HomeMy WebLinkAbout201192_Stormwater Design Report_2021-07-14
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. Drainage Area Maps for pre and post-development are included in Appendix A.
Calculations for each individual drainage area are included in Appendix B. Resources for C-values
used in the calculation of runoff are included in Appendix C. Groundwater information is provided
in Appendix D. A Stormwater Facilities Inspection and Maintenance Plan is included in Appendix
E. Excerpts from the Parklands at the Village Downtown Subdivision stormwater report are
included in Appendix F.
POST-DEVELOPMENT DRAINAGE AREAS
Drainage Area 1
Drainage Area 1 consists of the south half 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 528 cubic feet of
storage. 620 cubic feet of storage is provided in Retention Pond #1.
Drainage Area 2
Drainage Area 2 consists of the north half 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 541 cubic feet of storage.
614 cubic feet of storage is provided in Retention Pond #2.
Drainage Area 3
Drainage Area 3 consists of the entire building and 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 2,019 cubic feet of storage. 2,382 cubic feet
of storage is provided in the Underground Stormwater System including the surrounding washed
rock using a stone porosity of 40%. 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.
Drainage Area 4
Drainage Area 4 consists of the landscaped and concrete areas located between the building and
existing Right of Ways (ROW) of Village Crossing Way and Village Downtown Blvd. Runoff
from this drainage area flows into said ROWs and subsequently into an existing detention pond at
the end of Village Downtown Blvd. Said pond was recently re-designed and re-shaped for the new
Parklands Subdivision. The pond was designed with a volume of 7,726 cubic feet, when only 7,522
cubic feet was required. An excess of 204 cubic feet of storage exists in said pond. DA #4
contributes 196 cubic feet of stormwater during the design storm event (10-yr, 2-hr).
Drainage Area 5
Drainage Area 5 consists primarily of the area of the site that is undisturbed. Runoff from this
drainage area flows to the adjacent lots to the east. Historically, a large portion of the lot’s runoff
has flowed in a similar fashion. See Drainage Area Map – Pre-Development in Appendix A for a
visual reference. With the development of the lot, the runoff leaving the property is reduced from
1,205 cubic feet pre-development to 803 cubic feet post-development. Therefore, it is acceptable
not to provide any stormwater improvements for DA #5 and to allow this area to naturally drain
into adjacent lots to the east as it has done historically.
DEPTH TO GROUNDWATER
Groundwater monitoring was conducted on the subject property in May 2021 by TD&H
Engineering. It was discovered that groundwater is extremely deep at approximately 27 feet below
ground surface; therefore, the proposed retention ponds and underground Stormtech system are
higher than existing groundwater.
G:\C&H\20\201192\Design Reports\Storm\201192_Stormwater Design Report.Docx
APPENDIX A
DRAINAGE AREA MAPS
A4.011A4.02A3.01 1A3.012
1A4.001A4.001A4.01
2A4.002A4.00
2A4.012A4.01
1A4.02 2A4.022A4.02
A3.002A3.001
OPEN
AB
OV
E
APPENDIX B
DRAINAGE AREA,
INFILTRATION/RETENTION SYSTEM
CALCULATIONS
DRAINAGE AREA #1
RETENTION POND #1
1. Calculate Area and Weighted C Factor
Contributing Area DA #C Area (ft 2 )C * Area
Hardscape 1 0.90 7012 6310
Landscape 1 0.15 10118 1518
Total 17130 7828
A = Area (acres)0.39
C = Weighted C Factor 0.46
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.46
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres)0.39
Q = runoff (cfs)0.07
V = REQUIRED VOL (ft3)528
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 7012
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 0.42 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.41 decimal
A = Entire drainage area 0.39 acres
RRV = Runoff Reduction Volume 0.007 acre-ft
RRV = Runoff Reduction Volume 299 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 (528 cf).
