HomeMy WebLinkAbout018 - Appendix P - Stormwater Design ReportTable of Contents
1 Introduction ......................................................................................................................... 1
2 Proposed Development ....................................................................................................... 1
2.1 Project Location and Description ................................................................................ 1
2.2 Development Horizon ................................................................................................. 2
3 Existing Area Conditions ..................................................................................................... 2
3.1 Existing Land Cover and Slopes ................................................................................. 2
3.2 NRCS Soils ................................................................................................................ 2
3.3 Site Groundwater Levels ............................................................................................ 2
3.4 Existing Drainage Features......................................................................................... 2
4 Major Drainage Basins ........................................................................................................ 3
5 Methodologies .................................................................................................................... 4
5.1 Design Methodology ................................................................................................... 4
5.2 Design Storm Analyses .............................................................................................. 6
5.3 Storm Drain Capture and Conveyance ....................................................................... 6
5.4 Storm Water Treatment Facilities................................................................................ 7
5.5 Temporary Storage Ponds .......................................................................................... 8
5.6 Overflow ..................................................................................................................... 9
5.7 Irrigation Ditches/Waterways .....................................................................................10
5.8 Offsite Run-on ...........................................................................................................10
6 Maintenance ......................................................................................................................11
7 Conclusions .......................................................................................................................11
List of Exhibits
Exhibit A Vicinity Map
Exhibit ST-1 Pre-Development Drainage Basins
Exhibit ST-2 Post-Development Drainage Basins
List of Appendices
Appendix A NRCS Soil Report
Appendix B Geotechnical Report
Appendix C Groundwater Data
Appendix D Storm Water Calculations
Appendix E Operation and Maintenance Manual
Appendix F Irrigation Ditch Correspondence
Appendix G Infrastructure Phasing Exhibits
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1 INTRODUCTION
This report provides a basis of storm drain design for the Turnrow Subdivision project. The
storm drain system serving Turnrow Subdivision was designed and shall be installed in
accordance with the following:
· The City of Bozeman Design Standards and Specifications Policy (dated May 1, 2017)
o Including Addendum Numbers 1 thru 8
· Montana Public Works Standard Specifications (MPWSS) – 6th Edition
· The City of Bozeman Modifications to MPWSS
2 PROPOSED DEVELOPMENT
2.1 Project Location and Description
The proposed Turnrow Subdivision encompasses a total of 114.69 acres and is generally
located at the southwest corner of the intersection of Valley Center Road and Davis Lane. The
property is within the boundaries of the City of Bozeman, Montana and was annexed in
September of 2022. The property is zoned REMU (Residential Emphasis Mixed Use). See the
vicinity map (Exhibit A) for additional location details.
The following is the legal description for the property:
Three parcels of land as described in 2 Warranty Deeds: Include Parcel 1 and
Parcel 2 in Warranty Deed filed under document number 2736653, and the
parcel described in a Warranty Deed Under document number 2736654. Situated
in the NE 1/4 of Section 27, Township 1 South, Range 5 East, Principal
Meridian,Gallatin County, Montana
The property has historically been utilized for agricultural purposes. Groundwater is relatively
deep based on test pits and groundwater monitoring. Groundwater monitoring was completed in
2021 and results are included in Appendix C. There are two waterways crossing the property.
Cattail Creek flows through the northeast corner of the property after crossing Davis Lane. An
unnamed stream-ditch traverses through the southwest corner and then north along the western
boundary of the property.
Storm collection for the project will consist of grated curb inlets located within rights-of-way
(ROWs). Storm water will generally be conveyed to surface ponds or underground retention
systems using storm drain mains as designated on Exhibit ST-2. Storm water generated by
commercial or multifamily areas located in Block 1, Block 7, and Block 12, are proposed to be
treated using underground storage chambers (Stormtech® or approved equal). Design of these
treatment systems will occur during site plan design and are not included. Where outlet
conveyance pathways are available, ponds will be allowed to overflow to existing downstream
conveyance channels. Treated stormwater overflows will discharge north of the property, travel
within the roadside ditch south of Valley Center Road, and exit through existing downstream
conveyance facilities.
