HomeMy WebLinkAboutWPO202000053 Calculations 2020-12-11PLEASANT GREEN- PHASES II & III
STORMWATER MANAGEMENT CALCULATIONS PACKET
F Lfr�1
SCOTT R. LLINS;
No. 35791 a
11/30/20
� 1`�'1oNAL
Date of Calculations
NOVEMBER 30, 2020
PREPARED BY:
COLLINS =NGi,-4EERIN
200 GARRETT STREET, SUITE K
CHARLOTTESVILLE, VA 22902
434.293.3719 PH
434.293.2813 FX
www.collins-engineering.com
SCS TR-55 Calculations
Soils Table
(Source: NRCS Web Soil Survey Online Database)
Albemarle County, Virginia (VA003)
Map
Unit
Map Unit Name
Acres Percent of
Symbol
in AOI AOI
713
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: NCAA's National Weather Service, Hydrometerorological Design Studies Center, Atlas 14
Point Precipitation Frequency Estimates)
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U.S. Department of Agriculture
Natural Resources Conservation Service
TR 55 Worksheet 3: Time of Concentration (T,) 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, P, (in.)
5 Land slope, s (ft/ft)
6 Compute T, = [0.007(n*L)"aj / Pi .5 0.4
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
DAB
DA C
DA D
DAB
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 T,=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"sj/n
18 Flow length, L (ft)
19 T,= L / 3600*V
20 Watershed or subarea T, or T,
(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 !� I.F. * (Q Pre -Developed * IRV Pre -Developed I ) / (RV Developed J
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)
1.76 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.b.:
The point of discharge (the SWM facility outfall), "releases a postdevelopment peak flow
rate for the 10-year 24-hour storm event that is less than the predevelopment peak flow rate
from the 10-year 24-hour storm event. Downstream stormwater conveyance systems do not
require any additional analysis to show compliance with flood protection criteria if this
option is utilized."
10-Year, 24-Hour Pre -Development Subarea 1 Peak Flow Rate = 19.90 cfs
10-Year, 24-Hour Post -Development Subarea 1 Peak Flow Rate = 15.96 cfs
Section C.3 Limits of Analysis
The flood protection analysis terminates at the SWM facility since subsection C.2.b is met.
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 !� I.F. * (Q Pre -Developed * IRV Pre -Developed I ) / (RV Developed J
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 !� I.F. * (Q Pre -Developed * IRV Pre -Developed I ) / (RV Developed J
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)
0.30 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 Water Surface Elevation = 700.28 ft
SWM Facility'D' Top of Dam Elevation = 702.00 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."
Riprap Lined Level Spreader Calculations
LEVEL SPREADER W 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 m i 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 14, the Level Spreader should be designed per the following:
"Where the Conserved Open Space is a mix of grass and forest (or re -forested), establish the Level
Spreader length by computing a weighted average of the lengths required for each vegetation type."
75%+/- Native Grasses/groundcover below tree canopy at a LS length of 13 ft per 1 cfs
25%+/- Forest Litter/groundcover below tree canopy at a LS length of 40 ft per 1 cfs
Site Specific / Weighted Average LS length = 19.8 ft
LS W 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 14, the Level Spreader should be designed per the following:
"Where the Conserved Open Space is a mix of grass and forest (or re -forested), establish the Level
Spreader length by computing a weighted average of the lengths required for each vegetation type."
75%+/- Native Grasses/groundcover below tree canopy at a LS length of 13 ft per 1 cfs
25%+/- Forest Litter/groundcover below tree canopy at a LS length of 40 ft per 1 cfs
Site Specific / Weighted Average LS length = 19.8 ft
LS'C' Design Length= 93 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 14, the Level Spreader should be designed per the following:
"Where the Conserved Open Space is a mix of grass and forest (or re -forested), establish the Level
Spreader length by computing a weighted average of the lengths required for each vegetation type."
75%+/- Native Grasses/groundcover below tree canopy at a LS length of 13 ft per 1 cfs
25%+/- Forest Litter/groundcover below tree canopy at a LS length of 40 ft per 1 cfs
Site Specific / Weighted Average LS length = 19.8 ft
LS'D' Design Length= 64 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 peak 10-year post -development outfall is less than the peak
pre -development 10-year flow.)
