HomeMy WebLinkAbout012 Stormwater Design ReportStormwater Drainage Design Report
West Beall Townhomes Site Civil
1608 W. Beall St.
Bozeman, MT
05.20.2022
109 E. Main St., Bozeman, MT 59715 I OFFICE 406.728.4611 I EMAIL wgm@wgmgroup.com
WEST BEAL TOWNHOME SITE CIVIL
Storm Drainage Design Report
REPORT DATE:
May 20, 2022
AUTHOR:
Gary Fox, PE
Project Engineer
WGM Group, Inc.
Derek Dennehy, PE
Project Engineer
WGM Group, Inc.
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Storm Drainage Design Report
CONTENTS
1.0 LOCATION & DESCRIPTIONS ....................................................................................................... 1
1.1 LOCATION .................................................................................................................................. 1
1.2 DESCRIPTION OF PROPERTY .................................................................................................. 2
1.3 GENERAL PROJECT DESCRIPTION ......................................................................................... 2
1.4 CONSTRUCTION SCHEDULE .................................................................................................... 3
2.0 EXISTING DRAINAGE .................................................................................................................... 4
2.1 EXISTING DRAINAGE PATTERNS ............................................................................................. 4
3.0 STORMWATER DESIGN CRITERIA ............................................................................................... 5
3.1 REGULATIONS AND PROJECT DESIGN CRITERIA ................................................................. 5
3.2 HYDROLOGY .............................................................................................................................. 5
3.4 SOILS INFORMATION ................................................................................................................ 6
4.0 PROPOSED DESIGN ...................................................................................................................... 7
4.1 PRE-DEVELOPED RUNOFF RATES .......................................................................................... 7
4.2 POST DEVELOPMENT RUNOFF RATES ................................................................................... 7
4.3 POST-DEVELOPMENT DRAINAGE BASINS ............................................................................. 8
4.4 DRAINAGE BASIN DETENTION/RETENTION DESIGN UTILIZING LOW IMPACT
DEVELOPMENT (LID) ....................................................................................................................... 9
4.5 WATER QUALITY TREATMENT ............................................................................................... 10
4.6 HYDRAULIC DESIGN ............................................................................................................... 10
5.0 CONCLUSIONS ............................................................................................................................ 11
6.0 REFERENCES .............................................................................................................................. 12
APPENDICES
A – STORAGE CALCULATIONS
B – CONVEYANCE CALCULATIONS AND ANALYSIS
C – OPERATION AND MAINTENANCE PLAN
D – STORM PLANS AND DETAILS
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1.0 LOCATION & DESCRIPTIONS
1.1 LOCATION
Lot 2 of the Kirk 2nd Subdivision site is located at 1608 West Beall Street, approximately 400 feet west of
the West Beall and North 15th Avenue intersection. The site is located in Section 12, Township 2S, Range
5E. The development will be comprised of 11 townhome units and an open space lot. This project site is
located within the Bozeman Solvent Site.
FIGURE 1. VICINITY MAP
Project
Site
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FIGURE 2. PROJECT LOCATION
1.2 DESCRIPTION OF PROPERTY
The property is 0.721-acres in total size and is currently zoned B-2. An existing drainage swale is located
along the southern boundary of the property. It is our understanding that this may be the old "Walton
Ditch" which originally conveyed irrigation water but now only conveys occasional stormwater.
Existing conditions on the site are as shown in the aerial photo in Figure 2. Existing conditions includes
an old asphalt parking area, slab on grade, and small landscape areas at the front of the property.
Topography generally slopes to the north.
1.3 GENERAL PROJECT DESCRIPTION
The development will be comprised of 11 townhome units, a common open space area, water and sewer
main extensions, and a stormwater management system. The stormwater management system is
comprised of valley gutter, catch curb, spill curb, stormwater inlets, stormwater conveyance pipes, and
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an underground Contech infiltration chamber system located beneath the five additional parking spaces
near the east portion of the site.
1.4 CONSTRUCTION SCHEDULE
Project construction is anticipated to begin in the summer of 2022.
