HomeMy WebLinkAboutWPO201700037 Calculations 2020-03-20BROOKHILL STORMWATER MANAGEMENT
CALCULATIONS PACKET
(Town Center)
Date of Calculations
MARCH 16, 2020
PREPARED BY:
COLLINS ENGINEERING
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 Mapping (Source: NRCS Web Soil Survey Online Database)
Soils Table (Source: NRCS Web Soil Survey Online Database)
Albemarle County, Virginia (VA003)
Map Unit
MaUnit Name
p
percent of
symbol
ADI
27B
Elioak loam, 2
35.511/0
to 7 percent
slopes
27C
Elioak loam, 7
27.0%
to 15 percent
slopes
27D
Elioak loam,
2.6%
15 to 25
percent slopes
34C
Glenelg loam,
24.9%
7 to 15
percent slopes
34D
Glenelg loam,
9.911/0
15 to 25
percent slopes
Totals for Area of
100.00/0
Interest
Note: The soils' properties within this development are predominantly composed of hydrologic group type B.
Pre -Construction Aerial Photography
This aerial photograph was taken prior to recent disturbances (shown on the previous page). This
photograph is used soley for the purposes of establishing the pre -construction land covers (i.e. for
the pre -development CN values) for Subareas A & B. Subareas A & B are approximated to be 25%
woods and 75% grass in the pre -development state for this section of Brookhill. This is a more
conservative approach/land cover when compared to using the exposed earth shown on the previous
page.
Precipitation Data
(Source: NOAA's National Weather Service, Hydrometerorological Design Studies Center, Atlas 14
Point Precipitation Frequency Estimates)
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POINT PRECIPITATION FREQUENCY (PF) ESTIMATES
WITH 90%3 CONFIDENCE INTERVALS AND SUPPLEMENTARY INFORMATION
NOAA Atlas 14, Volume 2, Version 3
PD5-based precipitation frequency estimates with 90% confidence intervals [in inches]'
Average recurrence interval [years]
❑uration
1 2 5 7� 25 50 100 200 540 7�a0
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24hr
3.00
{2.6".37)
3.62
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4.63
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5.47
(4.88-6.14y
6.72
[5.95-7.51)
7.79
[6.86-6.69)
8.97
[7.83-9.99]
00.3
{8.97-11.4)
22.2
{10- -13.6)
33.9
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U.S. Department of Agriculture
Natural Resources Conservation Service
TR 55 Worksheet 3: Time of Concentration (Tt) or Travel Time (Tt)
Project: Brookhill
Location: Village Center
Check One: Present X Developed X
Check One: Tc X Tt
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, PZ (in.)
5 Land slope, s (ft/ft)
6 Compute Tt = [0.007(n*L)0.8) / Pzo.s so.a
Shallow Concentrated Flow:
Designed By: FGM, PE
Checked By: SRC, PE
Through subarea n/a
FL-ENG-21A
06/04
Date: 3/16/2020
Date: 3/16/2020
DA A
DA A
DA B
DA B
(Pre-Dev.)
(Post-Dev.)
(Pre-Dev.)
(Post-Dev.)
Dense
Dense
Dense
Dense
Grass
Grass
Grass
Grass
0.24
0.24
0.24
0.24
100
100
100
60
3.62
3.62
3.62
3.62
0.015
0.22
0.015
0.23
0.25
0.09
0.25
0.06
7 Surface description (paved or unpaved) Unpaved Unpaved
8 Flow Length, L (ft) 950 160
9 Watercourse slope, s (ft/ft) 0.011 0.013
10 Average velocity, V (Figure 3-1) (ft/s) 1.7 1.8
11 Tt= L / 3600*V 0.16 0.02
Channel Flow:
12 Cross sectional flow area, a (ft)
13 Wetted perimeter, PW (ft)
14 Hydraulic radius, r = a/PW (ft)
15 Channel Slope, s (ft/ft)
16 Manning's Roughness Coeff, n
17 V= [ 1.49r2/3s05 ) / n
18 Flow length, L (ft)
19 Tt L / 3600*V
20 Watershed or subarea Tc or Tt
(Add Tt in steps 6, 11 and 19)
Note: The shoretest SCS Tc is 0.10 hrs.