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
4442.00
446.2
748.00
6755.00
857.9
750.00
9497.00
1457.0
Start—Elevation(ft)
5 Outlet Structures
Outlet structure 0
Orifice
739.75 Vol.(cy) 0.00
name: Barrel
area (sf) 1.767
diameter or depth (in) 18.000
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) 739.750
multiple 1
discharge into riser
Page 1
Pleasant Green- Phases II and III SWM Facility B
Outlet structure 2
Orifice
name: Mid -Flow Orifices
area (sf) 0.785
diameter or depth (in) 12.000
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 745.670
multiple 4
discharge into riser
Outlet structure 3
Weir
name: Riser
diameter (in) 48.000
side angle 0.000
coefficient 3.300
invert (ft) 747.330
multiple 1
discharge into riser
transition at (ft) 1.215
orifice coef. 0.500
orifice area (sf) 12.566
Outlet structure 4
Weir
name: Emergency Spillway
length (ft) 25.000
side angle 0.000
coefficient 3.300
invert (ft) 748.330
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)
1.757 at
12.24 (hrs)
744.975 at
12.24 (hrs)
297.379
Peaks: 2-yr 24-hr SCS Design Storm
11.886 at
11.96 (hrs)
3.966 at
12.16 (hrs)
745.991 at
12.16 (hrs)
444.745
Peaks: 10-yr 24-hr SCS Design Storm
24.612 at
11.96 (hrs)
15.956 at
12.08 (hrs)
747.351 at
12.08 (hrs)
705.348
Peaks: 100-yr 24-hr
51.695 at
48.985 at
748.819 at
1078.830
Wed Nov 25 13:41:46 EST 2020
SCS Design Storm
11.96 (hrs)
12.00 (hrs)
12.00 (hrs)
Page 4
SWM Facility `C' Routing Calculations
(Confirms the 1-year outflow is compliant with energy balance computations
and the 10-year outfall is confined within the SWM 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 outfall is confined within the SWM Facility.)
Pleasant Green- Phases II and III SWM Facility D
BasinFlow printout
INPUT:
Basin: Pleasant Green- Phases II and III SWM Facility D
5 Contour Areas
Elevation(ft)
Area(sf)
Computed Vol.(cy)
695.00
1111.00
0.0
696.00
1600.00
49.9
698.00
2826.00
211.7
700.00
4376.00
476.4
702.00
6200.00
866.1
Start—Elevation(ft)
5 Outlet Structures
Outlet structure 0
Orifice
695.00 Vol.(cy) 0.00
name: Str. D-101, A High -Flow Bypass Barrel
area (sf)
3.142
diameter or depth (in)
24.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
696.240
multiple
1
discharge out
of riser
Outlet structure 1
Orifice
name: Str. D-103, A Low -Flow Barrel
area (sf)
0.196
diameter or depth (in)
6.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
694.840
multiple
1
discharge out
of riser
Page 1
Pleasant Green- Phases II and III SWM Facility D
Outlet structure 2
Orifice
name: Low -Flow Orifice
area (sf) 0.034
diameter or depth (in) 2.500
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 695.000
multiple 1
discharge into riser
Outlet structure 3
Weir
name: Str. D100, A Riser
diameter (in) 48.000
side angle 0.000
coefficient 3.300
invert (ft) 699.750
multiple 1
discharge into riser
transition at (ft) 1.215
orifice coef. 0.500
orifice area (sf) 12.566
Outlet structure 4
Weir
name: Emergency Spillway
length (ft) 20.000
side angle 0.000
coefficient 3.300
invert (ft) 700.750
multiple 1
discharge through dam
4 Inflow Hydrographs
Hydrograph 0
SCS
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
Page 2
Pleasant Green- Phases II and III SWM Facility D
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
Hydrograph 3
SCS
name: 100-yr
24-hr SCS Design Storm
Area (acres)
4.790
CN
79.000
Type 2
rainfall, P (in)
9.120
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)
32.800
peak time (hrs)
11.991
volume (cy)
4219.055
Page 3
Pleasant Green- Phases II and III SWM Facility D
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
6.244 at
11.98 (hrs)
0.295 at
12.86 (hrs)
699.744 at
12.86 (hrs)
435.895
Peaks: 2-yr 24-hr SCS Design Storm
8.692 at
11.98 (hrs)
5.231 at
12.12 (hrs)
699.992 at
12.12 (hrs)
475.034
Peaks: 10-yr 24-hr SCS Design Storm
16.589 at
11.98 (hrs)
16.354 at
12.02 (hrs)
700.281 at
12.02 (hrs)
523.132
Peaks: 100-yr 24-hr
32.637 at
30.113 at
700.907 at
636.333
Mon Nov 02 14:54:17 EST 2020
SCS Design Storm
11.98 (hrs)
12.04 (hrs)
12.04 (hrs)
Page 4
Riprap Lined Outlet Protection Calculations
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Water Quality Calculations for Pleasant Green, Using the DEQ
Virginia Runoff Reduction Method for New Developments
(Includes Phases I, II, III and the Connector Road)
NOTEWORTH WATER QUALITY ITEMS:
1. A water quality analysis for the entire Pleasant Green development was completed. The analysis
included approved WPO 201900003's phase I, the proposed connector road under WPO 201900027
and this plan's phases II and III. The analysis also used the total ESC limits of disturbance boundary for
the VRRM water quality boundary.
