HomeMy WebLinkAboutStormwater Design Report (Revised 2020-01-22)INTRODUCTION
The proposed development on Lots 5 & 6 of Gallatin Center Subdivision P.U.D. Phase 1 consists
of a new carwash business with a drive-thru carwash,parking area,office space, car wash detail
bays, and associated site landscaping and stormwater management features.The property is
located within the Bozeman city limits and is currently zoned B-2 commercial.A combination of
site grading, curb and gutter,drainage swales,and retention and detention ponds will be used to
manage stormwater runoff on the site.
EXISTING SITE &STORMWATER
The existing vacant lots have no on-site stormwater management facilities so any runoff generated
from Lots 5 and 6 currently sheet flows to East Catron Creek along the northwest side of the
property and west onto the adjacent Lot 7.The undeveloped site currently has a weighted C value
of 0.20 per the Bozeman Design Standards and Specifications Policy (DSSP).The NW side of the
property contains an 80’ wide stream corridor along Catron Street while the NE side of the
property has a 50’ wide green way corridor. Since neither of these corridors will be developed,
only the portion of the site that is proposed for development was included in this drainage design.
PROPOSED STORMWATER DESIGN
The site was divided into 4 drainage areas as shown on the Proposed Drainage Area Map included
in Appendix B. Each drainage area was assessed using the 10-year 2-hour storm event.Drainage
Area 1 is located in the SW corner of the site and conveys runoff from the southern portion of the
driveway into Retention Pond 1.The runoff coefficient for Drainage Area 1 is 0.61 over an area of
11,924 sf,which generates 493 cf of runoff during the 10-year 2-hour storm event.Retention Pond
1 has a proposed volume of 690 cf,a design depth of 1.0’,and 4:1 (H:V) side slopes.
Drainage Area 2 encompasses the interior of the site and will capture runoff in Detention Pond 2
located in the landscape island in the center of the site. Drainage Area 2 has a runoff coefficient of
0.79 over an area of 36,056 sf. Detention Pond 2 will release runoff through an outlet structure at
the pre-development flow rate into East Catron Creek. The pre-development time of concentration
for Drainage Area 2 was found to be 19 minutes. The pre-development runoff rate for Drainage
Area 2 was found to be 0.22 cfs (see Appendix A). To maintain this pre-development runoff rate
for Drainage Area 2, the outlet structure will have an internal weir with a slot width of 0.4”. The
pond volume needed to detain the design storm and maintain pre-development runoff rates is 777
cf.Detention Pond 2 has a volume of 810 cf, a design depth of 1.5’, and 4:1 (H:V) side slopes.
Supporting calculations for all drainage areas are included in Appendix A.
Drainage Area 3 is located along the perimeter of the development and has a runoff coefficient of
0.71 over an area of 30,395 sf.Runoff in Drainage Area 3 will be captured in Detention Pond 3 at
the north end of the site, which will release runoff through an outlet structure at the
pre-development flow rate into East Catron Creek.The pre-development time of concentration for
Drainage Area 3 was found to be 15 minutes. The pre-development runoff rate for Drainage Area 3
was found to be 0.22 cfs (see Appendix A). To maintain this pre-development runoff rate for
Drainage Area 3 the outlet structure will have a weir with a slot width of 0.4”.The pond volume
needed to detain the design storm and maintain pre-development runoff rates is 504 cf.Detention
Pond 3 has a volume of 675 cf,a design depth of 1.5’, and 4:1 (H:V) side slopes.Supporting
calculations for all drainage areas are included in Appendix A.
Drainage Area 4 includes the 18’ wide landscaped area along the west side of the site as well as the
western half of the car wash roof. Drainage Area 4 has a runoff coefficient of 0.41 over an area of
9,505 sf and generates 267 cf of runoff during the 10-year 2-hour storm event.Since over 70% of
this drainage area is comprised of landscaping, it is anticipated that runoff from the western
portion of the car wash roof will flow into this 18’ wide landscape strip and infiltrate.
Furthermore, a vegetated storm swale is proposed along the western side of the car wash building
in order to capture sheet flow runoff from the roof.This storm swale will also prevent stormwater
from migrating westward toward the property boundary.
