HomeMy WebLinkAboutWPO202000030 Calculations 2021-01-292415 IVY ROAD COMMERCIAL REDEVELOPMENT
FINAL STORMWATER MANAGEMENT CALCULATIONS PACKET
(WPO 202000030)
Date of Calculations
June 1, 2020
Revised on
January 11, 2021
APPROVED
by the Albemarle County
Community Development Department
Date 01/29/2021
File WP0202000030
PREPARED BY:
COLLINS _.AGINEERING
200 GARRETT STREET, SUITE K
CHARLOTTESVILLE, VA 22902
434.293.3719 PH
434.293.2813 FX
www.collins-engineering.com
Table of Contents
Section 1- DEQ Virginia Runoff Reduction Method for ReDevelopment Water Quality Calculations
Section 2- SCS TR-55 Calculations
Section 3- Channel and Flood Protection Compliance
Section 4- Pre -Development SCS Unit Hydrographs
Section 5- Post -Development SCS Unit Hydrographs
Section 6- Proposed Dry Detention Basin Routing Calculations
Section 1
DEQ Virginia Runoff Reduction Method for ReDevelopment
Water Quality Calculations
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Section 2
SCS TR-55 Calculations
Soils Mapping (Source: NRCS Web Soil Survey Online Database)
Soils Table (source: NRCS Web Soil Survey Online Database)
Albemarle County, Virginia (VA003)
Map Unit
Map Unit Name
percent of
Symbol
AOI
19B
Minnieville
3.0%
loam, 2 to 7
percent slopes
36C
Hayesville
4.1%
loam, 7 to 15
percent slopes
91 Urban land 92.9%
Totals for Area of 100.0010
Interest
The NRCS web soil survey defiaerialnes this development's soils as being primarily Urban Land. The following
calculations conservatively consider 'Urban Land' as having a hydrologic group type of D.
Precipitation Data
(Source: NOAA's National Weather Service, Hydrometerorological Design Studies Center, Atlas 14
Point Precipitation Frequency Estimates)
Map
Terrain
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POINT PRECIPITATION FREQUENCY (PF) ESTIMATES
WITH 90% CONFIDENCE INTERVALS AND SUPPLEMENTARY INFORMATION
NOAA Atlas 14, Volume 2, Version 3
PDS-based precipitation frequency estimates with 90% confidence intervals (in inches)'
Average recwr imemN (years)
cw;- 0+ OD to 25 ® too ®® t000
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Section 3
Channel and Flood Protection Compliance
9 VAC 25-870-66 (Onsite Subareas A & 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 (Calcuated(:5 1.F• * I (Q Pre -Developed * RV Pre -Developed) RV Developed )
Q Allowed (Calcuated( <
Where: QPre-Developed—
CN Pre -Developed —
S Pre -Developed —
RV Pre -Developed =
CN Post -Developed =
S Post -Developed =
RV Post-Developed—
I.F. _
Q Developed
7.25 cfs
9.89 cfs
(From Worksheet 4 Watershed Summary)
86
(From Worksheet 4 Watershed Summary)
1.62
(From SCS TR-55 eq. 2-4)
1.70 in.
(From SCS TR-55 eq. 2-3)
88
(From Worksheet 4 Watershed Summary)
1.36
(From SCS TR-55 eq. 2-4)
1.86 in.
(From SCS TR-55 eq. 2-3)
0.80
(Factor of Safety)
= 7.12 cfs
Section B.4 Limits of Analysis
The analysis terminates at the site outfalls (i.e. outfalls'A' &'B') since "...subsection 3 of this
subsection is utilized to show compliance with the channel protection criteria..."
9 VAC 25-870-66 (Onsite Subareas A & 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 (i.e. outfalls'A' &'B'), "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."
Pre -Development 10-yr 24-hr SCS TR-55 Design Storm Peak Total Flow = 23.12 cfs
Post -Development 10-yr 24-hr 5CS TR-55 Design Storm Peak Total Flow = 18.90 cfs
Section C.3 Limits of Analysis
The flood protection analysis terminates at the site outfall (i.e. outfalls 'A' & 'B') since
subsection C.2.b is met.
Section 4
Pre -Development SCS Unit Hydrographs
Worksheet 5b (Modified for Clarity): Basic watershed Data
1-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 (mi'-in)
Onsite DA A (Pre-Dev.) 0.16 0.16 0.10 0.0086
Hydrograph Time 1-yr Discharges
(hrs.)