The expansion of Valley Center Road and modifications of Davis Lane have been included in
this report. Valley Center Road will have a symmetrical 6-foot expansion of both sides of the
existing asphalt roadway. Davis Lane will be modified adjacent to Turnrow’s eastern boundary
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to allow a 3-lane configuration. This design report considers the full build out of Davis Lane (5-
lane section) that may occur in the future.
2.2 Development Horizon
The project will begin development a soon as the required approvals are in place and Phase 1
of the development is tentatively scheduled to begin construction in 2024.
3 EXISTING AREA CONDITIONS
3.1 Existing Land Cover and Slopes
The existing property is primarily agricultural land. The existing slopes range from 0% to 8%
and generally drain from south to north. There are four existing outfalls along Valley Center
Road shown on Exhibit ST-1.
3.2 NRCS Soils
Data on existing site soils is provided in the Gallatin County Area, Montana Soil Survey dated
November 14, 2022 through Web Soil Survey (WSS) operated by the United States Department
of Agriculture (USDA) Natural Resources Conservation Service (NRCS). According to
information obtained from WSS, the site contains three basic soil types –Blackmore Silt Loam,
Turner Loam, and Hyalite-Beaverton Complex. See Appendix A for the complete Soil Resource
Report.
3.3 Site Groundwater Levels
Groundwater levels beneath the proposed subdivision experience seasonal variations but are
generally relatively deep. While monitoring ground water levels from May 2021 to August 2021,
levels were 5.11 feet below existing grade or deeper. Complete groundwater observation data is
provided in Appendix C, including a map showing the locations of each well. Storm treatment
structures for the subdivision were designed with bottom elevations above the seasonal high
ground water level to ensure they will provide sufficient treatment volumes as required.
Stormwater treatment structures were designed with sufficient storage to treat the 10-year, 2-
hour design storm and Runoff Reduction Volume (first ½” over impervious areas) to meet the
State of Montana MS4-required levels prior to release. Overflow pathways for treatment basins
were considered to the extent practicable that will allow larger storm events sufficient drainage
pathways to prevent inundation of structures. The total volume released from all treatment
facilities is less than the total site pre-development rate.
3.4 Existing Drainage Features
An existing stream-ditch traverses the southwest corner of the property and continues north
along the western property line until it is conveyed beneath Valley Center Road. This facility is
proposed to largely remain undisturbed except for planned future roadway crossings where the
water will be conveyed using culverts.
Cattail Creek flows approximately 380’ across the northeast corner of the property into MDT
rights-of-ways and then approximately 610’ northwest along the southside of Valley Center
Road. This creek is proposed to largely remain undisturbed except for replacing and upsizing
culverts under Davis Lane and Quicksilver Road. A new culvert will also be placed under the
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proposed shared use path parallel to Valley Center Road. Cattail Creek will continue to outlet
north across Valley Center Road through the existing culverts.
An irrigation ditch located east of Cattail Creek located in MDT rights-of-way currently crosses
Davis Lane using an existing culvert prior to being further conveyed beneath Valley Center
Road via existing culvert. The culvert beneath Davis Lane will be replaced using a hydraulically
equivalent new culvert to accommodate the expansion of Davis Lane.
Turnrow ownership and engineering consultants have sent notice of development letters to
assumed water users regarding expected impacts of the above waterways and have not
received a response.
A fourth culvert location between the stream ditch and Cattail Creek drains across Valley Center
Road and the interstate where drainage is then conveyed along the south side of the frontage
road eventually meeting up with the stream-ditch.
4 MAJOR DRAINAGE BASINS
The pre-development drainage for Turnrow Subdivision was divided into four basins as shown
on Exhibit ST-1. The pre-development discharge also included runoff from Valley Center Road
and Davis Lane from the centerline of the roadways to the property and run-on from the
property south of the proposed development. A weighted rational coefficient for the pre-
development condition was calculated by using 0.9 for existing asphalt and 0.2 for the rights-of
way ditches and undeveloped agriculture land. In order to determine the pre-development
discharge, the hydraulically longest time of concentration was estimated based on the existing
contours and flow paths. Considering the relatively even grading of the site from south to north,
this method of analysis provides a reasonable estimation of the current pre-development
conditions. Total pre-development runoff from the site was estimated to be 12.68 cfs, and run-
on onto the site was estimated to 14.15 cfs totaling a discharge rate of 26.83 cfs (Appendix D).
