HomeMy WebLinkAboutStormwaterDesignReport_07262021Stormwater Drainage Design Report
Annie Subdivision Phase 4
Bozeman, MT
109 E. Main St. Suite B, Bozeman, MT 59715 I OFFICE 406.728.4611 I EMAIL wgm@wgmgroup.com
JULY 22, 2021
REPORT DATE:
July, 2021
AUTHOR:
Hunter Morrical, PE
Project Engineer
WGM Group, Inc.
1 Annie Subdivision Phase 4
Stormwater Drainage Plan
CONTENTS
1.0 LOCATION AND DESCRIPTION ............................. 2
1.1 LOCATION ................................................................................ 2
1.2 DESCRIPTION OF PROPERTY ............................................... 3
1.3 GENERAL PROJECT DESCRIPTION ...................................... 3
1.4 CONSTRUCTION SCHEDULE ................................................. 3
2.0 EXISTING DRAINAGE ............................................. 4
2.1 EXISTING DRAINAGE PATTERNS .......................................... 4
3.0 STORMWATER DESIGN CRITERIA ........................ 4
3.1 REGULATIONS ......................................................................... 4
3.2 HYDROLOGY............................................................................ 5
3.4 SOILS INFORMATION .............................................................. 5
4.0 PROPOSED DESIGN ............................................... 6
4.1 PRE-DEVELOPED RUNOFF RATES ....................................... 6
4.2 POST DEVELOPMENT RUNOFF RATES ................................ 6
4.3 POST DEVELOPMENT DRAINAGE BASINS…………………...7
4.4 DRAINAGE BASIN DESIGN ..................................................... 7
4.5 WATER QUALITY TREATMENT .............................................. 8
4.6 HYDRAULIC DESIGN………………………..…………………….9
5.0 CONCLUSIONS ........................................................ 9
6.0 REFERENCES .......................................................... 9
APPENDICES
A – STORAGE CALCULATIONS
B – CONVEYANCE CALCULATIONS & ANALYSIS
C – STORMWATER SYSTEM OPERATION AND MAINTENANCE
PLAN
D – STORM PLANS & DETAILS
2
Annie Subdivision Phase 4
Stormwater Drainage Plan
1.0 LOCATION AND DESCRIPTION
1.1 LOCATION
Lot 1 of the Annie Subdivision Phase 3C site is located at 2305 Durston Road, approximately
0.25 miles west of the Durston and North 19th Avenue intersection. The site is located in
Section 2, Township 2S, Range 5E. The development will be comprised of 16 townhome lots,
1 single family lot, 2 four-plex lots, and an open space/park lot.
FIGURE 1. VICINITY MAP
Project Site
3 Annie Subdivision Phase 4
Stormwater Drainage Plan
FIGURE 2. PROJECT LOCATION
1.2 DESCRIPTION OF PROPERTY
The property is 3.24-acres in total size and is zoned to be R-3. An existing stream is located
along the east edge of the property.
Existing conditions on the site are as shown in the aerial photo in Figure 2. Existing
development includes a home, storage building, driveway, garden, and a vehicle maintenance
parking area.
1.3 GENERAL PROJECT DESCRIPTION
The development will be comprised of approximately 19 residential lots, and an open
space/park lot, water and sewer extensions, and a stormwater management system. The
stormwater management system is comprised of dispersed treatment-conveyance-infiltration
features including cobble infiltration galleries along with standard closed storm drain conduits,
inlets, and a storm pond.
1.4 CONSTRUCTION SCHEDULE
Project construction is anticipated to begin in the Fall of 2021.
4 Annie Subdivision Phase 4
Stormwater Drainage Plan
2.0 EXISTING DRAINAGE
2.1 EXISTING DRAINAGE PATTERNS
The site is located on a relatively flat area adjacent to The West Fork of Catron Creek. The
majority of the site is gently sloped toward the north. A small portion of the property drains to
the west towards the adjacent townhomes on the west side of the property. Currently because
of the site being bounded by Durston, North 25th, and the creek, there is no evidence that
stormwater runs on to the property. Appendix D contains an exhibit showing pre-development
drainage basins. There are two major existing sub-basins for the site, and most drainage
currently flows north and east on slopes that are typically less than 5%.
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.
5 Annie Subdivision Phase 4
Stormwater Drainage Plan
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.
Two drainage basins have been combined into one drainage basin as part of the post
development plan. The basin delineation is shown in Appendix D along with existing discharge
points and basin size.
