HomeMy WebLinkAbout20 - Design Report - Cottonwood Subdivision - Stormwater
STORMWATER DESIGN REPORT
FOR:
COTTONWOOD SUBDIVISION
BOZEMAN, MT
Prepared By:
Madison Engineering
895 Technology Blvd Ste 203
Bozeman, MT 59718
(406) 586-0262
MARCH 2020
Cottonwood Subdivision
Stormwater Design Report
Page 1 of 5
COTTONWOOD SUBDIVISION
STORMWATER DESIGN REPORT
A. Introduction
This design report will give an overview of the stormwater management plan for the proposed
28.09-acre Cottonwood Subdivision. The site is currently undeveloped and vacant. There is one
jurisdictional wetland area of approximately 13,304 sf in the southwest portion of the property
that has been mitigated for off-site. Stormwater management within the subdivision will be
accomplished with the combination of surface/gutter flow, pipe conveyance, and detention and
retention facilities. Monolithic curb and gutters and valley gutters will be utilized to transfer
stormwater from the street to the drain inlets which will be connected to the closed conveyance
piping collection system.
The stormwater facilities and flow control structures will control and meter the discharge of the
increased flow to the 10-year pre-development flows. The facilities will help remove solids, silt,
oils, grease and other pollutants from the stormwater, with the first 0.5 inches of rainfall from a
24-hour storm event preceded by 48 hours of no measurable precipitation also captured.
The collection system was designed to convey the 25-year storm event. The retention and
detention facilities were designed to accommodate the 10-year storm event. The following
references were used in the preparation of this report:
a. COB Design Standards and Specifications Policy, 2004. Addendum #6
b. COB Modifications to Montana Public Works Standard Specifications (MPWSS).
Addendum #3
c. Bozeman Stormwater Master Plan – 1982
B. Peak Flow (Runoff) Calculations
The project area was divided into overall drainage areas as shown on Sheet SD1.1 (Appendix K)
and sub basins that contribute to individual curb inlets as shown on Sheet SD1.2 (Appendix L).
These areas were used to determine the stormwater runoff flows, which in turn were used to
determine the size of the storm drain pipes, gutter capacities, curb inlet sizes, and
detention/retention facility volumes. Peak flow calculations are provided in Appendix E.
C. Storm Drain Piping
Storm drain piping from the curb inlets to the detention facilities was sized to carry the 25-year
storm event peak runoff flow. All pipes are 15-inch or 18-inch diameter PVC. All storm drains
were sized using Manning's Equation for circular channels to determine the pipes capacity to flow
stormwater at specified grades. See pipe capacity calculations in Appendix F and an
accompanying map (Sheet SD 1.3) which identifies each pipe that was sized by number in
Appendix M. Additionally, two curb cuts with sidewalk chases were designed to the same
standard and using Manning’s Equation. These two curb cuts to sidewalk chases are located in the
Cottonwood Subdivision
Stormwater Design Report
Page 2 of 5
south and east portion of the project and could not be converted to curb inlets with underground
pipe given limitations due to pipe cover requirements, groundwater depth, and existing grades of
the adjacent property. The sidewalk chase capacity calculation is found in Appendix G.
D. Stormwater Drainage Areas
Pre-Development
The project area was delineated into two watersheds or “basins” (Basin 1 and Basin 2) based on
existing topography as shown on Sheet SD 1.0 (Appendix J). These two basins each have
separate, distinct outfalls. Basin 1, located in the northeast, flows north and ties into an existing
18-inch Reinforced Concrete Pipe (RCP) stormwater collection system in Cottonwood Road.
Basin 2, located in the southwest, flows into an existing 30-inch RCP culvert that drains under
Cottonwood Road and into Baxter Creek on the west side of Cottonwood Road. The pre-
development flow for the 10-year storm was calculated for each basin and used to size the outlet
structure weirs and detention facilities throughout the project.
Post-Development
The project area was divided into five post-development drainage areas based on the proposed
subdivision street and lot layout as shown on Sheet SD1.1 (Appendix K). Drainage Areas 5, 6 and
7 (DA 5, DA 6, DA 7) are located south of W. Garfield St. and Drainage Areas 8 and 9 (DA 8,
DA 9) are located north of W. Garfield St. In general, each basin will drain north. Four existing
drainage areas (DA 1, DA 2, DA 3, DA 4) in Cottonwood Road drain into two detention ponds
located on the subject property which were designed as a part of the Cottonwood Road
Infrastructure Improvements project in May 2019. These two detention ponds have been re-
analyzed and re-configured with the additional storm water runoff from the proposed 28.09-acre
subdivision and converted to underground R Tank HD detention facilities.