Post-development runoff estimates from public ROWs and interior lots were used to size the
proposed storm water treatment facilities. The site was divided into several drainage basins
based on the proposed topography and layout of collection and conveyance systems. A
coefficient and time of concentration was calculated for each drainage basin. Based on this
analysis, the expected total discharge rate from the full buildout of the site would be 65.14 for
the site and 16.17 cfs for run-on totaling a discharge rate of 81.32 cfs without any capture or
treatment. After treatment, the post-development run-off rate is 24.52 cfs.
The storm drain system was designed to capture and convey using a system of surface
drainage features, curb and gutter, inlets, and buried storm drain mains. This water will be
treated using stormwater ponds and underground retention systems. Overflows from the
treatment structure will generally discharge into the existing ditches south of Valley Center Road
(including Cattail Creek) and be conveyed to the existing outfall of Cattail Creek or the outfall of
the steam-ditch.
Road grades were set to minimize chance of discharging runoff offsite during the 10- and 25-
year design storm events. Please refer to Exhibit ST-2 for a visual representation of the major
drainage basins that were considered. This exhibit shows the general grading drainage
pathways assumed for the analysis.
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5 METHODOLOGIES
This section documents the methodologies and assumptions used to conduct the storm water
runoff analyses for the project. Drainage plan methodologies and analyses are based on the
City of Bozeman’s Design Standards and Specifications Policy (Standards).
5.1 Design Methodology
The storm water management system for the proposed subdivision utilizes a system of curb,
gutter, inlets, and piping to capture and convey stormwater where applicable. Subsurface
retention structures (e.g., Stormtech®) are planned to be located southwest of Valley Center
and Quicksilver intersection in a restricted lot (U-1) and another located southwest of Valley
Center and Davis Lan intersection in an open space (U-2) as shown on Figure ST-2. Surface
retention ponds are also proposed to treat storm water runoff from other ROWs and single
family residential lots within the site. Run-on from south of the development will be captured and
conveyed separately, bypassing the site and discharging at either Cattail Creek (Phase 1) or the
stream-ditch along the west boundary of the property.
Summaries of runoff estimates and treatment volumes are provided in the sections that follow.
The pre-development condition assumed a runoff coefficient of 0.2 based on undeveloped
agricultural land. Surface ponds and underground storage systems were designed to treat runoff
during the post-development condition for a 10-year design storm. A summary of the pre- and
post-development runoff volume calculations is provided in the tables below. Storm pipe sizes
were selected to convey the 25-year design storm without exceeding 75% of the full flow
capacity (d/D) and utilize the most efficient diameter. Further detail can be found in Appendix D.
Table 1: Pre- versus Post-Development Conditions (Cattail Creek outfall)
Sub-basin
Area
(acres)
Runoff
Coefficient
Tc
(min)
Stormwater
Discharge Rate
Expected during a
10-yr Storm
(cfs)
Treatment
Facility
Treatment
Facility Type
Cattail Creek Outfall Pre-Development
PD-1 51.98 0.22 82.8 5.94 NONE NONE
RUN-ON 1 79.97 0.20 129.2 6.22 NONE NONE
(Total) 131.95 12.15 NONE NONE
Cattail Creek Outfall Post-Development
1 17.72 0.59 27 0.00 P-1 RETENTION
2 1.05 0.67 6 0.00 U-1 RETENTION
3 2.79 0.88 21 0.00 U-2 RETENTION
6 17.95 0.33 36 5.28 NONE
FUTURE
SITE PLAN
RUN-ON 1 79.97 0.20 129 6.22 NONE BYPASS
RUN-ON 3 1.00 0.63 6 0.00 U-2 RETENTION
Total 120.48 0.34 120 11.5 VARIOUS VARIOUS
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Table 2: Pre- versus Post-Development Conditions (Stream-ditch outfall)
Sub-basin
Area
(acres)
Runoff
Coefficient
Tc
(min)
Stormwater
Discharge Rate
Expected during a
10-yr Storm
(cfs)
Treatment
Facility
Treatment