3.4 SOILS INFORMATION
Soil test pits were dug on March 19th, 2021 by Allied Engineering Services and documented in
the attachments to the Preliminary Plat application. Groundwater was not encountered in any
of the test pits, however test pit #7 was “wet” at 9.3’ below the ground surface. Monitor wells
were installed as to monitor groundwater through the typical high groundwater season. Test
pits generally identified a Silty Clay soil layer in the 0- to 3-feet below ground surface range,
and a sandy Gravel in the 3-10-foot soil range.
Soil infiltration values were obtained per Circular DEQ-8, Appendix C (see Table 3) and are
known to be conservative. A Ksat value 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 3. DESIGN INFILTRATION RATES PER DEQ-8
6 Annie Subdivision Phase 4
Stormwater Drainage Plan
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 Low to Medium Density Residential
Runoff Coefficient in the COB Design Standards (0.35) – as the site currently has a dwelling
unit along with a barn. As shown in the plan set in Appendix D, there are two existing sub-
basins for the site, and most drainage currently flows northward and eastward towards the
adjacent developments. 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 1 – C Factor = 0.35
Drainage Basin 2 – C Factor = 0.35
Drainage Basin 1 – Time of Concentration in Minutes = 31.48
Drainage Basin 2 – Time of Concentration in Minutes = 29.9
The calculations for existing Time of Concentrations for each 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 4. PRE-DEVELOPED HYDROLOGIC SUMMARY
DRAINAGE ID DRAINAGE
BASIN SIZE
(ACRES)
STORM
FREQUENCY
PRE-DEVELOPED FLOW
(cfs)
DB-1 0.978 10YR 0.35
DB-2 2.261 10YR 0.80
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 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 2A – Weighted C Factor = 0.50
7 Annie Subdivision Phase 4
Stormwater Drainage Plan
Drainage Basin 2A – Time of Concentration in Minutes = 8.24
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 pre-development rates that are shown in table 4 above for the 10-year storm.
4.3 POST-DEVELOPMENT DRAINAGE BASINS
A single drainage basin has been delineated that will route the stormwater to the site
stormwater pond – this drainage basin is listed as DB-2A throughout the remainder of this
report. The drainage basin is 3.24 acres in size. The single basin is defined by the main
development road (Daffodil Street), which will route stormwater from the west side of the
development all the way to the pond on the east side of the development. Final driveways and
general lot slope will be towards this road, as to collect the majority of stormwater runoff in the
development.
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/or infiltrate runoff
by means of a detention pond. A summary of each feature’s design and performance is
provided below.
DB-2A – Proposed Detention Pond
Drainage Basin 2A after development is a total of 3.24 acres as shown on the post-
development site plan in Appendix D. Proposed measures include retaining the first 0.5 in 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 retained. 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 2A are
included in Appendix A.
Volume to Retain/Infiltrate from first 0.5” of Rain = 2,556 Cubic Feet
Volume to Detain for 10-year, 2-hour Storm = 2,562 Cubic Feet
8 Annie Subdivision Phase 4
Stormwater Drainage Plan
Stormwater in Drainage Basin 2A will be conveyed along the internal road corridor using
standard curb and gutter. Capacity calculations for the curb and gutter show that the entire 25-
year flow in DB-2A could almost be conveyed with a single gutter line, so two curb lines will
easily convey the 25-year event. The curb will convey the runoff to two standard inlets as
shown on the attached post-development storm plan in Appendix D. Curb inlets will be placed
on grade, however final grading will place these inlets at a slight depression so they act as an
inlet in a sag, thus ensuring all stormwater will be conveyed through these inlets for the 25-
year event. These will be standard City storm inlets. Proposed 18” dia. pipe will convey the
stormwater to the proposed detention pond shown on the plans. Calculations in Appendix B
show that the 18” dia. Pipe has enough capacity to convey the 25-year storm event from DB-
2A.
The pond serving DB-2A was designed per COB standards, being that the maximum water
depth is to be 1.5-ft, the side slopes of the pond are to be 1V:4H, and the maximum pond
depth is 2.5-ft. Pond sizing calculations are shown in Appendix A. The bottom of the pond
gallery will be approximately 2-ft below the current ground surface, thus high water is not
anticipated to reduce capacity of the pond based on current groundwater monitoring data.
Additionally, the outlet of the pond will be positioned at an elevation so that the creek does not
backwater the pond during normal flows. The pond outlet will be positioned so that the first 0.5”
of stormwater is completely retained to be infiltrated – as required by COB guidelines. The
outlet will be above the creek elevation, and at an elevation that does not cause the storm pipe
to back up into Daffodil Road during a storm event where the pond outlet is activated. The total
storage capacity of the pond is as follows:
Capacity of the Pond (1.5’ depth) = 2,748 cubic feet
Total Storage Capacity (before pond overtops) = 5,783 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. When
the pond reaches a full volume depth of 1.4-ft, the outlet structure will then overflow using a
designed weir to release 0.80 cfs of stormwater as limited by the pre-development runoff rate.
