HomeMy WebLinkAbout07.2 Stormwater Management Design Report UNIVERSITY CROSSINGS – STORMWATER DESIGN REPORT
Dec 16, 2021
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TABLE OF CONTENTS
REPORT
Introduction ..........................................................................................................................2
Existing Site & Stormwater .................................................................................................2
Stormwater Design...............................................................................................................2
Groundwater Considerations ...............................................................................................3
Falling Head Percolation Tests ............................................................................................3
Drainage Areas.....................................................................................................................4
Infiltration Chamber Design ................................................................................................7
APPENDICES
Appendix A: Drainage Area Map
Appendix B: Storm Sewer Facilities Calculations
Appendix C: GW Monitoring Data
Appendix D: Stormwater Facility Maintenance Plans
UNIVERSITY CROSSINGS – STORMWATER DESIGN REPORT
Dec 16, 2021
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INTRODUCTION
This project includes the proposed development of Lot 1A-1, a portion of Lot 2A (undeveloped)
and Lot 3 (Crowley Fleck Law Office property). The proposed development includes the
construction of two apartment buildings (59 total units), surrounding parking lot, a fire lane
connecting to an existing parking lot, and demolition of an existing stormwater pond. The property
is located within the Bozeman city limits and is currently zoned B-2 commercial. A combination
of site grading, curb and gutter, storm chases, drywells, and underground infiltration chambers will
be used to manage stormwater runoff on the site.
EXISTING SITE
The property for the proposed development (Lot 1A-1) generally slopes from south to north and
is currently undeveloped. The existing retention pond is located on the north end of the property
along the existing drive approach. The existing law office building (Lot 3, Drainage Area 0) drains
to a drywell and the remainder of the site drains to the existing retention pond (V=4,094 ft3). The
existing pond will be removed and replaced with a StormTech infiltration system to account for
the lost storage volume.
STORMWATER DESIGN
Stormwater runoff from the proposed improvements will be conveyed via curb and gutter channel
flow to a series of curb chases and storm sewer inlets. It will then be routed through storm drainage
pipes to detention and retention facilities for storage. StormTech chambers A&B will act in
detention along with storing adequate runoff for the half inch requirement. These chambers will
release runoff through infiltration at a rate determined by the falling head percolation tests. The
time of concentration for all of the drainage areas were assumed to be less than 5 mins. A post
development Drainage Area Map is included in Appendix A, and calculations for pond/chamber
sizing for each of the post development drainage areas (total area, weighted C factor, required and
provided storage volumes, and discharge rates) are included in Appendix B. StormTech chambers
C & D will retain runoff from Kagy Boulevard (Drainage Areas 5 & 6).
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Storm sewer facilities were sized for the 25-yr storm using Manning’s Equation, and for each inlet,
the contributing area, gutter capacity, weighted C factor, and time of concentration were
calculated. These values were input into Manning’s Equation to check capacity and flow
characteristics for inlets, storm drain pipes, and curb gutters. All curb gutters were designed to
maintain 0.15’ freeboard per C.O.B. Design Manual Section IV.C.5.
GROUNDWATER CONSIDERATIONS
Three groundwater (GW) monitoring wells were installed across Lot 1A-1 and Lot 2A and have
been regularly inspected this past spring to record fluctuations in the groundwater elevation during
a typical peak season for groundwater. The GW monitoring results are included in Appendix C
for reference. The peak GW depth from the three wells found the seasonal high ground water to
be 4.59’ below ground surface (July 9, 2021).
FALLING HEAD PERCOLATION TESTS
In order to determine the infiltration rate of the soils underlying the project area a series of falling
head percolation tests were performed across the site. Each of these tests was performed on the
native gravels underlying the site, which is the proposed infiltrative surface for the proposed
StormTech infiltration systems. A total of six falling head percolation tests were performed at two
different locations. Attached is an aerial map showing the locations of the percolation tests.