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0.00
0.00
0.00
0.41 1 0.10 1 0.27 1 0.10
U.S. Department of Agriculture
Natural Resources Conservation Service
Project:
Brookhill
Location:
Village Center
Check One:
Present X Developed X
TR 55 Worksheet 4: Graphical Peak Discharge Method
Designed By: FGM, PE
Checked By: SRC, PE
Date: 3/16/2020
Date: 3/16/2020
1. Data
Drainage Area Description
Drainage Area Description
Drainage Area Description
Drainage Area Description
DA A
(Pre-Dev.)
DA A
(Post-Dev.)
DA B
(Pre-Dev.)
DA B
(Post-Dev.)
Drainage Area (Am) in mileS2 =
0.0254
0.0266
0.0113
0.0100
Runoff curve number CN=
60
86
60
90
Time of concentration (Tc)=
0.41
0.10
0.27
0.10
Rainfall distribution type--
II
II
II
II
Pond and swamp areas spread
throughout the watershed=
0
0
0
0
2. Frequency -years
1
2
10
1 2 10
1
2
10
1 2 10
3. Rainfall, P (24 hour)- inches
3.00
3.62
5.47
3.00 3.62 5.47
3.00
3.62
5.47
3.00 3.62 5.47
4. Initial Abstraction, la- inches
1.36
1.36
1.36
0.33 0.33 0.33
1.36
1.36
1.36
0.23 0.23 0.23
5. Compute la/P
0.45
0.38
0.25
0.11 0.09 0.06
0.45
0.38
0.25
0.08 0.06 0.04
6. Unit peak discharge, Qu- csm/in
350
425
525
975 1000 1000
425
525
650
1000 1000 1000
7. Runoff, Qfrom Worksheet 2- inches
0.32
0.56
1.55
1.66 2.20 3.91
0.32
0.56
1.55
1.95 2.52 4.29
8. Pond and Swamp adjustment factor, Fp
I 1
1
1
1 1 1 1
1 1
1
1
1 1 1
9. Peak Discharge, Qp- cfs
where Qp=Qu Am Q Fp
1 2.83
6.07
20.62
Routed Through
The SWM Facility
1.52
3.32
11.31
Routed Through
The SWM Facility
WATERSHED SUMMARY FOR SUBAREA A
CN Area, 1-year Flow, 2-year Flow, 10-year Flow,
ac. cfs cfs cfs
DA A (Pre-Dev.) 60 16.25 2.83 6.07 20.62
DA A (Post-Dev.) 86 17.04 0.34 2.17 54.09
WATERSHED SUMMARY FOR SUBAREA B
CN Area, 1-year Flow, 2-year Flow, 10-year Flow,
ac. cfs cfs cfs
DA B (Pre-Dev.) 60 7.20 1.52 3.32 11.31
DA B (Post-Dev.) 90 6.41 0.20 1.04 11.18
Overall Acreage Summary
Pre -Development
Post -Development
Area, ac.
Area, ac.
DA A
16.25
17.04
DA B
7.20
6.41
23.45 23.45
Channel & Flood Protection Compliance Verifications
9 VAC 25-870-66 (Point of Interest #1)
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 * RV Pre -Developed) / RV Developed ]
Q Allowed <
0.43 cfs
Where: Q Pre -Developed =
2.83 cfs
(From Worksheet 4 Watershed Summary)
CN Pre -Developed -
60
(From Worksheet 4 Watershed Summary)
S Pre -Developed -
6•81
(From SCS TR-55 eq. 2-4)
RV Pre -Developed =
0.32 in.
(From SCS TR-55 eq. 2-3)
CN Post -Developed-
86
(From Worksheet 4 Watershed Summary)
S Post -Developed -
1.63
(From SCS TR-55 eq. 2-4)
RV Post -Developed =
1.66 in.
