HomeMy WebLinkAbout009 Stormwater Management Design Report
STORMWATER MANAGEMENT
DESIGN REPORT
FOR:
Salvation Army
North 19th Avenue
Bozeman, MT
Prepared By:
WWC/Madison Engineering
895 Technology Drive, Suite 203
Bozeman, MT 59718
(406) 586-0262
May 2024
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895 Technology Boulevard, Suite 203, Bozeman, MT 59718 | 406.586.0262
Salvation Army
North 19th Avenue
Stormwater Design Report
Introduction
This design report will give an overview of the proposed stormwater system for the proposed site
of Salvation Army located off North 19th Avenue adjacent to North 22nd avenue. The Salvation
Army proposed building spans three individual lots that are currently in the process of being
aggregated to support the future development. The property is located at Homesites Subdivision
S01, T02 S, R05 E, Lot 1 of Plat E-11 and Maple Terrace Subdivision S01, T02 S, R05 E, Block
A, Lot 15 & 16 of Plat F-24.
The proposed Salvation Army building is a two-story commercial building development and will
be constructed in a single phase on R-O (Residential Office District) zoned property. The single
building will be constructed with associated on-site drive aisles, driveways, pedestrian
walkways, parking, and landscaping. Open space is not required for the site.
The stormwater system will include utilizing proposed curb chases, curb inlets, storm piping and
on-site underground detention storage. Water will be conveyed by a combination of sheet flow,
curb and gutter and piping that will direct stormwater to an on-site underground ADS stormtech
SC-740 detention/infiltration basin. The proposed basin will be excavated to native gravels to
permit maximum infiltration.
The following references were used in the preparation of this report:
a. City of Bozeman Modifications to Montana Public Works Standard Specifications. Sixth
edition, addendum No 3.
b. City of Bozeman Design Standards and Specifications Policy, March 2004. Addendum
#7
c. Circular DEQ 8, Montana Standards for Subdivision Storm Drainage, 2002 Edition
The proposed drainage and grading plan and ADS Stormtech basin detail are included in the
Appendix of this report.
Runoff and Basin Calculation
A 25-year, 2-hour design rainfall frequency was used for calculating the storm sewer inlet and
piping facilities. A 10-year, 2-hour storm intensity was used for calculating the basin sizing
utilizing the rational method in accordance with the COB design standards. A runoff coefficient
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C of 0.90 was used for impervious areas and a C value of 0.20 was used for landscaped areas.
The tables below illustrate the calculated runoff coefficients and areas for each as well as a
summary of the stormwater basin calculations.
Table 1: Stormwater Basin Run-Off Calculations
Basin Weighted C Total Area
(sf)
Impervious
Area (sf)
Landscaped
Area (sf)
A 0.75 40,729 31,981 8,748
Table 2: Stormwater Basin Calculations
Basin Required Storage (cf) Provided Storage (cf)
A 2,069 2,218
Native Soils and Groundwater Data
Soils on the subject property are typical for the area and are detailed in the Appendix D –
Geotechnical Report prepared by TD&H Engineering. Native poorly-graded gravels with clay
and sand – very dense, grayish brown & slightly moist were found at a depth of 5.3-6.7’,
overlain with Sandy Lean Clay – firm, brown and moist. The nearest test pit, B-1, is very close in
relation to the proposed underground ADS Stormtech. ADS Stormtech SC-740 infiltration
system was choose and consists of excavating down to native gravels to permit maximum
infiltration of the stormwater. Backfill above and below the ADS chambers consists of clean,
crushed angular stone and well-graded aggregate mixtures.
Groundwater depths are currently being monitored. On May 15th, 2024 groundwater depth on the
property to the North was measured at approximately 10’. This is roughly 8’ below existing
ground. No groundwater was observed in the monitoring well. The monitoring well on the
property to the North is within approximately one hundred feet of our proposed system. The
monitoring well will continue to be measured through June.
Conveyance Capacity
The proposed private stormwater collection and detention/infiltration system was designed to
convey the 25-year storm event per the City of Bozeman standards. The conveyance structures
include a curb inlet and piping that directs stormwater run-off to the on-site infiltration basins. A
twelve-inch storm sewer pipe is proposed to collect and convey the stormwater run-off to a
detention/infiltration basin. Stormwater piping calculations are included in Appendix C and Curb
and Curb Inlet calculations are included in Appendix E of this report.
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Stormwater Detention/Infiltration Basin
All of the stormwater runoff from the site is proposed to be conveyed to on-site
detention/infiltration basins and ultimately infiltrate into native gravels. Subsurface ADS
infiltration chambers are proposed for this project. Stormwater calculations are included in
Appendices B of this report.
Table 3 below shows the finished grade elevations of each ADS infiltrator basin and the depth to
the bottom of gravel for each basin.
Table 3: Groundwater Depth vs. Proposed FG Elevations
Basin A
Existing Ground Elevation: 4775.0
Elevation of Groundwater: 4767.0 ±
Proposed Finish Surface Grade: 4774.5
Bottom of ADS Basin Gravel Elevation: 4769.0
Bottom of Basin Gravel Above Groundwater 4770.5
As shown in Table 3 there is adequate separation between the bottom of the basin gravels and the
estimated seasonally high groundwater.