For each falling head percolation test, an excavator was used to excavate a test hole down to the
native gravel elevation. Next, 4-inch Diameter PVC was embedded 6 inches into the native gravel
layer in order to limit horizontal migration of the water in order to more accurately determine the
vertical infiltration rate of the gravel layer. Once the PVC pipe was installed approximately 5 to
10 gallons of water was poured into the pipe and allowed to infiltrate the gravel to pre-soak the
soil before taking actual measurements of the infiltration rate. Following the pre-soak, the PVC
pipe was filled with water and then the time to drain was measured. This process was repeated
three times at each test location. The following tables provide a summary of the testing results:
UNIVERSITY CROSSINGS – STORMWATER DESIGN REPORT
Dec 16, 2021
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Percolation Test Location #1
Test # Depth of Water
(in)
Time to Drain
(min)
Infiltration Rate
(in/min)
Infiltration Rate
(in/hr)
1 24 23.42 1.02 61.20
2 18 8.10 2.22 133.20
3 20 8.20 2.43 145.80
Percolation Test Location #2
Test # Depth of Water
(in)
Time to Drain
(min)
Infiltration Rate
(in/min)
Infiltration Rate
(in/hr)
1 20 23.0 0.86 51.60
2 12 32.5 0.36 21.60
3 5 18.2 0.27 16.20
Based on the percolation test data, Test Location #1 yielded an average infiltration rate of 113.4
inches per hour and Test Location #2 yielded an average infiltration rate of 29.8 inches per hour.
The first perc test failed to pass the DEQ Circular 8 testing requirements with the increasing
infiltration rate between successive trials, this test was not used in determining the design
infiltration rate. In order to determine the design infiltration rate, we have taken the average of the
second test results and then included a factor of safety of 2 to arrive at a value of 14.9 inches per
hour.
DRAINAGE AREAS
Drainage Area 1&2
Drainage Area 1 consists of a portion of the neighboring Lot 2A and Lot 3. The proposed
realignment of the drive aisle and curb and gutter will be graded. These existing lots flow north &
west along the curb & gutters to a storm chase draining to the existing retention pond. A proposed
fire lane connects the proposed parking lot and the existing Crowley Fleck parking lot. This
addition will alter the existing runoff volume from the neighboring lots. DA 1 includes all the areas
UNIVERSITY CROSSINGS – STORMWATER DESIGN REPORT
Dec 16, 2021
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of the neighboring lots that will drain to the proposed StormTech infiltration system (Chamber A)
located in DA 1. It was determined 1,750 cu ft of detention volume is required to meet the half
inch requirement (per DSS II.A.4) for Drainage Area 1&2.
Drainage Area 2 (DA 2.0 & 2.1) consists of the northern portion of Lot 1A-1, including the east
half of the northern apartment building as well as the surrounding sidewalks and a portion of the
parking lot. Runoff from DA 2 will drain via sheet flow, curb & gutter, curb chases, and roof
downspouts to the proposed StormTech system (Chamber A). The system will be located under
the proposed parking lot where the existing pond is located. To meet the half inch requirement 941
ft3 of runoff will need to be detained from Drainage Area 2.
The system was sized to detain the half inch requirement volume of 2,683 ft3 from DA 1, 2.0, &
2.1. The StormTech system will have a proposed storage volume of 2,729 ft3 and an inlet to capture
runoff from both drainage areas. Specific manufacturer specifications for this StormTech system
are included in the detail sheets (C5.1 &C5.2 of site plan set).
Drainage Area 3
Drainage Area 3 consists of the southern part of the parking lot, a portion of the south apartment
building, sidewalks, and landscaping along the building. Runoff from this DA will drain via sheet
flow, curb & gutter, curb chases, and roof downspouts to the proposed StormTech system Chamber
B. To meet the half inch requirement, 852 ft3 of storage is required for Drainage Area 3. The
proposed system is located on the west side of the parking lot and is sized to detain 935 ft3 of
runoff. Specific manufacturer specifications for this StormTech system are included in in the detail
sheets (C5.1 &C5.2 of site plan set). Curb inlet capacity calcs are included in Appendix B.
Drainage Area 4
Drainage Area 4 consists of a portion of the lot including the west half of the north building. Also
included is the landscaped area between the building and the west property line. The 10-year 2-
hour storm generates 365 ft3 of runoff, and the runoff from DA 4 will flow via sheet flow and 6 in.
downspouts towards Retention Pond 1 which provides 389 cf of storage. This pond was sized to
UNIVERSITY CROSSINGS – STORMWATER DESIGN REPORT
Dec 16, 2021
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retain the 10-year 2-hr storm runoff volume. The downspouts were designed to detain the 25-year
storm event.
Drainage Area 5& 6 (Kagy Blvd. Expansion)
As indicated in the COB Master Transportation Plan Kagy Boulevard is proposed to be widened
to meet the principle arterial roadway standards. The proposed expansion of Kagy Boulevard will
generate excess stormwater runoff that will need to be retained on-site. Drainage Areas 5&6
include the areas impacted by the expansion, runoff from these DAs will drain via curb & gutter
to curb inlets piped to proposed StormTech systems (Chambers C&D). The chambers were sized
according to City of Bozeman Design Standards to capture and retain the volume of the 10-year
2-hour storm event. DA 5 will produce 916 ft3 of runoff that will be conveyed through a 12 in.