2. Please see the following sheet for a summary of the required minimum phosphorous removal rate, as
well as a breakdown of how the Pleasant Green development meets this requirement.
SUMMARY OF PHOSPHOROUS REMOVAL RATE REQUIRED
Total Phosphorous Removal Rate Required for Proposed Pleasant Green Phases I, II, III and the Connector Road 24.57 Ibs/yr
Portion of Phosphorous Removal Rate Provided (By Others) for Connector Road's Impacts to SWM Forest & Open Space Esmt. 0.37 Ibs/yr
Total Phosphorous Removal Rate Required for the Proposed Pleasant Green Developer 24.20 Ibs/yr
SUMMARY OF PHOSPHOROUS REMOVAL RATE PROVIDED
Approved Phase I (WPO 201900003) Phosphorous Removal Rate Achieved from Nutrient Credits 6.97 Ibs/yr
Connector Road (WPO 201900027) Phosphorous Removal Rate Achieved from Level II Bioretention Basin (Subarea A) 1.17 Ibs/yr
Proposed Phosphorous Removal Rate Achieved from Phase It's Outfall to Conserved Open Spaces (Subarea B) 6.07 Ibs/yr
Proposed Phosphorous Removal Rate Achieved from Phase II's Outfall to Conserved Open Spaces (Subarea C) 7.6 Ibs/yr
Proposed Phosphorous Removal Rate Achieved from Phase 11 & III's Outfall to Conserved Open Spaces (Subarea D) 4.60 Ibs/yr
Approved Phase I (WPO 201900003) Phosphorous Removal Rate Achieved from the Level II Bioretention Basin (Subarea E) 1.42 Ibs/yr
Total Phosphorous Removal Rate Provided 27.83 Ibs/yr
Total Phosphorous Removal Rate Provided Onsite 20.86 Ibs/yr
Percentage of the Required Phosphorous Removal Rate Treated Onsite (20.86 / 24.20 x 100) 86 %
ml
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Runoff Volume and Curve Number Calculations
Enter design storm rainfall
depths (in):
1-year storm 2-year storm
10-year storm
3.09 373
561 —11
Use NOAA All., 14 pw,Vbds.nws oaa.9ou//'ds/pJdSQ
•Notes (see below):
[1] The curve numbers and rmwff volumes computed in this spreadsheet for
each drainage area are limited in their applicability for determining and demonstrating compliance with water quantity
requirements. See VRRM user's Guide and Documentation M additional information.
121 Runoff Volume (RV) for on, and pactdr-firpment drainage areas must be
in wlumetric units leg., acce-feet or cubic
been when using the Energy Balance Equation. Runoff measured in watershed-
nches and shown in the spreadsheet as RVlwatershed-in[hl can only be used
in the EneW Balance Equation when the
pre -and po4development drainage areas are equal. Otherwise RVleatershed-inc
must be multiplied by the drainage area.
13l Adjusted CNs are based on runoff reduction volumes as calculated in GA.
tabs. An alternative CN adjustment calculation
for Vegetated Roofs is included in BMp specification No. S.