It will take approximately 20 minutes for the pond to reach a maximum level before spilling out
of the pond for the 10-year storm event; during that first 20 minutes, it can be expected that
some water will infiltrate through the bottom of the pond, thus the pond and storm drain system
likely will have more capacity to handle an event larger than the 10-year.
4.5 WATER QUALITY TREATMENT
The first 0.5” of stormwater is to be retained and infiltrated through the use of a detention pond
in Drainage Basin 2A. 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.
9 Annie Subdivision Phase 4
Stormwater Drainage Plan
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.
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.
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.
10 Annie Subdivision Phase 4
Stormwater Drainage Plan
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 1 (TOC in Min.)
Pre-Development (from DEQ IDF
Spreadsheet)
Drainage Basin 1 (TOC in Min.)
Post-Development (from DEQ
IDF Spreadsheet)
31.48 N/A
Drainage Basin 2 (TOC in Min.)
Pre-Development (from DEQ IDF
Spreadsheet)
Drainage Basin 2A (TOC in Min.)
Post-Development (from DEQ
IDF Spreadsheet)
29.9 8.24
Time of Concentration Summary (From DEQ-8 Spreadsheet)
Drainage Basin 1 Pre-Development - Time of Concentration
Calculations
Drainage Basin 2 Pre-Development and 2A Post-Development - Time of
Concentration Calculations
Drainage Basin 2 Pre-Development
Drainage Basin 2A Post-Development
Land Use:Low to Med. Density ResidentialMin Duration (min)Intensity (in/hr)Future Runoff Rate (Q=CiA) (cfs)Runoff Volume (cf)Release Volume (cf)Required Storage (cf)Area (acres):2.26103.2185.260.000.000.00C:0.3553.2185.261577.110.001577.11TOC (min):29.9102.0513.352010.120.002010.12Design Storm Frequency:10 year151.5762.572316.620.002316.62Intensity at Tc:1.01201.3072.142562.020.002562.02Peak Runoff Rate (cfs):0.80251.1311.852770.14239.802530.34301.0041.642952.67479.612473.07350.8331.362857.27719.412137.86Land Use:Med. Density Residential 400.8331.363265.45959.212306.24Area (acres):3.24450.7721.263402.881199.022203.87C:0.50500.7211.183530.711438.822091.89TOC (min):8.24550.6771.113650.481678.621971.86Design Storm Frequency:10 year600.6401.053763.361918.421844.93Design Release Rate (cfs):0.80650.6080.993870.282158.231712.05Impervous Area Estimate (sf):61,336700.5540.903797.792398.031399.76Initial Rainfall to Retain (in)0.5750.5540.904069.062637.831431.23Volume to Retain/infiltrate (cf):2555.67Pre Development DB-2Minimum Volume TablePost Development DB-2Lot 1 of Annie Phase 3C - Drainage Basin 2A Runoff Calculations
Pond Report
Hydraflow Hydrographs Extension for Autodesk® Civil 3D® by Autodesk, Inc. v2020 Monday, 07 / 19 / 2021
Pond No. 1 - Storm Pond
Pond Data
Trapezoid -Bottom L x W = 85.0 x 14.0 ft, Side slope = 4.00:1, Bottom elev. = 4778.00 ft, Depth = 2.50 ft
Stage / Storage Table
Stage (ft) Elevation (ft) Contour area (sqft) Incr. Storage (cuft) Total storage (cuft)
0.00 4778.00 1,190 0 0
0.25 4778.25 1,392 323 323
0.50 4778.50 1,602 374 697
0.75 4778.75 1,820 428 1,124
1.00 4779.00 2,046 483 1,607
1.25 4779.25 2,280 541 2,148
1.50 4779.50 2,522 600 2,748
1.75 4779.75 2,772 662 3,410
2.00 4780.00 3,030 725 4,135
2.25 4780.25 3,296 791 4,925
2.50 4780.50 3,570 858 5,783
Culvert / Orifice Structures Weir Structures
[A] [B] [C] [PrfRsr] [A] [B] [C] [D]
Rise (in)= 0.00 0.00 0.00 0.00
Span (in)= 0.00 0.00 0.00 0.00
No. Barrels = 0 0 0 0
Invert El. (ft)= 0.00 0.00 0.00 0.00
Length (ft)= 0.00 0.00 0.00 0.00
Slope (%)= 0.00 0.00 0.00 n/a
N-Value = .013 .013 .013 n/a
Orifice Coeff.= 0.60 0.60 0.60 0.60
Multi-Stage = n/a No No No
Crest Len (ft)= 2.00 0.00 0.00 0.00
Crest El. (ft)= 4779.50 0.00 0.00 0.00
Weir Coeff.= 3.33 3.33 3.33 3.33
Weir Type = Rect --- --- ---
Multi-Stage = No No No No
Exfil.(in/hr)= 0.000 (by Contour)
TW Elev. (ft)= 0.00
Note: Culvert/Orifice outflows are analyzed under inlet (ic) and outlet (oc) control. Weir risers checked for orifice conditions (ic) and submergence (s).
