and create drainage problems. Specific maintenance tasks and schedules vary for
different devices. A maintenance schedule should be provided that considers all of the
manufacturer’s recommended maintenance activities. The volume of pollutants draining to a device
dictates the rate at which it reaches its capacity; therefore, the characteristics of the contributing
drainage area are often a primary factor in establishing the frequency of maintenance activities.
Frequent inspections throughout the first year of installation are recommended to understand how
often maintenance is needed.
Montana Post-Construction Storm Water BMP Design Guidance Manual
September 2017 | WC-1
Works Cited
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http://stormwater.pca.state.mn.us/images/0/01/Infiltration_Basin_INFILTRATION_BASIN_PLAN_
%26_PROFILE_%281%29.pdf.
20. Evaluation and Optimization of Bioretention Media for Treatment of Urban Storm Water Runoff.
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https://stormwater.pca.state.mn.us/index.php?title=Bioretention_combined.
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Impact Development Guidance Manual. s.l. : State of Washington Department of Ecology, June
2013.
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Bioretention Media: Nitrogen and Phosphorus Removal. Water Environment Research, 78(3)
(pg. 284-293). 2006.
29. Minnesota Stormwater Manual contributors. CADD images for individual best management
practices: All bioretention cadd images combined. Minnesota Stormwater Manual. [Online] April
24, 2017. [Cited: June 5, 2017.]
https://stormwater.pca.state.mn.us/index.php?title=File:All_bioretention_cadd_images_combined
.pdf.
30. Urban Drainage and Flood Control District. Chapter 4: Fact Sheet T-10.1 - Permeable
Interlocking Concrete Pavement. Urban Drainage Criteria Manual: Volume 3 - Best Management
Practices. Denver, CO : s.n., November 2010.
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33. Smith, David R. Permeable Interlocking Concrete Pavements, Fourth Edition. Herndon, VA :
Interlocking Concrete Pavement Institute, 2011.
34. AHBL, Inc. and HDR, Inc. Section 4.6: Permeable Pavement. Eastern Washington Low Impact
Development Guidance Manual. s.l. : State of Washington Department of Ecology, June 2013.
35. Tennessee Department of Environment and Conservation. Section 5.4.8: Permeable
Pavement. [book auth.] Division of Water Resources. Tennessee Permanent Stormwater and
Design Guidance Manual. December 2014.
Montana Post-Construction Storm Water BMP Design Guidance Manual
September 2017 | WC-3
Works Cited
36. Washington State Department of Transportation. Chapter 5: FC.01/FC.02 - Natural and
Engineered Dispersion. [book auth.] Development Division, Design Office Engineering and
Regional Operations. Highway Runoff Manual. April 2014.
37. West Virginia Department of Environmental Protection. Chapter 4.2.5: Grass Swale. [book
auth.] Inc. Center for Watershed Protection. West Virginia Stormwater Management and Design
Guidance Manual. November 2012.
38. Urban Drainage and Flood Control District. Chapter 4: T-2 - Grass Swale. Urban Storm
Drainage Criteria Manual: Volume 3 - Best Management Practices. Denver, CO : s.n.,
November, 2015.
39. AHBL, Inc. and HDR, Inc. Chapter 4.3: Dispersion. Eastern Washington Low Impact
Development Guidance Manual. s.l. : State of Washington Department of Ecology, June 2013.
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auth.] Development Devision, Design office Engineering and Regional Operations. Highway
Runoff Manual. February, 2016.
41. Tennessee Department of Environment and Conservation. Chapter 5.4.3: Vegetated Swale.
[book auth.] Division of Water Resources. Tennessee Permanenet Stormwater and Design
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42. City of Billings Public Works Department. Chapter 8.12: Swales. [book auth.] DOWL City of
Billings. Stormwater Management Manual. May, 2015.
43. U.S Environmental Protection Agency. Chapter 3.5: Biofiltration Swale. [book auth.] Tetra
Tech USEPA. Draft Montana Stormwater Criteria Manual. August, 2015.
44. Urban Drainage and Flood Control District. Chapter 4: Fact Sheet T-5 - Extended Detention
Basin. Urban Storm Drainage Criteria Manual: Volume 3 - Best Management Practices. Denver,
CO : s.n., November, 2015.
45. Schueler, Thomas R. Controlling Urban Runoff: A Practical Manual for Planning and Designing
Urban BMPs. Washington, DC : Metropolitan Washington Council of Governments, Department
of Environmental Programs, 1987.
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Detention Pond. The Stormwater Manager's Resource Center. [Online] [Cited: September 28,
2016.]
http://www.stormwatercenter.net/Assorted%20Fact%20Sheets/Tool6_Stormwater_Practices/Pon
d/Dry%20ED%20Pond.htm.
47. Virginia Department of Conservation and Recreation. Design Specification No. 15: Extended
Detention Pond - Version 2.0. Virginia DCR Stormwater Design Specification. January 2013.
48. Urban Drainage and Flood Control District. Chapter 4: Fact Sheet T-12 - Outlet Structures.
Urban Storm Drainage Criteria Manual: Volume 3 - Best Management Practices. Denver, CO :
s.n., August, 2013.
49. Minnesota Stormwater Manual contributors. Stormwater ponds combined. Minnesota
Stormwater Manual. [Online] March 27, 2013. [Cited: October 28, 2016.]
http://stormwater.pca.state.mn.us/index.php/Stormwater_ponds_combined.
50. Virginia Department of Conservation and Recreation. Figure 3.07 - 3a. Stormwater
Management Handbook: Volume 1. Richmond, VA : s.n., 199.
51. Schueler, T. R. A Current Assessment of Urban Best Management Practices. s.l. : Metropolitan
Washington Council of Governments, 1992.
52. Urban Drainage and Flood Control District. Chapter 4: Fact Sheet T-7 - Retention Pond.
Urban Storm Drainage Criteria Manual: Volume 3 - Best Management Practices. Denver, CO :
s.n., November, 2015.
53. United States Environmental Protection Agency. Storm Water Technology Fact Sheet: Wet
Detention Ponds. Washington, D.C. : Office ow Water, September 1999.
54. Tennessee Department of Environment and Conservation. Section 5.4.2: Wet Pond. [book
auth.] Division of Water Resources. Tennessee Permanent Stormwater and Design Guidance
Manual. December 2014.
55. Urban Drainage and Flood Control District. Chapter 4: Fact Sheet T-12 - Outlet Structures.
Urban Storm Drainage Criteria Manual: Volume 3 - Best Management Practices. Denver, CO :
s.n., November 2015.
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WC-4 | September 2017
Works Cited
56. —. Chapter 6: BMP Maintenance - Section 7.0. Urban Storm Drainage Criteria Manual: Volume
3 - Best Management Practices. Denver, CO : s.n., November, 2010.
57. Minnesota Stormwater Manual contributors. CADD images for individual best management
practices: Pond Plan & Profile. Minnesota Stormwater Manual. [Online] December 20, 2012.
[Cited: October 28, 2016.]
http://stormwater.pca.state.mn.us/index.php/CADD_images_for_individual_best_management_p
ractices.
58. Minnesota Stormwater Manual Contributors. Hydrodynamic Devices. Minnesota Stormwater
manual. [Online] September 22, 2014. [Cited: April 27, 2017.]
https://stormwater.pca.state.mn.us/index.php?title=Hydrodynamic_devices.
59. —. Filtration Devices. Minnesota Stormwater Manual. [Online] February 25, 2015. [Cited: April
27, 2017.] https://stormwater.pca.state.mn.us/index.php?title=Filtration_devices.
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Manual. [book auth.] Division of Water Resources. Tennessee Permanent Stormwater and
Design Guidance Manual. December 2014.
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Manual. [book auth.] Inc. Center for Watershed Protection. West Virginia Stormwater
Management and Design Guidance Manual. November 2012.
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Environmental Protection Agency. [Online] [Cited: December 15, 2016.]
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63. Montana Department of Environmental Quality. General Permit for Storm Water Discharges
Associated with Small Municipal Separate Storm Sewer Systems (MS4s). Issued November 30,
2016. Effictive January 1, 2017.
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Engineering Handbook: Part 630 Hydrology. Washington, DC : s.n., January 2009.
65. Natural Resources Conservation Service, Conservation Engineering Division. Urban
Hydrology for Small Watersheds, TR-55, Second Ed. Washington D.C. : United States
Department of Agriculture, June 1986.
66. Pitt, R. Small Storm Hydrology. Presented at design of stormwater quality management
practices. Madison, WI : University of Alabama - Birmingham. Unpublished manuscrpit, May 17-
19 1994.
67. California Stormwater Quality Association. Section 5: Fact Sheet TC-11 - Infiltration Basin.
Stormwater Best Management Practice Handbook: New Development and Redevelopment.
January 2003.
68. Atlanta Regional Commision. Chapter 4.15: Permeable Paver Systems. Georgia Stormwater
Management Manual. Atlanta, GA : s.n., 2016.
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Federal Highway Administration. TechBrief Publication Number FJWA-HIF-15-007. s.l. : U.S.
Department of Transportation, January 2015.
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auth.] Inc. Center for Watershed Protection. West Virginia Stormwater Management and Design
Guidance Manual. November 2012.
71. California Stormwater Quality Association. Section 5: Fact Sheet TC-22 - Extended Detention
Basin. Stormwater Best Management Practice Handbook: New Development and
Redevelopment. January 2003.