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2.0 EXISTING DRAINAGE
2.1 EXISTING DRAINAGE PATTERNS
The site is located on a relatively flat lot that drains to West Beall Street. The street drains to the east
toward N 15th Ave, where an inlet in the intersection of N. 15th and W. Beall collects this runoff into a
public stormwater pipe that drains to the north. Currently, the site slopes generally from the edge of an
existing ditch on the south portion of the site to the sidewalk in West Beall, so there is no evidence of off-
site flows entering the site. It is our understanding that this ditch is the old “Walton Ditch” which was once
used for irrigation but now only conveys stormwater during large storm events. Correspondence with the
ditch company along with the City of Bozeman has confirmed that the ditch is no longer used for irrigation
purposes but does carry stormwater runoff. The ditch drains to the east towards N. 11th Avenue and
continues north along N. 11th Avenue. Appendix D contains an exhibit showing the pre-development
drainage basin. There is one major existing sub-basin for the site, and most drainage currently flows
north and east on slopes that are typically less than 5%.
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3.0 STORMWATER DESIGN CRITERIA
3.1 REGULATIONS AND PROJECT DESIGN CRITERIA
The stormwater drainage plan for this project has been developed to exceed stormwater drainage design
criteria required by the City of Bozeman Design Standards and Specifications Policy (City Standards),
dated March 2004. Minimum design criteria per the design standards included:
Stormwater runoff from the development site shall be limited to the pre-development runoff rates.
Adequate on-site stormwater detention shall be provided for design storm runoff exceeding the
pre-development rate.
Stormwater storage and treatment facilities shall be designed to remove pollutants.
Storm sewer facilities shall be designed to handle a 25-year storm event.
The drainage plan shall include, to the greatest extent feasible, low impact development practices that
infiltrate, evapotranspire, or capture for reuse the runoff generated from the first 0.5 inches of rainfall from
a 24-hour storm preceded by 48 hours of no measurable precipitation.
The Design Standards reference a modified Rational Method to calculate detention storage for the 10-
year, 2-hour storm event and the standard Rational Method to calculate peak runoff of the 25-year storm
event.
3.2 HYDROLOGY
The design storms investigated for this site include the 10-year and 25-year recurrence intervals to
document that post-development runoff rates do not exceed that of pre-development conditions per COB
standards. The Modified Rational Method was used for evaluating storage, infiltration, and discharge
rates per COB design standards. Time of concentration values were obtained using MDEQ Circular-8
methodology which outlines type of flow, slope, and terrain the stormwater crosses on its drainage path.
The COB IDF data was used to calculate the estimated runoff amounts for the 10-year and 25-year storm
events. Minimum detention values were obtained using the same methodology as the Sizing Detention
Basins Sample Problem in the COB Design Standards Appendix.
Similar to the pre-development basin, there is one drainage basin for the post-development drainage
plan. The basin delineation is shown in Appendix D along with existing discharge points and basin size.
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3.4 SOILS INFORMATION
A geotechnical report for this property was prepared by CTL Thompson in December 2021. Four test pits
were dug throughout the site. According to the test pit logs obtained from the geotechnical report, clean
gravels were encountered from 4-7 feet. No evidence of groundwater was present in any of the four test
pits. There was also no evidence of groundwater present in any of the monitoring wells when they were
checked in May 2022. The underground stormwater retention system will be excavated to the clean gravel
layer.
Soil infiltration values were obtained per Circular DEQ-8, Appendix C (see Table 1) and are known to be
conservative. A Ksat value of 2.6 inches per hour (in/hr) is assumed for cobble/gravel infiltration galleries
based on the elevation of the bottom of the galleries in relation to the soil profiles.
TABLE 1. DESIGN INFILTRATION RATES PER DEQ-8
TEXTURE INFILTRATION RATE (PER HOUR)
Gravel, gravelly sand, or very coarse sand 2.6
Loamy sand, coarse sand (d) 1.05
Medium sand, sandy loam 0.9
Fine sandy loam, loam 0.7
Very fine sand, sandy clay loam, silt loam 0.7
Clay loam, silty clay loam 0.07
Sandy clay 0.07
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4.0 PROPOSED DESIGN
4.1 PRE-DEVELOPED RUNOFF RATES
Stormwater runoff from the development site is required to be limited to the pre-development runoff rates.