Stage / Storage / Discharge Table
Stage Storage Elevation Clv A Clv B Clv C PrfRsr Wr A Wr B Wr C Wr D Exfil User Total
ft cuft ft cfs cfs cfs cfs cfs cfs cfs cfs cfs cfs cfs
0.00 0 4778.00 --- --- --- --- 0.00 --- --- --- --- --- 0.000
0.25 323 4778.25 --- --- --- --- 0.00 --- --- --- --- --- 0.000
0.50 697 4778.50 --- --- --- --- 0.00 --- --- --- --- --- 0.000
0.75 1,124 4778.75 --- --- --- --- 0.00 --- --- --- --- --- 0.000
1.00 1,607 4779.00 --- --- --- --- 0.00 --- --- --- --- --- 0.000
1.25 2,148 4779.25 --- --- --- --- 0.00 --- --- --- --- --- 0.000
1.50 2,748 4779.50 --- --- --- --- 0.00 --- --- --- --- --- 0.000
1.75 3,410 4779.75 --- --- --- --- 0.83 --- --- --- --- --- 0.832
2.00 4,135 4780.00 --- --- --- --- 2.35 --- --- --- --- --- 2.355
2.25 4,925 4780.25 --- --- --- --- 4.33 --- --- --- --- --- 4.326
2.50 5,783 4780.50 --- --- --- --- 6.66 --- --- --- --- --- 6.660
11 Annie Subdivision Phase 4
Stormwater Drainage Plan
APPENDIX B – CONVEYANCE
CALCULATIONS & ANALYSIS
Area (Acres)C
Impervious 1.41 0.90
Pervious 1.83 0.20
Total 3.24 0.50
0.50
8.24
0.50
2.78
3.24
4.54
Intensity for 25 Year Storm ( I= 0.78Tc^-.64 (in/hr)) =
Area (acres) =
Q = Required Gutter Capacity (cfs) =
Drainage Basin 2A - Required Gutter Capacity
Weighted C Factor Calculation
Weighted C Factor =
Calculate Flow from the Rational Formula ( Q=CIA)
Time of Concentration (tc In minutes) for DB-1 =
Weighted C Factor (from above) =
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) = 0.005
Q (Actual Gutter Capacity) = 2.57
Actual Gutter Capacity - Drainage Basin 2A
Gutter Capacity - 0.15' below Top of Curb
Culvert Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc.Tuesday, Jul 20 2021
Annie - DB2 Transport Pipe
Invert Elev Dn (ft) = 4778.16
Pipe Length (ft) = 152.63
Slope (%) = 0.41
Invert Elev Up (ft) = 4778.78
Rise (in) = 18.0
Shape = Circular
Span (in) = 18.0
No. Barrels = 1
n-Value = 0.012
Culvert Type = Circular Corrugate Metal Pipe
Culvert Entrance = Headwall
Coeff. K,M,c,Y,k = 0.0078, 2, 0.0379, 0.69, 0.5
Embankment
Top Elevation (ft) = 4783.00
Top Width (ft) = 100.00
Crest Width (ft) = 100.00
Calculations
Qmin (cfs) = 3.00
Qmax (cfs) = 10.00
Tailwater Elev (ft) = 4779.5
Highlighted
Qtotal (cfs) = 5.00
Qpipe (cfs) = 5.00
Qovertop (cfs) = 0.00
Veloc Dn (ft/s) = 3.00
Veloc Up (ft/s) = 4.77
HGL Dn (ft) = 4779.50
HGL Up (ft) = 4779.64
Hw Elev (ft) = 4780.05
Hw/D (ft) = 0.85
Flow Regime = Inlet Control
12 Annie Subdivision Phase 4
Stormwater Drainage Plan
APPENDIX C – OPERATION &
MAINTENANCE PLAN
Stormwater Operation and Maintenance Plan
Annie Subdivision Phase 4
Bozeman, MT
109 E. Main St. Suite B, Bozeman, MT 59715 I OFFICE 406.728.4611 I EMAIL wgm@wgmgroup.com
ANNIE SUBDIVISION PH. 4 STORMWATER
Operation and Maintenance Manual
STORMWATER OPERATION &
MAINTENANCE PLAN
LOCATION
The project site is Lot 1 of the Annie Subdivision Phase 3C which is located at 2305 Durston Road,
approximately 0.25 miles west of the Durston and North 19th Avenue intersection. The site is located in
Section 2, Township 2S, Range 5E. The development will be comprised of townhome lots, a single
family lot, and multi-family lots, and an open space lot.