72. Virginia Department of Conservation and Recreation. Figure 3.07 - 3a. Stormwater
Management Handbook: Volume 1. Richmond, VA : s.n., 1999.
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Manufactured/Proprietary Treatment Devices. [book auth.] Division of Water Resources.
Tennessee Permanent Stormwater and Design Guidance Manual. December 2014.
Appendix A. Glossary
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Appendix A – Glossary
Appendix A: Glossary
Note: The definitions provided within this glossary are for the purposes of this manual, and are
focused on post-construction BMPs in accordance with Montana’s General Permit for Storm Water
Discharges Associated with Small Municipal Separate Storm Sewer Systems (General Permit).
Consult the local jurisdiction, General Permit, and/or Montana DEQ for regulatory definitions
related to storm water management.
Best Management Practice (BMP): In the context of this manual, BMP refers to a permanent
storm water management facility used to prevent or reduce the discharge or pollutants to state
waters.
Biofiltration Swale: A densely vegetated channel with a trapezoidal cross-section and low
longitudinal slopes which conveys runoff. The trapezoidal cross-section and low longitudinal slope
promotes shallow concentrated flow which allows for filtration of storm water by plants.
Bioretention Area: A shallow landscaped depression that captures and infiltrates or filters storm
water runoff through plants, engineered soil media, and often an underdrain system.
Cation Exchange Capacity (CEC): The total amount of positively charged elements that a soil can
hold. This value is typically expressed in milliequivalents per 100 grams (meq/100g) of soil.
Dispersion: A BMP that achieves runoff reduction by utilizing vegetation, soil, and gentle slopes
located adjacent to impervious surfaces to impede the velocity of storm water runoff and encourage
infiltration.
Extended Detention Basin (EDB): A sedimentation basin designed to detain and slowly release
storm water over an extended period of time following a rainfall event. These facilities are sometimes
referred to as “dry ponds” because they are designed to remain empty between runoff events.
Flood Control: Management of storm water runoff to reduce peak flows from a developed area.
This is often achieved using storm water management facilities which detain and slowly release
runoff.
Impaired Water Body: A water body or stream segment for which sufficient credible data shows
that the water body or stream segment is failing to achieve compliance with applicable water quality
standards (MCA 75-5-103).
Impervious Surface: A hard surface area (e.g., parking lot, roadway, rooftop, etc.) that prevents or
retards the infiltration of storm water, thus causing storm water to run off the surface in greater
quantities and at an increased rate of flow when compared to pervious areas.
Montana Post-Construction Storm Water BMP Design Guidance Manual
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Chapter 1 Appendix – Introduction A – Glossary to the Manual
Infiltration Basin: A constructed basin designed to collect and retain storm water runoff so that it
can infiltrate into underlying soils. These facilities remain dry between runoff events and often have
permanent vegetation ranging from grass to small shrubs.
Low Impact Development (LID): A multistep storm water management approach which utilizes
thoughtful site planning and manages rainfall at its source by using integrated and distributed small-
scale BMPs.
Micropool: A small permanent pool located in front of an outlet structure within an EDB. A
micropool is designed to prevent sediment resuspension, protect the low flow outlet riser orifices or
perforated plate from clogging, and reduce mosquito breeding areas.
Municipal Separate Storm Sewer System (MS4): A conveyance or system of conveyances
(including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches,
man-made channels, or storm drains) that discharges to surface waters and is owned or operated by
the state of Montana, a governmental subdivision of the state, a district, association, or other public
body created by or pursuant to Montana law, including special districts such as sewer districts, flood
control districts, drainage districts and similar entities, and designated and approved management
agencies under section 208 of the federal Clean Water Act, which has jurisdiction over disposal of
sewage, industrial wastes, storm water, or other wastes, and is:
a. Designed or used for collecting or conveying storm water;
b. Not a combined sewer; and
c. Not part of a publicly owned treatment works as defined in ARM Title 17, chapter 30,
subchapter 13.
Multi-use Facility: A facility that serves more than one purpose.
Offsite: A location not within the boundaries of the development or redevelopment area.
Offsite treatment: An approach that uses a regional facility to manage storm water runoff from
multiple development projects located within the same subwatershed.
Onsite: A location within the boundaries of the development or redevelopment area.
Onsite BMP: A BMP located within the boundary of a development or redevelopment area.
Permeable Pavement System: A general term to describe any one of several surfaces that allow
storm water runoff to filter through surface voids into an underlying aggregate reservoir for temporary
storage and/or infiltration.
Post-Construction Performance Standard: The BMP design requirement presented in Part
II.A.5.b.iii of the General Permit.
Pretreatment Forebay: A small hard-bottomed basin located immediately downstream of an inlet
within certain BMPs. These facilities are designed to trap incoming coarse sediments and other
gross solids so that they do not accumulate within the main treatment area of the BMP.
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Appendix A – Glossary
Proprietary Treatment Devices: Manufactured devices used to treat storm water runoff through
various processes which may include sedimentation, filtration, and sorption. These devices can
provide treatment for a variety of different pollutants.
Regulated New and/or Redevelopment Project: New development and redevelopment projects
that disturb greater than or equal to one acre, including projects less than one acre that are part of a
larger common plan of development or sale, that discharge into a permitted small MS4.
Runoff Reduction: Implementation of a BMP (or series of BMPs) designed to infiltrate,
evapotranspire, or capture for reuse the RRV.
Runoff Reduction Requirement: The portion of the Post-Construction Performance Standard
which requires that all regulated projects implement BMPs that are designed to infiltrate,
evapotranspire, and/or capture for reuse the post-construction runoff generated from the first 0.5
inches of rainfall from a 24-hour storm preceded by 48 hours of no measurable precipitation.
Runoff Reduction Volume (RRV): The volume of storm water runoff generated from the first 0.5
inches of rainfall from a 24-hour storm preceded by 48-hours of no measurable precipitation.
Runoff Treatment: Implementation of a BMP (or series of BMPs) expected to remove 80 percent
TSS from the RTV.
Runoff Treatment Requirement: The portion of the Post-Construction Performance Standard
which requires that for projects that cannot meet 100% of the Runoff Reduction Requirement, the
remainder of the runoff from the first 0.5 inches of rainfall must be treated using BMPs expected to
remove 80 percent total suspended solids.
Runoff Treatment Flow Rate (RTF): The peak flow rate associated with the RRV or RTV which is
used to size flow-based systems such as biofiltration swales and flow diversion structures for off-line
storm water management practices.
Runoff Treatment Volume (RTV): The remainder of the RRV which was not infiltrated,
evapotranspired, or captured for reuse onsite. This volume must be treated onsite or managed
offsite.
Storm Water Hotspot: An area which produces higher concentrations of pollutants than is normally
found in urban runoff. Examples include gas stations, vehicle maintenance/repair areas, and auto
recyclers.
Storm Water Management: The practice of managing storm water runoff to meet certain objectives
which may include runoff reduction, treatment, and flood control.
Treatment: The removal of pollutants from storm water runoff.
Treatment Train: A combination of two or more treatment BMPS connected in series.
Total Maximum Daily Load (TMDL): The sum of the individual waste load allocations for point
sources and load allocations for both nonpoint sources and natural background sources established
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Chapter 1 Appendix – Introduction A – Glossary to the Manual
at a level necessary to achieve compliance with applicable surface water quality standards (MCA 75-
5-103).
Total Suspended Solids (TSS): TSS refers to all particles suspended in storm water runoff which
typically have settling times that typically exceed one hour.
Turf Grass: Grasses which form a dense even turf which can be mowed and maintained.
Wet Detention Basin (WDB): A constructed basin designed to capture, detain, and slowly release
storm water runoff while maintaining a permanent pool of water in order to promote pollutant removal
through sedimentation and biological uptake.
Waste Load Allocation (WLA): The portion of a receiving water’s loading capacity that is allocated
to one of its existing or future point sources (MCA 75-5-103).
Appendix B. Additional Hydrology Information
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Appendix B – Additional Hydrology Information
Appendix B: Additional Hydrology Information
Table B-1. Roughness Coefficients for Sheet Flow
Surface Description n
1
Smooth surfaces (concrete, asphalt, gravel, or bare soil) 0.011
Fallow (no residue) 0.05
Cultivated Soils:
Residue cover ≤ 20%
Residue cover > 20%
0.06
0.17
Grass:
Short grass prairie
Dense grasses
2
Bermuda grass
0.15
0.24
0.41
Range (natural) 0.13
Woods
3
:
Light underbrush
Dense underbrush
0.40
0.80
1
The n values are a composite of information compiled by Engman (1986).
2
Includes species such as weeping lovegrass, bluegrass, buffalo grass, blue grama
grass, and native grass mixtures.
3
When selecting n, consider cover to a height of about 0.1 ft. This is the only part of
the plant cover that will obstruct sheet flow.