E. Stormwater Detention Facilities
There are four proposed underground R Tank HD detention facilities to capture and release
stormwater from the subdivision. Detention Facility 1 (DF 1), which drains the north half of the
subdivision (DA 8) and a portion of Cottonwood Road (DA 1), will release at the 10-year pre-
development rate into the existing 18-inch RCP stormwater collection system in Cottonwood
Road. Detention Facility 2 (DF 2) will release at the 10-year pre-development rate into the
existing 30-inch RCP culvert that drains into Baxter Creek on the west side of Cottonwood Road.
Detention Facilities 3 and 4 will release at the 10-year pre-development rate into an on-site PVC
stormwater collection system that ultimately drains into DF 2. The sizing of DF 2 and DF 3
accounts for the upstream facility draining at pre-development rate into it. This was done by
adding a column to the typical detention pond sizing spreadsheet called “Outlet from D.F. #X.”
This column is based on the outlet from the upstream facility and is added to the “Runoff volume”
column which is used in sizing the detention facility (see Detention Facility Calculations in
Appendix B).
Cottonwood Subdivision
Stormwater Design Report
Page 3 of 5
The depth to seasonal high groundwater table (from data collected in 2018) was checked at each
underground detention facility to ensure that groundwater remains below the bottom of the
underground R Tank HD systems. The bottom elevation of each underground R Tank HD system
was set above the seasonal high groundwater level (recorded on April 13, 2018) except for DF 2.
This facility is a converted detention basin from Cottonwood Road Improvements 2019 and was
constrained in its design by the elevation of the incoming storm drain pipe from Cottonwood
Road. As a result, DF 2 is proposed to be wrapped in PVC liner to prevent it from being
inundated with groundwater. Buoyancy calculations (Appendix I) were provided by the R Tank
supplier, ACF Environmental, to ensure the system does not float.
The half-inch requirement per Bozeman DSSP II.A.4 was accounted for the impervious areas
within the subdivision by elevating the weirs of Detention Facilities 1, 3 and 4. (Since Detention
Facility 2 is reconfigured from the previous Cottonwood Road Improvements project and was
previously approved without an elevated weir, it is not proposed to have an elevated weir.) The
impervious areas within the subdivision that were included in the calculations are the roads, and
65% of the area of each high-density lot. (65% impervious area for a high-density lot is an
estimate based on experience.) However, two lots were not included in the half-inch requirement;
Lot 1, Block 2 and Lot 1, Block 6. This is because if those lots had been included in the half-inch
calculation for their respective detention facility it would have required an elevated weir elevation
which exceeded the elevation of the top of the R Tank facility, rendering it no longer a detention
facility, but a retention facility, thereby fundamentally changing the design. Therefore, Lot 1,
Block 2 and Lot 1, Block 6 will need to address their respective half-inch requirement with their
site plan applications. Table 1 below summarizes the proposed R Tank detention facilities.
TABLE 1
R TANK DETENTION FACILITY VOLUMES
Facility ID Contributing
Drainage Areas*
10-yr, 2-hr Post
Development
Peak Runoff
(cfs)
Required
Volume
(C.F.)
Proposed
Volume
(C.F.)
1 1, 8 9.62 9,014 9,127
2 2, 3, 4 (outflow
from DF 3) 2.18 2,593 2,650
3
5 (outflow from
DF 4)
5.93 7,277 7,285
4 6 7.33 5,631 5,719
*See Appendix K Sheet SD 1.1 – Post-Development Delineation
Cottonwood Subdivision
Stormwater Design Report
Page 4 of 5
F. Stormwater Retention Facilities
There are three proposed retention facilities to capture stormwater from the subdivision. Retention
Facility 1 is an underground R Tank facility located under the pavement at the low point of the
alley near the approach onto Stafford Ave. at the north end of the project area. Retention Facilities
2 and 3 are typical at-grade basins with water design depths of 1-foot located on the east and west
sides of Resort Drive in the southeast portion of the project area. These two retention facilities
drain the east and west halves of Resort Drive via curb cuts, sidewalk chases and swales at the
low (north) end of the street before it terminates at the property line. This eliminates the need to
obtain a temporary stormwater easement on the neighbor’s property. Given the shallow depth of
these two at-grade ponds and the depth of groundwater in this location, it is not anticipated to
have standing groundwater in these facilities during the season of high groundwater. See
Retention Facility Calculations in Appendix C. Table 2 below summarizes the proposed retention
facilities.