The detention storage volumes were sized based on the 10-year 2-hour design rainfall frequency
per the City Design Standards. The rational method was used to determine post-development
stormwater flows. The calculations for all stormwater storage facilities are included in the
Appendix of this report. The proposed stormwater facilities reduce the post-development runoff
rate to zero. All impervious surface runoff is conveyed to the infiltration system and then
infiltrates into the ground.
Stormwater Facility Maintenance
The proposed storm drainage facilities will be privately operated and maintained by the property
owner of the on-site development. Included in Appendix G is a proposed maintenance program
for the stormwater collection and infiltration systems as well as the O&M Manual for the ADS
StormTech system.
Appendices
A. Grading/Drainage Plan
B. ADS Stormtech Specifications
C. Storm Piping Calculations
D. Geotechnical Report – TD&H.
E. Gutter Flow and Curb Inlet Calculations
F. ADS Stormtech SC-740 Chamber Design
G. Stormwater Maintenance Plan
APPENDICES
Appendix A – Grading/Drainage Plan
DNDNDNDNDNDNUPUPUP1:12
1:121:12SALVATION
ARMY
SHEETWWC ENGINEERING/MADISON895 TECHNOLOGY BLVD, SUITE 4BOZEMAN, MT 59718(406) 586-02621" =
0
SCALE
20'
40'10'20'
SALVATION ARMY
GRADING PLAN
C1.2SALVATION ARMYGRADING & DRAINAGE PLANBOZEMAN MTNORTH 19TH AVENUEPRIVATE DRIVE
SALVATION ARMY GRADING & DRAINAGE PLAN
SALVATION ARMY ADS SC-740 STORMTECH BASIN
Appendix B – ADS Stormtech
Specifications
Salvation Army
Stormwater Detention/Infiltration Calculations
Basin A
Calculation of Required Volume for Storm Water Retention & Infitration System
Design Rainfall Freq.10 10 Year - 2 Hour event
IDF coefficient a 0.64
IDF coefficient b
IDF coefficient n 0.65
Post-development Calculations:
Total Basin Area:40,729 sf
C
Areas (ft2):Roof Area 11,537 0.90
Impervious 20,444 0.90
Landscaped 8,748 0.20
Total:40,729 0.75
Retention Basin Calculation:
Q = CIA
C =0.75 (post-development)
I = 0.41 in/hr (10-yr, 2-hr storm)
A = 0.94 acres
Qpost =0.29 cfs
Required retention storage (ft3) =2,069 ft3 (10-yr, 2-hr storm)
Provided retention storage (ft3) =2,218 ft3
North 19th Avenue
ADS Basin - A.xls
Salvation Army
25 year Storm Event
Stormwater Run-off Calculation
Design Rainfall Freq.25 year
IDF coefficient a 0.78
IDF coefficient b
IDF coefficient n 0.64
Basin A
Pre-Development Calculations
C
Areas (ft2):Landscape 40,729 0.20
Impervious 0.90
Total:40,729
total area:0.935 acres
composite C:0.20
Overland tc
average slope:3.1 percent
travel distance:330 feet
tc:21 minutes
Total tc:21 minutes
intensity at tc (fig 23):1.53 in/hr
Peak runoff:0.29 cfs
Predevelopment Calcs.xls
Salvation Army
25 year Storm Event
Stormwater Run-off Calculation
Design Rainfall Freq.25 year
IDF coefficient a 0.78
IDF coefficient b
IDF coefficient n 0.64
Basin A
Post-Development Calculations
C
Areas (ft2):Landscape 8,748 0.20
Impervious 31,981 0.90
Total:40,729
total area:0.935 acres
composite C:0.75
Overland tc
average slope:3.1 percent
travel distance:330 feet
tc:8 minutes
Total tc:8 minutes
intensity at tc (fig 23):2.80 in/hr
Peak runoff:1.96 cfs
Storm Runoff Calculation - 25 Year Event.xls
Appendix C – Storm Piping Calculations
12" PVC
Checked at slope =0.46%
Appendix C
CIRCULAR CHANNEL
Manning's Eqn.1.486 A R2/3 S1/2
n
Diameter,do (in) =12 Enter Value
Diameter,do (ft) =1
Units =1.486
n =0.013
Slope, S (ft/ft)0.02
Depth, y (ft)Theta (rad)
Area, A
(ft2)
Wetted
Perimeter, P
(ft)
Hydraulic
Radius, R
(ft)
Top Width, T
(ft)
Hydraulic
Depth, D
(ft)
Section
Factor, Z
(ft5/2)Q (cfs)Q (gpm)
Q (gpd - 8
hour day)V (ft/s)
Energy, E
= V2/2g
(ft)
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.0 0.0 0.0 0.0 0.00
