HomeMy WebLinkAboutWPO202000053 Calculations 2021-02-02PLEASANT GREEN- PHASES II & III
STORMWATER MANAGEMENT CALCULATIONS PACKET
No. 35791 ,,
1/29/21
10NAI,
Date of Calculations
NOVEMBER 30, 2020
Revised on
JANUARAY 29, 2021
PREPARED BY:
COLLINS ENGIk ` mRING
200 GARRETT STREET, SUITE K
CHARLOTTESVILLE, VA 22902
434.293.3719 PH
434.293.2813 FX
www.collins-engineering.com
SCS TR-55 Calculations
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Soils Table
(Source: NRCS Web Soil Survey Online Database)
Albemarle County, Virginia (VA003)
Map
Unb
Map Unit Name
Acres Percent of
Symbol
in AOI AOI
7B
Braddock loam,
7.6 21.7%
2 to 7 percent
slopes
8C3
Braddock clay
6.7 19.1%
loam, 7 to 15
percent slopes,
severely eroded
17
Craigsville
4.4 12.6%
loam, 0 to 2
percent slopes,
frequently
Flooded
25B
Dyke silt loam,
13.2 37.7%
2 to 7 percent
slopes
36D
Hayesville
2.7 7.8%
loam, 15 to 25
percent slopes
37D3
Hayesville clay
0.4 1.1%
loam, 15 to 25
percent slopes,
severely eroded
Totals for Area of
35.0 100.0%
Interest
The NRCS web soil survey defines this development's soils as predominantly having a hydrologic group type B.
Precipitation Data
(Source: NOAA's National Weather Service, Hydrometerorological Design Studies Center, Atlas 14
Point Precipitation Frequency Estimates)
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l'DS-based precipitation frequency estimates with 90% confidence intervals (in inches)'
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U.S. Department of Agriculture
Natural Resources Conservation Service
TR 55 Worksheet 3: Time of Concentration (TJ or Travel Time (T,)
Project: Pleasant Green Phases II & III
Location: West of McComb Street
Check One: Present X Developed X
Check One: T, X T,
Segment ID:
Sheet Flow: (Applicable to T, only)
1 Surface description (Table 3-1)
2 Manning's roughness coeff., n (Table 3-1
3 Flow length, L (total L < 100) (ft)
4 Two-year 24-hour rainfall, Pi (in.)
5 Land slope, s (ft/ft)
6 Compute Tr = [0.007(n*L)"a] / Pis s"
Shallow Concentrated Flow
Designed By: FGM, PE
Checked By: SRC, PE
Through subarea n/a
FL-ENG-21A
06/04
Date: 11/30/2020
Date: 11/30/2020
DA B
DA C
DA D
DA B
DA C
DA D
(Pre-Dev.)
(Pre-Dev.)
(Pre-Dev.)
(Post-Dev.)
(Post-Dev.)
(Post-Dev.)
Dense
Dense
Woods -Light
Dense
Dense
Dense
Grass
Grass
Underbrush
Grass
Grass
Grass
0.24
0.24
0.4
0.24
0.24
0.24
100
100
100
100
100
100
3.73
3.73
3.73
3.73
3.73
3.73
0.06
0.05
0.04
0.06
0.05
0.03
0.14
0.15
0.25
0.14
0.15
0.19
7 Surface description (paved or unpaved)
Unpaved
Unpaved
Unpaved
Unpaved
Unpaved
Unpaved
8 Flow Length, L (ft)
535
150
400
300
150
225
9 Watercourse slope, s (ft/ft)
0.07
0.03
0.08
0.07
0.03
0.05
10 Average velocity, V (Figure 3-1) (ft/s)
4.2
2.8
4.5
4.2
2.8
3.6
11 Tr=L/3600*V
0.04
0.01
0.02
0.02
0.01
0.02
Channel Flow:
12 Cross sectional flow area, a (ft)
13 Wetted perimeter, P. (ft)
14 Hydraulic radius, r = a/P* (ft)
15 Channel Slope, s (ft/ft)
16 Manning's Roughness Coeff, n
17V=[1.49r2"s"s]/n
18 Flow length, L (ft)
19 Tr= L / 3600*V
20 Watershed or subarea T, or Tr
(Add T, in steps 6, 11 and 19)
Note: The shoretest SCS Tc is 0.10 hrs.