(cfs)
0.0
0.00
11.0
0.21
11.3
0.29
11.6
0.45
11.9
2.87
12.0
5.55
12.1
8.67
12.2
5.35
12.3
1.86
12.4
1.26
12.5
1.06
12.6
0.89
12.7
0.74
12.8
0.65
13.0
0.57
13.2
0.49
13.4
0.44
13.6
0.39
13.8
0.36
14.0
0.33
14.3
0.29
14.6
0.27
15.0
0.25
15.5
0.22
16.0
0.20
16.5
0.18
17.0
0.17
17.5
0.16
18.0
0.15
19.0
0.13
20.0
0.11
22.0
0.10
26.0
0.00
SCS TR-55 Pre -Development Unit
Hydrograph (Onsite DA A)
30.0
Worksheet 5b (Modified for Clarity): Basic watershed Data
2-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 (mil -in)
Onsite DA A (Pre-Dev.) 0.16 0.16 0.08 0.0113
Hydrograph Time 2-yr Discharges
(hrs.)
(cfs)
0.0
0.00
11.0
0.27
11.3
0.38
11.6
0.60
11.9
3.77
12.0
7.31
12.1
11.41
12.2
7.04
12.3
2.45
12.4
1.66
12.5
1.39
12.6
1.17
12.7
0.97
12.8
0.86
13.0
0.75
13.2
0.64
13.4
0.58
13.6
0.52
13.8
0.47
14.0
0.43
14.3
0.38
14.6
0.36
15.0
0.33
15.5
0.29
16.0
0.26
16.5
0.24
17.0
0.23
17.5
0.21
18.0
0.20
19.0
0.17
20.0
0.15
22.0
0.14
26.0
0.00
12.00
10.00
8.00
6.00
4.00
2.00
0.00
0.0
SCS TR-55 Pre -Development Unit
Hydrograph (Onsite DA A)
5.0 10.0 15.0 20.0 25.0 30.0
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)
Onsite DA A (Pre-Dev.) 0.16 0.16 0.05 0.0198
Hydrograph Time 10-yr Discharges
(hrs.)
(cfs)
0.0
0.00
11.0
0.47
11.3
0.67
11.6
1.05
11.9
6.60
12.0
12.78
12.1
19.95
12.2
12.31
12.3
4.29
12.4
2.90
12.5
2.43
12.6
2.05
12.7
1.70
12.8
1.50
13.0
1.30
13.2
1.13
13.4
1.01
13.6
0.91
13.8
0.83
14.0
0.75
14.3
0.67
14.6
0.63
15.0
0.57
15.5
0.51
16.0
0.45
16.5
0.41
17.0
0.40
17.5
0.38
18.0
0.36
19.0
0.30
20.0
0.26
22.0
0.24
26.0
0.00
SCS TR-55 Pre -Development Unit
Hydrograph (Onsite DA A)
30.0
Worksheet 5b (Modified for Clarity): Basic watershed Data
1-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 (mil -in)
Onsite DA B (Pre-Dev.) 0.16 0.16 0.16 0.0012
Hydrograph Time 1-yr Discharges
(hrs.)
(cfs)
0.0
0.00
11.0
0.03
11.3
0.04
11.6
0.06
11.9
0.40
12.0
0.78
12.1
1.22
12.2
0.75
12.3
0.26
12.4
0.18
12.5
0.15
12.6
0.13
12.7
0.10
12.8
0.09
13.0
0.08
13.2
0.07
13.4
0.06
13.6
0.06
13.8
0.05
14.0
0.05
14.3
0.04
14.6
0.04
15.0
0.04
15.5
0.03
16.0
0.03
16.5
0.03
17.0
0.02
17.5
0.02
18.0
0.02
19.0
0.02
20.0
0.02
22.0
0.01
26.0
0.00
SCS TR-55 Pre -Development Unit
Hydrograph (Onsite DA B)
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
0.0 5.0 10.0 15.0 20.0 25.0 30.0
Worksheet 5b (Modified for Clarity): Basic watershed Data
2-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 (mi'-in)
Onsite DA B (Pre-Dev.) 0.16 0.16 0.13 0.0017
Hydrograph Time 2-yr Discharges
(hrs.)
