HomeMy WebLinkAbout007 Stormwater Design Report - BDC
INTRODUCTION
The Bozeman Design Center proposes to develop an existing building pad site located on Lot 1,
Block 2, Ferguson Farm Subdivision. The proposed development consists of a two-story
commercial building, adjacent sidewalk, parking and landscaping improvements. Existing
stormwater infrastructure was installed with the overall subdivision street improvements in 2015.
Additional infrastructure was completed in 2018 with the Block 2 parking lot improvements. Both
stormwater design reports were referenced in this analysis to ensure adequate capacity in the
subdivision and parking lot exist. The Block 2 drainage report has been updated as well. It is
included with this site plan application. Within Block 2, Drainage Area 1 will drain to the
subdivision’s retention pond at the north end of the subdivision. Drainage Areas 2, 3 and 4 will
drain to the Block 2 parking lot retention ponds.
The proposed building’s roof will drain to a new underground Stormtech SC-740 chamber system
under the proposed parking stalls to the south of the building. Two dry wells were installed with
the Block 2 parking lot improvements and stubbed out to the Lot 1 building pad. These existing dry
wells and stubs are being abandoned with this project due to the influence of the high groundwater
table in the lower 1 to 2 feet of the drywells.
STORMWATER DESIGN The original Block 2 stormwater report calculated 2,755 cubic feet of runoff from the area outside
of the proposed building pad that drains into the right of way (Drainage Area #1 in the original
Block 2 stormwater report). With the revision of the Block 2 site plan for the Bozeman Design
Center, the runoff from this drainage area increased to 2,821 cubic feet, which is within the
allowable limit of the previously allocated amount of 2,840 cubic feet. (See updated Block 2
stormwater report included with this site plan application). Additionally, the drainage from the
area that contributes to Retention Pond #2 located on the west side of the parking lot (Drainage
Area #4 in the original Block 2 stormwater report) increased slightly due to site modifications for
the Bozeman Design Center. The increase in stormwater drainage was from 1,224 to 1,326 cubic
feet and is still within the originally allocated volume of the pond, which is 1,500 cubic feet.
The existing previously-installed drywells (approximately 8 feet deep) are going to be abandoned
in place due to the influence of the high groundwater table. Therefore, a Stormtech SC-740
chamber system is proposed to be installed under the parking stalls along the south side of the
Bozeman Design Center to capture the runoff from the building. The Stormtech chamber systems
are sized according to City of Bozeman Design Standards to capture and retain or detain the
volume of the 10-year 2-hour storm event. The proposed Stormtech infiltration chambers are
designed to detain stormwater runoff using the arch-shaped chambers and void space in the
surrounding washed rock, while the runoff infiltrates into the ground.
The footprint of these chambers will be over-excavated down to native gravels to remove any
existing non-draining material beneath the chambers. This excavation will be back-filled with a
well-draining gravel to ensure infiltration. The chambers were sized by applying an infiltration
rate for gravel subgrades to the footprint area of the chambers to determine the discharge (infiltrate
rate) from the system. The discharge rate was then compared to the proposed inflow rates from the
contributing areas to the systems during the 10-year 2-hour storm event to determine the required
detention volumes for each system.
The gravel infiltration rate used for these sizing calculations was determined to be 6 minutes per
inch (10 inches per hour) per Circular DEQ 4 Table 2.1-1. This infiltration rate is considered to be
conservative for native gravels based on the engineering experience of C&H Engineering in
Bozeman. Because the required volume when using the infiltration design was less than the Runoff
Reduction Volume (i.e. the “half inch requirement”) the system was sized for the larger. The
runoff reduction volume is 843 cubic feet and the proposed chamber system is 975 cubic feet.
GROUNDWATER A groundwater monitoring well exists within the building pad. Groundwater measurements were
taken in November 2021. Given a lack of monitoring data during peak runoff season, two feet was
added to the November measurements to estimate a high groundwater table elevation. (Based on
groundwater monitoring data for the nearby proposed Ferguson Farm II Subdivision to the east,
two feet was chosen as an average fluctuation in groundwater elevation for this site.) The
approximate high groundwater table elevation in the footprint of the proposed Stormtech chamber
system is 4,828.80 feet and the bottom of Stormtech gravel elevation is 4,831.13 feet.
Approximately 2.3 feet of separation between high groundwater table and bottom of Stormtech
system is provided.