Source: NRCS TR-55, Table 3-1
Table B-2. Roughness Coefficients for Open Channel Flow and Pipe Flow
Conduit Material n
1
Closed Conduits:
Concrete Pipe
Corrugated Metal Pipe
Plastic Pipe (smooth)
Plastic Pipe (corrugated)
0.011 –0.013
0.022 – 0.024
0.009 – 0.011
0.018 – 0.025
Pavement/gutter sections 0.012 – 0.016
Small Open Channels:
Concrete
Rubble or riprap
Vegetation
Bare Soil
0.011 – 0.013
0.020 – 0.035
0.020 – 0.150
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Appendix Chapter B 1 – – Additional Introduction Hydrology to the Manual Information
Figure B-1: Unit Peak Discharge (qu) for NRCS (SCS) Type I Distribution
Source: NRCS TR-55, Exhibit 4-I
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Appendix B – Additional Hydrology Information
Figure B-2: Unit Peak Discharge (qu) for NRCS (SCS) Type II Distribution
Source: NRCS TR-55, Exhibit 4-II
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Appendix C. Evaluating Soil Infiltration Rates
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Appendix C – Evaluating Soil Infiltration Rates
Appendix C: Evaluating Soil Infiltration Rates
Introduction
Infiltration is the process of moving water (or storm water runoff) from the ground surface into the
underlying soils. Infiltration of water into the underlying soils typically begins quickly (while the
underlying soils are becoming saturated) and then declines until a steady-state infiltration rate is
reached when the soils are saturated. Determining the steady-state infiltration rate is an essential
part of BMP design because this infiltration rate is a key factor in assessing the feasibility and sizing
of infiltration BMPs. Incorrect assumptions about the soils ability to infiltrate runoff can lead to failure
of BMPs.
A variety of on-site tests exist for determining the infiltration rate of native soil. Three test methods
are described within this appendix, each of which require excavation of a soil test pit, which is made
for the purpose of exposing and evaluating the soil profile and conducting a soil infiltration test at the
appropriate depth in the location of a proposed BMP. Tests should not be conducted during rainfall
events, within 24 hours following significant rainfall events (greater than 0.5 inches), or when the
temperature is below freezing. Laboratory tests for determining the infiltration rate of native soils are
strongly discouraged because the soils are disrupted during sample collection which may not
represent field conditions.
Note: When using BMPs which rely on infiltration, the field measured steady-state infiltration rate
typically declines due to factors such as compaction and clogging. In order to account for this and
mitigate failure of BMPs, this manual recommends dividing the field measured steady-state
infiltration rate by a factor of 2.0 in order to obtain the design infiltration rate.
Large Scale Pilot Infiltration Test
A PIT is a relatively large-scale soil test used to approximate infiltration rates for design of infiltration
BMPs. A PIT is recommended for use whenever adequate space and the necessary equipment are
available. Guidance for conducting both a large and small scale PIT is provided in this appendix.
(Guidance for when a small scale test maybe more appropriate is provided in the small scale PIT
procedures section below.) The PIT procedures have been adopted from the State of Washington
Department of Ecology’s Stormwater Management Manual for Western Washington, with minor
revisions which account for local considerations.
Both a large and small scale PIT involves digging a soil test pit near the proposed infiltrative surface
(e.g., bottom of infiltration basin or top of the native soil for a bioretention or permeable pavement
facility). Water is ponded to between 6 and 12 inches in depth and flow to the pit is maintained until
the infiltration rate has stabilized (a minimum of 6 hours). Then the water source is removed from the
pit and the rate of infiltration (the drop rate of the standing water) is measured and recorded.
Large Scale PIT Equipment
Excavating equipment
Water supply (with rigid pipe and splash plate)
Rota- or magnetic meters to measure flow rate into the pit
Stopwatch or timer
Measuring rod
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Appendix C – Evaluating Soil Infiltration Rates
Log sheets for recording data
Large Scale PIT Procedure
Excavate the test pit to the estimated surface elevation of the proposed infiltration facility.
Lay back the slopes sufficiently to avoid caving and erosion during the test. Alternatively,
consider shoring the sides of the test pit.
The horizontal surface area of the bottom of the test pit should be approximately 100 square
feet. Accurately document the size and geometry of the test pit.
Install a vertical measuring rod (minimum 5-ft. long) marked in half-inch increments in the
center of the pit bottom.
Use a rigid 6-inch diameter pipe with a splash plate on the bottom to convey water to the pit
and reduce side-wall erosion or excessive disturbance of the pond bottom. Excessive
erosion and bottom disturbance will result in clogging of the infiltration receptor and yield
lower than actual infiltration rates.
Add water to the pit at a rate that will maintain a water level between 6 and 12 inches above
the bottom of the pit. A rotameter can be used to measure the flow rate into the pit.
Note: The depth should not exceed the proposed maximum depth of water expected in the
completed facility. For infiltration facilities serving large drainage areas, designs with
multiple feet of standing water can have infiltration tests with greater than 1 foot of standing
water.
Every 15-30 min, record the cumulative volume and instantaneous flow rate in gallons per
minute necessary to maintain the water level at the same point on the measuring rod.
Keep adding water to the pit until one hour after the flow rate into the pit has stabilized
(constant flow rate; a goal of 5% variation or less variation in the total flow) while
maintaining the same pond water level. The total of the pre-soak time plus one hour after
the flow rate has stabilized should be no less than 6 hours.
After the flow rate has stabilized for at least one hour, turn off the water and record the rate
of infiltration (the drop rate of the standing water) in inches per hour from the measuring rod
data, until the pit is empty. Consider running this falling head phase of the test several times
to estimate the dependency of infiltration rate with head.
At the conclusion of testing, over-excavate the pit to see if the test water is mounded on
shallow restrictive layers or if it has continued to flow deep into the subsurface. The depth of
excavation varies depending on soil type and depth to hydraulic restricting layer, and is
determined by the engineer or certified soils professional. Mounding is an indication that a
mounding analysis may be necessary.
Large Scale PIT Data Analysis
Calculate and record the saturated hydraulic conductivity rate in inches per hour in 30
minutes or one hour increments until one hour after the flow has stabilized.
Note: Use statistical/trend analysis to obtain the hourly flow rate when the flow stabilizes.
This would be the lowest hourly flow rate.
Divide the saturated hydraulic conductivity by the recommended safety factor of 2.0 (or as
required by the local jurisdiction) to determine the site-specific design infiltration rate.
Small Scale Pilot Infiltration Test
A smaller-scale PIT is recommended in place of large-scale PIT in any of the following instances.
The drainage area to the infiltration site is less than 1 acre.
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Appendix C – Evaluating Soil Infiltration Rates
The testing is for bioretention or permeable pavement BMPs that either serve small
drainage areas and /or are widely dispersed throughout a project site.
The site has a high infiltration rate, making a full-scale PIT difficult, and the site geotechnical
investigation suggests uniform subsurface characteristics.
Small Scale PIT Equipment
See the guidance for large scale PIT equipment above.
Small Scale PIT Procedure
Excavate the test pit to the estimated surface elevation of the proposed infiltration facility. In
the case of bioretention, excavate to the estimated elevation at which the bioretention soil
media will lie on top of the underlying native soil. For permeable pavements, excavate to the
elevation at which the imported subgrade materials will contact the underlying native soil. If
the native soils (road subgrade) will have to meet a minimum subgrade compaction
requirement, compact the native soil to that requirement prior to testing. Note that
permeable pavement design guidance typically recommends compaction not exceed 90% -
92%. Finally, lay back the slopes sufficiently to avoid caving and erosion during the test.
Alternatively, consider shoring the sides of the test pit.
The horizontal surface area of the bottom of the test pit should be 12 to 32 square feet. It
may be circular or rectangular, but accurately document the size and geometry of the test
pit.
Install a vertical measuring rod adequate to measure the ponded water depth and that is
marked in half-inch increments in the center of the pit bottom.
Use a rigid pipe with a splash plate on the bottom to convey water to the pit and reduce
side-wall erosion or excessive disturbance of the pond bottom. Excessive erosion and
bottom disturbance will result in clogging of the infiltration receptor and yield lower than
actual infiltration rates. Use a 3-inch diameter pipe for pits on the smaller end of the
recommended surface area, and a 4-inch pipe for pits on the larger end of the
recommended surface area.
Pre-soak period: Add water to the pit so that there is standing water for at least 6 hours.
Maintain the pre-soak water level at least 12 inches above the bottom of the pit.
At the end of the pre-soak period, add water to the pit at a rate that will maintain a 6-12 inch
water level above the bottom of the pit over a full hour. The depth should not exceed the
proposed maximum depth of water expected in the completed facility.
Every 15 minutes, record the cumulative volume and instantaneous flow rate in gallons per
minute necessary to maintain the water level at the same point (between 6 inches and 1
foot) on the measuring rod. The specific depth should be the same as the maximum
designed ponding depth.
After one hour, turn off the water and record the rate of infiltration (the drop rate of the
standing water) in inches per hour from the measuring rod data, until the pit is empty.
A self-logging pressure sensor may also be used to determine water depth and drain-down.
At the conclusion of testing, over-excavate the pit to see if the test water is mounded on
shallow restrictive layers or if it has continued to flow deep into the subsurface. The depth of
excavation varies depending on soil type and depth to hydraulic restricting layer, and is
determined by the engineer or certified soils professional. A soils professional should judge
whether a mounding analysis is necessary.
Pilot Infiltration Test Data Analysis
See the explanation under the guidance for large scale PITs.