PVC pipe in Chamber C with a storage volume of 1,050 ft3. DA 6 will produce 1,010 ft3 of runoff
that will be conveyed through a 12 in. PVC pipe to Chamber D with a storage volume of 1,125 ft3.
The inlet pipes leading to the chambers were sized based off the 25-year storm event. Calculations
for sizing the chambers & pipes are included in Appendix B.
Drainage Area 7
Drainage Area 7 consists of a portion of the parking lot on the north end of the lot. Runoff from
this drainage area flows via sheet flow and curb & gutter to a curb inlet piped to Drywell #1. The
10-year 2-hour storm generates 183 ft3 of runoff and the drywell was sized to retain 194 ft3 of
runoff. The inlet pipe was sized to accommodate a 25-year storm event per COB standards. A 12
in. PVC pipe will adequately convey runoff to the drywell. Pipe and drywell sizing calculations
are included in Appendix B.
Drainage Area 8
Drainage Area 8 consists of a portion of the lot including the south half of the south building as
well as the surrounding landscaped and patio areas. The 10-year 2-hour storm generates 570 ft3 of
runoff, and the runoff from DA 8 will flow via sheet flow and 6 in. downspouts to Retention Pond
UNIVERSITY CROSSINGS – STORMWATER DESIGN REPORT
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2 which provides 753 cf of storage. The pond was sized to retain the 10-year 2-hr storm runoff
volume. The downspouts were designed to detain the 25-year storm event.
INFILTRATION CHAMBER DESIGN
The StormTech infiltration system (Chamber A) will be placed where the existing stormwater
pond is located for DA 1 & DA 2. The second system (Chamber B) will be placed at the low point
of the southwesterly part of the parking lot. The third system (Chamber C) will be placed at the
northwest corner of Proposed Lot 1A-1. The last system (Chamber D) will be placed at the
northwest corner of Lot 2A. 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. Systems A&B are StormTech SC-310 with
combination curb inlet Nyloplast drain basins. Systems C&D are StormTech SC-740 with curb
inlets piped to the Nyloplast drain basins.
The footprint of these chambers will be excavated down to native gravels and 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 each system. An infiltration rate of 35 in/hour was used based on the Falling Head
Percolation Tests performed on the lots. These discharge rates were 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. Calculations used to determine these
system sizes can be found in Appendix B.
APPENDIX A
Drainage Area Map
APPENDIX B
Storm Sewer Facilities Calculations
RUNOFF VOLUME FROM DA#1
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft
2) C * Area
Landscape 0.2 24434 4887
Hardscape 0.95 42716 40580
Total 67150 45467
A = Area (acres)1.54
C = Weighted C Factor 0.68
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.68
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 1.54
Q = RUNOFF (cfs)0.43
V = REQUIRED VOL (ft3)3065
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 42716
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 0.62 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.64 decimal
A = Entire drainage area 1.54 acres
RRV = Runoff Reduction Volume 0.0400 acre-ft
RRV = Runoff Reduction Volume 1742 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 larger runoff volume is used (3065 cf).
Check the quantity of stormwater allocated from DA 1 (Tc Method)
1. Calculate Tc (Time to Concentration) of DA 1
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 3.4% 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) 104 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)1.3
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 (%) 0.72%
L = length of gutter (ft) 321
V = mean velocity (ft/s) 2.54
Tc Gutter Flow (minutes) =2.1
Tc Total = 5.0 (5 minute minimum)
2. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.68 (calculated above)
I = .78 Tc-0.64 (in/hr)3.83 (25-yr storm)
A = area (acres) 1.54 (calculated above)
Q = Pro Flow (cfs) 3.99 (assuming no carry flow)
RUNOFF VOLUME FROM DA#2.0
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft
2) C * Area
Landscape 0.2 3718 744
Hardscape 0.95 12595 11965
Total 16313 12709
A = Area (acres)0.37
C = Weighted C Factor 0.78
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.78
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.37
Q = RUNOFF (cfs)0.12
V = REQUIRED VOL (ft3)857
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 12595
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 0.74 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.77 decimal
A = Entire drainage area 0.37 acres
RRV = Runoff Reduction Volume 0.0116 acre-ft
RRV = Runoff Reduction Volume 506 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 facility is sized to handle the larger volume (857 cf).