Facility Type
Stream-ditch Outfall Pre-Development
PD-2 57.81 0.21 112.7 5.16 NONE NONE
RUN-ON 5 38.83 0.20 84.3 3.98 NONE NONE
Total 96.64 9.14 NONE NONE
Stream-ditch Outfall Post-Development
4 10.27 0.65 15 0.00 P-2 RETENTION
5 43.06 0.65 30 0.00 P-3 RETENTION
7 20.94 0.21 40 3.66 NONE
NONE
(OPENSPACE)
RUN-ON 5 38.83 0.20 84.3 3.98 NONE BYPASS
Total 113.1 0.34 40 7.64 VARIOUS VARIOUS
Table 3:Pre- versus Post-Development Conditions (Interstate outfall)
Sub-basin
Area
(acres)
Runoff
Coefficient
Tc
(min)
Stormwater
Discharge Rate
Expected during a
10-yr Storm
(cfs)
Treatment
Facility
Treatment
Facility Type
Interstate Underpass Outfall Pre-Development
PD-3 9.91 0.21 62.4 1.30 NONE NONE
RUN-ON - - - - - -
Total 9.91 0.21 62.4 1.30 NONE NONE
Interstate Underpass Outfall Post-Development
8 5.28 0.25 36 1.18 NONE
FUTURE
SITE PLAN
Total 5.28 0.25 36 1.18 NONE NONE
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Table 4:Pre- versus Post-Development Conditions (Irrigation ditch outfall)
Sub-basin
Area
(acres)
Runoff
Coefficient
Tc
(min)
Stormwater
Discharge Rate
Expected during a
10-yr Storm
(cfs)
Treatment
Facility
Treatment
Facility Type
Irrigation Ditch Outfall Pre-Development
PD-4 0.24 0.55 9.3 0.28 NONE NONE
RUN-ON 9 15.81 0.25 30.2 3.95 NONE NONE
Total 16.05 4.24 NONE NONE
Irrigation Ditch Outfall Post-Development
9 0.05 0.20 5 0.03 NONE NONE
RUN-ON 9 15.64 0.26 29 4.17 NONE NONE
TOTAL 15.69 4.20 NONE NONE
5.2 Design Storm Analyses
Storm water runoff analyses were performed using the Rational Method. The analyses included
evaluations of the 10-year, 2-hour design storm for sizing of treatment facility volumes and the
25-year, 2-hour design storm for sizing capture and conveyance facilities. Runoff coefficients
were calculated based on expected surfacing and checked against general development
conditions expected as outlined in the City’s Standards.
Autodesk Storm and Sanitary Analysis (SSA) software was utilized for the major storm analysis.
Various excel spreadsheets were used to calculate rational coefficients, treatment volumes, and
time of concentrations. Autodesk Hydraflow Express software was used to determine the
maximum gutter conveyance/spread widths at inlets and the corresponding maximum flow to
aid in final design for locating inlets. SSA was used as it more accurately depicts the expected
performance of the system and considers overflow pathways. Detailed calculations and reports
are included in Appendix D.
5.3 Storm Drain Capture and Conveyance
The storm drain piping system for the proposed development is designed to have maximum
reliability of operation, minimal maintenance requirements, and to ensure that inlets function to
their design capacities while meeting necessary area drainage requirements. The 25-year
design storm was used as the basis for design for conveyance facilities pursuant to the City of
Bozeman requirements outlined in the City Standards.
Time of concentrations for each major drainage basin were calculated using the TR-55 method.
These time of concentration values were used to calculate peak flows for the 25-year storm and
size conveyance facilities. SSA analysis results for conveyance of the 25-year storm for Phase
1 of the project are provided in Appendix D. Conveyance calculations shall be provided for
future phases of the Turnrow Subdivision at the time they are being platted. The piping was
sized to convey expected flows at less than 75% capacity.
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The model incorporates inflows for low points in the curb line of the roads. Spacing of inlets
was designed to be sufficient to capture the expected flows and limit gutter spread. Based on
the road design profiles, maximum runoff flows for each road was determined to limit gutter flow
depth to 0.15’ below the top of the curb (spread widths less than 9.5 feet). A typical sag inlet
was modeled for the site as well. Inlets and pipes were designed to convey peak flows from a
25-year design storm. A summary table for potential inlet/gutter spread conditions with the
proposed road slopes is included below. Additional gutter spread reports are provided in
Appendix D.