TABLE 2
RETENTION FACILITY VOLUMES
Facility ID Contributing
Drainage Areas*
10-yr, 2-hr Post
Development
Peak Runoff
(cfs)
Required
Volume
(C.F.)
Proposed
Volume
(C.F.)
1 9 0.22 1,584 1,640
2 7.1 0.04 279 280
3 7.2 0.09 653 668
G. Gutter Flow and Curb Inlets
Curb inlets are proposed as required and where necessary. The following factors guided the layout
of the storm sewer system and the location of the 17 curb inlets and one valley gutter inlet within
the subdivision:
• 10-foot separation from water main,
• 5-foot separation from sanitary sewer main,
• the need for storm sewer stubs to serve the future development of the high-density lots,
and
• the multiple curves of the main north-south road, Stafford Ave.
Cottonwood Subdivision
Stormwater Design Report
Page 5 of 5
In consideration of all of these factors, a storm sewer system consisting of a more-than-average
number of curb inlets is proposed with this design.
A 25-year storm event was used to calculate the runoff flows. Based on these values, the curb
inlet capacities are more than adequate for carrying the 25-year storm event. The gutter-flow
capacity is also well above the designed runoff values to avoid overflow encroachment into the
drive lane. Standard 24”x36” inlets are proposed, except one non-typical inlet located at the alley
approach onto Stafford Ave where a curb inlet suitable for drop curb is proposed (Neenah Heavy
Duty R-3508-A2) and one non-typical inlet located at the low point of the alley grading (Neenah
Heavy Duty R-3508-B). Gutter flow and curb inlet calculations are provided in Appendix E. A
map of the individual drainage areas contributing to each inlet is found on Sheet SD1.2 in
Appendix L.
H. Discussion of C values and future site plan development
When sizing the stormwater infrastructure for the subdivision, a runoff coefficient (C) of 0.50 for
dense residential was not used per Table I-1 of the Bozeman DSSP. The project consists of high-
density apartment buildings and parking lots which typically yield higher C values than the city
standard of 0.5 for dense residential. Instead, a C value of 0.7 was used based on a C Coefficient
table from Appendix 20.A of the Civil Engineering Reference Manual 14th Edition found in
Appendix A of this report. By using a more accurate C value for the proposed high-density lots,
the stormwater design for the individual lot site plans will be simplified and will not require large
areas of developable land to be converted to stormwater facilities within each lot. Additionally, by
connecting the subdivision’s detention facilities to a PVC stormwater collection system (which is
stubbed into each future high-density lot), the individual lot grading plans will be simplified.
Appendix
A. Rational Method C-Coefficients Table from the Civil Engineering Reference Manual
B. Detention Facility Calculations
C. Retention Facility Calculations
D. Composite C Value calculations for Road Right of Ways
E. Peak Flow, Gutter and Inlet Capacity Calculations
F. Pipe Capacity Calculations
G. Sidewalk Chase Capacity Calculation
H. R-Tank HD Specifications
I. Buoyancy calcs for Detention Facility 2
J. Stormwater Drainage Exhibit – Pre-development: SD1.0
K. Stormwater Drainage Exhibit – Post-development: SD1.1
L. Stormwater Drainage Exhibit – Curb Inlet Drainage Areas: SD1.2
M. Stormwater Drainage Exhibit – Pipe Sizing Identification Map: SD1.3
Appendix A:
Rational Method C-Coefficients Table
from Civil Engineering Reference
Manual 14th Edition
Appendix B:
Detention Facility Calculations
Appendix C:
Retention Facility Calculations
Appendix D:
Composite C Value Calculations for Road
Right of Ways
Appendix E:
Peak Flow, Gutter and Inlet Capacity
Calculations
0.00
2.00
4.00
6.00
8.00
10.00
12.00
14.00
Grate Inlet Flow Rate (cfs)Depth of Water Above Grate (ft)
Sump Condition Capacity Chart R-3508-B
DISCLAIMER: Data provided herein is provided on an as-is basis. Neenah Foundry Company makes every effort possible to ensure the accurcy of the data, however Neenah Foundry Company does not
warrant, guaranty or assume liaibility for the use of the data provided herein. A design professional
familar with all aspects of the project should review the data for accuracy and application purposes, and
shall ulimtately be responsible for appplication of the data.