Check the quantity of stormwater allocated from DA 2 (Tc Method)
1. Calculate Tc (Time to Concentration) of DA 2
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 1.8% 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) 108 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)1.6
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 (%) 1.68%
L = length of gutter (ft) 100
V = mean velocity (ft/s) 3.89
Tc Gutter Flow (minutes) =0.4
Tc Total = 5.0 (5 minute minimum)
2. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.78 (calculated above)
I = .78 Tc-0.64 (in/hr)3.83 (25-yr storm)
A = area (acres) 0.37 (calculated above)
Q = Pro Flow (cfs) 1.12 (assuming no carry flow)
25- year storm runoff calc
Runoff From DA 2.0
1. Summary of Drainage Areas
Contributing Area DA # C Area (ft2 )C * Area
Landscape 2 0.20 3718 744
Hardscape 2 0.95 12595 11965
Total 16313 12709
A = Area (acres)0.37
C = Weighted C Factor 0.78
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 0.78 (calculated above)
I = 0.78 Tc-0.64 (in/hr)3.83 (25-yr storm)
A = area (acres)0.37 (calculated above)
Q 25-yr Flow Rate (cfs)=1.12
MANNING'S EQUATION for OPEN CHANNEL FLOW Curb Chase #1
Project:University Crossings Location: DA #2.1
By: JG Date: 12/1/2021
Chk By:DK Date: 12/1/2021
INPUT
z (sideslope)= 0
Mannings Formula z (sideslope)= 0
w (btm width, ft)= 1.5
Q = (1.486/n)ARh2/3S1/2 d (depth, ft)= 0.5
R = A/P S (slope, ft/ft) 0.015
A = cross sectional area n low =0.035
P= wetted perimeter n high =0.035
S = slope of channel V = (1.49/n)Rh2/3S1/2
n = Manning's roughness coefficient Q = V x A
Depth, ft Area, sf
Wetted
Perimete
r, ft
Hydraulic
Radius, ft Velocity, fps Flow, cfs
Velocity,
fps Flow, cfs
0.5 0.75 2.50 0.30 2.33 1.75 2.33 1.75 T = 1.50
Dm = 0.500
Sc low = 0.0444 Sc high = 0.0444
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 0.0311 0.0577 0.0311 0.0577
Low N High N
d
w
z
11
z
T
Clear Data
Entry Cells
RUNOFF VOLUME FROM DA#2.1
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft
2) C * Area
Landscape 0.2 1798 360
Hardscape 0.95 10896 10352
Total 12694 10711
A = Area (acres)0.29
C = Weighted C Factor 0.84
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.84
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.29
Q = RUNOFF (cfs)0.10
V = REQUIRED VOL (ft3)722
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 10896
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 0.82 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.86 decimal
A = Entire drainage area 0.29 acres
RRV = Runoff Reduction Volume 0.0100 acre-ft
RRV = Runoff Reduction Volume 435 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 facility is sized to handle the larger volume (722 cf).
Check the quantity of stormwater allocated from DA 2.1 (Tc Method)
1. Calculate Tc (Time to Concentration) of DA 2.1
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 3.4% 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) 90.5 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)1.2
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 (%) 2.00%
L = length of gutter (ft) 100
V = mean velocity (ft/s) 4.24
Tc Gutter Flow (minutes) =0.4
Tc Total = 5.0 (5 minute minimum)
2. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.84 (calculated above)
I = .78 Tc-0.64 (in/hr)3.83 (25-yr storm)
A = area (acres) 0.29 (calculated above)
Q = Pro Flow (cfs) 0.94 (assuming no carry flow)
25-year storm runoff calc
Runoff From DA 2.0 & 2.1
1. Summary of Drainage Areas
Contributing Area DA # C Area (ft2 )C * Area
Landscape 2&2.1 0.20 5516 1103
Hardscape 2&2.1 0.95 23491 22316
Total 29007 23420
A = Area (acres)0.67
C = Weighted C Factor 0.81
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 0.81 (calculated above)
I = 0.78 Tc-0.64 (in/hr)3.83 (25-yr storm)
A = area (acres)0.67 (calculated above)
Q 25-yr Flow Rate (cfs)=2.06
MANNING'S EQUATION for OPEN CHANNEL FLOW Curb Chase #2
Project:University Crossings Location: DA #2.1
By: JG Date: 12/1/2021
Chk By:DK Date: 12/1/2021
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) 0.015
A = cross sectional area n low =0.035
P= wetted perimeter n high =0.035
S = slope of channel V = (1.49/n)Rh2/3S1/2
n = Manning's roughness coefficient Q = 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 2.50 2.50 2.50 2.50 T =2.00
Dm = 0.500
Sc low = 0.0386 Sc high = 0.0386
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 0.0270 0.0502 0.0270 0.0502
Low N High N
d
w
z
11
z
T
Clear Data
Entry Cells
Chamber A
DA #1,2.0,2.1
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor (Post-Development)
Contributing Area DA C Area (ft2)C * Area
Hardscape 1,2,2.1 0.95 66207 62897