Pre-development condition is defined by the Commercial Downtown Runoff Coefficient in the COB
Design Standards (0.80) – as the site is currently at least 80% impervious. As shown in the plan set in
Appendix D, there is one drainage basin for the pre-existing site conditions with all the runoff draining to
West Beall. The project’s design objective to encourage infiltration will reliably contribute to the overall
development retaining stormwater during normal storm events. Pre-developed runoff rates were
calculated using existing Time of Concentration drainage paths. Runoff coefficients and Time of
Concentration’s for each drainage basin are shown below:
Drainage Basin 1A – C Factor = 0.8
Drainage Basin 1A – Time of Concentration in Minutes = 2.24 minutes
- Assuming 5 minute minimum
The calculations for existing Time of Concentrations for the pre-development drainage basin are included
in Appendix A. Pre-developed runoff rate calculations are included in Appendix A and show the use of
the Modified Rational Method as outlined in the City of Bozeman Design Standards detention basin sizing
example. Runoff rates were determined by multiplying the drainage basin size by the basin C factor, and
then multiplying by the intensity at the time of concentration value for each drainage basin – as outlined
by the modified rational method.
TABLE 2. PRE-DEVELOPED HYDROLOGIC SUMMARY
DRAINAGE ID
DRAINAGE
BASIN SIZE
(ACRES)
STORM
FREQUENCY
PRE-DEVELOPED FLOW
(CFS)
DB-1A 0.72 10YR 1.86
4.2 POST DEVELOPMENT RUNOFF RATES
Post development runoff rates were calculated using the Modified Rational Method as outlined by the
COB design standards. The 10-year and 25-year event were evaluated for detention system sizing and
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pipe & gutter capacity. Runoff coefficients and Time of Concentrations for each drainage basin are shown
below and calculations are included in Appendix A.
Drainage Basin 1B – Weighted C Factor = 0.64
Drainage Basin 1B – Time of Concentration in Minutes = 2.52 minutes
- Assuming 5 minute minimum
Post development time of concentrations were conservatively assumed to occur when no landscaping
vegetation is installed, as to represent a storm occurring during construction before landscaping and turf
has been installed. Post development runoff rates are to be limited to the pre-development rate that is
shown in Table 2 above for the 10-year storm. A summary of post-development runoff rates is shown
below in Table 3.
TABLE 3. POST-DEVELOPED HYDROLOGIC SUMMARY
DRAINAGE ID DRAINAGE BASIN
SIZE (ACRES) STORM FREQUENCY POST-DEVELOPED
FLOW (CFS)
DB-1B 0.72 10-YR 0.34
4.3 POST-DEVELOPMENT DRAINAGE BASINS
One drainage basin has been delineated that will route the stormwater to the underground stormwater
infiltration facility on site. Drainage Basin 1B refers to the post-development drainage basin and
encompasses the entire 0.72-acre site. As shown on the attached grading plan, stormwater will be
directed to a valley gutter located in the center of the asphalt parking lot. The valley gutter will direct
stormwater to curb inlets. From here the stormwater will flow via underground piping to an underground
infiltration system. There is also a curb inlet located on the west portion of the parking lot where water
will sheet flow to the inlet and be piped to the underground infiltration system. Per the geotechnical report,
groundwater was not encountered in any of the four test pits dug on-site. Downspouts from the north
building will be piped to an underground manifold that directs stormwater to one of the curb inlets located
in the center of the parking lot. Driveways and general lot slope will be towards this valley gutter and inlet,
both of which convey stormwater to the underground infiltration system. All stormwater runoff will be
retained on site. Stormwater runoff will either infiltrate or be directed to the underground system.
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4.4 DRAINAGE BASIN DETENTION/RETENTION
DESIGN UTILIZING LOW IMPACT DEVELOPMENT (LID)
City Standards specify that “the drainage plan shall include, to the greatest extent feasible, low impact
development practices that infiltrate, evapotranspire, or capture for reuse the runoff generated from the
first 0.5 inches of rainfall from a 24-hour storm preceded by 48 hours of no measurable precipitation.” To
accommodate this practice, the proposed development has included storage designs that utilize
infiltration to the greatest extent as to limit the amount of stormwater being discharged from the site. This
infiltration design also helps treated stormwater recharge the underlying groundwater aquifer as would
occur in a natural setting.
To optimize stormwater infiltration and associated environmental benefits, the site grading, drainage and
landscape plan was comprehensively designed to capture and infiltrate runoff by means of underground
infiltration chambers. All captured stormwater is designed to be infiltrated on-site.