Appendix F:
Pipe Capacity Calculations
Appendix G:
Sidewalk Chase Capacity Calculation
Appendix H:
R-Tank HD Specifications
8
800.448.3636 acfenvironmental.comPRODUCT SPECIFICATION
For more information about Stormwater Management, contact us at 800.448.3636
email at info@acfenv.com
PART 1 – GENERAL
1.01 Related Documents
A. Drawings, technical specification and general provisions of the Contract as modified herein apply to this section.
1.02 Description of Work Included
A. Provide excavation and base preparation per geotechnical engineer's recommendations and/or as shown on
the design drawings, to provide adequate support for project design loads and safety from excavation sidewall
collapse. Excavations shall be in accordance with the owner’s and OSHA requirements.
B. Provide and install R-TankLD, R-TankHD, R-TankSD, or R-TankUD system (hereafter called R-Tank) and all related
products including fill materials, geotextiles, geogrids, inlet and outlet pipe with connections per the
manufacturer’s installation guidelines provided in this section.
C. Provide and construct the cover of the R-Tank system including; stone backfill, structural fill cover, and
pavement section as specified.
D. Protect R-Tank system from construction traffic after installation until completion of all construction activity in
the installation area.
1.03 Quality Control
A. All materials shall be manufactured in ISO certified facilities.
B. Installation Contractor shall demonstrate the following experience:
1. A minimum of three R-Tank or equivalent projects completed within 2 years; and,
2. A minimum of 25,000 cubic feet of storage volume completed within 2 years.
3. Contractor experience requirement may be waived if the manufacturer’s representative provides on-site
training and review during construction.
C. Installation Personnel: Performed only by skilled workers with satisfactory record of performance on bulk
earthworks, pipe, chamber, or pond/landfill construction projects of comparable size and quality.
D. Contractor must have manufacturer’s representative available for site review if requested by Owner.
1.04 Submittals
A. Submit proposed R-Tank layout drawings. Drawings shall include typical section details as well as the required
base elevation of stone and tanks, minimum cover requirements and tank configuration.
B. Submit manufacturer's product data, including compressive strength and unit weight.
C. Submit manufacturer’s installation instructions.
D. Submit R-Tank sample for review. Reviewed and accepted samples will be returned to the Contractor.
E. Submit material certificates for geotextile, geogrid, base course and backfill materials.
F. Submit required experience and personnel requirements as specified in Section 1.03.
G. Any proposed equal alternative product substitution to this specification must be submitted for review and
approved prior to bid opening. Review package should include third party reviewed performance data that meets
or exceeds criteria in Table 2.01 B.
1.05 Delivery, Storage, and Handling
A. Protect R-Tank and other materials from damage during delivery, and store UV sensitive materials under tarp
to protect from sunlight when time from delivery to installation exceeds two weeks. Storage of materials should
be on smooth surfaces, free from dirt, mud and debris.
B. Handling is to be performed with equipment appropriate to the materials and site conditions, and may include
hand, handcart, forklifts, extension lifts, etc.
C. Cold weather:
1. Care must be taken when handling plastics when air temperature is 40 degrees or below as plastic becomes
brittle.
2. Do not use frozen materials or materials mixed or coated with ice or frost.
3. Do not build on frozen ground or wet, saturated or muddy subgrade.
1.06 Preinstallation Conference.
A. Prior to the start of the installation, a preinstallation conference shall occur with the representatives from the
design team, the general contractor, the excavation contractor, the R-Tank installation contractor, and the
manufacturer’s representative.
1.07 Project Conditions
A. Coordinate installation for the R-Tank system with other on-site activities to eliminate all non-installation related
construction traffic over the completed R-Tank system. No loads heavier than the design loads shall be allowed
over the system, and in no case shall loads higher than a standard AASHTO HS20 (or HS25, depending on
design criteria) load be allowed on the system at any time.
B. Protect adjacent work from damage during R-Tank system installation.
C. All pre-treatment systems to remove debris and heavy sediments must be in place and functional prior to
operation of the R-Tank system. Additional pretreatment measures may be needed if unit is operational during
construction due to increased sediment loads.
D. Contractor is responsible for any damage to the system during construction.
PART 2 – PRODUCTS
2.01 R-Tank Units
A. R -Tank - Injection molded plastic tank plates assembled to form a 95% void modular structure of predesigned
height (custom for each project).