Landscape 1,2,2.1 0.20 29950 5990
Total 96157 68887
A = Area (acres)2.21 Storm
C = Weighted C Factor 0.72 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 (in/hour) =14.9 51 to 100 1.25
Infiltration Rate (ft/sec) =0.00034
Infiltration Length (ft) =84.92
Infiltration Width (ft) =22.83
Infiltration Area (sf) =1938.724
Total Area (acres) =2.21 acres
Weighted C =0.72
Discharge Rate (cfs) =0.67 cfs
Duration(min)Duration
(hrs)
Intensity
(in/hr)Qin (cfs)Runoff
Volume
Release
Volume
Required
Storage
(ft3)
18 0.30 1.40 2.21 2391 722 1668.52
19 0.32 1.35 2.14 2436 762 1674.07
20 0.33 1.31 2.07 2480 802 1678.08
21 0.35 1.27 2.00 2523 843 1680.69
22 0.37 1.23 1.94 2565 883 1681.99
23 0.38 1.19 1.89 2605 923 1682.08
24 0.40 1.16 1.84 2644 963 1681.05
25 0.42 1.13 1.79 2682 1003 1678.98
26 0.43 1.10 1.74 2719 1043 1675.93
27 0.45 1.08 1.70 2755 1083 1671.96
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 66207
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)0.69 decimal
A = Entire drainage area 2.21 acres
RRV = Runoff Reduction Volume 0.0616 acre-ft
RRV = Runoff Reduction Volume 2683 cubic feet
PROVIDED VOLUME (ft3)2,729
1/2" VOLUME REQ.=2683
DETENTION VOLUME REQ.=1682.08
DRAINAGE AREA # 3
RUNOFF VOLUME FROM DA#3
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft
2) C * Area
Landscape 0.2 1895 379
Hardscape 0.95 21421 20350
Total 23315 20729
A = Area (acres)0.54
C = Weighted C Factor 0.89
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.89
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.54
Q = RUNOFF (cfs)0.19
V = REQUIRED VOL (ft3)1397
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 21421
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 0.88 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.92 decimal
A = Entire drainage area 0.54 acres
RRV = Runoff Reduction Volume 0.0196 acre-ft
RRV = Runoff Reduction Volume 852 cubic feet
Check the quantity of stormwater allocated from DA 3 (Tc Method)
1. Calculate Tc (Time to Concentration) of DA 3
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 3.4% Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.95 2 to 10 1
Cf = Frequency Adjustment Factor 1 11 to 25 1.1
D = Length of Basin (ft) 170 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)2.4
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 (%) 2.00%
L = length of gutter (ft) 100
V = mean velocity (ft/s) 4.24
Tc Gutter Flow (minutes) =0.4
Tc Total = 5.0 (5 minute minimum)
2. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.89 (calculated above)
I = .78 Tc-0.64 (in/hr)3.83 (25-yr storm)
A = area (acres) 0.54 (calculated above)
Q = Pro Flow (cfs) 1.82 (assuming no carry flow)
Chamber B
DA # 3
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor (Post-Development)
Contributing Area DA C Area (ft2)C * Area
Hardscape 3 0.95 21421 20350
Landscape 3 0.20 1895 379
Total 23315 20729
A = Area (acres) 0.54 Storm
C = Weighted C Factor 0.89 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 (in/hour) =14.9 51 to 100 1.25
Infiltration Rate (ft/sec) =0.00034
Infiltration Length (ft) = 37
Infiltration Width (ft) = 16
Infiltration Area (sf) = 592
Total Area (acres) =0.54 acres
Weighted C =0.89
Discharge Rate (cfs) =0.20 cfs
Duration(min)Duration
(hrs)
Intensity
(in/hr)Qin (cfs)Runoff
Volume
Release
Volume
Required
Storage
(ft3)
17 0.28 1.45 0.69 705 208 496.86
18 0.30 1.40 0.67 719 221 498.86
19 0.32 1.35 0.64 733 233 500.35
20 0.33 1.31 0.62 746 245 501.38
21 0.35 1.27 0.60 759 257 501.99
22 0.37 1.23 0.58 772 270 502.20
23 0.38 1.19 0.57 784 282 502.05
24 0.40 1.16 0.55 796 294 501.56
25 0.42 1.13 0.54 807 306 500.76
26 0.43 1.10 0.52 818 319 499.66
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 21421
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 0.88 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.92 decimal
A = Entire drainage area 0.54 acres
RRV = Runoff Reduction Volume 0.0196 acre-ft
RRV = Runoff Reduction Volume 852 cubic feet
PROVIDED VOLUME (ft3)935
1/2" VOLUME REQ.=852
DETENTION VOLUME REQ.=501.99
Input
Type of Grate 2'x3' Curb Inlet High Flow
Head (ft) 0.3
Properties
Orifice Flow Area (in)425.88
Orifice Flow Area (ft)2.96
Weir Flow Perimeter (in)124.32
Weir Flow Perimeter (ft)10.36
Solution
Capacity (cfs)5.67
Capacity (gpm)2544.13
REV 2.1.21
Nyloplast Inlet Capacity Table
DISCLAIMER: SAFETY FACTORS ARE NOT INCLUDED IN THESE CALCULATIONS. ACTUAL CALCULATIONS SHOULD BE CARRIED OUT AND VERIFIED BY THE
DESIGN ENGINEER TAKING INTO ACCOUNT ALL LOCAL CONDITIONS. NYLOPLAST RECOMMENDS USING A MINIMUM SAFETY FACTOR OF 1.25 FOR
PAVED AREAS AND 2.0 FOR TURF AREAS. ADS/NYLOPLAST IS NOT RESPONSIBLE FOR MISUSE OF THIS TOOL.