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Appendix C – Evaluating Soil Infiltration Rates
Double Ring Infiltrometer
A double ring infiltrometer test is recommended when lack of space and/or equipment preclude the
use of a large or small scale PIT. A double ring infiltrometer test (ASTM D 3385) consists of two
concentric metal rings which are driven into the ground and filled with water which infiltrates into the
underlying soil. The inner ring (12 inches in diameter) is used to measure the infiltration rate and the
outer ring (24 inches in diameter) helps to prevent divergent flow. There are two operational
techniques which can be used with this test, the constant head method and the falling head method.
ASTM D 3385 mandates the use of the constant head method. Reference ASTM D 3385 for a
detailed description of the testing procedure.
Note: This appendix provides recommended soil infiltration evaluation methods; however, the local
jurisdiction may have different recommendations or requirements. Coordinate with the local
jurisdiction for more information on soil testing requirements.
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Appendix C – Evaluating Soil Infiltration Rates
Works Cited
1. Washington State Department of Ecology. Volume III, Chapter 3.3.6 - Design Saturated
Hydraulic Conductivity - Guidelines and Criteria. 2012 Stormwater Management Manual for Western
Washington, Amended. 2014.
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Appendix D. Soil Amendments
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Appendix D – Soil Amendments
Appendix D: Soil Amendments
Soil Suitability Criteria: Composition, Characteristics, and
Configuration
Bioretention soil media mixes, often referred to as soil amendments, are composed of a mixture of
sand (including top soil) and organic matter. The exact composition, characteristics, and
configuration of bioretention soil media mixes may vary depending on local climate characteristics,
availability of material, and performance objectives (i.e., target pollutants). This Appendix provides
guidance for the design of bioretention soil media mixes. Selection of a mixture should be made
considering its primary function as part of the bioretention area which includes: provide adequate
drainage, reduce pollutant levels, and support plant growth. TSS removal rates of 80% or higher
have been documented from bioretention soil media mixes with the characteristics described in this
Appendix.
Mix Composition
The bioretention soil media mix composition is defined by either weight or volume. Recommended
compositions include:
Characterized by weight:
- 3 to 8% organic matter
- 95 to 97% sand (1)
Characterized by volume:
- 20 to 40% organic matter
- 80 to 60% sand (1) (2)
Sand
The sand portion of a bioretention soil media mix typically includes both sand and top soil. The
gradation of sands is important to achieve the primary bioretention functions described above.
Recommended characteristics of the sand portion of a bioretention soil media mix include:
Soils that are classified as sandy loam and loamy sand on the USDA Textural Triangle (1)
Soils with less than 8-12% fines (silt and clays) (3)
A gradation that aligns with washed medium sand or ASTM C-33 Standard Specification for
Concrete Aggregates (4)
A coefficient of uniformity, Cu, of less than 4
Cu=
D60
D10
A coefficient of curvature, Cc, greater than or equal to 1 and less than or equal to 3 (2)
Cc=
(D30)2
D10
D60
Organic Matter
Organic matter consists of materials such as compost and peat moss. The purpose of organic matter
is to support plant growth. Organic matter has several characteristics which are important
considerations when developing a bioretention soil media mixture.
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Chapter Appendix 1 – Introduction D – Soil Amendments to the Manual
Cation Exchange Capacity (CEC)
The CEC of a soil refers to the total amount of positively charged elements that a soil can hold,
which means the higher the CEC of the soils the higher the capacity of the soils to retain storm water
pollutants. In addition, the CEC is known to remove dissolved metals from storm water runoff. CEC
is commonly expressed in milliequivalents per 100 grams (meq/100g) of soil. The CEC of the soil is
a function of the amount of clay, humus, and/or organic matter present. A minimum CEC of 5
meq/100g is recommended (5).
Phosphorus and Nitrogen Management
Bioretention researchers have reported that the phosphorus and nitrogen content in organic
materials can leach into storm water runoff as it flows through the BSM mix (6). Recommendations
for reducing nutrient leaching include:
Only locate organic materials in the root zone of the plants which is typically the top 6-inches
of the bioretention soil media mix
Use organic materials with a lower nutrient content: 1000 mg/kg of nitrogen and a
phosphorus index (P-index) between 10 and 30 (7) (8)
For locations where bioretention areas will discharge to a receiving water body, use organic
materials with a available phosphorus content of less than 100 mg/kg (7)
Additional Organic Matter Recommendations
Select organic matter with a relatively neutral pH (4)
Compost feedstocks should be limited to organic materials such as yard debris and crop
residues
The toxicity equivalent of the compost should not exceed 9.0 ng/Kg-dry (9)
Depth and Configuration
Bioretention soil media mixture depths range from 18 to 36-inches. Considerations when designing
the depth are as follows:
An 18-inch depth is recommended when TSS and dissolved metals are the target pollutants
since pollutant removal typically occurs within the top 6-inches
For locations where phosphorus and nitrogen are also a targeted for removal, depths of 24-
inches and 36-inches respectively are recommended (3)
The potential leaching of nutrients from organic matter can be reduced by locating organic
matter only in the top 6-inches and placing only sand in the Bottom 12” (8) (see Figure D-1)
Figure D-1. Recommended Bioretention Cell Configuration (Full Infiltration Section)
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Appendix D – Soil Amendments
Infiltration Rates
The infiltration rate defines the rate at which ponded water will infiltrate into the bioretention soil
media mix. The infiltration rate must be balanced between being too low, which results in reduced
capacity, and too high, which reduces treatment capabilities. Recommended infiltration
characteristics are as follows:
The long-term infiltration rate ranges from 1 to 6-inches per hour (2) (7)
- Higher rates can allow for management of more runoff
- Lower rates facilitate better pollutant removal and support plant growth
For BSM mixes with lower infiltration rates, more frequent maintenance is recommended to
maintain proper function (specifically remove and replace the top 2-3-inches of the mix)
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Chapter Appendix 1 – Introduction D – Soil Amendments to the Manual
Works Cited
1. Influence of Planting Soil Mix Characteristics on Bioretention Cell Design and Performance (pg.
404-416). Hallam, D.D. Carpenter and L. 15, s.l. : Journal of Hydrologic Engineering, June 2010.
2. Washington State Department of Ecology. Chapter 7: BMP T7.30 - Bioretention Cells, Swales,
and Planter Boxes. 2012 Stormwater Management Manual for Western Washington, Amended.
2014.
3. Bioretention Performance, Design, Construction, and Maintenance. Lord, W.F. Hunt and W.G.
North Carolina : North Carolina Cooperative Extension Service, 2006.
4. AHBL, Inc. and HDR, Inc. Chapter 4.4: Bioretention. Eastern Washington Low Impact
Development Guidance Manual. s.l. : State of Washington Department of Ecology, June 2013.
5. Washington State Department of Transportation. Chapter 5: RT.08 - Bioretention Area. [book
auth.] Development Division, Design Office Engineering and Regional Operations. Highway Runoff
Manual. February, 2016.
6. Hydrologic and Pollutant Removal Performance of Stormwater Biofiltration Systems at the Field
Scale. B.E. Hatt, T.D. Fletcher, & A. Deletic. s.l. : Journal of Hydrology (pg. 310-321), 2009.
7. Adoption Guidelines for Stormwater Biofiltration Systems. Facility for Advance Water
Biofiltration. s.l. : Monash University, 2009.
8. Minnesota Stormwater Manual Contributors. Design Criteria for Bioretention. Minnesota
Stormwater Manual. [Online] September 22, 2014.
http://stormwater.pca.state.mn.us/index.php/Design_criteria_for_bioretention.
9. Washington State Legislature. WAC 173-350-220 Composting Facilities. 2013.
10. A.P. Davis, M. Shokouhian, H. Sharma, & C. Minami. Water Quality Improvement Through
Bioretention Media: Nitrogen and Phosphorus Removal. Water Environment Research, 78(3) (pg.
284-293). 2006.
Appendix E. Standard Forms
Note: This appendix contains an offsite treatment evaluation form template and
a plan review checklist template. These templates may be adapted and/or
adopted by a local jurisdiction to assist with the implementation of their storm
water management program.
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Offsite Treatment Evaluation Form Template Page 1
CITY OF [NAME]
[NAME OF DEPARTMENT, IF APPLICABLE]
OFFSITE TREATMENT EVALUATION FORM
Template Instructions: In accordance with the MS4 General Permit, each MS4 must develop criteria to be
considered before allowing offsite storm water treatment for new and redevelopment projects which are
subject to Part II.A.5.b.iii of the Permit. The following template contains example criteria which may be
used to evaluate projects for eligibility to utilize offsite treatment. The process, criteria, and other
considerations provided within the template are provided as examples and should be modified and/or
expanded upon to meet the local jurisdictions specific needs and goals.