B. R-Tank units shall meet the following Physical & Chemical Characteristics:
PROPERTY DESCRIPTION R-TankLD
VALUE
R-TankHD
VALUE
R-TankSD
VALUE
R-TankUD
VALUE
Void Area Volume available for
water storage 95% 95% 95% 95%
Surface Void Area Percentage of exterior
available for infiltration 90% 90% 90% 90%
Compressive Strength ASTM D 2412 /
ASTM F 2418 30.0 psi 33.4 psi 42.9 psi 134.2 psi
HS-20 Minimum Cover Cover required to support
HS-20 loads N/A 20” 18” 12”
(Stone Backfill)
HS-25 Minimum Cover Cover required to support
HS-25 loads N/A 24” 19” 15”
(Stone Backfill)
Maximum Cover Maximum allowable
cover depth 3 feet < 7 feet < 10 feet 5 feet
Unit Weight Weight of plastic per
cubic foot of tank 3.29 lbs / cf 3.62 lbs/cf 3.96 lbs / cf 4.33 lbs / cf
Rib Thickness Thickness of
load-bearing members 0.18 inches 0.18 inches 0.18 inches N/A
Service Temperature Safe temperature range
for use -14 – 167° F -14 – 167° F -14 – 167° F -14 – 167° F
C. Supplier: ACF Environmental 2831 Cardwell Road Richmond, VA 23234
(T): 800-448-3636; (F): 804-743-7779 www.acfenvironmental.com
2.02 Geosynthetics
A. Geotextile. A geotextile envelope is required to prevent backfill material from entering the R-Tank modules.
1. Standard Application: The standard geotextile shall be an 8 oz per square yard nonwoven geotextile (ACF
N080 or equivalent).
2. Infiltration Applications: When water must infiltrate/exfiltrate through the geotextile as a function of the
system design, a woven monofilament (ACF M200 or equivalent) shall be used.
B. Geogrid. For installations subject to traffic loads and/or when required by project plans, install geogrid (ACF BX12
or equivalent) to reinforce backfill above the R-Tank system. Geogrid is not always required for R-TankUD
installations, and is often not required for non-traffic load applications.
2.03 Backfill & Cover Materials
A. Bedding Materials: Stone (angular and smaller than 1.5” in diameter) or soil (GW, GP, SW, or SP as classified
by the Unified Soil Classification System) shall be used below the R-Tank system (3” minimum). Material must
be free from lumps, debris, and any sharp objects that could cut the geotextile. Material shall be within 3 percent
of the optimum moisture content as determined by ASTM D698 at the time of installation. For infiltration
applications bedding material shall be free draining.
B. Side and Top Backfill: Free draining material shall be used adjacent to (24” minimum) and above (for the first
12”) the R-Tank system. Material must be free from lumps, debris and any sharp objects that could cut the
geotextile. Material shall be within 3 percent of the optimum moisture content as determined by ASTM D698 at
the time of installation.
1. For LD, HD, and SD modules, backfill materials shall be free draining stone (angular and smaller than 1.5”
in diameter) or soil (GW, GP, SW, or SP as classified by the Unified Soil Classification System).
2. For UD modules in traffic loaded (HS-20) applications with less than 14” of top cover, backfill materials
shall be free draining stone (angular and smaller than 1.5” in diameter). The use of soil backfill on the sides
and top of the UD module is not permitted unless the modules are installed outside of traffic areas or with
cover depths of 14” or more. Top backfill material (from top of module to bottom of pavement base or 12”
maximum) must be consistent with side backfill.
C. Additional Cover Materials: Structural Fill shall consist of granular materials meeting the gradational
requirements of SM, SP, SW, GM, GP or GW as classified by the Unified Soil Classification System. Structural
fill shall have a maximum of 25 percent passing the No. 200 sieve, shall have a maximum clay content of 10
percent and a maximum Plasticity Index of 4. Material shall be within 3 percent of the optimum moisture content
as determined by ASTM D698 at the time of installation.
2.04 Other Materials
A. Utility Marker: Install metallic tape at corners of R-Tank system to mark the area for future utility detection.
PART 3 - EXECUTION
3.01 Assembly of R-Tank Units
A. Assembly of modules shall be performed in accordance with the R-Tank Installation Manual, Section 2.
3.02 Layout and Excavation
A. Installer shall stake out, excavate, and prepare the subgrade area to the required plan grades and dimensions,
ensuring that the excavation is at least 2 feet greater than R-Tank dimensions in each direction allowing for
installation of geotextile filter fabric, R-Tank modules, and free draining backfill materials.
B. All excavations must be prepared with OSHA approved excavated sides and sufficient working space.
C. Protect partially completed installation against damage from other construction traffic by establishing a
perimeter with high visibility construction tape, fencing, barricades, or other means until construction is
complete.
D. Base of the excavation shall be uniform, level, and free of lumps or debris and soft or yielding subgrade areas.
A minimum 2,000 pounds per square foot bearing capacity is required.
1. Standard Applications: Compact subgrade to a minimum of 95% of Standard Proctor (ASTM D698)
density or as required by the Owner’s engineer.
