HomeMy WebLinkAboutWPO202100034 Plan - VSMP 2021-12-17 (3)A � PRE -DEVELOPMENT SWM PLAN:
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GRAPHIC SCALE RUNOFF ANALYSIS
50 0 25 50 100 200
CURVE NUMBER ANALYSIS
1. Runoff curve Number (CN)
Cover description
Soil name and
CN (weighted) _
Drainage Area
(Covertype, treatment, and hydrologic condition;
Area
Product of CN x
Calculated
h
CN
tototalal
Description
percent impervious; unconnected/ connected impervious
(Acres)
Area
'S' Value
A)
(Appendix A)
(Appendix
ar a
total area
area ratio)
DAB
Impervious Areas
98
0.00
0.0
(Pre-Dev.)
B
Lawns in Good Condition (25%+/-)
61
0.10
6.1
58
7.10
Sheetflow
Woods Grass Good Condition
58
1 0.53
1 30.7
DAC
Impervious Areas
98
0.00
0.0
(Post-Dev.)
B
Lawns in Good Condition (25%+/-)
61
0.24
14.6
61
6.39
Sheetflow
Woods Grass Good Condition
58
0.00
0.0
EXISTING 2- AND io-YEAR HYDROGRAPHS
W orksheet 5b (Modified for Clarity): Basic watershed Data
2-yr. 24-hr.SCSTR-55 Method Unit Hydrograph for the la/P &Tc Listed Below
Subarea Tc(hrs.) Tt(hr.) la/P Am*Q (mi2-in)
DA B 0.10 0.10 0.39 0.0005
(Pre-Dev.)
Sheetflow
Hydrograph Time
(hrs.)
2-yr
Discharges
(cfs)
0.0
0.00
11.0
0.00
11.3
0.00
11.6
0.04
11.9
0.18
22.0
0.39
12.1
0.24
12.2
0.11
12.3
0.09
12.4
0.08
12.5
0.07
12.6
0.06
12.7
0.05
12.8
0.05
13.0
0.04
13.2
0.04
13.4
0.04
13.6
0.03
13.8
0.03
14.0
0.03
14.3
0.03
14.6
0.02
15.0
0.02
15.5
0.02
16.0
0.02
16.5
0.02
17.0
0.02
17.5
0.02
18.0
0.01
19.0
0.01
20.0
0.01
22.0
0.00
26.0
0.00
SCS TR-55 Existing Unit
Hydrograph (ON DA B Sheetflow)
6
s
4
3
2
1
0
0 5 10 15 20 25 30
Worksheet 5b (Modified for Clarity): Basic watershed Data
10-yr. 24-hr. SCS TR-55 Method Unit Hydrograph for the la/P & Tc Listed Below
Subarea Tc(hrs.) Tt(hr.) la/P Am*Q(mi2-in)
DA B 0.10 0.10 0.26 0.0015
(Pre-Dev.)
Sheetflow
Hydrograph Time
(hrs.)
10-yr
Discharges
(cfs)
0.0
0.00
11.0
0.02
11.3
0.03
11.6
0.04
11.9
0.37
12.0
0.89
12.1
1.42
12.2
0.84
12.3
0.32
12.4
0.23
12.5
0.20
12.6
0.17
12.7
0.14
12.8
0.12
13.0
0.11
13.2
0.10
13.4
0.09
13.6
0.08
13.8
0.07
14.0
0.07
14.3
0.06
14.6
0.06
15.0
0.05
15.5
0.05
16.0
0.04
16.5
0.04
17.0
0.04
17.5
0.03
18.0
0.03
19.0
0.03
20.0
0.02
22.0
0.02
26.0
0.00
( IN FEET )
1 inch = 50 11.
SCS TR-55 Existing Unit
Hydrograph (ON DA B Sheetflow)
6
5
4
3
2
1
0
0 5 30 15 20 25 30
PROPOSED 2- AND io-YEAR HYDROGRAPHS
Worksheet 5b (Modified for Clarity): Basic watershed Data
2-yr. 24-hr. SCS TR-55 Method Unit Hydrograph forthe la/P & Tc Listed Below
Subarea Tc(hrs.) Tt(hr.) la/P Am*Q(mi2-in)
DAC 0.10 0.10 0.35 0.0002
( Post-Dev. )
Sheetflow
Hydrograph Time
(hrs.)