DB-1B – PROPOSED UNDERGROUND INFILTRATION SYSTEM
Drainage Basin 1B encompasses the entire site area of 0.72 acres as shown on the post-development
site plan in Appendix D. Proposed measures include retaining or infiltrating the first 0.5-inches of
stormwater from a 24-hour storm and to also detain stormwater from the 10-year event while releasing a
volume equal to the pre-development runoff – after the treatment water volume has been infiltrated.
Retention and detention volumes are shown below and calculations that outline the volume of stormwater
to be detained for the 10-year storm in Drainage Basin 1B are included in Appendix A.
Volume to Retain/Infiltrate from first 0.5” of Rain = 817 Cubic Feet
Volume to Detain for 10-year, 2-hour Storm = 1,348 Cubic Feet
Proposed 12” dia. pipe will convey the stormwater to the proposed underground stormwater system. A
Contech CMP system with perforated pipe has been designed to capture and infiltrate stormwater flows
exceeding the volume required per the 10-year, 2-hour storm as shown above. Calculations in Appendix
B show that the 12” dia. pipe has enough capacity to convey the 25-year storm event from DB-1B.
Washed 1-1/2” minus base course will be used as the material bottom to allow the stormwater runoff to
infiltrate quickly to the native infiltrative soil layer. The Contech system will be dug to native gravels to
encourage maximum infiltration of stormwater.
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The infiltration chambers are currently specified to be Contech CMP Chambers. For this site, 8 chambers
(30-in. diameter) are proposed: four rows of two chambers encased in washed 1-1/2” minus to allow for
water to infiltrate into the surrounding soils. The chambers and stone encasement envelope is referred
to as the “infiltration gallery” in this report. Contech’s Design Software was used to size the chambers
and infiltration gallery. The design calculations are shown in Appendix A. The bottom of the infiltration
gallery will be approximately 4 ft. below the existing ground surface, thus high water is not anticipated to
reduce capacity of the system based on data from the geotechnical report. The total storage capacity of
the Contech system is as follows:
Capacity of the Infiltration Chambers (as reported from Contech) = 804 cubic feet
Total Storage Capacity of Backfill Storage Volume (assuming 40% voids) = 605 cubic feet
Total Storage Capacity of Chamber & Backfill Storage Volume = 1,409 cubic feet
This storage volume will ensure that the first 0.5” of stormwater is captured and retained/infiltrated, and
the maximum volume from the 10-year storm event is detained. If the system were to surcharge during
a high-intensity storm event, runoff would flow into West Beall Street – which mimics existing drainage
patterns.
4.5 WATER QUALITY TREATMENT
The first 0.5” of stormwater is to be retained and infiltrated through the use of an underground infiltration
system. This will enhance stormwater management and allow it to be treated through means of ground
filtration before reaching the water table. Evapotranspiration was not evaluated as a mean for stormwater
loss, as this design promotes underground infiltration to promote a healthy aquifer recharge to mimic
native and undeveloped alluvial conditions.
4.6 HYDRAULIC DESIGN
Inlets and storm drain pipes are sized to handle the 25-year storm as calculated using the Rational
Method per City Standards. Conveyance calculations conservatively assume that no peak flow
attenuation is provided by upgradient landscaping features that the home developments will likely have.
Hydraulic calculations are included in Appendix B.
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5.0 CONCLUSIONS
All infrastructure is adequately sized to convey the runoff from the 25-year storm event and detain or
retain the volume from the 10-year storm event. Runoff release rates from the project site during proposed
conditions do not exceed pre-developed rates in receiving conveyances for the 10-year storm event.
Excess stormwater from an event larger than the 10-year will be conveyed by the storm infrastructure so
that home sites are not affected by the stormwater.
The constructed project will improve site drainage through runoff reduction, implementation of a designed
stormwater management system, and treatment for pollutant removal. The proposed site improvements
are detailed in the preliminary storm plan sheets developed for this site. To the best of our knowledge
the Storm Drainage Plan has been assembled per the requirements defined within the City of Bozeman
Design Standards and Specifications Policy.
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6.0 REFERENCES
City of Bozeman, 2004. Design Standards and Specifications Policy. City Engineering Department.