Project Information
Project name:
Description of work:
Subdivision name (if applicable):
Site area (acres):
Impervious surface created or altered (acres):
Runoff reduction volume (acre feet): Runoff reduction flow (cfs):
Project classification (check all that apply):
New Development Redevelopment Residential Commercial
Regional Facility Considerations
Basin name:
Regional treatment facility to be utilized:
Design capacity of regional treatment facility:
Does the regional treatment facility have adequate capacity? Yes No
Technical Considerations
(Are following criteria considered within the technical report to provide reasoning for use of offsite treatment)
Topography (Steep Slopes) Yes No Space available Yes No
Soil infiltration rate Yes No Shallow bedrock Yes No
Contaminated soils Yes No Prohibitive costs Yes No
High groundwater Yes No Down-gradient structures Yes No
City code/ordinance Yes No Community development rules Yes No
Water quality benefits Yes No [Insert Other] Yes No
Additional Information
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Post-Construction Stormwater Management
Plan Review Checklist Template Page 1
DATE RECEIVED
CITY OF [NAME]
[NAME OF DEPARTMENT, IF APPLICABLE]
POST-CONSTRUCTION STORMWATER MANAGEMENT
PLAN REVIEW CHECKLIST
NAME OF PROJECT PROJECT FILE NO. ADDRESS
TOTAL PROJECT ACRES
TOTAL DISTURBED ACRES
Latitude: Longitude:
GPS LOCATION OF CONSTRUCTION SITE
APPLICANT
ADDRESS
PHONE NUMBER
OWNER (If different from Applicant) ADDRESS
PHONE NUMBER
Review History
First Review
Plan Received on: Approved/Denied:
Review Completed on: Comments:
Reviewed by:
Second Review
Plan Received on: Approved/Denied:
Review Completed on: Comments:
Reviewed by:
Third Review
Plan Received on: Approved/Denied:
Review Completed on: Comments:
Reviewed by:
REPORT OF TECHNICAL REVIEW
The Stormwater Management Plan for the above named project or activity includes the necessary post-
construction controls in order to comply with the State and local post-construction stormwater requirements (as
identified within the attached checklist).
The Stormwater Management Plan for the above named project or activity does not include the necessary
post-construction controls in order to comply with the State and local post-construction stormwater requirements
(as identified within the attached checklist) through failure to include the following:
Review by: _________________________________________
Signature: __________________________________________ Date: ___________________
Post-Construction Stormwater Management
Plan Review Checklist Template Page 2
Project Name: Applicant:
General Information
Complete
Incomplete
N/A
1. Location
a. Address, subdivision name, legal description, etc�
2. Type of development (residential, commercial, etc...)
3. Areas (ac)
a. Total disturbed area
b. Existing impervious area
c. Post-development impervious area
4. Drainage basin maps are provided which clearly label the following:
a. Existing basin boundaries
b. Existing time of concentration flowpaths for each basin
c. Post-development basin boundaries
d. Post-development time of concentration flowpaths for each basin
e. Discharge location(s)
f. Receiving waters within 200 feet of project are identified
5. Montana Licensed Engineer Stamp
Drainage Plan Content
1. Topographic map of existing and finished grade contours at 2-foot max intervals
2. Location of each permanent stormwater control
3. Plan and profile of each permanent stormwater control
4. Invert elevations, slopes, and lengths of storm drain facilities
5. Size, types, invert elevations and lengths of all culverts and pipe systems
6. Discharge points clearly labeled
7. Receiving surface waters identified
8. Existing on-site natural resources identified and protected
9. FEMA floodplains identified
Calculations and Design Documentation
1. Hydrology calculations
a. State runoff method used (rational, SCS, etc�)
b. State modeling constants and assumptions
c. Description of design storms (frequency, depth, duration)
d. Existing and post-development land uses
Post-Construction Stormwater Management
Plan Review Checklist Template Page 3
Project Name: Applicant:
Calculations and Design Documentation (Continued)
Complete
Incomplete
N/A
e. Existing and post-development peak runoff rate for each design storm
f. Existing and post-development runoff volume for each design storm
2. Post-construction BMP sizing calculations
a. State design requirements (0.5-inch requirement, TSS removal, or other)
b. Required permanent controls capacities, flow rates, and operating levels
c. Sizing calculations with results
d. A statement documenting compliance with design requirements
e. If 0.5-inch or TSS removal requirements are not met, provide documentation showing
the impracticability of infiltration, evapotranspiration, capture for reuse, and treatment.
3. Culvert and pipe system capacities and outlet velocities
4. Ditch capacities and velocities
Additional Information
1. Permits, easements, setbacks, and discharge agreements
2. Floodplain maps
3. Operations and Maintenance Manual for each permanent stormwater control
a. Identify the owner
b. Identify the party responsible for long-term O&M
c. A schedule of inspection and maintenance for routine and non-routine maintenance
tasks to be conducted
d. System failure and replacement criteria to define the structure's performance
requirements
4. Geotechnical Report
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Appendix F. Example Inspection Forms
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Example Inspection Form – Infiltration Basin Page 1 September 2017
Example Inspection Form – Infiltration Basin
General Information
Site Name (if Applicable): Type of BMP:
Location (Physical Address):
Site Owner: Phone Number:
Responsible Party: Phone Number:
Date of Inspection: Start/End Time:
Inspector’s Name: Inspector’s Title:
Inspector’s Contact Information (phone):
Type of Inspection:
Routine, Dry Weather Routine, Wet Weather Complaint Response
Other: ________________
Weather Information
Weather at time of this inspection:
Clear Cloudy Raining Sleet Fog Snowing High Winds
Other: ________________ Temperature: ________________
Do you suspect that any physical changes or damages to the BMP may have occurred since the last
inspection? Yes No
If yes, provide description of physical changes or damages:
Are there any storm water discharges at the time of inspection (i.e., discharge from an outlet)? Yes No
If yes, provide location(s) and a description of storm water discharged from the site (presence of suspended
sediment, turbid water, discoloration and/or oil sheen, odor, etc…).
Prohibited Discharges
Are there any prohibited discharges at the time of inspection and/or any signs of prohibited discharges since
the last inspection (i.e., chemicals, oils, or other illicit discharges flowing into the BMP)? Yes No
If yes, provide location(s) and a description:
Inspector’s Signature: ________________________________ Date: _________________
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – Infiltration Basin Page 2 September 2017
Required Corrective Action/Notes
Maintenance
Needed?
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Desired Conditions
Maintenance access to the basin is not
obstructed in any way.
Trash and debris is not accumulated
within or around the basin and there is
no evidence of oil, gasoline,
contaminants, or other pollutants.
The contributing drainage area is
stabilized and not contributing excessive
amounts of sediment.
There is no damage to the fence or gate
that permits entry to the facility.
If applicable, diversion structures are
operating as intended and adequately
conveying storm water runoff.
The inlet structure and energy
dissipation area show no signs of
erosion.
The inlet structure is not damaged,
clogged, or defective.
The outlet structure of the pretreatment
facility is not damaged, clogged, or
defective.
There is no standing water in the
pretreatment facility.
There is no evidence of excessive
sediment deposition in the pretreatment
facility.
There is no damage that impacts the
performance of the pretreatment facility.
Inspection
Item
Accessibility
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – Infiltration Basin Page 3 September 2017
Required Corrective Action/Notes
Maintenance
Needed?
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Desired Conditions
Embankments and side slopes are
stabilized and not contributing sediment
to the basin.
There is no evidence of rodent holes or
water piping through dam or berm.
There is no standing water in the basin.
There is no evidence of excessive
sediment deposition on the bottom of the
basin.
Vegetation is healthy and basin is not
overrun by weeds.
Excessive vegetation is not present
within the vicinity of the basin that could
drop leaf litter, fruits, and other
vegetative materials.
The spillway does not show signs of
damage or erosion.
Structural components such as
concrete, riprap, or concrete block mats
are not damaged or defective.
The trash rack, orifice plate, outlet pipe,
and/or riser are free of sediment, debris
and/or dead vegetation.
The outlet structure is not damaged,
leaking, or showing signs of erosion or
undercutting.
Permanent erosion control measures at
the outfall of the structure are not
damaged or contributing excess
sediment to the downstream channel.
Inspection
Item
This page is intentionally left blank.
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – Bioretention Page 1 September 2017
Example Inspection Form – Bioretention
General Information
Site Name (if Applicable): Type of BMP:
Location (Physical Address):
Site Owner: Phone Number:
Responsible Party: Phone Number:
Date of Inspection: Start/End Time:
Inspector’s Name: Inspector’s Title:
Inspector’s Contact Information (phone):
Type of Inspection:
Routine, Dry Weather Routine, Wet Weather Complaint Response
Other: ________________
Weather Information
Weather at time of this inspection:
Clear Cloudy Raining Sleet Fog Snowing High Winds
Other: ________________ Temperature: ________________
Do you suspect that any physical changes or damages to the BMP may have occurred since the last
inspection? Yes No
If yes, provide description of physical changes or damages:
Are there any storm water discharges at the time of inspection (i.e., discharge from an outlet)? Yes No
If yes, provide location(s) and a description of storm water discharged from the site (presence of suspended
sediment, turbid water, discoloration and/or oil sheen, odor, etc…).
Prohibited Discharges
Are there any prohibited discharges at the time of inspection and/or any signs of prohibited discharges since
the last inspection (i.e., chemicals, oils, or other illicit discharges flowing into the BMP)? Yes No
If yes, provide location(s) and a description:
Inspector’s Signature: ________________________________ Date: _________________
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – Bioretention Page 2 September 2017
Required Corrective Action/Notes
Maintenance
Needed?
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Desired Conditions
Maintenance access to the bioretention
area is not obstructed in any way.
Trash and debris are not accumulated
within or around the basin and there is
no evidence of oil, gasoline,
contaminants, or other pollutants.
The contributing drainage area is
stabilized and not contributing
excessive amounts of sediment.
The bioretention area’s structural
components are not damaged.
If applicable, diversion structures are
operating as intended.
The inlet structure and energy
dissipation area show no signs of
erosion.
The inlet structure is not damaged,
clogged, or defective.
There is no standing water in the
pretreatment facility.