2. Infiltration Applications: Subgrade shall be prepared in accordance with the contract documents.
Compaction of subgrade should not be performed in infiltration applications.
E. Unsuitable Soils or Conditions: All questions about the base of the excavation shall be directed to the owner’s
engineer, who will approve the subgrade conditions prior to placement of stone. The owner’s engineer shall
determine the required bearing capacity of the R-Tank subgrade; however in no case shall a bearing capacity
of less than 2,000 pounds per square foot be provided.
1. If unsuitable soils are encountered at the subgrade, or if the subgrade is pumping or appears excessively
soft, repair the area in accordance with contract documents and/or as directed by the owner’s engineer.
2. If indications of the water table are observed during excavation, the engineer shall be contacted to provide
recommendations.
3. Do not start installation of the R-Tank system until unsatisfactory subgrade conditions are corrected and
the subgrade conditions are accepted by the owner’s engineer.
3.03 Preparation of Base
A. Place a thin layer (3” unless otherwise specified) of bedding material (Section 2.03 A), over the subgrade to
establish a level working platform for the R-Tank modules. Level to within ½” (+/- ¼”) or as shown on the plans.
Native subgrade soils or other materials may be used if determined to meet the requirements of 2.03 A and are
accepted by the owner’s engineer.
1. Standard Applications: Static roll or otherwise compact bedding materials until they are firm and
unyielding.
2. Infiltration Applications: Bedding materials shall be prepared in accordance with the contract documents.
B. Outline the footprint of the R-Tank system on the excavation floor using spray paint or chalk line to ensure a 2’
perimeter is available around the R-Tank system for proper installation and compaction of backfill.
3.04 Installation of the R-Tanks
A. Where a geotextile wrap is specified on the stone base, cut strips to length and install in excavation, removing
wrinkles so material lays flat. Overlap geotextile a minimum 12” or as recommended by manufacturer.
B. Where an impervious liner (for containment) is specified, install the liner per manufacturer’s recommendations
and the contract documents. The R-Tank units shall be separated from impervious liner by a non-woven
geotextile fabric installed accordance with Section 3.04A.
C. Install R-Tank modules by placing side by side, in accordance with the design drawings. No lateral connections
are required. It is advisable to use a string line to form square corners and straight edges along the perimeter
of the R-Tank system. The modules are to be oriented as per the design drawing with required depth as shown
on plans..
1. For LD, HD, and SD installations, the large side plate of the tank should be placed on the perimeter of the
system. This will typically require that the two ends of the tank area will have a row of tanks placed
perpendicular to all other tanks. If this is not shown in the construction drawings, it is a simple field
adjustment that will have minimal effect on the overall system footprint. Refer to R-Tank Installation Guide
for more details
2. For UD installations, there is no perpendicular end row required.
D. Wrap the R-Tank top and sides in specified geotextile. Cut strips of geotextile so that it will cover the sides and
top, encapsulating the entire system to prevent backfill entry into the system. Overlap geotextile 12” or as
recommended by manufacturer. Take great care to avoid damage to geotextile (and, if specified, impervious
liner) during placement.
E. Identify locations of inlet, outlet and any other penetrations of the geotextile (and optional liner). These
connections should be installed flush (butted up to the R-Tank) and the geotextile fabric shall be cut to enable
hydraulic continuity between the connections and the R-Tank units. These connections shall be secured using
pipe boots with stainless steel pipe clamps. Support pipe in trenches during backfill operations to prevent pipe
from settling and damaging the geotextile, impervious liner (if specified) or pipe. Connecting pipes at 90 degree
angles facilitates construction, unless otherwise specified. Ensure end of pipe is installed snug against R-Tank
system.
F. Install Inspection and Maintenance Ports in locations noted on plans. At a minimum one maintenance port shall
be installed within 10’ of each inlet & outlet connection, and with a maximum spacing of one maintenance port
for every 2,500 square feet. Install all ports as noted in the R-Tank Installation Guide.
G. If required, install ventilation pipes and vents as specified on drawings to provide ventilation for proper hydraulic
performance. The number of pipes and vents will depend on the size of the system. Vents are often installed
using a 90 degree elbow with PVC pipe into a landscaped area with ‘U” bend or venting bollard to inhibit the
ingress of debris. A ground level concrete or steel cover can be used.
3.05 Backfilling of the R-Tank Units
A. Backfill and fill with recommended materials as follows:
1. Place freely draining backfill materials (Section 2.03 B) around the perimeter in lifts with a maximum
thickness of 12”. Each lift shall be placed around the entire perimeter such that each lift is no more than 24”
higher than the side backfill along any other location on the perimeter of the R-Tank system. No fill shall be
placed over top of tanks until the side backfill has been completed.