GROUNDWATER AND INFILTRATION CONSIDERATIONS
From previous geotechnical investigations in the vicinity of the site, native gravels are expected to
be approximately 3’-4’ below existing grade,with a silt/clay layer above the gravel. Groundwater
is typically encountered at the top of native gravel elevation, but during periods of high
groundwater it could rise as much as a foot above this gravel elevation.The bottoms of Pond 1
and Pond 3 are less than 2’below existing grade and the bottom of Pond 2 is approximately 2.5’
below existing grade,so groundwater inundation of these ponds is not anticipated.
Since a poorly draining silt/clay layer likely exists above the native gravel elevation, the proposed
design for Retention Pond 1 includes over-excavating the pond footprint down to the native gravel
depth.This excavation will be backfilled with a well-draining pit run gravel to provide a conduit
for the stormwater to infiltrate through.Above the gravel drain,the pond will be finish graded with
6” of topsoil and seeded.
APPENDIX A
STORMWATER CALCULATIONS
DRAINAGE AREA # 1 (POND 1)
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Hardscape 0.95 6684 6350
Landscape 0.2 5240 1048
Total 11924 7398
A = Area (acres)0.27
C = Weighted C Factor 0.62
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.62
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.27
Q = runoff (cfs) 0.07
V = REQUIRED STORAGE VOL. (ft3)501
PROVIDED VOLUME (ft3)690
DA 1 SIDEWALK CHASE 25-YR OUTFLOW RATE
REQUIRED CAPACITY
REQUIRED GUTTER CAPACITY
1. Calculate Area and Weighted C Factor
A = Area (acres)0.03
C = Weighted C Factor 0.95
2. Calculate Rainfall Intensity (Duration = Max Tc from Contributing Drainage Areas)
i = 0.78x-0.64 (25-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs)0.04
i = rainfall intensity (in./hr.)5.91
3. Calculate 25-yr Pond Outflow Rate
Q = CiA
C = Rational Method Runoff Coefficient 0.95 (calculated above)i = rainfall intensity (in./hr.)5.91 (calculated above)
A = Area (acres)0.03 (calculated above)
Q 25-yr Chase Flow Rate (cfs)=0.17
Stormwater Chase DA 1 Sizing
Determine slope of stormwater chase
slope (ft/ft)0.015
Calculate chase capacity
chase depth (feet)0.50Chase width (feet)1.00
Mannings #, n (for concrete)0.013
X-sect. area (ft2)0.500
Perimeter (ft)2.000Hydr. Radius, R (ft)0.2500
slope (ft/ft)0.015
Qfull (cfs)2.78
DRAINAGE AREA # 2
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
Hardscape 0.95 20520 19494
Landscape 0.2 7884 1577
Roof 0.95 7654 7272
Total 36058 28343
A = Area (acres)0.83
C = Weighted C Factor 0.79
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%)1.19 Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.95 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft)114 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)1.88
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 (ft/ft)0.0088
L = length of gutter (ft)110
V = mean velocity (ft/s)2.82
Tc Gutter Flow (minutes) =0.65
Tc Total =2.53
DRAINAGE AREA # 3
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
Hardscape 0.95 16538 15711
Landscape 0.2 9888 1978
Roof 0.95 3967 3769
Total 30393 21458
A = Area (acres)0.70
C = Weighted C Factor 0.71
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%)1.68 Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.95 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft)103 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)1.60