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Channel and Flood Protection Computations
9 VAC 25-870-66 (Concentrated Outfall from Subarea B)
Section B: Channel Protection
"Concentrated stormwater flow shall be released into a stormwater conveyance system and shall meet
the criteria in subdivision 1, 2 or 3 of this subsection..."
Section B.1: Manmade stormwater conveyance systems
"When stormwater from a development is discharged to a manmade stormwater conveyance system,
following the land -disturbing activity, either..." a. or b. shall be met:
Section B.1.b.
"The peak discharge requirements for concentrated stormwater flow to natural stormwater conveyance
systems in subdivision 3 of this subsection shall be met."
Section B.3
"... the maximum peak flow rate from the one-year 24-hour storm following the land -disturbing activity
shall be calculated either.." a. or b."
Section B.3.a.
In accordance with the following methodology:
Q Allowed !� LF• * (Q Pre -Developed * [RV Pre -Developed I ) / (RV Developed )
Q Allowed
Where: Q Pre -Developed =
CN Pre -Developed =
S Pre -Developed =
RV Pre -Developed=
CN Post -Developed=
S Post -Developed =
RV Post -Developed =
I.F. _
Q Developed
< 1.92 cfs
Section B.4 Limits of Analysis
4.31 cfs
(From Worksheet 4 Watershed Summary)
64
(From Worksheet 4 Watershed Summary)
5.60
(From SCS TR-55 eq. 2-4)
0.51 in.
(From SCS TR-55 eq. 2-3)
73
(From Worksheet 4 Watershed Summary)
3.68
(From SCS TR-55 eq. 2-4)
0.92 in.
(From SCS TR-55 eq. 2-3)
0.80
(Factor of Safety)
0.90 cfs
The analysis terminates at the SWM facility since subsection B.3.a is met.
9 VAC 25-870-66 (Concentrated Outfall from Subarea B)
Section C: Flood Protection
"Concentrated stormwater flow shall be released into a stormwater conveyance system and shall
meet one of the following criteria as demonstrated by use of acceptable hydrologic and hydraulic
methodologies:"
Section C.2.a.:
The point of discharge (the SWM facility outfall), "Confines the postdevelopment peak flow rate
from the 10-year 24-hour storm event within the stormwater conveyance system [the SWM
facility] to avoid the localized flooding. Detention of stormwater or downstream improvements
may be incorporated into the approved land -disturbing activity to meet this criterion...."
10-Year, 24-Hour Design Storm Peak Back Water Surface Elevation = 748.31 ft
SWM Facility'B' Top of Dam Elevation =
(No overtopping Occurs)
750.00 ft
Section C.3 Limits of Analysis
....stormwater conveyance systems shall be analyzed for compliance with flood protection
criteria to a point where:" a, b or c are met.
Section C.3.c.:
"The stormwater conveyance system [the SWM Facility outfall] enters a mapped floodplain or
other flood -prone area, adopted by ordinance, of any locality."
9 VAC 25-870-66 (Concentrated Outfall from Subarea Q
Section B: Channel Protection
"Concentrated stormwater flow shall be released into a stormwater conveyance system and shall meet
the criteria in subdivision 1, 2 or 3 of this subsection..."
Section B.1: Manmade stormwater conveyance systems
"When stormwater from a development is discharged to a manmade stormwater conveyance system,
following the land -disturbing activity, either..." a. or b. shall be met:
Section B.1.b.
"The peak discharge requirements for concentrated stormwater flow to natural stormwater conveyance
systems in subdivision 3 of this subsection shall be met."
Section B.3
"... the maximum peak flow rate from the one-year 24-hour storm following the land -disturbing activity
shall be calculated either.." a. or b."
Section B.3.a.
In accordance with the following methodology:
Q Allowed !� LF• * (Q Pre -Developed * [RV Pre -Developed I ) / (RV Developed )
Q Allowed
Where: Q Pre -Developed =
CN Pre -Developed =
S Pre -Developed =
RV Pre -Developed=
CN Post -Developed=
S Post -Developed =
RV Post -Developed =
I.F. _
Q Developed
< 1.41 cfs
Section B.4 Limits of Analysis
3.86 cfs
(From Worksheet 4 Watershed Summary)
63
(From Worksheet 4 Watershed Summary)
5.83
(From SCS TR-55 eq. 2-4)
0.48 in.