There is no evidence of excessive
sediment deposition.
There is no damage that impacts the
performance of the pretreatment facility.
Side slopes are stabilized and not
contributing sediment to the surface
ponding area.
There is no evidence of rodent holes,
sinkholes, or instability.
Inspection
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – Bioretention Page 3 September 2017
Required Corrective Action/Notes
Maintenance
Needed?
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Desired Conditions
There is no standing water in the surface
ponding area.
If applicable, the irrigation system is
intact and functioning as designed.
There is no evidence of excessive
sediment deposition within or around the
surface ponding area.
The mulch layer is fully intact (2- to 3-
inches thick) with no bare areas.
Vegetation is healthy and matches the
original design.
Weeds are not growing within the
surface ponding area.
Underdrains and cleanouts are free of
sediment and debris.
There is no damage that impacts the
performance of the underdrain system.
Overflow structures are free of sediment,
debris and/or dead vegetation.
Permanent erosion control measures at
the overflow structure are not damaged
or contributing excess sediment to the
downstream channel.
The structure is not damaged or
showing signs of erosion.
Inspection
Item
Drainage
Irrigation
Sediment
Surface Cover
Vegetation
This page is intentionally left blank.
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – PICPs Page 1 September 2017
Example Inspection Form
Permeable Interlocking Concrete Pavers
General Information
Site Name (if Applicable): Type of BMP:
Location (Physical Address):
Site Owner: Phone Number:
Responsible Party: Phone Number:
Date of Inspection: Start/End Time:
Inspector’s Name: Inspector’s Title:
Inspector’s Contact Information (phone):
Type of Inspection:
Routine, Dry Weather Routine, Wet Weather Complaint Response
Other: ________________
Weather Information
Weather at time of this inspection:
Clear Cloudy Raining Sleet Fog Snowing High Winds
Other: ________________ Temperature: ________________
Do you suspect that any physical changes or damages to the BMP may have occurred since the last
inspection? Yes No
If yes, provide description of physical changes or damages:
Are there any storm water discharges at the time of inspection (i.e., discharge from an outlet)? Yes No
If yes, provide location(s) and a description of storm water discharged from the site (presence of suspended
sediment, turbid water, discoloration and/or oil sheen, odor, etc…).
Prohibited Discharges
Are there any prohibited discharges at the time of inspection and/or any signs of prohibited discharges since
the last inspection (i.e., chemicals, oils, or other illicit discharges flowing onto the BMP)? Yes No
If yes, provide location(s) and a description:
Inspector’s Signature: ________________________________ Date: _________________
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – PICPs Page 2 September 2017
Required Corrective Action/Notes
Maintenance
Needed?
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Desired Conditions
Maintenance access to the permeable
surface is not obstructed in any way.
Trash and debris are not accumulated
within or around the basin and there is
no evidence of oil, gasoline,
contaminants, or other pollutants.
The contributing drainage area is
stabilized and not contributing
excessive amounts of sediment.
Structural components within and
around the area are not damaged or
defective.
Vegetation around the perimeter of the
permeable surface area is healthy and
not overrun by excessive vegetation or
weeds.
Perimeter barriers are not leaking or
negatively impacting the adjacent areas.
There is no evidence of cracking,
settling, or deterioration.
There is no standing water on the
surface of the PICP area.
There is no evidence of excessive
sediment deposition within the PICP
area or within the PICP joints.
The pavers are not deteriorating,
cracked, settling, or misaligned. Paver
joint material is not missing.
Vegetation is not growing in between
pavers.
Inspection
Item
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – PICPs Page 3 September 2017
Required Corrective Action/Notes
Maintenance
Needed?
Yes
No
Yes
No
Yes
No
Desired Conditions
Underdrains and overflow structures are
free of sediment, debris and/or dead
vegetation.
Permanent erosion control measures at
the overflow structure are not damaged
or contributing excess sediment to the
downstream channel.
The structure is not damaged and is
operating as intended.
Inspection
Item
Clogging
Erosion
Control
Structural
Damage
Primary
Components
Underdrain &
Overflow System
(If applicable)
This page is intentionally left blank.
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – Dispersion Page 1 September 2017
Example Inspection Form – Dispersion
General Information
Site Name (if Applicable): Type of BMP:
Location (Physical Address):
Site Owner: Phone Number:
Responsible Party: Phone Number:
Date of Inspection: Start/End Time:
Inspector’s Name: Inspector’s Title:
Inspector’s Contact Information (phone):
Type of Inspection:
Routine, Dry Weather Routine, Wet Weather Complaint Response
Other: ________________
Weather Information
Weather at time of this inspection:
Clear Cloudy Raining Sleet Fog Snowing High Winds
Other: ________________ Temperature: ________________
Do you suspect that any physical changes or damages to the BMP may have occurred since the last
inspection? Yes No
If yes, provide description of physical changes or damages:
Are there any storm water discharges at the time of inspection? Yes No
If yes, provide location(s) and a description of storm water discharged from the site (presence of suspended
sediment, turbid water, discoloration and/or oil sheen, odor, etc…).
Prohibited Discharges
Are there any prohibited discharges at the time of inspection and/or any signs of prohibited discharges since
the last inspection (i.e., chemicals, oils, or other illicit discharges flowing into the BMP)? Yes No
If yes, provide location(s) and a description:
Inspector’s Signature: ________________________________ Date: _________________
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – Dispersion Page 2 September 2017
Required Corrective Action/Notes
Maintenance
Needed?
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Desired Conditions
Maintenance access to the dispersion
area is not obstructed in any way.
Trash and debris are not accumulated
within or around the dispersion area
and there is no evidence of oil, gasoline,
contaminants, or other pollutants.
The contributing drainage area is
stabilized and not contributing
excessive amounts of sediment.
Structural components within and
around the dispersion area are not
damaged or defective.
The level spreader is not filled with
sediment or other debris.
Gravel within the level spreader is intact
and not displaced due to traffic or
erosion.
If applicable, the diversion structure is
operating as intended and adequately
conveying storm water runoff.
Erosion is not occurring along the
boundary of the flow spreader.
The flow spreader is not damaged,
clogged, or defective.
The dispersion area is maintaining a
uniform slope to promote sheet flow.
There is no evidence of erosion such as
rills, gullies, or channels.
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – Biofiltration Swale Page 1 September 2017
Example Inspection Form – Biofiltration Swale
General Information
Site Name (if Applicable): Type of BMP:
Location (Physical Address):
Site Owner: Phone Number:
Responsible Party: Phone Number:
Date of Inspection: Start/End Time:
Inspector’s Name: Inspector’s Title:
Inspector’s Contact Information (phone):
Type of Inspection:
Routine, Dry Weather Routine, Wet Weather Complaint Response
Other: ________________
Weather Information
Weather at time of this inspection:
Clear Cloudy Raining Sleet Fog Snowing High Winds
Other: ________________ Temperature: ________________
Do you suspect that any physical changes or damages to the BMP may have occurred since the last
inspection? Yes No
If yes, provide description of physical changes or damages:
Are there any storm water discharges at the time of inspection (i.e., discharge from an outlet)? Yes No
If yes, provide location(s) and a description of storm water discharged from the site (presence of suspended
sediment, turbid water, discoloration and/or oil sheen, odor, etc…)
Prohibited Discharges
Are there any prohibited discharges at the time of inspection and/or any signs of prohibited discharges since
the last inspection (i.e., chemicals, oils, or other illicit discharges flowing into the BMP)? Yes No
If yes, provide location(s) and a description:
Inspector’s Signature: ________________________________ Date: _________________
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – Biofiltration Swale Page 2 September 2017
Required Corrective Action/Notes
Maintenance
Needed?
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Desired Conditions
Maintenance access to the swale is not
obstructed in any way.
Trash and debris are not accumulated
within or around the swale. There is no
evidence of oil, gasoline, contaminants,
or other pollutants.
There is no evidence of erosion such as
sloughing, rills, or gullies.
The contributing drainage area is
stabilized and not contributing
excessive amounts of sediment.
Structural components within and
around the swale are not damaged or
defective.
If applicable, diversion structures are
operating as intended and adequately
conveying storm water runoff.
The inlet structure and energy
dissipation area show no signs of
erosion.
The inlet structure is not damaged,
clogged, or defective.
The outlet structure is not damaged,
clogged, or defective.
There is no standing water in the
pretreatment facility.
There is no evidence of excessive
sediment deposition.
There is no damage that impacts the
performance of the pretreatment facility.
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – Biofiltration Swale Page 3 September 2017
Required Corrective Action/Notes
Maintenance
Needed?
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Desired Conditions
Side slopes are stabilized and not
contributing sediment to the swale.
There is no evidence of rodent holes,
sinkholes, or instability.
There is no standing water in the swale.
There is no evidence of excessive
sediment deposition within the bottom of
the swale.
Level spreaders, check dams, and/or
energy dissipaters are not damaged and
are operating as intended.
Vegetation is thick, healthy, and
matches to original design.
Swale area is not overrun by weeds.
The outlet structure is free of sediment,
debris and/or dead vegetation.
Permanent erosion control measures at
the outlet structure are not damaged or
contributing excess sediment to the
downstream channel.
The structure is not damaged and
operating as intended.
Inspection
Item
Erosion
Control
Structural
Damage
Drainage
Sediment
Structural
Damage
Vegetation
Vegetation
This page is intentionally left blank.
