HomeMy WebLinkAboutStormwater Management 05-19-2020
DESIGN REPORT
STORMWATER MANAGEMENT
DELTA GAMMA SITE PLAN
Prepared for:
J&D Customs, Inc.
6550 Falcon Lane, Suite 4
Bozeman, MT 59718
Prepared by:
C&H Engineering and Surveying, Inc.
1091 Stoneridge Drive, Bozeman, MT 59718
(406) 587-1115
Project Number: 200029
May 2020
INTRODUCTION
The proposed Delta Gamma site plan is a parking lot development located on a 0.64-acre parcel in
Section 13, Township 2 South, Range 5 East of P.M.M., Gallatin County, City of Bozeman. A
combination of site grading, curb and gutter, curb cuts, swales, and retention ponds will be used
to manage stormwater runoff on the site. Supporting stormwater calculations are attached to this
report. A Drainage Area Map is included in Appendix A. Calculations for each individual drainage
area (total area, weighted C factor, and time of concentration) are included in Appendix B.
RETENTION POND DESIGN
All ponds have been sized according to City of Bozeman Design Standards. Retention facilities
are sized to capture the entire volume of the 10-year 2-hour storm event. The retention ponds are
designed with an effective water depth of 1.5 feet, and maximum side slope of 4:1. Calculations
used for sizing each pond can be found in Appendix C.
STORM SEWER FACILITIES DESIGN
Storm sewer facilities (curb cut and swale) were sized for the 25-yr storm using Manning’s
Equation. The calculation assumed the cut and swale is at minimum slope (1%). The calculated
peak flow contributing to the cut is 0.46 cfs. The 2’ wide curb cut has a minimum capacity of 5.50
cfs and can therefore adequately handle the runoff from this site. Curb cut calculations are included
in Appendix D.
Proposed Retention Pond
The proposed retention pond is located just east of the proposed parking lot. It receives runoff from
Drainage Area 1, totaling 0.39 acres. Runoff from Drainage Area 1 is conveyed via surface flow
and gutters to the proposed curb cut. All curb cuts drain to swales which drain to the proposed
pond. The required pond volume was calculated to be 572 cubic feet. The proposed volume of the
pond is 705 cubic feet and has an effective depth of 1.5’.
APPENDIX A
DRAINAGE AREA MAP
APPENDIX B
DRAINAGE AREA CALCULATIONS
DRAINAGE AREA #1
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
Hardscape 0.95 6830 6488
Landscape 0.20 9986 1997
Total 16816 8486
A = Area (acres) 0.3861
C = Weighted C Factor 0.50
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 2.00% Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.35 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 95 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)10.3
Tc Gutter Flow
Tc = L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient 0.013
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 1.20%
L = length of gutter (ft) 50
V = mean velocity (ft/s) 3.28
Tc Gutter Flow (minutes) =0.3
Tc Total = 10.6 (5 minute minimum)
APPENDIX C
RETENTION FACILTIES CALCULATIONS
RETENTION POND # 1
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Hardscape 0.95 6830 6488
Landscape 0.20 9986 1997
Total 16816 8486
C=Weighted C Factor 0.50
2. Calculate Additional Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.50
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.39
Q = runoff (cfs) 0.08
V = REQUIRED VOL (ft3)572
APPENDIX D
STORM SEWER FACILITIES
CALCULATIONS
CURB CUT/SWALE
REQUIRED CAPACITY
1. Calculate Area and Weighted C Factor (Post-Development)
Contributing Area C Area (ft 2 )C * Area
Hardscape 0.95 6830 6488
Landscape 0.20 9986 1997
Total 16816 8486
A = Area (acres) 0.3861
C = Weighted C Factor 0.50
2. Calculate Rainfall Intensity (Duration = Max Tc from Contributing Drainage Areas)
i = 0.78x-0.64 (10-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs) 0.18 (DA #1)
i = rainfall intensity (in./hr.) 2.37
3. Calculate 25-yr Pond Outflow Rate
Q = CiA
C = Rational Method Runoff Coefficient 0.50 (calculated above)
i = rainfall intensity (in./hr.) 2.37 (calculated above)
A = Area (acres) 0.39 (calculated above)
Q = 25-yr Flow Rate (cfs) 0.46
MANNING'S EQUATION for OPEN CHANNEL FLOW
Project: Delta Gamma Sorority Location: Bozeman, MT
By: LRS Date: 5/5/2020
Chk By: Date:
INPUT
z (sideslope)= 0
Mannings Formula z (sideslope)= 0
w (btm width, ft)= 2
Q = (1.486/n)ARh2/3S1/2 d (depth, ft)= 0.5
R = A/P S (slope, ft/ft) 0.01
A = cross sectional area n low =0.013
P= wetted perimeter n high =0.013
S = slope of channel V = (1.49/n)Rh2/3S1/2
n = Manning's roughness coeffiQ = V x A
Depth, ft Area, sf
Wetted
Perimete
r, ft
Hydraulic
Radius, ft Velocity, fps Flow, cfs
Velocity,
fps Flow, cfs
0.5 1.00 3.00 0.33 5.50 5.50 5.50 5.50 T = 2.00
Dm = 0.500
Sc low = 0.0053 Sc high = 0.0053
sc =critical slope ft / ft
T = top width of the stream .7 Sc 1.3 Sc .7 Sc 1.3 Sc
dm =a/T = mean depth of flow 0.0037 0.0069 0.0037 0.0069
Low N High N
d
w
z
11
z
T
Clear Data
Entry Cells