(From SCS TR-55 eq. 2-3)
75
(From Worksheet 4 Watershed Summary)
3.25
(From SCS TR-55 eq. 2-4)
1.05 in.
(From SCS TR-55 eq. 2-3)
0.80
(Factor of Safety)
1.40 cfs
The analysis terminates at the SWM facility since subsection B.3.a is met.
9 VAC 25-870-66 (Concentrated Outfall from Subarea C)
Section C: Flood Protection
"Concentrated stormwater flow shall be released into a stormwater conveyance system and shall
meet one of the following criteria as demonstrated by use of acceptable hydrologic and hydraulic
methodologies:"
Section C.2.a.:
The point of discharge (the SWM facility outfall), "Confines the postdevelopment peak flow rate
from the 10-year 24-hour storm event within the stormwater conveyance system [the SWM
facility] to avoid the localized flooding. Detention of stormwater or downstream improvements
may be incorporated into the approved land -disturbing activity to meet this criterion...."
10-Year, 24-Hour Design Storm Peak Back Water Surface Elevation = 729.98 ft
SWM Facility'C' Crown of Pipe Elevation at High Point = 729.86 ft
SWM Facility'C' Rim Elevation at Lowest Access MH / Weir Plate = 737.75 ft
(No overtopping Occurs)
Section C.3 Limits of Analysis
....stormwater conveyance systems shall be analyzed for compliance with flood protection
criteria to a point where:" a, b or c are met.
Section C.3.c.:
"The stormwater conveyance system [the SWM Facility outfall] enters a mapped floodplain or
other flood -prone area, adopted by ordinance, of any locality."
9 VAC 25-870-66 (Concentrated Outfall from Subarea D)
Section B: Channel Protection
"Concentrated stormwater flow shall be released into a stormwater conveyance system and shall meet
the criteria in subdivision 1, 2 or 3 of this subsection..."
Section B.1: Manmade stormwater conveyance systems
"When stormwater from a development is discharged to a manmade stormwater conveyance system,
following the land -disturbing activity, either..." a. or b. shall be met:
Section B.1.b.
"The peak discharge requirements for concentrated stormwater flow to natural stormwater conveyance
systems in subdivision 3 of this subsection shall be met."
Section B.3
"... the maximum peak flow rate from the one-year 24-hour storm following the land -disturbing activity
shall be calculated either.." a. or b."
Section B.3.a.
In accordance with the following methodology:
Q Allowed !� LF• * (Q Pre -Developed * [RV Pre -Developed I ) / (RV Developed )
Q Allowed
Where: Q Pre -Developed =
CN Pre -Developed =
S Pre -Developed =
RV Pre -Developed=
CN Post -Developed=
S Post -Developed =
RV Post -Developed =
I.F. _
Q Developed
< 0.32 cfs
Section B.4 Limits of Analysis
1.29 cfs
(From Worksheet 4 Watershed Summary)
61
(From Worksheet 4 Watershed Summary)
6.53
(From SCS TR-55 eq. 2-4)
0.38 in.
(From SCS TR-55 eq. 2-3)
79
(From Worksheet 4 Watershed Summary)
2.68
(From SCS TR-55 eq. 2-4)
1.25 in.
(From SCS TR-55 eq. 2-3)
0.80
(Factor of Safety)
0.31 cfs
The analysis terminates at the SWM facility since subsection B.3.a is met.
9 VAC 25-870-66 (Concentrated Outfall from Subarea D)
Section C: Flood Protection
"Concentrated stormwater flow shall be released into a stormwater conveyance system and
shall meet one of the following criteria as demonstrated by use of acceptable hydrologic and
hydraulic methodologies:"
Section C.2.a.:
The point of discharge (the SWM facility outfall), "Confines the postdevelopment peak flow
rate from the 10-year 24-hour storm event within the stormwater conveyance system [the
SWM facility & outfall pipe] to avoid the localized flooding. Detention of stormwater or
downstream improvements may be incorporated into the approved land -disturbing activity
to meet this criterion...."
10-Year, 24-Hour Design Storm Peak Back Water Surface 705.28 ft
SWM Facility'D' Crown of Pipe Elevation at High Point = 705.87 ft
SWM Facility'D' Rim Elevation at Lowest Access MH / Weir Plate 707.00 ft
(No overtopping Occurs)
Section C.3 Limits of Analysis
....stormwater conveyance systems shall be analyzed for compliance with flood protection
criteria to a point where:" a, b or c are met.