(From SCS TR-55 eq. 2-3)
I.F. =
0.80
(Factor of Safety)
Q Developed =
0.34 cfs
Section B.4 Limits of Analysis
The analysis terminates at Approved 'Wet Pond 4' since subsection B.3.a is met.
9 VAC 25-870-66 (Point of Interest #1)
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:
"Concentrated stormwater flow to stormwater conveyance systems that currently
experience localized flooding [downstream floodplain] during the 10-year 24-hour storm
event: The point of discharge either" must meet a. or b.
Section C.2.a:
"Confines the postdevelopment peak flow rate from the 10-year 24-hour storm event within
the stormwater conveyance system [Wet Pond] to avoid the localized flooding. Detention of
stormwater..." was incorporated.
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. is met.
Section C.3.c
The limits of analysis ends when "The stormwater conveyance system enters a mapped
floodplain or other flood -prone area..."
9 VAC 25-870-66 (Point of Interest #2)
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 * RV Pre -Developed) / RV Developed ]
Q Allowed <
0.20 cfs
Where: Q Pre -Developed =
1.52 cfs
(From Worksheet 4 Watershed Summary)
CN Pre -Developed -
60
(From Worksheet 4 Watershed Summary)
S Pre -Developed -
6•81
(From SCS TR-55 eq. 2-4)
RV Pre -Developed =
0.32 in.
(From SCS TR-55 eq. 2-3)
CN Post -Developed-
90
(From Worksheet 4 Watershed Summary)
S Post -Developed -
1.16
(From SCS TR-55 eq. 2-4)
RV Post -Developed =
1.95 in.
(From SCS TR-55 eq. 2-3)
I.F. =
0.80
(Factor of Safety)
Q Developed =
0.20 cfs
Section B.4 Limits of Analysis
The analysis terminates at Wet Pond B since subsection B.3.a is met.
9 VAC 25-870-66 (Point of Interest #2)
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:
"Concentrated stormwater flow to stormwater conveyance systems that currently
experience localized flooding [downstream floodplain] during the 10-year 24-hour storm
event: The point of discharge either" must meet a. or b.
Section C.2.16:
The proposed Wet Pond 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."
Section C.3: Limits of Analysis
The limits of analysis ends with Wet Pond B since subdivsion 2b is met.
'Wet Pond 4' Routing Calculations
This plan only proposes one change to the approved 'Wet Pond 4'. This plan proposes to modify the
approved riser structure's two SWM orifices, which are currently plugged for erosion & sediment
control / construction purposes. The approved 6" low -flow and 9" mid -flow orifices are being
replaced with new 3" low -flow and 8" mid -flow orifices. The following routing calculations otherwise
simply show the Approved 'Wet Pond 4' can adequately handle Block 5 and the future Town Center
conceptually shown.
BH Town Center- Wet Pond 4 Routings
BasinFlow printout
INPUT:
Basin: BH Town Center- Wet Pond 4 Routings
9 Contour Areas
Elevation(ft)
Area(sf)
Computed Vol.(cy)
358.00
3690.00
0.0
360.00
5715.00
345.6
362.00
8315.00
862.2
364.00
11483.00
1592.3
365.00
14332.00
2069.4
366.00
18294.00
2672.1
368.00
29044.00
4410.1
370.00
33823.00
6736.3
372.00
38733.00
9421.5
Start—Elevation(ft)
5 Outlet Structures
Outlet structure 0
Orifice
366.00 Vol.(cy) 2672.13
name: APPROVED Barrel
area (sf)
19.635
diameter or depth (in)
60.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
363.300
multiple
1
discharge out of
riser
Outlet structure 1
Orifice
name: MODIFIED Low -Flow Orifice
area (sf)
0.049
diameter or depth (in)
3.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
366.000
multiple
1
discharge into
riser
Page 1
Outlet structure 2
Orifice
BH Town Center- Wet Pond 4 Routings
name: MODIFIED Mid -Flow Orifice
area
(sf)
0.349
diameter or depth
(in)
8.000
width for rect.
