HomeMy WebLinkAboutAppendix D Stormwater Design Report 02-22-2021
DESIGN REPORT
STORMWATER MANAGEMENT
PALISADE AT 130 VILLAGE CROSSING WAY
LOT 5
MINOR SUBDIVISION NO. 344
Prepared for:
SMA Architects
109 E. Oak St, Suite 2E
Bozeman, MT 59715
Prepared by:
Project Number: 201192
February 2021
INTRODUCTION
The Palisade at 130 Village Crossing Way site plan consists of a multi-story residential building
with a footprint of approximately 33,500 sq. ft. and associated improvements located on Lot 5 of
Minor Subdivision 344 in Bozeman, Montana. A combination of site grading, curb and gutter, at-
grade retention ponds and underground Stormtech MC-3500 storage chambers will be used to
manage stormwater runoff for the development of the site. The proposed underground Stormtech
MC-3500 stormwater storage system and retention ponds were sized for the 10-year, 2-hour storm
and checked for the half-inch requirement. Supporting stormwater calculations are attached to this
report. A Drainage Area Map is included in Appendix A. Calculations for each individual drainage
area are included in Appendix B. A Stormwater Facilities Inspection and Maintenance Plan is
included in Appendix C.
DRAINAGE AREAS
Drainage Area 1
Drainage Area 1 consists of the southeast side of the parking lot and adjacent landscaping. Runoff
from this drainage area flows via curb and gutter to Retention Pond #1. The proposed retention
pond was sized for the 10-year, 2-hour storm and was checked for the half-inch requirement. It
was found that the 10-year, 2-hour storm governed the design and required 543 cubic feet of
storage. 570 cubic feet of storage is provided in Retention Pond #1.
Drainage Area 2
Drainage Area 2 consists of the northwest side of the parking lot and adjacent landscaping. Runoff
from this drainage area flows via curb and gutter to Retention Pond #2. The proposed retention
pond was sized for the 10-year, 2-hour storm and was checked for the half-inch requirement. It
was found that the 10-year, 2-hour storm governed the design and required 516 cubic feet of
storage. 525 cubic feet of storage is provided in Retention Pond #2.
Drainage Area 3
Drainage Area 3 consists of the entire building and the adjacent courtyard. Runoff from this
drainage area flows via a system of downspouts and pipes designed by others to the proposed
Stormtech MC-3500 infiltration chambers located to the east of the building above the steep slope.
The chambers are designed to retain stormwater runoff using the arch-shaped chambers and void
space in the surrounding washed rock while the runoff infiltrates into the ground. The chambers
were sized for the 10-year, 2-hour storm and was checked for the half-inch requirement. It was
found that the 10-year, 2-hour storm governed the design and required 2,127 cubic feet of storage.
2,382 cubic feet of storage is provided in the Underground Stormwater System including the
surrounding washed rock using a void ratio of 0.4. The system is equipped with an overflow pipe
that daylights to the adjacent hillside to the east. This overflow is set to the top of the chambers so
it only overflows in the unlikely event that the system is full.
DEPTH TO GROUNDWATER
Groundwater monitoring has not been conducted on the subject property. Given that the wetland
area on the far east end of the lot is approximately 20' lower in elevation than the bottom of the
Stormtech chambers, it is assumed that the underlying groundwater table will not interfere with
the function of the subsurface stormwater system. The developer is planning to dig test pits on site
in the coming months and will install groundwater monitoring wells to verify this assumption.
G:\C&H\20\201192\Design Reports\Storm\201192_Stormwater Design Report.Docx
APPENDIX A
DRAINAGE AREA MAP
5758
APPENDIX B
DRAINAGE AREA,
INFILTRATION/RETENTION SYSTEM
CALCULATIONS
DRAINAGE AREA #1
RUNOFF VOLUME FROM DA #1
1. Calculate Area and Weighted C Factor
Contributing Area DA #C Area (ft 2 )C * Area
Hardscape 1 0.95 7255 6892
Landscape 1 0.20 5829 1166
Total 13084 8058
A = Area (acres)0.30
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)0.30
Q = runoff (cfs)0.08
V = REQUIRED VOL (ft3)543
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #2
Contributing Area Area (ft 2)
Hardscape 7255
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 0.55 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.55 decimal
A = Entire drainage area 0.41 acres
RRV = Runoff Reduction Volume 0.009 acre-ft
RRV = Runoff Reduction Volume 406 cubic feet
Because the runoff volume from the 10-yr, 2-hr storm (for flood control) is
greater than the runoff volume produced by the half inch rainfall (for water quality)
the proposed retention pond #1 is sized to handle the larger volume (543 cf).
DRAINAGE AREA #2
RUNOFF VOLUME FROM DA #2
1. Calculate Area and Weighted C Factor
Contributing Area DA #C Area (ft 2 )C * Area
Hardscape 2 0.95 7816 7425
Landscape 2 0.20 1166 233
Total 8982 7658
A = Area (acres)0.21
C = Weighted C Factor 0.85
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.85
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres)0.21
Q = runoff (cfs)0.07
V = REQUIRED VOL (ft3)516
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #2
Contributing Area Area (ft 2)
Hardscape 7816
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 0.83 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.87 decimal
A = Entire drainage area 0.21 acres
RRV = Runoff Reduction Volume 0.007 acre-ft
RRV = Runoff Reduction Volume 312 cubic feet
Because the runoff volume from the 10-yr, 2-hr storm (for flood control) is
greater than the runoff volume produced by the half inch rainfall (for water quality)
the proposed retention pond #1 is sized to handle the larger volume (516 cf).