March 2004. Adopted April 5, 2004, Ordinance 1611.
Montana Department of Environmental Quality, 2017. Montana Standards for Subdivision Storm Water
Drainage, Circular DEQ-8.
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Storm Drainage Design Report
APPENDIX A
STORAGE CALCULATIONS
Time (min).Time (hrs)Intensity 2 yrIntensity 5 yrIntensity 10 yrIntensity 25 yrIntensity 50 yrIntensity 100yr00.0000n/an/an/an/an/an/a50.08331.598862.550823.218463.826234.742945.33793100.16671.054861.636892.051062.455343.001703.35492150.25000.827061.262761.575861.894142.296932.55683200.33330.695951.050421.307101.575621.899712.10859250.41670.608740.910621.130621.365931.639561.81578300.50000.545660.810331.004271.215501.453681.60698350.58330.497450.734200.908521.101311.313051.44930400.66670.459150.674060.832991.011101.202291.32526450.75000.427820.625120.771600.937681.112371.22470500.83330.401620.584360.720520.876541.037641.14123550.91670.379290.549780.677240.824670.974381.07063601.00000.360000.520000.640000.780000.920001.01000651.08330.343120.494030.607550.741050.872660.95726701.16670.328200.471150.578980.706720.831000.91089751.25000.314890.450800.553590.676200.794010.86974801.33330.302930.432560.530850.648830.760900.83294851.41670.292110.416090.510340.624140.731060.79978901.50000.282260.401150.491720.601720.703990.76973951.58330.273250.387500.474740.581260.679310.742351001.66670.264970.374990.459180.562490.656700.717271051.75000.257320.363460.444840.545190.635890.694201101.83330.250240.352800.431590.529200.616660.672901151.91670.243650.342900.419300.514360.598830.653151202.00000.237510.333690.407860.500540.582250.63479City of Bozeman IDF Curve Table Values
Drainage Basin 1A (TOC in Min.)
Pre-Development (from DEQ IDF
Spreadsheet)
Drainage Basin 1B (TOC in Min.)
Post-Development (from DEQ
IDF Spreadsheet)
5 5
Time of Concentration Summary (From DEQ-8 Spreadsheet)
Return to SummaryRainfall Intensity for DEQ 81. Location Data:Latitude:45.686164Longitude:-111.069172Distance to closest station (km)3.25Closest meteorological stationBOZEMAN MONTANA ST U2-hour, 24-hour precipitation (in)1.162. Pre-development Hydraulic Path:Flow Type Surface Description Flow Length (ft) Land Slope (ft/ft)Culvert Diameter (in)Depth of Flow in Channel (in) or Culvert (in)Channel - Top Width (ft.)Channel - Bottom Width (ft.)Cross Sectional Flow Area (ft^2)Wetted Perimeter (ft)nAverage Velocity (ft/s)Tt (hr) Tt (min)SheetSmooth Asphalt1000.0230.011n/a0.031.90ShallowPaved1050.0160.0125.170.010.34Appendix F: http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1044171.pdf3. Post-development Hydraulic Path:Flow Type Surface Description Flow Length (ft) Land Slope (ft/ft)Culvert Diameter (in)Depth of Flow in Channel (in) or Culvert (in)Channel - Top Width (ft.)Channel - Bottom Width (ft.)Cross Sectional Flow Area (ft^2)Wetted Perimeter (ft)nAverage Velocity (ft/s)Tt (hr) Tt (min)SheetSmooth Concrete200.020.012n/a0.010.60ShallowPaved1200.0050.0122.890.010.69Appendix F: http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1044171.pdf4. Time of Concentration and Rainfall Intensity (24-hour storm event):Pre-development PathTotal Time of Concentration (min)2.24Rainfall Intensity (in/hr), 2 Year1.70Rainfall Intensity (in/hr), 10 Year3.11Rainfall Intensity (in/hr), 100 Year5.13Post-development PathTotal Time of Concentration (min)1.29Rainfall Intensity (in/hr), 2 Year1.92Rainfall Intensity (in/hr), 10 Year3.52Rainfall Intensity (in/hr), 100 Year5.800.000.501.001.502.002.503.003.504.004.500 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160 165 170 175 180Rainfall Intensity in Inches per HourDuration in MinutesRainfall Intensity-Duration Frequency Curve for:2-yr10-yr100-yrPre-development Time of ConcentrationPost-development Time of ConcentrationBOZEMAN MONTANA ST UTime of Concentration - DB-1A and DB-1B