2. Each lift shall be compacted at the specified moisture content to a minimum of 95% of the Standard Proctor
Density until no further densification is observed (for self-compacting stone materials). The side lifts must
be compacted with walk behind compaction equipment. Even when “self-compacting” backfill materials are
selected, a walk behind vibratory compactor must be used.
3. Take care to ensure that the compaction process does not allow the machinery to come into contact with
the modules due to the potential for damage to the geotextile and R-Tank units.
4. No compaction equipment is permissible to operate directly on the R-Tank modules.
5. Top Backfill:
a. Typical Applications: Install a 12” (or as shown on plans) lift of freely draining material (Section
2.03 B) over the R-Tank Units, maintaining 12” between equipment tracks and R-Tank System.
b. Shallow Applications (< 18” total cover): Install top backfill in accordance with plans.
Lightly compacted using a walk-behind trench roller. Alternately, a roller (maximum gross vehicle weight of
6 tons) may be used. Roller must remain in static mode until a minimum of 24” of cover has been placed
over the modules. Sheep foot rollers should not be used.
6. If required, install a geogrid as shown on plans. Geogrid shall extend a minimum of 3 feet beyond the limits
of the excavation wall.
7. Following placement and compaction of the initial cover, subsequent lifts of structural fill (Section 2.03 C)
shall be placed at the specified moisture content and compacted to a minimum of 95% of the Standard
Proctor Density and shall cover the entire footprint of the R-Tank system. During placement of fill above the
system, unless otherwise specified, a uniform elevation of fill shall be maintained to within 12” across the
footprint of the R-Tank system. Do not exceed maximum cover depths listed in Table 2.01 B.
8. Place additional layers of geotextile and/or geogrid at elevations as specified in the design details. Each
layer of geosynthetic reinforcement placed above the R-Tank system shall extend a minimum of 3 feet
beyond the limits of the excavation wall.
B. Only low pressure tire or track vehicles shall be operated over the R-Tank system during construction. No
machinery should drive on top of the tank until a minimum of 18” of backfill and compaction is achieved. Dump
Trucks and Pans shall not be operated within the R-Tank system footprint at any time. Where necessary the
heavy equipment should unload in an area adjacent to the R-Tank system and the material should be moved
over the system with tracked equipment.
C. Ensure that all unrelated construction traffic is kept away from the limits of excavation until the project is
complete and final surface materials are in place. No non-installation related loading should be allowed over
the R-Tank system until the final design section has been constructed (including pavement).
D. Place surfacing materials, such as groundcovers (no large trees), or paving materials over the structure with
care to avoid displacement of cover fill and damage to surrounding areas.
E. Backfill depth over R-Tank system must be within the limitations shown in the table in Section 2.01 B. If the
total backfill depth does not comply with this table, contact engineer or manufacturer’s representative for
assistance.
PART 4 – USING THE SYSTEM
4.01 Maintenance Requirements
A. A routine maintenance effort is required to ensure proper performance of the R-Tank system. The Maintenance
program should be focused on pretreatment systems. Ensuring these structures are clean and functioning
properly will reduce the risk of contamination of the R-Tank system and stormwater released from the site. Pre-
treatment systems shall be inspected yearly, or as directed by the regulatory agency and by the manufacturer (for
proprietary systems). Maintain as needed using acceptable practices or following manufacturer’s guidelines (for
proprietary systems).
B. Inspection and/or Maintenance Ports in the R-Tank system will need to be inspected for accumulation of
sediments at least quarterly through the first year of operation and at least yearly thereafter. This is done by
removing the cap of the port and using a measuring device long enough to reach the bottom of the R-Tank system
and stiff enough to push through the loose sediments, allowing a depth measurement.
C. If sediment has accumulated to the level noted in the R-Tank Maintenance Guide or beyond a level acceptable
to the Owner’s engineer, the R-Tank system should be flushed.
D. A flushing event consists of pumping water into the Maintenance Port and/or adjacent structure, allowing the
turbulent flows through the R-Tank system to re-suspend the fine sediments. If multiple Maintenance Ports have
been installed, water should be pumped into each port to maximize flushing efficiency. Sediment-laden water can
be filtered through a Dirtbag or approved equivalent if permitted by the locality.