CHAMBER A&B NYLOPLAST INLETS
RUNOFF VOLUME FROM DA#4
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft2)C * Area
Landscape 0.2 7394 1479
Hardscape 0.95 4149 3941
Total 11543 5420
A = Area (acres)0.26
C = Weighted C Factor 0.47
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.47
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.26
Q = RUNOFF (cfs)0.05
V = REQUIRED VOL (ft3)365
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #4
Contributing Area Area (ft 2)
Hardscape 4149
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 0.37 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.36 decimal
A = Entire drainage area 0.26 acres
RRV = Runoff Reduction Volume 0.0041 acre-ft
RRV = Runoff Reduction Volume 180 cubic feet
PROVIDED VOLUME (ft3)389
1/2" VOLUME REQ.=180
RETENTION VOLUME REQ.=365.38
Check the quantity of stormwater allocated from DA 4 (Tc Method)
1. Calculate Tc (Time to Concentration) of DA 4
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%)3.4% 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)90.5 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)1.2
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 (%)2.00%
L = length of gutter (ft)100
V = mean velocity (ft/s)4.24
Tc Gutter Flow (minutes) =0.4
Tc Total =5.0 (5 minute minimum)
2. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.47 (calculated above)
I = .64 Tc-0.65 (in/hr)3.22 (25-yr storm)
A = area (acres)0.26 (calculated above)
Q = Pro Flow (cfs)0.40 (assuming no carry flow)
DRAINAGE AREA # 4
ROOF DOWNSPOUT PIPE ROUTED TO RETENTION POND 1
1. Summary of Roof Area and C Factor
Contributing Area DA # C Area (ft2 )C * Area
LANDSCAPE 4 0.2 7394 1479
HARDSCAPE 4 0.95 4149 3941
Total 11543 5420
A = Area (acres) 0.26
C = Weighted C Factor 0.47
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 0.47 (calculated above)
I = 0.78 Tc-0.64 (in/hr)3.83 (25-yr storm)
A = area (acres) 0.26 (calculated above)
Q 25-yr Flow Rate (cfs)=0.48
MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY)
Pipe: Pond #1 Location: Pond #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
RUNOFF VOLUME FROM DA#5
1. Calculate Weighted C Factor for ROW
Component C Width
ROW Landscape 0.2 9
ROW Hardscape 0.95 51
C = Weighted C Factor 0.84
2. Calculate Area and Weighted C Factor
Contributing Area C Area (ft
2) C * Area
Landscape 0.2 0 0Hardscape 0.95 0 0
Composite ROW 0.84 16232 13594
Total 16232 13594
A = Area (acres)0.37
C = Weighted C Factor 0.84
3. Calculate Required VolumeQ = CIA
V=7200Q
C = Weighted C Factor 0.84
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)A = Area (acres) 0.37
Q = RUNOFF (cfs)0.13
V = REQUIRED VOL (ft3)916
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #5
Contributing Area Area (ft2)Hardscape 16232
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 0.37 acres
RRV = Runoff Reduction Volume 0.0147 acre-ft
RRV = Runoff Reduction Volume 642 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 system is sized to handle the larger volume (916 cf).