DRAINAGE AREA #2
RETENTION POND #2
1. Calculate Area and Weighted C Factor
Contributing Area DA #C Area (ft 2 )C * Area
Hardscape 2 0.90 8279 7451
Landscape 2 0.15 3821 573
Total 12100 8025
A = Area (acres)0.28
C = Weighted C Factor 0.66
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.66
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres)0.28
Q = runoff (cfs)0.08
V = REQUIRED VOL (ft3)541
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 8279
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 0.67 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.68 decimal
A = Entire drainage area 0.28 acres
RRV = Runoff Reduction Volume 0.008 acre-ft
RRV = Runoff Reduction Volume 336 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 (541 cf).
DRAINAGE AREA #3
UNDERGROUND STORMWATER SYSTEM
1. Calculate Area and Weighted C Factor
Contributing Area DA #C Area (ft 2 )C * Area
Hardscape 3 0.90 33284 29956
Landscape 3 0.15 0 0
Total 33284 29956
A = Area (acres)0.76
C = Weighted C Factor 0.90
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.90
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres)0.76
Q = runoff (cfs)0.28
V = REQUIRED VOL (ft3)2019
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #3
Contributing Area Area (ft 2)
Hardscape 33284
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 1317 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 (2,019 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 0.90 33284 29956
Total 33284 29956
A = Area (acres)0.76
C = Weighted C Factor 0.90
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 0.90 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft)70 26 to 50 1.251 to 100 1.25
Tc Overland Flow (minutes)1.3
Tc Total =5.0 (5 minute minimum)
3. Calculate Flow (Rational Formula)Q = CIA
C = Weighted C Factor 0.90 (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)=2.63
MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY)
Pipe: 1 Location:Roof Downspout pipe
INPUT
D=12 inchesd=11.26 inchesMannings Formula n=0.013 mannings
θ=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
θ
D
DRAINAGE AREA #4
EXISTING SUBDIVISION POND
1. Calculate Area and Weighted C Factor
Contributing Area DA #C Area (ft 2)C * Area
Hardscape 4 0.90 2280 2052
Landscape 4 0.15 5709 856
Total 7989 2908
A = Area (acres)0.18
C = Weighted C Factor 0.36
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.36
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres)0.18
Q = runoff (cfs)0.03
V = REQUIRED VOL (ft3)196
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #3
Contributing Area Area (ft 2 )
Hardscape 2280
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 0.31 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.29 decimal
A = Entire drainage area 0.18 acres
RRV = Runoff Reduction Volume 0.002 acre-ft
RRV = Runoff Reduction Volume 102 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 capacity
of the existing subdivision pond was checked for the larger volume (196 cf).
The capacity of the existing subdivision detention pond located at the end of the
Village Downtown Blvd (and recently redesigned for the Parklands Subdivision) is
7,522 cf. The pond was designed with a volume of 7,726 cf leaving a remainder of 204 cf. The stormwater drainage from DA 4 contributes 196 cf which ultimately runoffs
into said existing pond. Therefore, the existing pond has sufficient volume for
the development of Lot 5.
DRAINAGE AREA #5
VERIFY RUNOFF TO ADJACENT PROPERTIES DOES NOT EXCEED HISTORICAL
1. Calculate Area and Weighted C Factor
Contributing Area DA #C Area (ft 2 )C * Area
Hardscape 5 0.90 272 245
Landscape 5 0.15 77734 11660
Total 78006 11905
A = Area (acres)1.79
C = Weighted C Factor 0.153
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.15
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)A = Area (acres)1.79
Q = runoff (cfs)0.11
V = REQUIRED VOL (ft3)803
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #3
Contributing Area Area (ft 2 )
Hardscape 272
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 0.05 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.00 decimal
A = Entire drainage area 1.79 acres
RRV = Runoff Reduction Volume 0.004 acre-ft
RRV = Runoff Reduction Volume 173 cubic feet
Because the runoff volume from the 10-yr, 2-hr storm (for flood control) is greater thanthe runoff volume produced by the half inch rainfall (for water quality) the historical
lot drainage was checked for the larger volume (803 cf).