END OF SECTION
Appendix I:
Buoyancy Calculations for
Detention Facility 2
Appendix J:
Stormwater Drainage Exhibit –
Pre-development: SD1.0
LP
PHNPHN
PHN
BASIN 2
BASIN 1
MADISON ENGINEERING
895 TECHNOLOGY BLVD SUITE 203
BOZEMAN, MT 59718
(406) 586-0262
SHEET
SD1.0
Pre-development
Cottonwood Subd.COTTONWOOD SUBDIVISION
BOZEMAN, MT
STORM WATER DRAINAGE EXHIBIT
PRE-DEVELOPMENT DELINEATION
SUMMARY TABLE
1 inch =
0
SCALE
200'
400100200
Appendix K:
Stormwater Drainage Exhibit –
Post-development: SD1.1
LP
PHNPHN
PHN
LP
DA 5DA 1DA 3DA 4DA 8
DA 7
DA 6
DA 9
DA 2
MADISON ENGINEERING
895 TECHNOLOGY BLVD SUITE 203
BOZEMAN, MT 59718
(406) 586-0262
SHEET
SD1.1
Post-development
Cottonwood Subd.COTTONWOOD SUBDIVISION
BOZEMAN, MT
STORM WATER DRAINAGE EXHIBIT
POST-DEVELOPMENT DELINEATION
1 inch =
0
SCALE
200'
400100 200
Appendix L:
Stormwater Drainage Exhibit –
Curb Inlet Drainage Areas: SD1.2
PHN
DA 6.01
DA 5.1
DA 6.10
DA 6.06
DA 7.2
DA 8.2
DA 8.4
DA 8.7
COTTONWOOD ROADMADISON ENGINEERING
895 TECHNOLOGY BLVD SUITE 203
BOZEMAN, MT 59718
(406) 586-0262
SHEET
SD1.2
Post-development
Cottonwood Subd.COTTONWOOD SUBDIVISION
BOZEMAN, MT
STORM WATER DRAINAGE EXHIBIT
CURB INLET DRAINAGE AREA DELINEATION
1 inch =
0
SCALE
200'
400100200
SUMMARY TABLE
Appendix M:
Stormwater Drainage Exhibit –
Pipe Sizing Identification Map: SD1.3
PHN
MADISON ENGINEERING
895 TECHNOLOGY BLVD SUITE 203
BOZEMAN, MT 59718
(406) 586-0262
SHEET
SD1.3
Post-development
Cottonwood Subd.COTTONWOOD SUBDIVISION
BOZEMAN, MT
STORM WATER DRAINAGE EXHIBIT
PIPE SIZING IDENTIFICATION MAP
1 inch =
0
SCALE
200'
400100200
SUMMARY TABLE
Appendix N:
Stormwater Maintenance Plan
G:\MADISON ENGINEERING\PROJECTS\2017\17-149 Cottonwood Sub\Reports\Stormwater Management\Cottonwood
Subd\STORMWATER MAINTENANCE PLAN.doc
STORMWATER MAINTENANCE PLAN
Cottonwood Subdivision
Owner’s responsibility for routine inspection and maintenance
1. Keep the outlet and inlets of the facility free of leaves, rocks, and other debris.
2. The storm water retention basins are to be mowed regularly. During the summer,
approximately once every two weeks, the grass is to be mowed and the cuttings are
to be promptly removed and disposed of. Unless visibly tainted, dispose of lawn
clippings in the same manner as yard waste. Otherwise, bag and take to a sanitary
landfill.
3. Remove sediment by hand with a flat bottom shovel during the summer months
whenever sediment covers vegetation. Have the grass cut short in that particular
location so that the bed can be made as level as possible.
5. Re-sod damaged or maintained areas immediately, or use grass plugs from the
adjacent up-slope area.
6. Inspect the facilities periodically, especially after heavy rains (preferably monthly
and after each storm that delivers 0.5 inches of rainfall).
7. Inspect flow control outlet semi-annually. Clean outlet when soil and vegetation
buildup interfere with flow introduction.
9. See that litter and other debris are removed from retention basins and swales.
10. Maintenance of the underground R Tank systems are as follows:
• Minimum required maintenance includes a quarterly inspection during the
first year of operation and a yearly inspection thereafter. Utilize inspection
ports for inspections.
• The inspection ports can be used to pump water into the system and re-
suspend accumulated sediment so that is may be pumped out. Flush and
pump as inspections deem necessary.
• Utilize the adjacent upstream storm sewer manhole or cleanout for removal
(vacuuming) of debris from the end of the storm drain at the connection
point with the R Tank chamber.
11. Owner to maintain and fund Operation and Maintenance of stormwater facilities.
_______________________________
Patrick Boel, Owner’s Representative