Drainage Area Curve Numbers and
Runoff Depths*
Curve numbers (CN, Cli and rapi depths (RV,) are completed with and without reduction pmctii
Drainage Area
ASolis BSoils
CSoils I DSoils TatalArea(acres): 1.10
Forert/Open Space— undisturbed, protected Area(-mi
brest/open space comfntated land CN
Managed Turf— disturbed, gaoled for yards or other WH Area(acres)
non o.0a
30 55
0.59 0.00
crop Crap Runoa ReduRbn
70 ]T Volume its: 1,664
crop Crap
towmrwed/mans d CN
Impervious Cover Area(anes)
39 61
0.51 0.00
74 80
O.ap coup
CN
98 98
98 98
r ili
66
_
1-year storm 2-year storm
10.year storm
RVDevemised (Ropleco ed inch) with no Runoff Reduction•
os9 a.93
216
_
Info eF•d (wmemed-inch) with Runoff Reduction•
C.v os1
1.T4
Mitered! EN•
I 53 5T
a
'See Notes above
Drainage Area
Forest/Open Space— undisturbed, protected 1 Area(acres)
ASolls BSolls
coup 000
CSolls DSoils Total AMA jacres): 7.97
Crop nor RunoRReductbn
forest/open space .-greased land CN
30 55
70 A Volume its: 9669
Managed Turf—diUurbed,gadedforyaNsorotherW Arca(acres)
0.00 5.36
Crop Oran
townowed/managed CN
39 61
74 80
Impervious Cover Area(acres)
0.00 2.61
scram 0.00
CN
98 98
98 98
[NIA
T,
RVDeveloped (wh tershed-inch) with no Runoff Reduction'
RVp„„yred (watershed -inch) with Runoff Reducton' i
AdjustedN' C
1-year storm 2-year storm
1 1.11 1.34
osB ioa
66 67
10.year storm
2.77
2A3
6s
'See Notes above
Drainage AreaC
F.de./Open Space— undisturbed, puddecled Area a
fares!/ reforested lantl CNcre
Managed Turf— disturbed, graded for yards or honer to prea laces)
ASoils
0.00
0.00
BSoils
DAD
55
I.AB
CSoils
00
o.O
70
0.00
DSoils
CLOD
])
ODD
Total Area laden):
RunoRReduction
Volume h'i
9.01
12114
.bemowed/mana d CN
39
61
74
80
Impervious Cover Arealacres)
0.00
3.53
0.00
0.0D
CN
98
98
98
98
CNIa.Aq
RVoevelolua(watershed-rcAwwraftfflimo Runoff Reducion•
RVaawaa (svntershed-isrcM1unoff Reduction'
...
iAtllusted EN
1-year storm
1D2
0.65
67
2-year storm
1.4T
1.1a
69
1(l-year storm
255
2sg
Ti
TS
•See Notes above
Drainage Area
Forest/Open Space-- undisturbed, protected Arealed res)
ASoils
0.00
BSoils
DAD
CSoils
Drop
DSoils
ODO
Total Area lacres):
RunoRReduclion
4.79
farest/open space or reforesed land CN
Managed Turf-- disturbed, graded for yartls or Other N Arealacres)
30
0.00
55
2AR
70
0.00
]T
oD0
Volume ha:
7325
to be ne—djmana d CN
39
61
74
80
Impervious Cover Arealacres
ODD
2.31
ODD
.1
CN
98
98
98
98
[Mara ul
19
Wow,,d (watershed -inch) with no Runoff Redu[[ion'
RVo„a d (watershed -inch) with Runoff Reduction'
_..
Adlusted CN'
1-year storm
125
o.g3
TS
2-year storm
1.T5
1.33
T3
I0.Vear storm
3 33
2.91
T4
•See Notes above
Drainage Area
ASoils
BSoils
CSoils
DSoils
Total Area (aores):
1.66
Forest/Open Space— undisturbed, protected Arealacres)
faresth nn space or reforested land CN
Managed Turf— disturbed, graded for yartls or other to Arealacres)
to be mowed/mana ed CN
Impervious [over Arealacres)
O.DD
30
0.00
39
0.00
DAD
55
1.18
61
OAR
om
70
oD0
74
0.0D
oD0
D
oD0
80
oD0
RunoRReduRbn
Volume ha:
2,010
CN
98
98
98
98
CNIaa.I
13
1-year storm
2-year storm
10.year storm
_ _- -
- -RVpp (watershed -inch) with no Runoff Reducion•
RVoaagea (watershed -inch) wish Runoff Reduction'
iAdlusted CN'
Dss
os3
65
1.2T
p.94
66
zsa
n34
0
*See Notes above
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