APPENDIX A
BOZEMAN DESIGN CENTER
DRAINAGE AREA MAP
APPENDIX B
SUPPORTING CALCULATIONS
RUNOFF VOLUME FROM DA#1
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Landscape 0.2 0 0
Hardscape 0.95 7092 6738
Total 7092 6738
A = Area (acres)0.16
C = Weighted C Factor 0.95
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.95
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.16
Q = RUNOFF (cfs)0.06
V = REQUIRED VOL (ft3)454
RUNOFF VOLUME FROM DA#2.0
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Landscape 0.2 0 0
Hardscape 0.95 7083 6728
Total 7083 6728
A = Area (acres)0.16
C = Weighted C Factor 0.95
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.95
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.16
Q = RUNOFF (cfs)0.06
V = REQUIRED VOL (ft3)454
RUNOFF VOLUME FROM DA#3.0
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Landscape 0.2 0 0
Hardscape 0.95 7130 6773
Total 7130 6773
A = Area (acres)0.16
C = Weighted C Factor 0.95
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.95
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres)0.16
Q = RUNOFF (cfs)0.06
V = REQUIRED VOL (ft3)457
Chamber A
DA #1&2&3
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor (Post-Development)
Contributing Area DA C Area (ft 2)C * Area
Hardscape 1&2&3 0.95 21305 20239
Landscape 1&2&3 0.20 0 0
Total 21305 20239
A = Area (acres)0.49 Storm
C = Weighted C Factor 0.95 Return(yrs)Cf
2 to 10 1
2. Calculate Infiltration Rate 11 to 25 1.1
Existing Soil Condition =Gravel 26 to 50 1.2
Infiltration Rate (min/in) =6 Circular DEQ 4 (Table 2.1-1)50 to 100 0.25
Infiltration Rate (in/hour) =10
Infiltration Rate (ft/sec) =0.00023
Infiltration Area (sf) =499
Total Area (acres) =0.49 acres
Weighted C =0.95
Discharge Rate (cfs) =0.12 cfs
Duration(min)Duration
(hrs)
Intensity
(in/hr)Qin (cfs)Runoff
Volume
Release
Volume
Required Storage
(ft3)47 0.78 0.75 0.35 983 326 657.05
48 0.80 0.74 0.34 990 333 657.39
49 0.82 0.73 0.34 997 340 657.63
50 0.83 0.72 0.33 1004 347 657.77
51 0.85 0.71 0.33 1011 353 657.83
52 0.87 0.70 0.33 1018 360 657.79
53 0.88 0.69 0.32 1025 367 657.67
54 0.90 0.69 0.32 1032 374 657.47
55 0.92 0.68 0.31 1038 381 657.19
56 0.93 0.67 0.31 1045 388 656.83
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #1&2
Contributing Area Area (ft 2)
Hardscape 21305
2. Calculate 1/2" runoff volume over hardscape
(aka Runoff Reduction Volume [RRV] as calculated in Montana Post-
Construction Storwater BMP Manual - Equation 3-1)
RRV = [P*Rv*A]/12
P = Water quality rainfall depth 0.50 inches
Rv = Dimensionless runoff coefficient 0.95 0.05 + 0.9*I
I = Percent impervious cover (decimal)1.00 decimal
A = Entire drainage area 0.49 acres
RRV = Runoff Reduction Volume 0.0194 acre-ft
RRV = Runoff Reduction Volume 843 cubic feet
DRAINAGE AREA # 1
ROOF DOWNSPOUT PIPE ROUTED TO INFILTRATION SYSTEM #1
1. Summary of Roof Area and C Factor
Contributing Area DA # C Area (ft 2)C * Area
Hardscape 1 0.95 7092 6738
Total 7092 6738
A = Area (acres)0.16
C = Weighted C Factor 0.95
2. Calculate Tc (Time to Concentration)
Tc Total = 5.0 (Assume 5 minute minimum)
3. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.95 (calculated above)
I = 0.78 Tc-0.64 (in/hr)3.83 (25-yr storm)
A = area (acres) 0.16 (calculated above)
Q 25-yr Flow Rate (cfs)=0.59
MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY)
Pipe: Downspout #1
INPUT
D= 6 inches
d= 5.63 inches
Mannings Formula n= 0.013 mannings
57.7 degrees
Q=(1.486/n)ARh2/3S1/2 S= 0.01 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 Wetted
Perimeter, ft
Hydraulic
Radius, ft velocity ft/s flow, cfs PVC 0.013
0.19 1.32 0.15 3.28 0.63 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
D
DRAINAGE AREA # 2
ROOF DOWNSPOUT PIPE ROUTED TO INFILTRATION SYSTEM #1
1. Summary of Roof Area and C Factor
Contributing Area DA # C Area (ft 2)C * Area
Hardscape 2 0.95 7083 6728
Total 7083 6728
A = Area (acres)0.16
C = Weighted C Factor 0.95
2. Calculate Tc (Time to Concentration)
Tc Total = 5.0 (Assume 5 minute minimum)
3. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.95 (calculated above)
I = 0.78 Tc-0.64 (in/hr)3.83 (25-yr storm)
A = area (acres) 0.16 (calculated above)
Q 25-yr Flow Rate (cfs)=0.59
MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY)
Pipe: Downspout #1
INPUT
D= 6 inches
d= 5.63 inches
Mannings Formula n= 0.013 mannings
57.7 degrees
Q=(1.486/n)ARh2/3S1/2 S= 0.01 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 Wetted
Perimeter, ft
Hydraulic
Radius, ft velocity ft/s flow, cfs PVC 0.013
0.19 1.32 0.15 3.28 0.63 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
D
DRAINAGE AREA # 3
ROOF DOWNSPOUT PIPE ROUTED TO INFILTRATION SYSTEM #1
1. Summary of Roof Area and C Factor
Contributing Area DA # C Area (ft 2)C * Area
Hardscape 3 0.95 7130 6773
Total 7130 6773
A = Area (acres)0.16
C = Weighted C Factor 0.95
2. Calculate Tc (Time to Concentration)
Tc Total = 5.0 (Assume 5 minute minimum)
3. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.95 (calculated above)
I = 0.78 Tc-0.64 (in/hr)3.83 (25-yr storm)
A = area (acres) 0.16 (calculated above)
Q 25-yr Flow Rate (cfs)=0.59
MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY)
Pipe: Downspout #1
INPUT
D= 6 inches
d= 5.63 inches
Mannings Formula n= 0.013 mannings
57.7 degrees
Q=(1.486/n)ARh2/3S1/2 S= 0.01 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 Wetted
Perimeter, ft
Hydraulic
Radius, ft velocity ft/s flow, cfs PVC 0.013
0.19 1.32 0.15 3.28 0.63 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
D
APPENDIX C
STORMTECH MAINTENANCE PLAN
An company
2
THE MOST ADVANCED NAME IN WATER MANAGEMENT SOLUTIONS TM
ECCENTRICHEADER
MANHOLEWITHOVERFLOWWEIR
STORMTECHISOLATOR ROW
OPTIONAL PRE-TREATMENT
OPTIONAL ACCESS STORMTECH CHAMBERS
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