0.05 0.90 0.01 0.45 0.03 0.44 0.03 0.00 0.0 10.9 5212.3 1.6 0.04
0.10 1.29 0.04 0.64 0.06 0.60 0.07 0.01 0.1 47.2 22661.4 2.6 0.10
0.15 1.59 0.07 0.80 0.09 0.71 0.10 0.02 0.2 109.9 52762.0 3.3 0.17
0.20 1.85 0.11 0.93 0.12 0.80 0.14 0.04 0.4 198.0 95052.4 3.9 0.24
0.25 2.09 0.15 1.05 0.15 0.87 0.18 0.06 0.7 309.8 148683.9 4.5 0.31
0.30 2.32 0.20 1.16 0.17 0.92 0.22 0.09 1.0 442.8 212560.7 5.0 0.38
0.35 2.53 0.24 1.27 0.19 0.95 0.26 0.12 1.3 594.6 285402.3 5.4 0.45
0.40 2.74 0.29 1.37 0.21 0.98 0.30 0.16 1.7 762.0 365776.0 5.8 0.52
0.45 2.94 0.34 1.47 0.23 0.99 0.34 0.20 2.1 941.9 452113.0 6.1 0.58
0.50 3.14 0.39 1.57 0.25 1.00 0.39 0.25 2.5 1130.7 542714.1 6.4 0.64
0.55 3.34 0.44 1.67 0.26 0.99 0.44 0.30 3.0 1324.5 635747.1 6.7 0.69
0.60 3.54 0.49 1.77 0.28 0.98 0.50 0.35 3.4 1519.2 729234.3 6.9 0.73
0.65 3.75 0.54 1.88 0.29 0.95 0.57 0.41 3.8 1710.5 821027.0 7.1 0.77
0.70 3.96 0.59 1.98 0.30 0.92 0.64 0.47 4.2 1893.3 908761.4 7.2 0.80
0.75 4.19 0.63 2.09 0.30 0.87 0.73 0.54 4.6 2062.0 989777.7 7.3 0.82
0.80 4.43 0.67 2.21 0.30 0.80 0.84 0.62 4.9 2210.4 1060970.1 7.3 0.83
0.85 4.69 0.71 2.35 0.30 0.71 1.00 0.71 5.2 2330.1 1118469.1 7.3 0.83
0.90 5.00 0.74 2.50 0.30 0.60 1.24 0.83 5.4 2410.1 1156846.5 7.2 0.81
0.95 5.38 0.77 2.69 0.29 0.44 1.77 1.02 5.4 2429.8 1166308.4 7.0 0.77
1.00 6.28 0.79 3.14 0.25 0.00 5.0 2262.1 1085795.2 6.4 0.64
Q =
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0.00 0.20 0.40 0.60 0.80 1.00 1.20
Depth (ft)
Q (CFS)
V (ft/s)
E (ft)ydoT
THETA
Appendix D – See Document 019:
Geotech Report by TD&H
Appendix E – Gutter Flow and Curb Inlet
Calcs
Appendix E
Gutter Section
Given:T =10.0 feet
W =1.50 feet
Ts =8.50 feet
Sw =0.08 ft/ft
Sx =0.02 ft/ft
a =1.14 inches
d =2.40 inches
n =0.015
So =0.013 Pryor Curb Inlet
So =0.033 Meriwether Curb Inlet
So =0.005 Floyd Curb Inlet
Gutter Capacity Summary
Pryor Q = 3.43 cfs
Meriwether Q = 5.40 cfs
Floyd Q = 2.11 cfs
Capacity for Inlets
Inlet #1
Based on Neenah inlet R-3067-L
Weir Equation
Where:P =perimeter (ft)
h =Head (ft)
P:5.90 feet
h:0.35 feet
Qweir =4.03 cfs
Qweir > Q25
Salvation Army
Inlet Capacity Calculations
The maximum flow (Q) anticipated at any inlet in this project during a 25-yr storm event is 1.96
cfs (see peak flow calculations in appendix B). A single curb inlet will accommodate the gutter
flow in all cases.
1.53.3P(h)Q =
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Appendix F – ADS Stormtech SC-740
Chamber Design
Appendix G – Stormwater Maintenance
Plan
K:\Bozeman\Springer Group Architects PC\2024089 Salvation Army\08Reports\Storm\004 Stormwater Maintenance Plan.doc
STORMWATER MAINTENANCE PLAN
Salvation Army
Owner’s responsibility for routine inspection and maintenance
1.Keep the inlets of the facilities free of leaves, rocks, and other debris.
2.See that litter and other debris are removed from inlets, swales, and vegetated and
paved areas.
3.Maintenance of the underground systems are as follows:
•Per StormTech Maintenance Recommendations
•Minimum required maintenance includes a quarterly inspection during the
first year of operation and a yearly inspection thereafter. Utilize inspection
ports for inspections.
•The inspection ports can be used to pump water into the system and re-
suspend accumulated sediment so that is may be pumped out. Flush and
pump as inspections deem necessary.
•Utilize the adjacent upstream storm sewer manhole for removal (vacuuming)
of debris from the end of the storm drain at the connection point with the
underground chamber.
4.Owner to maintain and fund Operation and Maintenance of stormwater facilities.
_______________________________
Owner
D. Kerry NickersonS2K/Miller Babcock, LLC