Section C.3.c.:
"The stormwater conveyance system [the SWM Facility outfall] enters a mapped floodplain
or other flood -prone area, adopted by ordinance, of any locality."
Riprap Lined Level Spreader Calculations
LEVELSPREADER'B' DESIGN
Per VSMH Chpt. 11, Section 11.5.3:
Step 1 Area = 7.97 ac.
(Eqn 11.11) Tv= 12892 cf
Q,= 0.45 watershed inches
Adjusted CN = 84
Step 2 Tc =
0.18 hrs
Step 3 la =
0.38
la/P=
0.38
q =
625 csm/in
A =
0.012 mi Z
q pTv =
3.5 cfs
Note, q pTv is the Treatment Volume peak discharge, or the peak flow rate for the 1-inch rainfall event.
Per the VA DEQ Specification #2, page 3, the Level Spreader should be designed per the following:
LS length = 13 ft per 1 cfs
25%+/- Forest Litter/groundcover below tree canopy at a LS length of 40 ft per 1 cfs
75%+/- Native groundcover below tree canopy at a LS length of 13 ft per 1 cfs
Site Specific / Weighted Average LS length = 19.8 ft
LS'B' Design Length= 68 ft
Note:
Max. LS Length = 130 ft
Min. LS Length = 13 ft
LEVEL SPREADER 'C' DESIGN
Per VSMH Chpt. 11, Section 11.5.3:
Step 1 Area = 9.01 ac.
(Eqn 11.11) Tv= 16152 cf
Q,= 0.49 watershed inches
Adjusted CN = 88
Step 2 Tc =
0.19 hrs
Step 3 la =
0.27
la/P=
0.27
q =
675 csm/in
A =
0.014 m i Z
q PTv=
4.7 cfs
Note, q PTvis the Treatment Volume peak discharge, or the peak flow rate for the 1-inch rainfall event.
Per the VA DEQ Specification #2, page 3, the Level Spreader should be designed per the following:
LS length = 13 ft per 1 cfs
LS'C' Design Length= 61 ft
Note:
Max. LS Length = 130 ft
Min. LS Length = 13 ft
LEVEL SPREADER 'D' DESIGN
Per VSMH Chpt. 11, Section 11.5.3:
Step 1 Area = 4.79 ac.
(Eqn 11.11) Tv= 9767 cf
Q,= 0.56 watershed inches
Adjusted CN = 96
Step 2 Tc =
0.23 hrs
Step 3 la =
0.09
la/P=
0.09
q =
775 csm/in
A=
0.007 mi'
q PTV=
3.3 cfs
Note, q pTvis the Treatment Volume peak discharge, or the peak flow rate for the 1-inch rainfall event.
Per the VA DEQ Specification #2, page 3, the Level Spreader should be designed per the following:
LS length = 13 ft per 1 cfs
LS'D' Design Length= 42 ft
Note:
Max. LS Length = 130 ft
Min. LS Length = 13 ft
SWM Facility'B' Routing Calculations
(Confirms the 1-year outflow is compliant with energy balance computations
and the 10-year post -development storm is contained within the facility.)
Pleasant Green- Phases II and III SWM Facility B
BasinFlow printout
INPUT:
Basin: Pleasant Green- Phases II and III SWM Facility B
7 Contour Areas
Elevation(ft)
Area(sf)
Computed Vol.(cy)
739.75
1.00
0.0
740.00
320.00
1.0
742.00
1188.00
53.5
744.00
2573.00
189.5
746.00
6576.00
517.0
748.00
10276.00
1136.1
750.00
14693.00
2056.0
Start—Elevation(ft)
4 Outlet Structures
Outlet structure 0
Orifice
746.00 Vol.(cy) 517.00
name: Barrel
area (sf) 3.142
diameter or depth (in) 24.000
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 739.480
multiple 1
discharge out of riser
Outlet structure 1
Orifice
name: Low -Flow Orifice
area (sf) 0.196
diameter or depth (in) 6.000
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 746.000
multiple 1
discharge into riser
Page 1
Pleasant Green- Phases II and III SWM Facility B
Outlet structure 2
Weir
name: Riser
diameter (in)
48.000
side angle
0.000
coefficient
3.300
invert (ft)
747.670
multiple
1
discharge
into riser
transition at (ft)
1.215
orifice coef.