Stormtech Chamber C Volume
PROVIDED VOLUME (ft3)1,050
1/2" VOLUME REQ.=642
RETENTION VOLUME REQ.=916.43
Check the quantity of stormwater allocated from DA 5 (Tc Method)
1. Calculate Tc (Time to Concentration) of DA 5
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 3.0% 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)49 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)0.9
Tc Gutter FlowTc = 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 (%) 0.66%
L = length of gutter (ft) 243
V = mean velocity (ft/s) 2.44
Tc Gutter Flow (minutes) =1.7
Tc Total =5.0 (5 minute minimum)
2. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.84 (calculated above)
I = .78 Tc-0.64 (in/hr)3.83 (25-yr storm)
A = area (acres) 0.37 (calculated above)
Q = Pro Flow (cfs) 1.19 (assuming no carry flow)
DRAINAGE AREA # 5
CATCH BASIN PIPE ROUTED TO INFILTRATION SYSTEM C
1. Summary of Roof Area and C Factor
Contributing Area DA # C Area (ft2 )C * Area
Composite ROW 5 0.84 16232 13594
Total 16232 13594
A = Area (acres) 0.37
C = Weighted C Factor 0.84
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 0.84 (calculated above)
I = 0.78 Tc-0.64 (in/hr)3.83 (25-yr storm)
A = area (acres) 0.37 (calculated above)
Q 25-yr Flow Rate (cfs)=1.19
MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY)
Pipe: Chamber C Location: Chamber C
INPUT
D= 12 inches
d= 11.26 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
0.77 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
D
RUNOFF VOLUME FROM DA#6
1. Calculate Weighted C Factor for ROW
Component C Width
ROW Landscape 0.2 9
ROW Hardscape 0.95 51
C = Weighted C Factor 0.84
2. Calculate Area and Weighted C Factor
Contributing Area C Area (ft
2) C * Area
Landscape 0.2 0 0Hardscape 0.95 0 0
Composite ROW 0.84 17895 14987
Total 17895 14987
A = Area (acres)0.41
C = Weighted C Factor 0.84
3. Calculate Required VolumeQ = CIA
V=7200Q
C = Weighted C Factor 0.84
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)A = Area (acres) 0.41
Q = RUNOFF (cfs)0.14
V = REQUIRED VOL (ft3)1010
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #5
Contributing Area Area (ft2)Hardscape 17895
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 0.41 acres
RRV = Runoff Reduction Volume 0.0163 acre-ft
RRV = Runoff Reduction Volume 708 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 facility is sized to handle the larger volume (1010 cf)
Stormtech Chamber D Volume
PROVIDED VOLUME (ft3)1,125
1/2" VOLUME REQ.=708
RETENTION VOLUME REQ.=1010
Check the quantity of stormwater allocated from DA 6 (Tc Method)
1. Calculate Tc (Time to Concentration) of DA 6Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 2.0% 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)40 26 to 50 1.251 to 100 1.25
Tc Overland Flow (minutes)0.9
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 (%) 0.66%
L = length of gutter (ft) 145
V = mean velocity (ft/s) 2.44
Tc Gutter Flow (minutes) =1.0
Tc Total =5.0 (5 minute minimum)
2. Calculate Flow (Rational Formula)
Q = CIA
C = Weighted C Factor 0.84 (calculated above)
I = .78 Tc-0.64 (in/hr)3.83 (25-yr storm)
A = area (acres) 0.41 (calculated above)
Q = Pro Flow (cfs) 1.32 (assuming no carry flow)
DRAINAGE AREA # 6
CATCH BASIN PIPE ROUTED TO INFILTRATION SYSTEM D
1. Summary of Roof Area and C Factor
Contributing Area DA # C Area (ft2 )C * Area
Composite ROW 6 0.84 17895 14987
Total 17895 14987
A = Area (acres) 0.41
C = Weighted C Factor 0.84
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 0.84 (calculated above)
I = 0.78 Tc-0.64 (in/hr)3.83 (25-yr storm)
A = area (acres) 0.41 (calculated above)
Q 25-yr Flow Rate (cfs)=1.32
MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY)
Pipe: Chamber D Location: Chamber D
INPUT
D= 12 inches
d= 11.26 inches
Mannings Formula n= 0.013 mannings
57.7 degrees
Q=(1.486/n)ARh2/3S1/2 S= 0.005 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 3.68 2.82 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
RUNOFF VOLUME FROM DA#7
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft
2) C * Area