(in)
0.000
coefficient
0.500
invert
(ft)
369.250
multiple
6
discharge
into riser
Outlet structure 3
Weir
name:
APPROVED
Riser
diameter (in)
72.000
side angle
0.000
coefficient
3.300
invert (ft)
369.800
multiple
1
discharge
into riser
transition at (ft)
1.823
orifice coef.
0.500
orifice area (sf)
28.274
Outlet structure 4
Weir
name: Approved APPROVED Emergency Spillway
length (ft)
30.000
side angle
0.000
coefficient
3.300
invert (ft)
370.200
multiple
1
discharge
through dam
4 Inflow Hydrographs
Hydrograph 0
SCS
name: 1-yr 24-hr TR-55 SCS Design Storm
Area (acres)
17.040
CN
86.000
Type 2
rainfall, P (in)
3.000
time of conc. (hrs)
0.1000
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
35.946
peak time (hrs)
11.917
volume (cy)
3805.675
Page 2
Hydrograph 1
SCS
BH Town Center- Wet Pond 4 Routings
name: 2-yr 24-hr TR-55 SCS Design Storm
Area (acres)
17.040
CN
86.000
Type 2
rainfall, P (in)
3.620
time of conc. (hrs)
0.1000
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
47.675
peak time (hrs)
11.917
volume (cy)
5047.342
Hydrograph 2
SCS
name: 10-yr 24-hr TR-55 SCS Design Storm
Area (acres)
17.040
CN
86.000
Type 2
rainfall, P (in)
5.470
time of conc. (hrs)
0.1000
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
84.496
peak time (hrs)
11.917
volume (cy)
8945.592
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)
100-yr 24-hr TR-55 SCS Design Storm
17.040
86.000
2
8.970
0.1000
0.0200
30.000
1.00
true
157.290
11.917
16652.429
Page 3
BH Town Center- Wet Pond 4 Routings
OUTPUT:
Routing Method: storage -indication
Hydrograph 0
Routing Summary
of
Peaks: 1-yr 24-hr
TR-55
SCS Design Storm
inflow
(cfs)
35.778
at
11.92
(hrs)
discharge
(cfs)
0.344
at
18.50
(hrs)
water level
(ft)
369.169
at
18.74
(hrs)
storage
(cy)
5727.177
Hydrograph 1
Routing Summary
of
Peaks: 2-yr 24-hr
TR-55
SCS Design Storm
inflow
(cfs)
47.451
at
11.92
(hrs)
discharge
(cfs)
2.165
at
13.14
(hrs)
water level
(ft)
369.535
at
13.14
(hrs)
storage
(cy)
6163.090
Hydrograph 2
Routing Summary
of
Peaks: 10-yr
24-hr
TR-55
SCS Design Storm
inflow
(cfs)
84.100
at
11.92
(hrs)
discharge
(cfs)
54.087
at
12.02
(hrs)
water level
(ft)
370.456
at
12.02
(hrs)
storage
(cy)
7316.336
Hydrograph 3
Routing Summary
of
Peaks: 100-yr
24-hr
TR-55
SCS Design Storm
inflow
(cfs)
156.553
at
11.92
(hrs)
discharge
(cfs)
143.265
at
11.96
(hrs)
water level
(ft)
370.915
at
11.96
(hrs)
storage
(cy)
7919.961
Wed Mar 04 13:26:37 EST 2020
Page 4
'Wet Pond B' Routing Calculations
BH Town Center- Wet Pond B Routings
BasinFlow printout
INPUT:
Basin: BH Town Center- Wet Pond B Routings
8 Contour Areas
Elevation(ft)
Area(sf)
Computed Vol.(cy)
386.00
433.00
0.0
388.00
1387.00
64.1
390.00
3568.00
241.3
391.00
5310.00
404.7
392.00
8605.00
659.9
394.00
13155.00
1459.9
396.00
18066.00
2611.5
398.00
23607.00
4150.3
Start—Elevation(ft)
5 Outlet Structures
Outlet structure 0
Orifice
392.00 Vol.(cy) 659.93
name: Barrel
area (sf) 1.227
diameter or depth (in) 15.000