PURPOSE
Stormwater runoff is a significant source of water pollution in urbanizing 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 are: a detention pond located in the open space on the east
side of the site, pipes and inlets utilized to divert runoff from the curb to the detention pond, and
cobble infiltration galleries on the west side 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.
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
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.
ANNIE SUBDIVISION PH. 4 STORMWATER
Operation and Maintenance Manual
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.
· 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.
· The stormwater pond should be inspected every three (3) months and following large storm
events. Accumulated trash should be removed. Sediment removal should be performed
frequently with hand tools to reduce or eliminate the need for sediment removal via larger
equipment.
· 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
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.
ANNIE SUBDIVISION PH. 4 STORMWATER
Operation and Maintenance Manual
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.
13 Annie Subdivision Phase 4
Stormwater Drainage Plan
APPENDIX D – STORM PLANS &
DETAILS
NPRELIMINARYPLOTTED:SAVED:7/20/217/20/212308 DURSTON ROADPARCEL B COS 1817603 EMILY DR.LOT 1 - 3, PT OF TRACT 1APLAT J-135WILLOWBROOK CONDOTRACT 1 COS 1855WILLOWBROOK CONDOTRACT 1 COS 1855WEST PARK MANOR SUB.PLAT J-184LOT 4 ANNIE SUB. PH 3CLOTS 2 & 3ANNIE SUB. PH 3CLOT 17 ANNIE SUB PH 3ALOTS 1 - 6 ANNIESUB. PH 3ACOMMON OPENSPACE, ANNIESUB PH. 3APOST DEVELOPMENTDRAINAGE BASIN 2A3.59 ACRESWGMGROUPWWW.WGMGROUP.COMPRE & POST DEVELOPMENT DRAINAGE BASINS
ANNIE SUBDIVISION, PHASE 4
BOZEMAN, MONTANAJULY 2021STM1PRE-DEVELOPMENTDRAINAGE BASIN 10.978 ACRESPRE-DEVELOPMENTDRAINAGE BASIN 22.261 ACRESPRE-DEVELOPMENTDRAINAGE BASIN 10.978 ACRESPRE-DEVELOPMENTDRAINAGE BASIN 22.261 ACRESPOST DEVELOPMENTDRAINAGE BASIN 2A3.59 ACRESTIME OF CONCENTRATION FOR DB-2TERMINATES AT STORM INLET TOREMAIN CONSERVATIVEPRE-DEVELOPMENT DB-2 TIMEOF CONCENTRATIONFLOWPATHPRE-DEVELOPMENT DB-1 TIMEOF CONCENTRATIONFLOWPATHPRE-DEVELOPMENT DRAINAGEBASIN DELINEATION BOUNDARYPOST DEVELOPMENT DB-2 TIMEOF CONCENTRATIONFLOWPATH
LEGEND-PROPOSEDNPRELIMINARYPLOTTED:SAVED:7/20/217/20/21WGMGROUPWWW.WGMGROUP.COMSTORM DRAINAGE INFRASTRUCTURE PLAN
ANNIE SUBDIVISION, PHASE 4
BOZEMAN, MONTANAJULY 2021STM2
NPRELIMINARYPLOTTED:SAVED:7/20/217/20/21WGMGROUPWWW.WGMGROUP.COMSTORM CONVEYANCE PLAN & PROFILE
ANNIE SUBDIVISION, PHASE 4
BOZEMAN, MONTANAJULY 2021STM3
PRELIMINARYPLOTTED:SAVED:7/20/217/20/21WGMGROUPWWW.WGMGROUP.COMSTORM PLAN DETAILS
ANNIE SUBDIVISION, PHASE 4
BOZEMAN, MONTANAJULY 2021STM4
PRELIMINARYPLOTTED:SAVED:7/20/217/20/21WGMGROUPWWW.WGMGROUP.COMSTORM PLAN DETAILS
ANNIE SUBDIVISION, PHASE 4
BOZEMAN, MONTANAJULY 2021STM5