Table 5: Inlet and Gutter Summary Table
Road
Road
Running
Slope
(%)
Gutter
Slope
(%)
Road
Cross
Slope
(%)
Maximum
Depth
(ft)
Qmax at Inlets
for Maximum
Allowable
Gutter Spread
(ft)
NA - Typical Sag N/A 6.25% 3.0% 0.30 2.25
Quicksilver - On-grade 0.70% 6.25% 3.0% 0.30 2.79
Gunsmoke - On-grade 0.50% 6.25% 3.0% 0.30 2.36
Turnrow – On-grade 1.30% 6.25% 3.0% 0.30 3.45
Durango - On-grade 0.60% 6.25% 3.0% 0.30 2.59
Davis Lane - On-grade 0.90% 6.25% 3.0% 0.30 3.17
Storm drain piping was designed to have a minimum velocity of 3.0 feet per second (fps) when
flowing full to prevent sediment deposition. Due to the maximum pond depth of 2.5-feet, the
minimum cover depth of 2 feet per the COB Standards could not be met in all cases, however
the piping selected has been designed with adequate cover to meet HS-20 loading
requirements (or equivalent HL-93 loading). Piping crossing major traffic areas shall be
specified to be class IV reinforced concrete pipe if cover is less than 2 feet. Conveyance pipe
sizing is tabulated in Appendix D
5.4 Storm Water Treatment Facilities
Stormwater treatment will be performed using surface retention ponds with overflow piping and
underground retention systems. All treatment facilities were designed to store runoff from a 10-
year, 2-hour storm event. These facilities will be owned and maintained by the Property Owner’s
Association (POA) in accordance with the operation and maintenance manual and shall be
accessible using the proposed network of roads or additional access routes as required. These
facilities are further described below.
Surface Treatment Ponds
Treatment of stormwater from roadways, buildings, lawns, and parking area for Basins 1, 4 and
5 will be performed using surface ponds as shown on Exhibit ST-2. The maximum water depth
during a 10-year storm was designed to be 1.5 feet per the City of Bozeman Standards.
Underground Retention Systems
Treatment of stormwater from roadways, buildings, lawns, and parking area for Basins 2 and 3
will be performed using underground retention systems. These systems will be sized to capture
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runoff for a 10-year, 2-hour storm. Stormwater will percolate through a gravel filter layer before
infiltrating into the native soil beneath. The native soil underneath the gravel filter layer is a well-
draining gravel, cobble and sand mix (based on the geotechnical report). This sandy gravel
layer exists at various elevations (~3-feet to 7-feet below existing grade) throughout the site.
The bottom of the chambers will be set to ensure that groundwater does not impact the storage
volume. Groundwater monitoring was performed on the site to measure the high groundwater
(Appendix C). The system will be installed above the high groundwater and will ensure
connection to the underlying gravel strata.
Table 6: Storage Facilities
Sub-
Basin
Area
(Acres)
Adjusted
C
First 1/2"
Facility (cf)
Peak
Flow
(Q10-
cfs)
Storage
type
Required
Storage
(cf)
Storage
Provided
(cf)
Treatment
Facility
1 97.69 0.27 18,517 10.93 Retention
Pond 30,709 31,711 P-1
2 1.05 0.67 1,276 0.29 Underground
Retention 2,064 2,300 U-1
3 3.79 0.82 6,058 1.26 Underground
Retention 9,087 9,229 U-2
4 10.27 0.65 12,031 2.72 Retention
Pond 19,612 22,083 P-2
5 81.89 0.65 49,789 11.34 Retention
Pond 82,194 82,424 P-3
Future Site Plans
Future lots located in Blocks 1, 7, and 12 are to have individual treatment systems that will be
separate from the subdivision infrastructure. These sites are to be designed during site plan
submittal process for those future lots.
5.5 Temporary Storage Ponds
Temporary stormwater ponds shall be utilized for interim stages of the subdivision. It is
anticipated that Phase 1 will require temporary retention ponds based on proposed
development extents. Phase 1 is compromised of 4 sub-phases (see attached Phasing Plan-
Appendix G).
During Phase 1.1, Pond T-5.3 will be placed within the future ROW for Turnrow Trail between
Quicksilver Drive and Temerity Lane. This pond will be sized as full retention for the 10-year, 2-
hour design storm and will include an overflow directed to the west into the undeveloped
property. This pond will be abandoned as required during future development of the subdivision.