2-yr
Discharges
(cfs)
0.0
0.00
11.0
0.00
11.3
0.00
11.6
0.03
11.9
0.11
12.0
0.20
12.1
0.12
12.2
0.05
12.3
0.04
12.4
0.04
12.5
0.03
12.6
0.03
12.7
0.02
12.8
0.02
13.0
0.02
13.2
0.02
13.4
0.02
13.6
0.01
13.8
0.01
14.0
0.01
14.3
0.01
14.6
0.01
15.0
0.01
15.5
0.01
16.0
0.01
16.5
0.01
17.0
0.01
17.5
0.01
18.0
0.01
19.0
0.01
20.0
0.01
22.0
0.00
26.0
0.00
SHEETFLOW ANALYSIS NARRATIVE:
F=N _ POST -DEVELOPMENT SWM PLAN: TH pF
SCALE 1"=50'
THE PURPOSE OFTHIS EXHIBIT IS TO ADDRESS CONCERNS OF
SUBAREA AREA'C' IN THE PROPOSED CONDITION DRAINING
FURTHER DOWNSTREAM OF THE LIMITS OF ANALYSIS AS WSCOTT R. I
SHOWN IN THE SWM PLAN (SHEET 8). THIS PORTION OF O� No. 35791
=R SUBAREA'C' SHEETFLOWS OFF THE SITE ALONG THE
=� DOWNSTREAM EDGE OF THE DRAINAGE AREA, FROM THE °po I 12/16/21 naN ADJACENT SITE TO THE NORTH TO BERKMAR DRIVE. PER ��ssl0 Etc
SECTION D OF 9VAC25-870-66, "INCREASED VOLUMES OF SHEET NAL
\ \\ Eby FLOW RESULTING FROM PERVIOUS OR DISCONNECTED
IMPERVIOUS AREAS, OR FROM PHYSICAL SPREADING OF
CONCENTRATED FLOW THROUGH LEVEL SPREADERS, MUST BE
IDENTIFIED AND EVALUATED FOR POTENTIAL IMPACTS ON
DOWN -GRADIENT PROPERTIES OR RESOURCES."
1 /
AS SHOWN ON THE PRE -DEVELOPMENT MAP, AN AREA WITHIN
SUBAREA'B' SHEETFLOWS OFFSITE UNCAPTURED AND ONTO
z - - EITHER BERKMAR DRIVE OR THE ADJACENT PROPERTY TO THE
\\\\ PORTION OF�SUBAREA C NORTH DOWNSTREAM OF THE LIMITS OF ANALYSIS. IN THE
(SHEETFLOW DOWNSTREAM OF POST DEVELOPED CONDITION, A PORTION OF THE SITE STILL
LIMIT§01' ANALYSIS) SHEETFLOWS OFFSITE UNCAPTURED DOWNSTREAM OF THE
�'24ac
LIMITS OF ANALYSIS.
CN=61 BELOW THE MAPS IS AN ANALYSIS OF THE RUNOFF VOLUMES r r
_10 w w
AND FLOWS OF THESE SHEETFLOW AREAS IN THE
1 \\\ PREDEVELOPED CONDITION AND THE PROPOSED CONDITION. Z o o
ANALYSES INCLUDE CURVE NUMBER CALCULATIONS, O a � ou o
\ 1 1 \\ \\\\1\ DRAINAGE AREA FLOW CALCULATIONS, AND VOLUMES tun
- - - - -- CALCULATIONS. AS SHOWN, THE VOLUME OF SHEETFLOW ~ w � M �
V A x -- o m
1 \\\ \ DECREASES FROM THE PREDEVELOPED CONDITION MEANING z < o 0
/ 1 THIS AREA OF CONCERN ACTUALLY DOES NOT HAVE A IH o F
1 1 \ \\\\ \ NEGATIVE IMPACT ON EROSION OR FLOODING AS COMPARED > > z w w
TO THE PREDEVELOPED CONDITION, SO NO FURTHER w
\ 1 \
ANALYSIS OF THIS AREA IS REQUIRED. �W/ a a
THE LIMITS OF ANALYSIS SHOWN ON SHEET 8 WAS CHOSEN AS
1 1 \ IT ENCOMPASSES THE "SITE'S CONTRIBUTING DRAINAGE
y ae1\' \X�� \\ \ AREA." AND TO BE CONSERVATIVE IN THE ANALYSIS THIS
PORTION OF SUBAREA'C' IN THE POST DEVELOPED CONDITION
WAS TAKEN INTO ACCOUNT FOR THE CHANNEL AND FLOOD
PROTECTION EVEN THOUGH IT IS SHEETFLOW. MOVING THE
LIMITS OFANALYSIS DOWNSTREAM WOULD ENLARGEN THE
DRAINAGE AREA, REQUIRING THE DEVELOPMENTTO PROVIDE
CHANNEL AND FLOOD PROTECTION FOR OFFSITE PROPERTIES
THAT ARE NOT APART OF THIS DEVELOPMENT.