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – EDB Page 1 September 2017
Example Inspection Form – Extended Detention Basin
General Information
Site Name (if Applicable): Type of BMP:
Location (Physical Address):
Site Owner: Phone Number:
Responsible Party: Phone Number:
Date of Inspection: Start/End Time:
Inspector’s Name: Inspector’s Title:
Inspector’s Contact Information (phone):
Type of Inspection:
Routine, Dry Weather Routine, Wet Weather Complaint Response
Other: ________________
Weather Information
Weather at time of this inspection:
Clear Cloudy Raining Sleet Fog Snowing High Winds
Other: ________________ Temperature: ________________
Do you suspect that any physical changes or damages to the BMP may have occurred since the last
inspection? Yes No
If yes, provide description of physical changes or damages:
Are there any storm water discharges at the time of inspection (i.e., discharge from an outlet)? Yes No
If yes, provide location(s) and a description of storm water discharged from the site (presence of suspended
sediment, turbid water, discoloration and/or oil sheen, odor, etc…)
Prohibited Discharges
Are there any prohibited discharges at the time of inspection and/or any signs of prohibited discharges since
the last inspection (i.e., chemicals, oils, or other illicit discharges flowing into the BMP)? Yes No
If yes, provide location(s) and a description:
Inspector’s Signature: ________________________________ Date: _________________
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – EDB Page 2 September 2017
Required Corrective Action/Notes
Maintenance
Needed?
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Desired Conditions
Maintenance access to the EDB is not
obstructed in any way.
Trash and debris are not accumulated
within or around the EDB and there is no
evidence of oil, gasoline, contaminants,
or other pollutants.
Trickle channel, upstream/downstream
channels, and berms show no signs of
erosion.
The contributing drainage area is
stabilized and not contributing
excessive amounts of sediment.
There is no damage to the fence or gate
that permits entry to the facility.
The inlet structure and energy
dissipation area show no signs of
erosion.
The inlet structure is not damaged,
clogged, or defective.
The forebay outlet structure is not
damaged, clogged, or defective.
There is no standing water in the
pretreatment forebay.
There is not excessive sediment
deposition in the pretreatment forebay.
There is no damage that impacts the
performance of the pretreatment
forebay.
Inspection
Item
Accessibility
Contaminants
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – EDB Page 3 September 2017
Required Corrective Action/Notes
Maintenance
Needed?
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Desired Conditions
Embankment and side slopes are
stabilized and not contributing sediment
to the EDB.
There is no evidence of rodent holes or
water piping through dam or berm.
There is no overgrowth of weeds or
nuisance vegetation within or around the
EDB.
Trees are not growing on or near berms
or embankments.
All dead vegetation has been removed
from the EDB.
The spillway does not show signs of
damage or erosion.
Erosion control measures such as
concrete, riprap, or concrete block mats
are not damaged.
The trash rack, orifice plate, outlet pipe,
and/or riser are free of sediment, debris
and/or dead vegetation.
Permanent erosion control measures at
the outfall of the structure are not
damaged or contributing excess
sediment to the downstream channel.
The outlet structure is not damaged,
leaking, or showing signs of erosion or
undercutting.
Inspection
Item
Erosion
Control
Structural
Damage
Overgrowth
This page is intentionally left blank.
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – WDB Page 1 September 2017
Example Inspection Form – Wet Detention Basin
General Information
Site Name (if Applicable): Type of BMP:
Location (Physical Address):
Site Owner: Phone Number:
Responsible Party: Phone Number:
Date of Inspection: Start/End Time:
Inspector’s Name: Inspector’s Title:
Inspector’s Contact Information (phone):
Type of Inspection:
Routine, Dry Weather Routine, Wet Weather Complaint Response
Other: ________________
Weather Information
Weather at time of this inspection:
Clear Cloudy Raining Sleet Fog Snowing High Winds
Other: ________________ Temperature: ________________
Do you suspect that any physical changes or damages to the BMP may have occurred since the last
inspection? Yes No
If yes, provide description of physical changes or damages:
Are there any storm water discharges at the time of inspection (i.e., discharge from an outlet)? Yes No
If yes, provide location(s) and a description of storm water discharged from the site (presence of suspended
sediment, turbid water, discoloration and/or oil sheen, odor, etc…)
Prohibited Discharges
Are there any prohibited discharges at the time of inspection and/or any signs of prohibited discharges since
the last inspection (i.e., chemicals, oils, or other illicit discharges flowing into the BMP)? Yes No
If yes, provide location(s) and a description:
Inspector’s Signature: ________________________________ Date: _________________
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – WDB Page 2 September 2017
Required Corrective Action/Notes
Maintenance
Needed?
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Desired Conditions
Maintenance access to the WDB is not
obstructed in any way.
Trash and debris are not accumulated
within or around the WDB and there is
no evidence of oil, gasoline,
contaminants, or other pollutants.
Upstream/downstream channels and
berms show no signs of erosion.
The contributing drainage area is
stabilized and not contributing
excessive amounts of sediment.
There is no damage to the fence or gate
that permits entry to the facility.
The inlet structure and energy
dissipation area show no signs of
erosion.
The inlet structure is not damaged,
clogged, or defective.
There is not excessive sediment
deposition in the pretreatment forebay.
There is no damage that impacts the
performance of the pretreatment
forebay.
Embankments and side slopes are
stabilized and not contributing sediment
to the WDB.
There is no evidence of rodent holes or
water piping through embankment or
berm.
Inspection
Item
Accessibility
Montana Post-Construction Storm Water BMP Design Guidance Manual
Example Inspection Form – WDB Page 3 September 2017
Required Corrective Action/Notes
Maintenance
Needed?
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Yes
No
Desired Conditions
A permanent pool of water is maintained
within the wetpool cell.
If applicable, the pond drain is not
damaged, clogged, or defective.
The shoreline area including the safety
wetland bench is stabilized and erosion
is not present.
There is no evidence of undesirable
vegetation or excessive algae blooms
within the wetpool cell.
The spillway does not show signs of
damage or erosion.
Erosion control measures such as
concrete, riprap, or concrete block mats
are not damaged.
The trash rack, orifice plate, outlet pipe,
and/or riser are free of sediment, debris
and/or dead vegetation.
Permanent erosion control measures at
the outfall of the structure are not
damaged or contributing excess
sediment to the downstream channel.
The outlet structure is not damaged,
leaking, or showing signs of erosion or
undercutting.
Inspection
Item
Base Flow
Dewatering
Erosion
Control
Vegetation
Structural
Damage
Structural
Damage
Clogging
Erosion
Control
Structural
Damage
Primary
Components
Wetpool Cell
Spillway
Outlet Structure
Contaminants
& Pollution
Erosion
Control
Sedimentation
Structural
Damage
Erosion
Control
Structural
Damage
Sedimentation
Structural
Damage
Erosion
Control
Structural
Damage
Primary
Components
General
Storm Water
Inlets
Pretreatment
Forebay
Embankments
&
Side Slopes
Tree Growth
Vegetation
Structural
Damage
Structural
Damage
Clogging
Erosion
Control
Structural
Damage
Primary
Components
Embankments
&
Side Slopes
Vegetation
Spillway
Outlet Structure
& Pollution
Erosion
Control
Sedimentation
Structural
Damage
Erosion
Control
Structural
Damage
Clogging
Drainage
Sedimentation
Structural
Damage
Primary
Components
General
Storm Water
Inlets
Pretreatment
Forebay
Clogging
Erosion
Control
Structural
Damage
Primary
Components
Side Slopes
Swale Area
Outlet Structure
(If applicable)
Inspection
Item
Accessibility
Contaminants
& Pollution
Erosion
Control
Sedimentation
Structural
Damage
Diversion
Structures
Erosion
Control
Structural
Damage
Clogging
Drainage
Sedimentation
Structural
Damage
Primary
Components
General
Storm Water
Inlets &
Conveyance
Pretreatment
Facilities
Vegetation is healthy and the area does
not have excessive weeds.
Trees are not growing within the
vegetation dispersion area.
Inspection
Item
Accessibility
Contaminants &
Pollution
Sedimentation
Structural
Damage
Sedimentation
Structural
Damage
Diversion
Structure
Erosion
Control
Structural
Damage
Drainage
Erosion
Vegetation
Vegetation
Primary
Components
General
Level Spreader
(If applicable)
Flow Spreader
(If applicable)
Dispersion
Area
Accessibility
Contaminants
& Pollution
Sedimentation
Structural
Damage
Vegetation
Drainage
Structural
Damage
Drainage
Sediment
Structural
Damage
Vegetation
Primary
Components
General
Perimeter Barrier
(If applicable)
Pavers
Vegetation
Clogging
Structural
Damage
Clogging
Erosion
Control
Structural
Damage
Primary
Components
Surface Ponding
Area (Filter Area)
Underdrain
(If Applicable)
Surface Overflow
Structures
(If applicable)
Item
Accessibility
Contaminants &
Pollution
Sedimentation
Structural
Damage
Diversion
Structures
Erosion
Control
Structural
Damage
Drainage
Sedimentation
Structural
Damage
Erosion
Control
Structural
Damage
Primary
Components
General
Storm Water
Inlets &
Conveyance
Pretreatment
Facilities
Side Slopes
Erosion
Control
Structural
Damage
Drainage
Sediment
Vegetation
Vegetation
Erosion
Control
Structural
Damage
Clogging
Structural
Damage
Erosion
Control
Primary
Components
Embankment &
Side Slopes
Infiltration Cell
Spillway
Outlet Structure
(If applicable)
Contaminants &
Pollution
Sedimentation
Structural
Damage
Diversion
Structures
Erosion Control
Structural
Damage
Clogging
Drainage
Sedimentation
Structural
Damage
Primary
Components
General
Storm Water
Inlets &
Conveyance
Pretreatment
Facility
0.016 – 0.025
Grass:
Short grass prairie
Dense grasses
2
Bermuda grass
0.15
0.24
0.41
Natural Channels
Fairly regular section
Irregular section with pools
0.025 – 0.050
0.040 – 0.150
1
Lower values are usually for well-constructed and maintained (smoother) pipes and
channels.