During Phase 1.1, Pond T-5.4 will be placed within the future ROW for Boomerang Road
between Quicksilver Drive and Temerity Lane. This pond will be sized as full retention for the
10-year, 2-hour design storm and will include an overflow directed to the west into the
undeveloped property. This pond will be abandoned as required during future development of
the subdivision.
During Phase 1.3, Pond T-1.3 will be placed within the ROW for Durango Lane between
Quicksilver Drive and Gunsmoke Avenue. This pond will be sized as full retention for the 10-
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year, 2-hour design storm and will include an overflow directed to the west into the undeveloped
property. This pond will be abandoned during sub-phase 1.4 construction.
During Phase 1.4, Pond T-5.0 will be placed within the future ROW for Westlake Road just west
of the Quicksilver Road and Westlake Road intersection. This pond will be sized as full retention
for the 10-year, 2-hour design storm and will include an overflow directed to the west into the
undeveloped property. This pond will be abandoned as required during future development of
the subdivision.
During Phase 1.4, Pond T-5.1 will be placed within the future ROW for Rowdy Road between
Quicksilver Drive and Muley Way. This pond will be sized as full retention for the 10-year, 2-
hour design storm and will include an overflow directed to the west into the undeveloped
property. This pond will be abandoned as required during future development of the subdivision.
During Phase 1.4, Pond T-5.2 will be placed within the future ROW for Durango Lane between
Quicksilver Drive and Ferguson Avenue. This pond will be sized as full retention for the 10-year,
2-hour design storm and will include an overflow directed to the west into the undeveloped
property. This pond will be abandoned as required during future development of the subdivision.
Table 7: Temporary Pond Summary
Name
Phase
Installed
Area
Served
(AC)
Weighted
Runoff
Coefficient
Required
Volume
(cf)
Storage
Provide
(cf) Area Served
T-1.3 1.3 0.28 0.72 599 632 Durango Lane
T-5.0 1.4 0.80 0.44 1,384 1,409
South half of Quicksilver Drive
block between Rowdy Rd and
Westlake. West half of
Westlake Road
T-5.1 1.4 0.43 0.62 770 1,499 West half of Quicksilver Drive
(Westlake to Rowdy Road)
T-5.2 1.4 0.57 0.57 958 1,499 West half of Quicksilver Drive
(Rowdy Road to Durango Lane)
T-5.3 1.1 .58 0.55 938 1,499
West half of Quicksilver Drive
(Durango Lane to Turnrow
Trail)
T-5.4 1.1 1.29 0.56 2,133 2,789 Alley 6.1 (Turnrow Trail to
Boomerang Road)
5.6 Overflow
As indicated previously, overflow pathways have been provided for treatment facilities during
storm events exceeding the 10-year, 2-hour design storm using outfall piping. These pipes
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were sized with sufficient capacity to convey the 25-year storm at less than 75% of the total
capacity.
For larger storm events, overflow from storage system facilities P-1, U-1, and U-2 will overflow
at the outlet structure and be conveyed to Cattail Creek using storm piping. Overflow from
storage system facilities P-2 and P-3 will be conveyed to the stream ditch using storm piping
and existing drainage swales. For the temporary pond of Phase 1, the ponds will overflow on
the northwest corner of the ponds and drain to a natural drainage swale to Cattail Creek
Total offsite discharge during the 10-year storm is proposed to be 10.15 cfs for the site and
16.14 cfs for run-on, totaling 24.52 cfs, which is less than the pre-existing discharge of 26.83
cfs. Calculations of flows received by outfalls, as well as pipe sizes and peak flow conveyance
calculations are provided in Appendix D.
5.7 Irrigation Ditches/Waterways
An existing stream-ditch traverses the southwest corner of the property and continues along the
western property line until it is conveyed beneath Valley Center Road. This irrigation facility is
believed to be fed by waters originating from the Farmers Canal Company and is defined as an
“Unnamed tributary to Buster Gulch”. The ditch is proposed to be impacted during future
phases when connections are made to Boomerang Road, Durango Lane, and Ferguson Avenue
from the adjacent property. These proposed impacts would include one culverts at each of the
proposed road connections.