0.500
orifice area (sf)
12.566
Outlet structure 3
Weir
name: Emergency Spillway
length (ft)
30.000
side angle
0.000
coefficient
3.300
invert (ft)
748.670
multiple
1
discharge
through
dam
4 Inflow Hydrographs
Hydrograph 0
SCS
name: 1-yr
24-hr SCS Design Storm
Area (acres)
7.970
CN
73.000
Type 2
rainfall, P (in)
3.090
time of conc. (hrs)
0.1800
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
8.119
peak time (hrs)
11.970
volume (cy)
977.568
Page 2
Pleasant Green- Phases II and III SWM Facility B
Hydrograph 1
SCS
name: 2-yr
24-hr SCS Design Storm
Area (acres)
7.970
CN
73.000
Type 2
rainfall, P (in)
3.730
time of conc. (hrs)
0.1800
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
11.886
peak time (hrs)
11.970
volume (cy)
1431.046
Hydrograph 2
SCS
name: 10-yr
24-hr SCS Design Storm
Area (acres)
7.970
CN
73.000
Type 2
rainfall, P (in)
5.610
time of conc. (hrs)
0.1800
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
24.612
peak time (hrs)
11.970
volume (cy)
2963.281
Hydrograph 3
SCS
name: 100-yr
24-hr SCS Design Storm
Area (acres)
7.970
CN
73.000
Type 2
rainfall, P (in)
9.120
time of conc. (hrs)
0.1800
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
51.696
peak time (hrs)
11.970
volume (cy)
6224.170
Page 3
Pleasant Green- Phases II and III SWM Facility B
OUTPUT:
Routing Method: storage -indication
Hydrograph 0
Routing Summary of
inflow (cfs)
discharge (cfs)
water level (ft)
storage (cy)
Hydrograph 1
Routing Summary of
inflow (cfs)
discharge (cfs)
water level (ft)
storage (cy)
Hydrograph 2
Routing Summary of
inflow (cfs)
discharge (cfs)
water level (ft)
storage (cy)
Hydrograph 3
Routing Summary of
inflow (cfs)
discharge (cfs)
water level (ft)
storage (cy)
Peaks: 1-yr 24-hr SCS Design Storm
8.119 at
11.96 (hrs)
0.897 at
12.38 (hrs)
747.549 at
12.44 (hrs)
971.910
Peaks: 2-yr 24-hr SCS Design Storm
11.886 at
11.96 (hrs)
5.211 at
12.12 (hrs)
747.887 at
12.12 (hrs)
1093.703
Peaks: 10-yr 24-hr SCS Design Storm
24.612 at
11.96 (hrs)
22.200 at
12.02 (hrs)
748.307 at
12.02 (hrs)
1256.359
Peaks: 100-yr 24-hr
51.695 at
46.508 at
748.887 at
1503.604
Fri ]an 15 19:42:49 EST 2021
SCS Design Storm
11.96 (hrs)
12.02 (hrs)
12.02 (hrs)
Page 4
SWM Facility `C' Routing Calculations
(Confirms the 1-year outflow is compliant with energy balance computations
and the 10-year post -development storm is contained within the facility.)
Pleasant Green- Phases II and III SWM Facility C
BasinFlow printout
INPUT:
Basin: Pleasant Green- Phases II and III SWM Facility C
0 Contour Areas
Elevation(ft) Area(sf) Computed Vol.(cy)
1 Storage Pipes
Storage pipe 0
name: UGD System
diameter (in) 120.000
length (ft) 250.000
invert (ft) 719.420
angle 0.287
volume (cy) 727.221
Start—Elevation(ft) 719.42 Vol.(cy) 0.00
5 Outlet Structures
Outlet structure 0
Orifice
name: Low -Flow Barrel, Str. C-101
area (sf)
1.767
diameter or depth (in)
18.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
719.400
multiple
1
discharge out
of riser
Page 1
Pleasant Green- Phases II and III SWM Facility C
Outlet structure 1
Orifice
name: Low -Flow Orifice
area (sf) 0.136
diameter or depth (in) 5.000
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 719.420
multiple 1
discharge into riser
Outlet structure 2
Orifice
name: Overflow / Emergency Spillway in Weir Plate
area (sf) 3.839
diameter or depth (in) 26.530
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 725.920
multiple 1
discharge into riser
Outlet structure 3
Weir
name:
Access Riser
diameter (in)
48.000
side angle
0.000
coefficient
3.300
invert (ft)
737.750
multiple
1
discharge
into riser
transition at (ft)
1.215
orifice coef.