Land Use:
Low to Med. Density
Residential
Min Duration
(min)
Intensity
(in/hr)
Future Runoff Rate
(Q=CiA) (cfs)
Runoff
Volume (cf)
Release
Volume (cf)
Required
Storage (cf)
Area (acres): 0.720 0 3.218 1.48 0.00 0.00 0.00
C: 0.8 5 3.218 1.48 443.18 101.11 342.07
TOC (min): 5 10 2.051 0.94 564.86 202.22 362.64
Design Storm Frequency:10 year 15 1.576 0.72 650.99 303.33 347.66
Intensity at Tc: 3.218 20 1.307 0.60 719.95 404.44 315.51
Peak Runoff Rate (cfs): 1.854 25 1.131 0.52 778.43 505.56 272.88
30 1.004 0.46 829.73 606.67 223.06
35 0.833 0.38 802.92 707.78 95.14
Land Use:Med. Density Residential 40 0.833 0.38 917.62 808.89 108.73
Area (acres): 0.72 45 0.772 0.35 956.24 910.00 46.24
Weighted C: 0.64 50 0.721 0.33 992.16 1011.11 -18.95
TOC (min): 5 55 0.677 0.31 1025.82 1112.22 -86.41
Design Storm Frequency:10 year 60 0.640 0.29 1057.54 1213.33 -155.80
Design Release Rate from infiltration (cfs): 0.34 65 0.608 0.28 1087.58 1314.44 -226.86
Impervous Area Estimate (sf): 19,602 70 0.554 0.25 1067.21 1415.56 -348.34
Initial Rainfall to Retain (in) 0.5 75 0.554 0.25 1143.44 1516.67 -373.23
Volume to Retain/infiltrate (cf):816.75 80 0.510 0.23 1124.37 1617.78 -493.41
85 0.510 0.23 1194.64 1718.89 -524.24
90 0.475 0.22 1176.70 1820.00 -643.30
West Beall Townhomes - Runoff Calculations
Pre Development DB-1A Minimum Volume Table
Post Development DB-1B
Calculation of Required Volume for Stormwater Retention and Infiltration System
Design Rainfall Frequency:10 year - 2 Hour event
IDF coefficient a:0.64
IDF coefficient b:
IDF coefficient n:0.65
Post Development Calculations:
Area (Acres)C
Impervious 0.45 0.90
Pervious 0.27 0.20
Total 0.72 0.64
Retention Basin Calculation:
Q = CIA
C = 0.64 (post development)
I =0.41 in/hr (10-yr, 2-hr storm)
A =0.72 acres
Q =0.19 cfs
Required retention storage:1348 cubic feet (10-yr, 2-hr storm)
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APPENDIX B
CONVEYANCE CALCULATIONS & ANALYSIS
Area (Acres)C
Impervious 0.45 0.90
Pervious 0.27 0.20
Total 0.72 0.64
0.64
5
0.64
3.83
0.72
1.76Q = Required Gutter Capacity (cfs) =
Weighted C Factor =
Weighted C Factor Calculation
Calculate Flow from the Rational Formula ( Q=CIA)
Drainage Basin 1B - Curb Gutter Capacity
Time of Concentration (tc In minutes) for DB-1 =
Weighted C Factor (from above) =
Intensity for 25 Year Storm ( I= 0.78Tc^-.64 (in/hr)) =
Area (acres) =
n (mannings coefficient) = 0.013
A (Area ft2) =1.24 (0.15" below TOC)
P (Wetted Perimeter ft) = 9.23 (0.15" below TOC)
R (Hydraulic Radius ft) = 0.13 (0.15" below TOC)
S (Slope ft/ft) = 0.05
Q (Actual Gutter Capacity) = 8.13
Actual Gutter Capacity - Drainage Basin 1-B
Gutter Capacity - 0.15' below Top of Curb
Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Thursday, Dec 16 2021
12-inch RCP @ 0.5%
Circular
Diameter (ft) = 1.00
Invert Elev (ft) = 6.21
Slope (%) = 0.50
N-Value = 0.013
Calculations
Compute by: Q vs Depth
No. Increments = 10
Highlighted
Depth (ft) = 0.80
Q (cfs) = 2.462
Area (sqft) = 0.67
Velocity (ft/s) = 3.65
Wetted Perim (ft) = 2.22
Crit Depth, Yc (ft) = 0.68
Top Width (ft) = 0.80
EGL (ft) = 1.01
0 1 2 3
Elev (ft)Depth (ft)Section
5.50 -0.71
6.00 -0.21
6.50 0.29
7.00 0.79
7.50 1.29
8.00 1.79
Reach (ft)
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Storm Drainage Design Report
APPENDIX C
OPERATION & MAINTENANCE PLAN
Stormwater Operation and Maintenance Plan
Beall St. Townhomes
Bozeman, MT
109 E. Main St. Suite B, Bozeman, MT 59715 I OFFICE 406.728.4611 I EMAIL wgm@wgmgroup.com
BEALL ST. TOWNHOMES STORMWATER
Operation and Maintenance Manual
STORMWATER OPERATION &
MAINTENANCE PLAN
LOCATION
The project location is Lot 2 of Block 3 of Kirk 2nd Subdivision. The address of the site is 1608 W. Beall
St.