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 386.000
multiple 1
discharge out of riser
Outlet structure 1
Orifice
name: Low -Flow Orifice
area (sf) 0.030
diameter or depth (in) 2.330
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 392.000
multiple 1
discharge into riser
Page 1
BH Town Center- Wet Pond B Routings
Outlet structure 2
Orifice
name: Mid -Flow Orifices
area (sf) 0.196
diameter or depth (in) 6.000
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 395.000
multiple 4
discharge into riser
Outlet structure 3
Weir
name: Riser
diameter (in) 24.000
side angle 0.000
coefficient 3.300
invert (ft) 395.750
multiple 1
discharge into riser
transition at (ft) 0.608
orifice coef. 0.500
orifice area (sf) 3.142
Outlet structure 4
Weir
name: Emergency Spillway
length (ft) 100.000
side angle 75.960
coefficient 3.300
invert (ft) 396.750
multiple 1
discharge through dam
4 Inflow Hydrographs
Hydrograph 0
SCS
name: 1-yr 24-hr TR-55 SCS Design Storm
Area (acres) 6.410
CN 90.000
Type 2
rainfall, P (in) 3.000
time of conc. (hrs) 0.1000
time increment (hrs) 0.0200
time limit (hrs) 30.000
fudge factor 1.00
routed true
peak flow (cfs) 16.138
peak time (hrs) 11.917
volume (cy) 1708.549
Page 2
Hydrograph 1
SCS
BH Town Center- Wet Pond B Routings
name: 2-yr 24-hr TR-55 SCS Design Storm
Area (acres)
6.410
CN
90.000
Type 2
rainfall, P (in)
3.620
time of conc. (hrs)
0.1000
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
20.826
peak time (hrs)
11.917
volume (cy)
2204.861
Hydrograph 2
SCS
name: 10-yr 24-hr TR-55 SCS Design Storm
Area (acres)
6.410
CN
90.000
Type 2
rainfall, P (in)
5.470
time of conc. (hrs)
0.1000
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
35.226
peak time (hrs)
11.917
volume (cy)
3729.354
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)
100-yr 24-hr TR-55 SCS Design Storm
6.410
90.000
2
8.970
0.1000
0.0200
30.000
1.00
true
63.133
11.917
6683.914
Page 3
BH Town Center- Wet Pond B Routings
OUTPUT:
Routing Method: storage -indication
Hydrograph 0
Routing Summary
of
Peaks: 1-yr 24-hr
TR-55
SCS Design Storm
inflow
(cfs)
16.063
at
11.92
(hrs)
discharge
(cfs)
0.200
at
15.52
(hrs)
water level
(ft)
394.931
at
17.00
(hrs)
storage
(cy)
1951.037
Hydrograph 1
Routing Summary
of
Peaks: 2-yr 24-hr
TR-55
SCS Design Storm
inflow
(cfs)
20.729
at
11.92
(hrs)
discharge
(cfs)
1.040
at
13.10
(hrs)
water level
(ft)
395.268
at
13.10
(hrs)
storage
(cy)
2147.215
Hydrograph 2
Routing Summary
of
Peaks: 10-yr
24-hr
TR-55
SCS Design Storm
inflow
(cfs)
35.060
at
11.92
(hrs)
discharge
(cfs)
11.175
at
12.08
(hrs)
water level
(ft)
396.269
at
12.08
(hrs)
storage
(cy)
2794.944
Hydrograph 3
Routing Summary
of
Peaks: 100-yr
24-hr
TR-55
SCS Design Storm
inflow
(cfs)
62.837
at
11.92
(hrs)
discharge
(cfs)
57.244
at
11.96
(hrs)
water level
(ft)
396.999
at
11.96
(hrs)
storage
(cy)
3329.007
Wed Mar 04 13:27:35 EST 2020
Page 4
`Wet Pond B' Drought Tolerance and
Riprap Outlet Protection Calculations
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