The existing lot's historical drainage pattern is delineated on the 'Drainage Area Map - Pre Development' located in Appendix A. Said map shows
that the existing lot naturally drains to adjacent parcels to the east and the Village
Downtown Blvd ROW due to the existing grade. A volume of 1,295 cf is produced
during the 10-yr, 2-hr storm and flows off site to adjacent lots to the east. After the lot
is developed the runoff to the adjacent lots is reduced to 803 cf; therefore, no
stormwater improvements are proposed within DA #5.
APPENDIX C
RATIONAL METHOD C VALUES
Fact Sheet-5.1.3
The Clean Water Team Guidance Compendium for Watershed Monitoring and Assessment
State Water Resources Control Board 5.1.3 FS-(RC) 2011
1
Runoff Coefficient (C) Fact Sheet
What is It?
The runoff coefficient (C) is a dimensionless coefficient relating the amount of runoff to the
amount of precipitation received. It is a larger value for areas with low infiltration and high
runoff (pavement, steep gradient), and lower for permeable, well vegetated areas (forest, flat
land).
Why is It Important?
It is important for flood control channel construction and for possible flood zone hazard
delineation. A high runoff coefficient (C) value may indicate flash flooding areas during storms
as water moves fast overland on its way to a river channel or a valley floor.
How is It Measured?
It is measured by determining the soil type, gradient, permeability and land use. The values are
taken from the table below. The larger values correspond to higher runoff and lower infiltration.
Land C Land C Use Use
Busin
Do
Ne
0
0
Lawn
San
San
San
Hea
Hea
Hea
.05
0
15
.13
8
.25
ess:
wntown areas
ighborhood areas
0.7
0.5
- 0.95
- 0.70
s:
dy soil, flat, 2%
dy soil, avg., 2-7%
dy soil, steep, 7%
vy soil, flat, 2%
vy soil, avg., 2-7%
vy soil, steep, 7%
0
0.1
0.
0
0.1
0
- 0.10
- 0.15
- 0.20
- 0.17
- 0.22
- 0.35
Resid
Sin
Mu
Mu
Sub
0.25
Agric
Bar
*Sm
*Ro
Cult
*He
*H
*Sa
*Sa
Pas
*H
*Sa
Wo
0.30
0.20
30
0
20
0
0.15
0.05
0.05
ential:
gle-family areas
lti units, detached
nti units, attached
urban
0.30
0.40
0.60
- 0.50
- 0.60
- 0.75
- 0.40
ultural land:
e packed soil
ooth
ugh
ivated rows
avy soil, no crop
eavy soil, with crop
ndy soil, no crop
ndy soil, with crop
ture
eavy soil
ndy soil
odlands
0.
0.2
0.
0.1
- 0.60
- 0.50
- 0.60
- 0.50
- 0.40
- 0.25
- 0.45
- 0.25
- 0.25
Fact Sheet-5.1.3
The Clean Water Team Guidance Compendium for Watershed Monitoring and Assessment
State Water Resources Control Board 5.1.3 FS-(RC) 2011
2
Indu
Lig
He
0.50
0.60
Stree
Asp
Con
Bri
0.70
0.80
0.70
strial:
ht areas
avy areas
- 0.80
- 0.90
ts:
haltic
crete
ck
- 0.95
- 0.95
- 0.85
Parks .10 Unim 0.10 , cemeteries 0 - 0.25 proved areas - 0.30
Playg 0.20 Drive 0.75 rounds - 0.35 s and walks - 0.85
Railr 20 Roof 0.75 oad yard areas 0.- 0.40 s - 0.95
Note: The designer must use judgment to select the appropriate "C" value within the range.
Generally, larger areas with permeable soils, flat slopes and dense vegetation should have the
lowest "C" values. Smaller areas with dense soils, moderate to steep slopes, and sparse
vegetation should assigned the highest "C" values.