PURPOSE
Stormwater runoff is a significant source of water pollution in the City of Bozeman and in urban areas.
In addition, increasing impervious area causes increased runoff flow rates and runoff volumes
discharged to receiving drainageways. The proposed stormwater facilities help mitigate negative
effects by providing treatment for pollutant removal as well as storage and infiltration functions to help
control release rates downstream.
Properly maintained stormwater facilities are effective at removing certain pollutants and providing
necessary management of stormwater volumes during larger storm events. Improperly maintained
facilities can increase the discharge of pollutants downstream, increase the risk of flooding
downstream, increase the instability of downstream channels, and lead to aesthetic and nuisance
problems.
STORMWATER SYSTEM OVERVIEW
The stormwater features used at this site include curb inlets, valley gutters, and underground storage
and infiltration chambers. The parking lot is graded to direct stormwater to the curb inlets, which then
direct stormwater to the underground chambers. Landscaping throughout the site will encourage
infiltration. The underground chambers are located below the five parking stalls on the east portion of
the site.
In combination with each other, the site’s stormwater facilities have the following general objectives:
Efficient internal conveyance of stormwater runoff to final infiltration and treatment facilities.
Treatment and infiltration of runoff to improve water quality.
Protection of the downstream (i.e. offsite) drainage system through reduction of runoff peak
flow rates and volumes via storage and infiltration.
Protection of groundwater and providing consideration to the relatively shallow and potentially
fluctuating groundwater table.
RESPOSIBILITY TO MAINTAIN
Designation of a responsible party is important to assure proper operation of your stormwater
detention and control features. In this instance it is a responsibility of the property owner for the
correct operation and proper maintenance of the facilities. The owner shall designate a qualified
professional entity or individual to perform all monitoring and maintenance. The City of Bozeman
Public Works Department is not responsible for the maintenance of the facilities located outside of the
right-of-way including the water quality devices, flow control structures, and outlet pipes. Additionally,
the City of Bozeman is not responsible for maintenance of the infiltration galleries within the right-of-
way. However, the City of Bozeman Public Works Department does have the authority to inspect and
BEALL ST. TOWNHOMES STORMWATER
Operation and Maintenance Manual
review maintenance activities to ensure the viability of your facilities. The owner’s representative shall
be required to keep a log of all required inspections and maintenance required. The log shall be
made available to the City of Bozeman Public Works Department for review as requested.
INSPECTION & MAINTENANCE
The onsite stormwater facilities have unique requirements for maintenance activities and intervals.
The facility size, type and characteristics of the tributary area, and facility locations will all influence
maintenance requirements. This section outlines maintenance activities, intervals, and procedures for
the stormwater system. The site’s maintenance program should be adjusted as necessary to keep the
stormwater system functioning properly.
Maintenance of the underground chambers should be per the manufacturer’s specifications. At
the time of this writing, that is assumed to be Contech via their detention storage system.
Accumulated sediment in the chambers should be cleaned whenever an inspection reveals
sediment or trash is clogging the discharge orifice. Annual inspections are the best practice for
all underground systems. Refer to Contech guidance for additional maintenance practices.