DRAINAGE AREA #3
UNDERGROUND STORMWATER SYSTEM
1. Calculate Area and Weighted C Factor
Contributing Area DA #C Area (ft2 )C * Area
Hardscape 3 0.95 33207 31547
Landscape 3 0.20 0 0
Total 33207 31547
A = Area (acres)0.76
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)0.76
Q = runoff (cfs)0.30
V = REQUIRED VOL (ft3)2127
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #1
Contributing Area Area (ft 2 )
Hardscape 33207
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 0.76 acres
RRV = Runoff Reduction Volume 0.030 acre-ft
RRV = Runoff Reduction Volume 1314 cubic feet
Because the runoff volume from the 10-yr, 2-hr storm (for flood control) is greater than
the runoff volume produced by the half inch rainfall (for water quality) the proposed
Underground Stormwater System is sized to handle the larger volume (2,127 cf).
User Inputs
Chamber Model: MC-3500
Outlet Control Structure: No
Project Name: Lot 5 - Palisade
Engineer: Tim Staub
Project Location:
Measurement Type: Imperial
Required Storage Volume: 2127 cubic ft.
Stone Porosity: 40%
Stone Foundation Depth: 48 in.
Stone Above Chambers: 12 in.
Average Cover Over Chambers: 48 in.
Design Constraint Dimensions:(25 ft. x 40 ft.)
Results
System Volume and Bed Size
Installed Storage Volume: 2381.85 cubic ft.
Storage Volume Per Chamber: 109.90 cubic ft.
Number Of Chambers Required: 7
Number Of End Caps Required: 4
Chamber Rows: 2
Maximum Length:38.35 ft.
Maximum Width: 15.33 ft.
Approx. Bed Size Required: 538.43 square ft.
System Components
Amount Of Stone Required: 143.79 cubic yards
Volume Of Excavation (Not Including
Fill):
174.49 cubic yards
DRAINAGE AREA #3
ROOF DOWNSPOUT PIPE ROUTED TO STORMTECH AND OVERFLOW PIPE
1. Summary of Roof Area and C Factor
Contributing Area DA #C Area (ft2 )C * Area
Hardscape 3 0.95 33207 31547
Total 33207 31547
A = Area (acres)0.76
C = Weighted C Factor 0.95
2. Calculate Tc (Time to Concentration)
Tc Overland Flow (on roof with pitch of 1/4" to 12")
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%)2.1%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)70 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)1.2
Tc Total =5.0 (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)0.76 (calculated above)
Q 25-yr Flow Rate (cfs)=2.77
MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY)
Pipe: 1 Location:Roof Downspout 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=0.01 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
0.77 2.64 0.29 5.21 3.99 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 C
STORMWATER FACILITIES INSPECTION
AND MAINTENANCE PLAN
INSPECTION AND MAINTENANCE FOR STORMWATER MANAGEMENT FACILITIES
The owner shall be responsible for the maintenance of the stormwater drainage facilities within The Palisade at 130 Village Crossing Way development on Lot 5, Minor Subdivision No. 344. Storm Water Facilities:
1. Underground Stormtech MC-3500 Infiltration System collect storm water runoff and store the water until it infiltrates into the ground. 2. Retention Ponds collect storm water runoff and store the water until it evaporates and/or infiltrates into the ground. 3. Pipe Networks convey storm water to different discharge locations underground.
Post Construction Inspection: 1. Use the attached Stormtech Isolator Row Operation & Maintenance manual to determine if maintenance is required on the system after construction is completed.
2. Observe drain time in retention ponds for a storm event after completion of the facility to confirm that the desired drain time has been obtained. If excessively slow infiltration rates are observed then excavate a minimum 5 ft by 5 ft drain to native gravels (or native well-draining material) and
backfill with well-draining material (pit-run). Semi-Annual Inspection:
1. Use the attached Stormtech Isolator Row Operation & Maintenance manual to determine if maintenance is required on the system semi-annually. 2. Check for grass clippings, litter, and debris in retention ponds. Flush and/or vacuum storm water pipes if excessive material is observed in the facilities. Standard Maintenance: 1. Inspect and remove debris from catch basins. Use a vacuum truck to clean catch basins and Stormtech system. 2. Inspect for the following issues: differential accumulation of sediment, drain time, signs of
petroleum hydrocarbon contamination (odors, oil sheen in pond water), standing water, trash and debris. 3. Monitor health of vegetation and revegetate as necessary to maintain full vegetative cover.
4. Check retention ponds three days following a storm event exceeding ¼ inch of precipitation. Failure
for water to percolate within this time period indicates clogging or poor-draining soils. Sediment accumulation:
In most cases, sediment in a catch basin or a retention system does not contain toxins at levels posing a hazardous concern. However, sediments should be tested for toxicants in compliance with current disposal requirements and if land uses in the drainage area include commercial or industrial zones, or if visual or olfactory indications of pollution are noticed. Sediments containing high levels of
pollutants should be disposed of in accordance with applicable regulations and the potential sources of contamination should be investigated and contamination practices terminated.
An company
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THE MOST ADVANCED NAME IN WATER MANAGEMENT SOLUTIONS TM
ECCENTRICHEADER
MANHOLEWITHOVERFLOWWEIR
STORMTECHISOLATOR ROW
OPTIONAL PRE-TREATMENT
OPTIONAL ACCESS STORMTECH CHAMBERS
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