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 (ft/ft)0.014
L = length of gutter (ft)359
V = mean velocity (ft/s)3.54
Tc Gutter Flow (minutes) =1.69
Tc Total =3.29
DRAINAGE AREA # 4
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
Landscape 0.2 6793 1359
Roof 0.95 2712 2576
Total 9505 3935
A = Area (acres)0.22
C = Weighted C Factor 0.41
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.41
I = intensity (in/hr)0.41 (10 yr, 2hr storm)
A = Area (acres)0.22
Q = runoff (cfs)0.04
V = REQUIRED STORAGE VOL. (ft3)267
POND 2 - DETENTION
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor (Post-Development)
Contributing Area C Area (ft2 )C * Area
Hardscape 0.95 20520 19494
Landscape 0.20 7884 1577
Roof 0.95 7654 7272
Total 36058 28343
A = Area (acres)0.83 Storm
C = Weighted C Factor 0.79 Return (yrs)Cf
2 to 10 1
2. Calculate T c (Pre-Development)11 to 25 1.1
Tc Overland Flow 26 to 50 1.2
Tc = 1.87 (1.1-CCf)D1/2/S1/3 51 to 100 1.25
S = Slope of Basin (%)2.713
C = Rational Method Runoff Coefficient 0.2
Cf = Frequency Adjustment Factor 1.1
D = Length of Basin (ft)265
Tc (Pre-Development) (minutes)19
3. Calculate Rainfall Intensity (Duration = Pre-Development Tc)
i = 0.64x-0.65 (10-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs)0.32 (Tc Pre-Development)
i = rainfall intensity (in./hr.)1.34
4. Calculate Runoff Rate (Pre-Development)
Q = CiA
C = Rational Method Runoff Coefficient 0.2 (open land)
i = rainfall intensity (in./hr.)1.34 (calculated above)
A = Area (acres)0.83 (calculated above)
Q = Runoff Rate (Pre-Development) (cfs)0.22
5. Calculate Required Pond Volume
Total Area (acres) =0.83 acres
Weighted C =0.79
Discharge Rate (cfs) =0.22 cfs (Equal to Pre-Development Runoff Rate)
Duration(min)Duration(h
rs)
Intensity
(in/hr)Qin (cfs)Runoff
Volume
Release
Volume
Required
Storage
(ft3)
25 0.42 1.13 0.74 1103 333 770
26 0.43 1.10 0.72 1119 347 772
27 0.45 1.08 0.70 1134 360 774
28 0.47 1.05 0.68 1148 373 775
29 0.48 1.03 0.67 1162 387 776
30 0.50 1.00 0.65 1176 400 776
31 0.52 0.98 0.64 1190 413 777
32 0.53 0.96 0.63 1203 427 776
33 0.55 0.94 0.61 1216 440 776
34 0.57 0.93 0.60 1229 453 776
PROVIDED VOLUME (ft3)810
OUTLET STRUCTURE SLOT
Q=CLH3/2
Q = Discharge (cfs)0.22
C = Weir Coefficient 3.33 (per COB Design Standards)
H = Head (ft)1.5
L = Horizontal Length (ft)0.04
L = Slot Width (inches)0.4
POND 3 - DETENTION
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor (Post-Development)
Contributing Area C Area (ft2 )C * Area
Hardscape 0.95 16538 15711
Landscape 0.20 9888 1978
Roof 0.95 3967 3769
Total 30393 21458
A = Area (acres)0.70 Storm
C = Weighted C Factor 0.71 Return (yrs)Cf
2 to 10 1
2. Calculate T c (Pre-Development)11 to 25 1.1
Tc Overland Flow 26 to 50 1.2
Tc = 1.87 (1.1-CCf)D1/2/S1/3 51 to 100 1.25
S = Slope of Basin (%)4.02
C = Rational Method Runoff Coefficient 0.2
Cf = Frequency Adjustment Factor 1.1
D = Length of Basin (ft)199
Tc (Pre-Development) (minutes)15
3. Calculate Rainfall Intensity (Duration = Pre-Development Tc)
i = 0.64x-0.65 (10-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs)0.24 (Tc Pre-Development)
i = rainfall intensity (in./hr.)1.60
4. Calculate Runoff Rate (Pre-Development)
Q = CiA
C = Rational Method Runoff Coefficient 0.2 (open land)