(cfs)
0.0
0.00
11.0
0.04
11.3
0.06
11.6
0.09
11.9
0.56
12.0
1.08
12.1
1.68
12.2
1.04
12.3
0.36
12.4
0.24
12.5
0.20
12.6
0.17
12.7
0.14
12.8
0.13
13.0
0.11
13.2
0.09
13.4
0.08
13.6
0.08
13.8
0.07
14.0
0.06
14.3
0.06
14.6
0.05
15.0
0.05
15.5
0.04
16.0
0.04
16.5
0.03
17.0
0.03
17.5
0.03
18.0
0.03
19.0
0.02
20.0
0.02
22.0
0.02
26.0
0.00
SCS TR-55 Pre -Development Unit
Hydrograph (Onsite DA B)
1.80
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
0.0
5.0
10.0
15.0
20.0
25.0
30.0
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 (mil -in)
Onsite DA B (Pre-Dev.) 0.16 0.16 0.09 0.0031
Hydrograph Time 10-yr Discharges
(hrs.)
(cfs)
0.0
0.00
11.0
0.08
11.3
0.11
11.6
0.17
11.9
1.05
12.0
2.03
12.1
3.17
12.2
1.95
12.3
0.68
12.4
0.46
12.5
0.39
12.6
0.33
12.7
0.27
12.8
0.24
13.0
0.21
13.2
0.18
13.4
0.16
13.6
0.14
13.8
0.13
14.0
0.12
14.3
0.11
14.6
0.10
15.0
0.09
15.5
0.08
16.0
0.07
16.5
0.07
17.0
0.06
17.5
0.06
18.0
0.06
19.0
0.05
20.0
0.04
22.0
0.04
26.0
0.00
SCS TR-55 Pre -Development Unit
Hydrograph (Onsite DA B)
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
0.0 5.0 10.0 15.0 20.0 25.0 30.0
Section 5
Post -Development SCS Unit Hydrographs
Worksheet 51b (Modified for Clarity): Basic watershed Data
1-yr. 24-hr. SCS TR-55 Method Unit Hydrograph for the la/P & Tc Listed Belov
Subarea Tc (hrs.) Tt (hr.) la / P Am*Q (mi2-in)
Onsite DA B (Post-Dev.) 0.15 0.15 0.09 0.0017
Hydrograph Time 1-yr Discharges
(hrs.)
(cfs)
0.0
0.00
11.0
0.04
11.3
0.06
11.6
0.09
11.9
0.57
12.0
1.10
12.1
1.72
12.2
1.06
12.3
0.37
12.4
0.25
12.5
0.21
12.6
0.18
12.7
0.15
12.8
0.13
13.0
0.11
13.2
0.10
13.4
0.09
13.6
0.08
13.8
0.07
14.0
0.06
14.3
0.06
14.6
0.05
15.0
0.05
15.5
0.04
16.0
0.04
16.5
0.04
17.0
0.03
17.5
0.03
18.0
0.03
19.0
0.03
20.0
0.02
22.0
0.02
26.0
0.00
2.00
1.80
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
0.0
SCS TR-55 Post -Development Unit
Hydrograph (Onsite DA 13)
5.0 10.0 15.0 20.0 25.0
30.0
Worksheet 51b (Modified for Clarity): Basic watershed Data
2-yr. 24-hr. SCS TR-55 Method Unit Hydrograph for the la/P & Tc Listed Belov
Subarea Tc (hrs.) Tt (hr.) la / P Am*Q (mi2-in)
Onsite DA B (Post-Dev.) 0.15 0.15 0.07 0.0022
Hydrograph Time 2-yr Discharges
(hrs.)
(cfs)
0.0
0.00
11.0
0.05
11.3
0.08
11.6
0.12
11.9
0.74
12.0
1.44
12.1
2.24
12.2
1.38
12.3
0.48
12.4
0.33
12.5
0.27
12.6
0.23
12.7
0.19
12.8
0.17
13.0
0.15
13.2
0.13
13.4
0.11
13.6
0.10
13.8
0.09
14.0
0.08
14.3
0.08
14.6
0.07
15.0
0.06
15.5
0.06
16.0
0.05
16.5
0.05
17.0
0.04
17.5
0.04
18.0
0.04
19.0
0.03
20.0
0.03
22.0
0.03
26.0
0.00
2.50
2.00
1.50
1.00
0.50
0.00
0.0
SCS TR-55 Post -Development Unit
Hydrograph (Onsite DA 13)
5.0 10.0 15.0 20.0 25.0 30.0
Worksheet 51b (Modified for Clarity): Basic watershed Data
10-yr. 24-hr. SCS TR-55 Method Unit Hydrograph for the la/P & Tc Listed Belov
Subarea Tc (hrs.) Tt (hr.) la / P Am*Q (mi2-in)
Onsite DA B (Post-Dev.) 0.15 0.15 0.05 0.0038
Hydrograph Time 10-yr Discharges
(hrs.)