Source: Adapted from FHWA HEC-22, Table 3-4
Erosion and embankment stabilization repair
Specialized equipment and training
1 Pollutant removal efficiencies vary for different units.
2 Maintenance requirements vary for different devices. See manufacturer’s recommendations for maintenance requirements specific
to each device.
Provide a landscaping plan
Provide an operations and maintenance plan
Pretreatment forebay
Volume is 10% of RTV (minimum)
Depth between 4 and 6 feet
Hard bottom
Provide maintenance access
Armored barrier or berm separating pretreatment
forebay and wet pool cell
Wet pool cell
Depth between 4 and 12 feet
Safety-wetland bench (4 feet wide, 6 to 12 inches
deep)
Outlet structure
48-hour minimum drain time for RTV
Provide trash rack
Provide pond drain (or other method to drain wet
pool cell)
1 This table presents the minimum design criteria for satisfying
the Runoff Treatment Requirement as defined in Section 1.3.2
of this manual.
Vegetation management
Erosion and embankment stabilization repair
Specialized equipment and training
Provide an operations and maintenance plan
Pretreatment forebay
Volume is 10% of RTV (minimum)
Depth between 4 and 6 feet
Hard bottom
Provide maintenance access
Trickle channel
Flow capacity equal to forebay outlet structure
discharge capacity
Micropool
Depth of at least 2.5 feet
Surface area of 10 square feet (minimum)
Hard bottom
Outlet structure
48-hour minimum drain time for RTV
Provide trash rack
1 This table presents the minimum design criteria for satisfying
the Runoff Treatment Requirement as defined in Section 1.3.2
of this manual.
Irrigation
Vegetation management
Erosion and embankment stabilization repair
Specialized equipment and training
y = Design flow depth for RTF (ft)
Calculate the bottom width of the swale using Equation 5.6-2. If the calculated value for b is less
than 2 feet, then set the bottom swale width to 2 feet. Biofiltration swales are limited to a maximum
Figure 5.6-3. Trapezoid Dimensions
Swale length is ≥100 feet
Provide maintenance access
Provide a landscaping plan
Provide an operations and maintenance plan
1 This table presents the minimum design criteria for satisfying
the Runoff Treatment Requirement as defined in Section 1.3.2
of this manual.
nitrogen.
Provide maintenance access
Provide a landscaping plan
Provide an operations and maintenance plan
Sheet flow dispersion
Provide level spreader for lateral slopes greater
than 6:1 (H:V)
Minimum length as presented in Table 5.5-1
Channelized dispersion
Provide a flow spreader sized to evenly distribute
the design flow rate over the dispersion area
Flow spreader and diversion structure (if
applicable) design flow rate is ≥ RTF
Minimum length as presented in Table 5.5-2
1 This table presents the minimum design criteria for satisfying
the Post-Construction Performance Standard as defined in
Section 1.3.2 of this manual.
100% of run-on flows across a pretreatment BMP
Provide maintenance access
Provide an operations and maintenance plan
Full infiltration section
Native soil is capable of infiltrating the design
volume within 48 hours (with safety factor of 2)
Partial infiltration section
Underdrain system sized to release the design
volume within 48 hours
No infiltration section
Underdrain system sized to release the design
volume within 48 hours
Impermeable liner
1 This table presents the minimum design criteria for satisfying
the Runoff Reduction Requirement as defined in Section 1.3.2
of this manual.
Specialized equipment and training
1TMDL considerations listed are for facilities with a full infiltration section and do not apply to facilities with an underdrain.
18 inches
Pretreatment facility applicable to inlet
configuration
Provide maintenance access
Provide a landscaping plan
Provide an operations and maintenance plan
Full infiltration section
Native subsurface soil is capable of infiltrating the
design volume within 48 hours (with safety factor
of 2)
Partial infiltration section
Underdrain sized to release the design volume
within 48 hours
Underdrain aggregate layer
No infiltration section
Underdrain sized to release the design volume
within 48 hours
Underdrain aggregate layer
Impermeable liner
1 This table presents the minimum design criteria for satisfying
the Post-Construction Performance Standard as defined in
Section 1.3.2 of this manual.
Erosion and embankment stabilization repair
Specialized equipment and training
1 TMDL considerations listed are for facilities with an underdrain. Bioretention is preferred for all pollutants when using a full
infiltration section.
General
Infiltration cell is sized for 100% of RRV (minimum)
Provide maintenance access
Provide a landscaping plan
Provide an operations and maintenance plan
Pretreatment forebay (if applicable)
Volume is 10% of RTV (minimum)
Depth between 4–6 feet
Hard bottom
Provide maintenance access
Armored barrier or berm separating pretreatment
forebay and infiltration cell
Infiltration cell
72-hour drain time for the RRV (maximum)
1 This table presents the minimum design criteria for satisfying
the Runoff Reduction Requirement as defined in Section 1.3.2
of this manual.
Erosion and embankment stabilization repair
Specialized equipment and training
2.5 acres
or less
2:1 ratio
Limit sheet
flow to
150 feet
5 acres
or less
5 acres to
1 square
mile
10 acres
minimum
Pollutant Removal Considerations
Fecal
Coliform
Preferred
—
Preferred
Preferred
—
—
—
Varies for different units
Metals
Preferred
Preferred
Preferred
Preferred
—
—
—
Temperature
Preferred
Preferred
Preferred
Preferred
—
Avoid
Avoid
Total
Nitrogen
Preferred
Avoid
Preferred
Preferred
—
—
—
Total
Phosphorus
Preferred
Avoid
Preferred
Preferred
—
—
Preferred
TSS3
Preferred
Preferred
Preferred
Preferred
Preferred
Preferred
Preferred
Primary Function
Runoff
Treatment2
Runoff
Reduction1
4
BMP
Infiltration
Basin
Bioretention
Permeable
Pavement
Systems
Dispersion
Biofiltration
Swale
Extended
Detention
Basin
Wet Detention
Basin
Proprietary
Treatment
Devices
solids). Adsorption is the adherence or bonding of a pollutant onto
the surface of media or soil particles.
Pollutants: dissolved phosphorus, dissolved metals, oil
(hydrocarbons), and organics
Bioretention
Dispersion
Source: West Virginia Department of Environmental Protection (11)
1 Some proprietary treatment BMPs use filtration. See Section 5.9 for more information.
2 Permeable surfaces use infiltration when there is not an underdrain system. See Section 5.4 for more information.
4.3 Screening Factors
Screening factors are characteristics and constraints of the project site and surrounding areas that
should be reviewed to help evaluate which BMPs are suitable for use. This section discusses some
of the screening factors that should be reviewed and considered during the BMP selection process.
4.3.1 Land Use
The first screening factor in BMP selection is identifying the land use from which the BMP will
receive runoff. This is because typical land use characteristics can provide the designer with
important information, such as the pollutants to be expected in storm water runoff from the site and
design strategies and limitations that should be considered. This section provides a general
description of each land use, along with strategies and limitations for BMP selection (11).
landscapes
Create multifunctional landscapes using
BMPs that provide filtration, treatment, and
infiltration. Design features that function as
open space, wildlife habitat, and snow
storage areas, in addition to storm water
management.
Improved aesthetics
Reduced cost of storm water
infrastructure
by rule, which means they can operate without an individual permit as long as the
injection does not endanger underground sources of drinking water and the
owner/operator submits basic inventory information to US EPA. Additional
information can be found on US EPA’s UIC webpage
(https://www.epa.gov/uic/stormwater-drainage-wells).
Section 438 of the Energy
Independence and Security
Act (EISA)
US EPA
Section 438 of EISA states that “the sponsor of any development or redevelopment
project involving a Federal Facility with a footprint that exceeds 5,000 ft
2
shall use
site planning, design, construction, and maintenance strategies for the property to
maintain or restore, to the maximum extent technically feasible, the predevelopment
hydrology of the property with regard to temperature, rate, volume, and duration of
flow.” In 2009, US EPA issued technical guidance for implementing this provision of
EISA (1).
Dam Safety Program
Montana Department of
Natural Resources and
Conservation (DNRC)
To build a new dam or alter an existing dam, either of which has an impoundment
capacity of 50 acre-feet or more, the owner must apply to the DNRC Dam Safety
Program for a hazard classification. Dams classified as high hazard and containing
over 50 acre-feet of water are required to obtain additional permits from the DNRC
Dam Safety Program.
1 This table is not intended to communicate every potential regulatory requirement. The project owner and/or design professional are
responsible for determining the applicable regulatory requirements and associated permits for each individual project.