http://water.me.vccs.edu/courses/CIV246/table2b.htm accessed 11/19/09
APPENDIX D
GROUNDWATER INFORMATION
130 Village Crossing Way Ground Water Monitoring
GW Well 5/6/2021 5/10/2021 5/17/2021
B3 27.73 27.64 27.63
Depth to Ground Water (ft)
10.00
12.00
14.00
16.00
18.00
20.00
22.00
24.00
26.00
28.00
30.00
5/4/2021 5/6/2021 5/8/2021 5/10/2021 5/12/2021 5/14/2021 5/16/2021 5/18/2021
Depth (Ft)Date (Month, Day, Year)
Ground Water Depths
B3
J:\2021\B21-027 130 Village Crossing Way\GEOTECH\GROUNDWATER MONITORING\Village Crossing Way GW 2021.xlsx 1 OF 1
APPENDIX E
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 trailer-mounter Vermeer VX50-800
vacuum excavator or similar (see Figure 1 below) to clean Stormtech pre-treatment manhole
with sump. To accomplish the vacuuming procedure, the trailer-mounted vac system should
be pulled into the parking garage to access the pre-treatment manhole with sump. There will
need to be coordination with the building management to ensure no cars are parked in the
parking spaces on the east wall of the parking garage to allow for the trailer-mounted vac
system and truck to pull into the area to access said manhole.
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.
Figure 1 Trailer-mounted Vermeer VX50-800 vacuum excavator.
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
2
THE MOST ADVANCED NAME IN WATER MANAGEMENT SOLUTIONS TM
ECCENTRICHEADER
MANHOLEWITHOVERFLOWWEIR
STORMTECHISOLATOR ROW
OPTIONAL PRE-TREATMENT
OPTIONAL ACCESS STORMTECH CHAMBERS
)(
APPENDIX F
EXCERPTS FROM PARKLANDS
SUBDIVISION STORMWATER REPORT
DETENTION POND #1
REQUIRED VOLUME
1. Calculate Weighted C Factor for Right-of-Way
Component Width C
ROW Hardscape 64 0.95
ROW Landscape 16 0.2
Weighted C Factor = 0.80
2. Calculate Area and Weighted C Factor (Post-Development)
Contributing Area C Area (ft 2 )C * Area
Composite ROW 0.80 186132 148906
OS 0.2 8601 1720
Low-Med Residential 0.35 7888 2761
Dense Residential 0.5 52799 26400
Total 255421 179787
A = Area (acres) 5.8637
C = Weighted C Factor 0.70
3. Calculate Tc (Pre-Development)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 3.50% Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.2 2 to 10 1
Cf = Frequency Adjustment Factor 1 11 to 25 1.1
D = Length of Basin (ft) 532 26 to 50 1.2
51 to 100 1.25
Tc (Pre-Development) (minutes) 26
4. 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) 0.43 (Tc Pre-Development)
i = rainfall intensity (in./hr.) 1.11
5. Calculate Runoff Rate (Pre-Development)
Q = CiA
C = Rational Method Runoff Coefficient 0.2 (open land)
i = rainfall intensity (in./hr.) 1.11 (calculated above)
A = Area (acres) 5.86 (calculated above)
Q = Runoff Rate (Pre-Development) (cfs) 1.31
6. Calculate Required Pond Volume
Total Area (acres) = 5.86 acres
Weighted C = 0.70
Discharge Rate (cfs) = 1.31 cfs (Equal to Pre-Development Runoff Rate)
Duration(min) Duration(hrs)
Intensity
(in/hr)Qin (cfs)Runoff
Volume
Release
Volume
Required
Storage (ft3)
30 0.50 1.00 4.14 7461 0 7461
30.5 0.51 0.99 4.10 7504 0 7504
31 0.52 0.98 4.06 7547 39 7508
32 0.53 0.96 3.97 7631 118 7514
33 0.55 0.94 3.90 7714 196 7518
34 0.57 0.93 3.82 7795 274 7521
35 0.58 0.91 3.75 7875 353 7522
36 0.60 0.89 3.68 7953 431 7521
37 0.62 0.88 3.62 8029 510 7520
38 0.63 0.86 3.55 8104 588 7516
OUTLET STRUCTURE SLOT
Q=CLH3/2
Q = Discharge (cfs) 1.31 (calculated above)
C = Weir Coefficient 3.33 (per COB Design Standards)
H = Head (ft) 1
L = Horizontal Length (ft) 0.39
L = Slot Width (inches) 4.7