Another existing irrigation ditch is located primarily east of Davis Lane, and crosses Davis Lane
near the northeast corner of the property. This facility will be preserved as close to existing
conditions as possible except at the crossing location along Davis Lane. The ditch is believed to
convey water across the property owned by Bozeman Development LLC. During Phase 1, the
culvert and ditch will be modified to accommodate the Davis Lane expansion. A hydraulically
equivalent culvert will be used. The existing downstream culvert that crosses Valley Center
Road will remain largely preserved, with a small extension to accommodate the expansion of
Valley Center Lane and the shared us path.
Cattail Creek flows along the northeast corner of the property and to the south of Valley Center
Road. This creek is proposed to primarily remain undisturbed except for the proposed crossings
at Davis Lane, Quicksilver Road, and the shard use pathway. Culverts for Cattail Creek were
sized per the flood hazard analysis to prevent inundation during a 100-year storm event. Cattail
Creek will continue to outlet north across Valley Center Road through the existing culverts. For
larger stormwater events, overflow stormwater from underground retention facilities U-1 and U-2
will outfall into cattail creek.
Permitting may be required for impacts and culverts for the above waterways. This shall be
further coordinated with the governing agencies for the development.
5.8 Offsite Run-on
This analysis considered run-on from three separate basins. Portions of the pre-development
basins which run onto the project site were considered for conveyance through the site. Run-on
from south of the property is divided into two sub-basins. The run on from PD-1 predevelopment
will be captured south of the intersection of Westlake Road and Gunsmoke Ave. This run-on is
then by passed through the site conveyed north along Gunsmoke Ave and then Quicksilver
Avenue discharging into Cattail Creek. The run-on from PD-2 will be captured south of Westlake
Road, between Ferguson Ave and Quicksilver Avenue and conveyed west along Westlake
Road discharging into the Stream-ditch.
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Run-on east of the property was also calculated for the irrigation ditch and the off-site
improvements for the 10’ asphalt pathway along Valley Center Road for pre-development and
post-development. With the new design for the intersection of Valley Center Road and Davis
Lane capturing stormwater and discharging into storm facility U-2, the post-development is less
than the pre-development even with the off-site improvements of the pathway.
The conveyance system also considers run-on from Davis Lane for future development. Storm
facility U-2 is sized to collect storm drainage from Davis Lane at a potential future 5-lane section
and can accommodate run-on approximately 200 feet south of the Davis Lane and Westlake
Road intersection.
6 MAINTENANCE
Storm drain inlets, catch basins, and piping will be inspected following large storm events and at
least once per year. Any necessary repair or maintenance should be prioritized and scheduled
through the spring, summer, and fall. These items may include inspecting for any damage,
removing blockages, cleaning and flushing of pipes, establishing vegetation on bare slopes, and
sediment removal.
Maintenance of the surface ponds is essential. General objectives of maintenance are to
prevent clogging, standing water and the growth of weeds and wetland plants. This requires
frequent unclogging of the outlets, inlets, and mowing. Removal of sediment with heavy
equipment may also be necessary in 10 to 20 years. An Operation and Maintenance Manual is
included in Appendix E which indicates common maintenance activities and frequencies.
Maintenance of underground storage systems shall be performed by the owner, according to
the manufacturer’s maintenance guidelines.
Maintenance of the private run-on conveyance systems shall be maintained by POA. If
maintenance requires destruction of the public sidewalk, it is the POA’s responsibility to restore
the sidewalk to existing or better conditions.
Access easements will be provided to the City and Property Owner’s Association (POA) for
facilities treating stormwater from ROWs. The POA shall be responsible for maintenance of all
onsite stormwater facilities serving the subdivision.
7 CONCLUSIONS
Runoff from the Turnrow Subdivision will be captured, conveyed, and treated according to City
of Bozeman Standards. Additionally, the stormwater design has accommodated expected
runoff from Davis Lane. The facilities are designed to provide maximum reliability of operation
and minimal maintenance requirements. Accessible routes to any storm facilities not
immediately accessible from the City ROWs will have separate accesses provided.
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TURNROW SUBDIVISION – STORM WATER DESIGN REPORT
Exhibit A
Vicinity Map
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TURNROW SUBDIVISION – STORM WATER DESIGN REPORT
Exhibit ST-1
Pre-Development Drainage Basins
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TURNROW SUBDIVISION – STORM WATER DESIGN REPORT
Exhibit ST-2
Post-Development Drainage Basins
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TURNROW SUBDIVISION – STORM WATER DESIGN REPORT
Appendix A
NRCS Soils Report