Regular maintenance activities should focus on the care of upstream pre-treatment facilities (i.e.
catch basins and rain gardens) to protect the viability of larger downstream treatment features.
This maintenance includes seasonal removal of any trash, debris, and sediment that
accumulates within pipes, catch basins, curb and gutters, drywells, and infiltration
galleries. Excessive accumulations of trash, debris, and sediment may cause clogging in the
system and result in reduced lifespan of infiltration facilities, overflow and flooding, and damage
risk to nearby structures.
Inlet grates should remain free of obstructions to flow from debris. Clogged inlets can lead to
flooding risk and damage to onsite and offsite structures. Inlet grates are particularly prone to
clogging from leaf litter in the fall and from road sanding/de-icing material in the spring. Inlets
should be inspected four times per year and following large storm events and sumps should be
cleaned annually or whenever basin sumps become filled with sediment to half of the depth.
DO NOT ENTER confined spaces such as manholes and underground infiltration facilities for
inspections. These facilities should be inspected from the surface.
Keeping underground infiltration facilities and drywells clear of accumulating sediment and
debris is critical for their proper function and lifespan. If the underlying gravel pack and soils
become clogged with fine sediments, the cobble galleries and drywells will not function as
designed and may require complete replacement. Drywells should be inspected frequently and
cleaned as necessary via hydraulic vacuum truck.
All piping should be inspected annually and following large storm events. Pipe should be
cleaned and flushed as determined in the field.
All paved areas shall be swept twice a year, scheduled in the spring and fall.
Routine Maintenance (seasonally to bi-annually):
1) Vegetation Management: When mowing, collect grass clippings and all other
clippings/trimmings and take offsite for disposal or dispose in trash on site. Do not leave
BEALL ST. TOWNHOMES STORMWATER
Operation and Maintenance Manual
clippings in an area that will clog catch basin inlet grates, ponds, or pipes. Limit the use of
fertilizers and pesticides around channels and rain gardens to minimize entry into subsequent
downstream waters.
2) Snow Storage Management: Snow shall be stockpiled in locations where accumulations
won’t limit or restrict the ability of the infiltration facilities and storm structures to convey runoff.
3) Trash, Debris and Litter Removal: Removal of any trash, etc. causing any obstructions to the
stormwater facilities during periodic inspections and especially after every significant runoff
event. In general, pick up trash, etc. in and around the facilities during all inspections. Inspect
all catch basins and drywell structures and clean floating debris as necessary.
4) Structural Component Check: Perform structural inspection of catch basins, drywells, inlets,
grates, pipes, and culverts on a regular basis.
Structural Repair/Replacement: Eventually structural components may need to be repaired or
replaced to ensure proper function. Monitor metal structures for excessive rust and corrosion.
Monitor concrete structures for large cracking and/or spalling. Repair and replace as necessary.
WEST BEAL TOWNHOME SITE CIVIL
Storm Drainage Design Report
APPENDIX D
STORM PLANS & DETAILS
W. BEALL STREETWGMGROUPWWW.WGMGROUP.COMPRE & POST DEVELOPMENT DRAINAGE BASINS
WEST BEALL TOWNHOMES
BOZEMAN, MONTANADECEMBER 2021STRMPRELIMINARYPLOTTED:SAVED:5/20/225/20/22NDB-1A:Pre-developedDrainage Basin0.72 ACDB-1B:Post-developedDrainage Basin0.72 ACTIME OFCONCENTRATIONFLOW PATH FOR DB-1ATIME OFCONCENTRATIONFLOW PATH FOR DB-1B
W. BEALL STREET
FF ELEV= 4808.4FF ELEV= 4807.0WGMGROUPWWW.WGMGROUP.COMGRADING AND DRAINAGE PLAN
THE LOFTS ON BEALL
BOZEMAN, MONTANAMAY 2022C3.0PRELIMINARYPLOTTED:SAVED:5/25/225/25/22LEGEND-GRADINGGENERAL CONSTRUCTION NOTES: (THIS SHEET)N
WGMGROUPWWW.WGMGROUP.COMUNDERGROUND STORMWATER SYSTEM
1608 W. BEALL TOWNHOMES
BOZEMAN, MTMAY 2022S.1PRELIMINARYPLOTTED:SAVED:5/25/22----