i = rainfall intensity (in./hr.)1.60 (calculated above)
A = Area (acres)0.70 (calculated above)
Q = Runoff Rate (Pre-Development) (cfs)0.22
5. Calculate Required Pond Volume
Total Area (acres) =0.70 acres
Weighted C =0.71
Discharge Rate (cfs) =0.22 cfs (Equal to Pre-Development Runoff Rate)
Duration(min)Duration
(hrs)
Intensity
(in/hr)Qin (cfs)Runoff
Volume
Release
Volume
Required
Storage
(ft3)
15 0.25 1.58 0.78 699 201 497
16 0.27 1.51 0.74 715 215 500
17 0.28 1.45 0.72 730 228 502
18 0.30 1.40 0.69 745 242 503
19 0.32 1.35 0.67 759 255 504
20 0.33 1.31 0.64 773 269 504
21 0.35 1.27 0.62 786 282 504
22 0.37 1.23 0.61 799 295 503
23 0.38 1.19 0.59 811 309 503
24 0.40 1.16 0.57 824 322 501
PROVIDED VOLUME (ft3)675
OUTLET STRUCTURE SLOT
Q=CLH3/2
Q = Discharge (cfs)0.22
C = Weir Coefficient 3.33 (per COB Design Standards)
H = Head (ft)1.5
L = Horizontal Length (ft)0.04
L = Slot Width (inches)0.4
PIPE A (OS A Outlet Pipe)25-YR OUTFLOW RATE
REQUIRED CAPACITY
1. Calculate Area and Weighted C Factor
A = Area (acres)0.83
C = Weighted C Factor 0.79
2. Calculate Rainfall Intensity (Duration = Max Tc from Contributing Drainage Areas)
i = 0.78x-0.64 (25-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs)0.04
i = rainfall intensity (in./hr.)5.91
3. Calculate 25-yr Pond Outflow Rate
Q = CiA
C = Rational Method Runoff Coefficient 0.79 (calculated above)
i = rainfall intensity (in./hr.)5.91 (calculated above)
A = Area (acres)0.83 (calculated above)
Q 25-yr Pipe Flow Rate (cfs)=3.85
MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY)
Pipe: A Location: OS A Outlet Pipe
INPUT
D= 12 inches
d= 11.26 inches
Mannings Formula n= 0.013 mannings
q=57.7 degrees
Q=(1.486/n)ARh2/3S1/2 S= 0.011 slope in/in
R=A/P
A=cross sectional area
P=wetted perimeter V=(1.49/n)Rh2/3S1/2
S=slope of channel Q=V x A
n=Manning's roughness coefficient
Solution to Mannings Equation
Area,ft2 WettedPerimeter, ft HydraulicRadius, ft velocity ft/s flow, cfs
PVC 0.013
0.77 2.64 0.29 5.25 4.02 PE (<9"dia) 0.015
PE (>12"dia) 0.02
PE(9-12"dia) 0.017
CMP 0.025
ADS N12 0.012
HCMP 0.023
Conc 0.013
Manning's n-values
d
q
D
PIPE B (OS B Outlet Pipe)25-YR OUTFLOW RATE
REQUIRED CAPACITY
1. Calculate Area and Weighted C Factor
A = Area (acres)0.70
C = Weighted C Factor 0.71
2. Calculate Rainfall Intensity (Duration = Max Tc from Contributing Drainage Areas)
i = 0.78x-0.64 (25-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs)0.05
i = rainfall intensity (in./hr.)5.00
3. Calculate 25-yr Pond Outflow Rate
Q = CiA
C = Rational Method Runoff Coefficient 0.71 (calculated above)
i = rainfall intensity (in./hr.)5.00 (calculated above)
A = Area (acres)0.70 (calculated above)
Q 25-yr Pipe Flow Rate (cfs)=2.46
MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY)
Pipe: B Location: OS B Outlet Pipe
INPUT
D= 12 inches
d= 11.26 inches
Mannings Formula n= 0.013 mannings
q=57.7 degrees
Q=(1.486/n)ARh2/3S1/2 S= 0.02 slope in/in
R=A/P
A=cross sectional area
P=wetted perimeter V=(1.49/n)Rh2/3S1/2
S=slope of channel Q=V x A
n=Manning's roughness coefficient
Solution to Mannings Equation
Area,ft2 WettedPerimeter, ft HydraulicRadius, ft velocity ft/s flow, cfs
PVC 0.013
0.77 2.64 0.29 7.08 5.42 PE (<9"dia) 0.015
PE (>12"dia) 0.02
PE(9-12"dia) 0.017
CMP 0.025
ADS N12 0.012
HCMP 0.023
Conc 0.013
Manning's n-values
d
q
D
APPENDIX B
DRAINAGE AREA MAP
W