Landscape 0.2 878 176
Hardscape 0.95 2676 2542
Total 3554 2718
A = Area (acres)0.08
C = Weighted C Factor 0.76
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.76
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.08
Q = RUNOFF (cfs)0.03
V = REQUIRED VOL (ft3)183
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #7
Contributing Area Area (ft
2)
Hardscape 2676
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 0.73 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.75 decimal
A = Entire drainage area 0.08 acres
RRV = Runoff Reduction Volume 0.0025 acre-ft
RRV = Runoff Reduction Volume 108 cubic feet
4. Calculate Drywell Volume
Void Ratio of Media 30.00%
Gravel Offset Dist. From Drywell (ft) 5 (see Circular DEQ 4,
Table 2.1-1)
Gravel Void Volume
Gravel Bed Depth (below MH) 0.00
Gravel Volume (ft3)463.23
Gravel Storage Volume (ft3)138.97
Manhole Volume
Manhole Depth (ft) 3.00 *use 4' manhole (only using top 3' out of groundwater for retention calc)
Manhole Volume (ft3)54.97
Provided Volume (ft3)194
CURB INLET PIPE ROUTED TO DRYWELL 1
1. Summary of Roof Area and C Factor
Contributing Area DA # C Area (ft2 )C * Area
LANDSCAPE 7 0.2 878 176
HARDSCAPE 7 0.95 2676 2542
Total 3554 2718
A = Area (acres) 0.08
C = Weighted C Factor 0.76
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 0.76 (calculated above)
I = 0.78 Tc-0.64 (in/hr)3.83 (25-yr storm)
A = area (acres) 0.08 (calculated above)
Q 25-yr Flow Rate (cfs)=0.24
MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY)
Pipe: DW 1 Pipe Location: DW 1
INPUT
D= 12 inches
d= 11.26 inches
Mannings Formula n= 0.013 mannings
57.7 degrees
Q=(1.486/n)ARh2/3S1/2 S= 0.009 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 5.00 3.82 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
RUNOFF VOLUME FROM DA#8
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Landscape 0.2 5316 1063
Hardscape 0.95 7787 7398
Total 13103 8461
A = Area (acres)0.30
C = Weighted C Factor 0.65
2. Calculate Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.65
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.30
Q = RUNOFF (cfs)0.08
V = REQUIRED VOL (ft3)570
Check the half inch requirement (per DSSP II.A.4)
1. Determine Area of Hardscape within Drainage Area #4
Contributing Area Area (ft 2 )
Hardscape 7787
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 0.58 0.05 + 0.9*I
I = Percent impervious cover (decimal)0.59 decimal
A = Entire drainage area 0.30 acres
RRV = Runoff Reduction Volume 0.0073 acre-ft
RRV = Runoff Reduction Volume 319 cubic feet
PROVIDED VOLUME (ft3)753
1/2" VOLUME REQ.=319
RETENTION VOLUME REQ.=570.39
DRAINAGE AREA # 8
ROOF DOWNSPOUT PIPE ROUTED TO RETENTION POND 2
1. Summary of Roof Area and C Factor
Contributing Area DA # C Area (ft2 )C * Area
Hardscape 8 0.95 7787 7398
Total 7787 7398
A = Area (acres) 0.18
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) 0.18 (calculated above)
Q 25-yr Flow Rate (cfs)=0.65
MANNING'S EQUATION FOR PIPE FLOW (PROVIDED CAPACITY)
Pipe: Pond #3 Location: Pond #3
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= 0.015 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 4.02 0.77 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
Groundwater Monitoring Results
PERC TEST
PERC TEST
GW-1
GW-2
#NAME?
Project Engineer:Drew Kirsch
Project:19TH & KAGY
Well Information:bgs = below ground surface ags = above ground surface
MW-1 MW-2 MW-32.33 2.43 3.50
4920.81 4923.79 4924.50
4918.48 4921.36 4921.00
Groundwater Information:
MW-1 MW-2 MW-37.57 dry drydrydrydry
7.12 7.22 dry
dry dry dry7.12 7.22 dry
6.48 6.52 dry
6.42 6.47 dry5.95 5.99 dry
6.15 6.26 dry
dry dry dry
dry 7.17 dry5.90 5.82 dry
6.01 5.99 dry
4.70 4.59 dry
5.74 5.85 dry
05/07/21
Monitor Well Data
Depth to Ground Water (feet-bgs)
04/23/21
04/30/21
Project Number:210228
UNIVERSITY CROSSINGS
Project Location:
05/07/21
04/30/21
Well ID
Well Depth (Feet)
Top of Well (Elevation)Ground Elevation
Date
05/14/2105/21/2105/28/21
06/04/21
06/11/2106/18/2106/24/21
07/01/21
07/09/21
07/16/21
APPENDIX D
Stormwater Facility Maintenance Plans