0.500
orifice area (sf)
12.566
Outlet structure 4
Orifice
name: High Flow Bypass Barrel, Str. C-109
area (sf) 1.767
diameter or depth (in) 18.000
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 725.920
multiple 1
discharge into riser
Page 2
Pleasant Green-
3 Inflow Hydrographs
Hydrograph 0
SCS
Phases II and III SWM Facility C
name: 1-yr 24-hr SCS Design Storm
Area (acres)
9.010
CN
75.000
Type 2
rainfall, P (in)
3.090
time of conc. (hrs)
0.1900
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
10.126
peak time (hrs)
11.953
volume (cy)
1234.455
Hydrograph 1
SCS
name: 2-yr
24-hr SCS Design Storm
Area (acres)
9.010
CN
75.000
Type 2
rainfall, P (in)
3.730
time of conc. (hrs)
0.1900
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
14.562
peak time (hrs)
11.953
volume (cy)
1775.231
Hydrograph 2
SCS
name: 10-yr
24-hr SCS Design Storm
Area (acres)
9.010
CN
75.000
Type 2
rainfall, P (in)
5.610
time of conc. (hrs)
0.1900
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
29.307
peak time (hrs)
11.953
volume (cy)
3572.773
Page 3
Pleasant Green- Phases II and III SWM Facility C
OUTPUT:
Routing Method: storage -indication
Hydrograph 0
Routing Summary
of
Peaks: 1-yr 24-hr
SCS Design Storm
inflow
(cfs)
10.122
at
11.96
(hrs)
discharge
(cfs)
1.402
at
12.34
(hrs)
water level
(ft)
725.960
at
12.34
(hrs)
storage
(cy)
447.477
Hydrograph 1
Routing Summary
of
Peaks: 2-yr 24-hr
SCS Design Storm
inflow
(cfs)
14.556
at
11.96
(hrs)
discharge
(cfs)
10.114
at
12.08
(hrs)
water level
(ft)
726.776
at
12.08
(hrs)
storage
(cy)
520.067
Hydrograph 2
Routing Summary
of
Peaks: 10-yr 24-hr
SCS Design Storm
inflow
(cfs)
29.295
at
11.96
(hrs)
discharge
(cfs)
22.224
at
12.06
(hrs)
water level
(ft)
729.981
at
12.06
(hrs)
storage
(cy)
722.259
Mon Nov 30 09:40:03 EST 2020
Page 4
SWM Facility'D' Routing Calculations
(Confirms the 1-year outflow is compliant with energy balance computations
and the 10-year post -development storm is contained within the facility.)
Pleasant Green- Phases II and III SWM Facility D
BasinFlow printout
INPUT:
Basin: Pleasant Green- Phases II and III SWM Facility D
0 Contour Areas
Elevation(ft) Area(sf) Computed Vol.(cy)
1 Storage Pipes
Storage pipe 0
name: UGD System 'D'
diameter (in) 120.000
length (ft) 177.750
invert (ft) 695.500
angle 0.287
volume (cy) 517.054
Start—Elevation(ft) 695.50 Vol.(cy) 0.00
5 Outlet Structures
Outlet structure 0
Orifice
name: Low -Flow Barrel
area (sf)
1.227
diameter or depth (in)
15.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
694.470
multiple
1
discharge out
of riser
Outlet structure 1
Orifice
name: Low -Flow Orifice in Weir Plate
area (sf)
0.028
diameter or depth (in)
2.250
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
695.500
multiple
1
discharge into
riser
Page 1
Pleasant Green- Phases II and III SWM Facility D
Outlet structure 2
Orifice
name: Overflow / Emergency Spillway in Weir Plate
area (sf)
2.796
diameter or depth (in)
22.640
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
703.500
multiple
1
discharge into
riser
Outlet structure 3
Weir
name:
Access MH Below System
diameter (in)
48.000
side angle
0.000
coefficient
3.300
invert (ft)
707.000
multiple
1
discharge
into riser
transition at (ft)
1.215
orifice coef.