(cfs)
0.0
0.00
11.0
0.09
11.3
0.13
11.6
0.20
11.9
1.28
12.0
2.48
12.1
3.87
12.2
2.38
12.3
0.83
12.4
0.56
12.5
0.47
12.6
0.40
12.7
0.33
12.8
0.29
13.0
0.25
13.2
0.22
13.4
0.20
13.6
0.18
13.8
0.16
14.0
0.15
14.3
0.13
14.6
0.12
15.0
0.11
15.5
0.10
16.0
0.09
16.5
0.08
17.0
0.08
17.5
0.07
18.0
0.07
19.0
0.06
20.0
0.05
22.0
0.05
26.0
0.00
4.50
400
3.50
3.00
2.50
2.00
1.50
1.00
0.50
0.00
0.0
SCS TR-55 Post -Development Unit
Hydrograph (Onsite DA B)
5.0 10.0 15.0 20.0 25.0
30.0
Section 6
Proposed Dry Detention Basin Routing Calculations
Dry Detention Basin
BasinFlow printout
INPUT:
Basin: Dry Detention Basin
7 Contour Areas
Elevation(ft)
Area(sf)
Computed Vol.(cy)
557.19
272.00
0.0
558.00
549.00
12.1
559.00
1140.00
42.7
560.00
1983.00
99.8
561.00
3063.00
192.5
562.00
4517.00
332.0
562.25
3260.00
367.9
Start—Elevation(ft)
3 Outlet Structures
Outlet structure 0
Orifice
557.19 Vol.(cy) 0.00
name: Pipe 11- Barrel Discharging to Outfall 'A'
area (sf)
3.142
diameter or depth (in)
24.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
557.190
multiple
1
discharge out
of riser
Outlet structure 1
Orifice
name: Low -Flow Orifice in Str. 16
area (sf)
0.785
diameter or depth (in)
12.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
557.190
multiple
1
discharge into
riser
Page 1
Dry Detention Basin
Outlet structure 2
Weir
name: Str.
16- Standpipe with Trashrack
diameter (in)
48.000
side angle
0.000
coefficient
3.300
invert (ft)
559.450
multiple
1
discharge
into riser
transition at (ft)
1.215
orifice coef.
0.500
orifice area (sf)
12.566
4 Inflow Hydrographs
Hydrograph 0
SCS
name: 1-yr
24-hr SCS Design Storm Event
Area (acres)
3.090
CN
88.000
Type 2
rainfall, P (in)
3.040
time of conc. (hrs)
0.1500
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
6.670
peak time (hrs)
11.930
volume (cy)
769.521
Hydrograph 1
SCS
name: 2-yr
24-hr SCS Design Storm Event
Area (acres)
3.090
CN
88.000
Type 2
rainfall, P (in)
3.670
time of conc. (hrs)
0.1500
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
8.722
peak time (hrs)
11.930
volume (cy)
1006.327
Page 2
Hydrograph 2
SCS
name:
Area (acres)
CN
Type
rainfall, P (in)
time of conc. (hrs)
time increment (hrs)
time limit (hrs)
fudge factor
routed
peak flow (cfs)
peak time (hrs)
volume (cy)
Hydrograph 3
SCS
name:
Area (acres)
CN
Type
rainfall, P (in)
time of conc. (hrs)
time increment (hrs)
time limit (hrs)
fudge factor
routed
peak flow (cfs)
peak time (hrs)
volume (cy)
Dry Detention Basin
10-yr 24-hr SCS Design Storm Event
3.090
88.000
2
5.550
0.1500
0.0200
30.000
1.00
true
15.089
11.930
1740.833
100-yr 24-hr SCS Design
3.090
88.000
2
9.110
0.1500
0.0200
30.000
1.00
true
27.546
11.930
3178.084
Storm Event
Page 3
Dry Detention Basin
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 Event
6.664 at
11.94 (hrs)
5.404 at
12.02 (hrs)
559.540 at
12.02 (hrs)
69.643
Peaks: 2-yr 24-hr SCS Design Storm Event
8.714 at
11.94 (hrs)
8.443 at
11.96 (hrs)
559.661 at
11.96 (hrs)
76.901
Peaks: 10-yr 24-hr SCS Design Storm Event
15.075 at
11.94 (hrs)
15.025 at
11.96 (hrs)
559.848 at
11.96 (hrs)
89.026
Peaks: 100-yr 24-hr
27.520 at
22.044 at
561.251 at
222.515
Mon Jan 11 10:34:49 EST 2021
SCS Design Storm Event
11.94 (hrs)
12.04 (hrs)
12.04 (hrs)
Page 4