Drainage Area
Description
Drainage Area
Description
DA B
(Pre-Dev.)
Sheetflow
DA C
(Post-Dev.)
Sheetflow
Drainage Area (Am) in miles2=
0.0010
0.0004
Runoff curve numberCN=
58
61
Time of concentration (Tc)=
0.10
0.10
Rainfall distribution type=
Type II
Type II
Pond and swamp areas spread
throughout the watershed=
0
0
2. Frequency -years
2 10
2 10
3. Rainfall, P (24 hour)- inches
3.63 5.48
3.63 5.48
4. Initial Abstraction, la- inches
1.42 1.42
1.28 1.28
5. Compute la/P
0.39 0.26
0.35 0.23
6. Unit peak discharge, Qu- csm/in
See
Hydrographs
See
Hydrographs
7. Runoff, Qfrom Worksheet 2- inches
0.52 1.48
0.63 1.67
8. Pond and Swamp adjustment factor, Fp
1 1
1 1
9. Peak Discharge, Qp- cfs
where Qp=Qu Am C. Fp
0.39
1.42
0.20
0.59
SCS TR-55 Post -Development Unit
Hydrograph (ON DA C)
6
5
4
3
2
1
0
0 5 10 15 20 25 30
Worksheet 5b (Modified for Clarity): Basic watershed Data
SHEETFLOW VOLUME ANALYSIS PER 9 VAC 25-870-66 SECTION D
9 VAC 25-870-66
Section D:
"Increased volumes of sheet flow resulting from pervious or disconnected
impervious areas, or from physical spreading of concentrated flow through level
spreaders, must be identified and evaluated for potential impacts on down -gradient
properties or resources. Increased volumes of sheet flow that will cause or
contribute to erosion, sedimentation, or flooding of down gradient properties or
resources shall be diverted to a stormwater management facility or a stormwater
conveyance system that conveys the runoff without causing down -gradient erosion,
sedimentation, orflooding. If all runoff from the site is sheet flow and the conditions
of this subsection are met, no further water quantity controls are required."
Am
(square
Volume
Q(in)
miles)
(acre-ft)
Pre -Development Onsite DAB Runoff Volume for 2-yr Storm (acre-ft)
0.52
0.00098
0.0275
Past -Development Onsite DA C Runoff Volume for 2-yr Storm (acre-ft)
0.63
0.00038
0.0126
Pre -Development Onsite DAB Runoff Volume for 10-yr Storm (acre-ft)
1.48
0.00098
0.0775
Past -Development Onsite DA C Runoff Volume for 10-yr Storm (acre-ft)
1.67
0.00038
0.0333
There is a decrease in sheetflow volume indicating there is no effect on erosion, sedimentation, or
flooding of down gradient properties for DA'C'.
10-yr. 24-hr. SCS TR-55 Method Unit Hydrograph for the la/P & Tc Listed Below
Subarea Tc(hrs.) Tt(hr.) la/P Am*Q(mi2-in)
DAC 0.10 0.10 0.23 0.0006
(Post-Dev.)
Sheetflow
Hydrograph Time
(hrs.)
10-yr
Discharges
(cfs)
0.0
0.00
11.0
0.00
11.3
0.00
11.6
0.01
11.9
0.12
12.0
0.36
12.1
0.59
12.2
0.34
12.3
0.14
12.4
0.10
12.5
0.09
12.6
0.08
12.7
0.06
12.8
0.06
13.0
0.05
13.2
0.05
13.4
0.04
13.6
0.04
13.8
0.03
14.0
0.03
14.3
0.03
14.6
0.03
15.0
0.02
15.5
0.02
16.0
0.02
16.5
0.02
17.0
0.02
17.5
0.02
18.0
0.02
19.0
0.01
20.0
0.01
22.0
0.01
26.0
0.00
SCS TR-55 Post -Development Unit
Hydrograph (ON DA C)
6
5
4
3
z
1
0
0 5 10 15 20 25 30
0
z
W
W
N
c
0
0
ti
M
M
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N
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JOB NO.
202196
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