0.500
orifice area (sf)
12.566
Outlet structure 4
Orifice
name: High Flow Bypass Barrel
area (sf)
1.767
diameter or depth (in)
18.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
703.500
multiple
1
discharge into
riser
Page 2
Pleasant Green-
3 Inflow Hydrographs
Hydrograph 0
SCS
Phases II and III SWM Facility D
name: 1-yr 24-hr SCS Design Storm
Area (acres)
4.790
CN
79.000
Type 2
rainfall, P (in)
3.090
time of conc. (hrs)
0.2300
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
6.275
peak time (hrs)
11.991
volume (cy)
807.137
Hydrograph 1
SCS
name: 2-yr
24-hr SCS Design Storm
Area (acres)
4.790
CN
79.000
Type 2
rainfall, P (in)
3.730
time of conc. (hrs)
0.2300
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
8.735
peak time (hrs)
11.991
volume (cy)
1123.618
Hydrograph 2
SCS
name: 10-yr
24-hr SCS Design Storm
Area (acres)
4.790
CN
79.000
Type 2
rainfall, P (in)
5.610
time of conc. (hrs)
0.2300
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
16.671
peak time (hrs)
11.991
volume (cy)
2144.431
Page 3
Pleasant Green- Phases II and III SWM Facility D
OUTPUT:
Routing Method: storage -indication
Hydrograph 0
Routing Summary
of
Peaks: 1-yr 24-hr
SCS Design Storm
inflow
(cfs)
6.244
at
11.98
(hrs)
discharge
(cfs)
0.311
at
12.70
(hrs)
water level
(ft)
703.479
at
12.88
(hrs)
storage
(cy)
417.579
Hydrograph 1
Routing Summary
of
Peaks: 2-yr 24-hr
SCS Design Storm
inflow
(cfs)
8.692
at
11.98
(hrs)
discharge
(cfs)
5.584
at
12.12
(hrs)
water level
(ft)
704.158
at
12.12
(hrs)
storage
(cy)
454.010
Hydrograph 2
Routing Summary
of
Peaks: 10-yr 24-hr
SCS Design Storm
inflow
(cfs)
16.589
at
11.98
(hrs)
discharge
(cfs)
15.707
at
12.04
(hrs)
water level
(ft)
705.278
at
12.04
(hrs)
storage
(cy)
501.424
Fri Jan 29 10:26:17 EST 2021
Page 4
Riprap Lined Outlet Protection Calculations
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NOTEWORTHY WATER QUALITY ITEMS:
1. The following water quality analysis is for phases I, II and III. The boundary for the ESC limits of
disturbance (for Approved Phase I and Proposed Phases II & III) was used for the VRRM boundary. The
total limits of disturbance for analysis #1 is 30.14 acres. This is equal to approved phase 1 (13.15 acres)
and proposed phases II & III (16.99 acres). A more detailed summary of the groundcovers and the
removal rates follow this sheet.
2. The approved Phase I (WPO 201900003) and the proposed Phases 11 & III result in a surplus in water
quality credit. This credit will be applied to the Connector Road VSMP plan (WPO 201900027), making
that plan's approval contingent upon the prior approval of this plan. Please note, a separate VSMP
plan (Amendment #1 to WPO 201600069) is required prior to the approval of the Connector Road
VSMP Plan (WPO 201900027), however this proposed Phases 11 & III plan is not contingent upon the
approval of Amendment #1 to WPO 201600069.
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Virginia Runoff Reduction Method for New Developments
(Analysis #2- Includes Phases I, II, III and the Connector Road)
NOTEWORTHY WATER QUALITY ITEMS:
1. The following water quality analysis is for phases I, II, III and the Connector Road. The boundary for the
ESC limits of disturbance (for Approved Phase I, Proposed Phases II & III and the Connector Road) was
used for the VRRM boundary. The total limits of disturbance for analysis #2 is 32.13 acres. This is
equal to approved phase I (13.15 acres), proposed phases II & III (16.99 acres) and the Connector Road
(1.99 ac.) proposed under WPO 201900027. A more detailed summary of the groundcovers and the
removal rates follow this sheet.
2. The approved Phase I (WPO 201900003) and the proposed Phases II & III result in a surplus in water
quality credit, as seen in the following calculations. This credit will be applied to the Connector Road
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plan. Please note, a separate VSMP plan (Amendment #1 to WPO 201600069) is required prior to the
approval of the Connector Road VSMP Plan (WPO 201900027), however this proposed Phases II & III
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this plan's proposed Phases II & IIII SWM Forest & Open Space Easements.
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