HomeMy WebLinkAboutWPO201600054 Calculations 2021-12-13Oki FTAI ► 1 • • 111• ■ F.AVIVS1 foll]1l 1
MANAGEMENT CALCULATIONS PACKET
Date of Approved WPO 201600054 Calculations
AUGUST 12, 2016
Date of Amendment #1 Calculations
OF
gl
soll o Q'
� VIC 913120 �tiCV
SEPTEMBER 3, 2020
APPROVED
by the Albemarle County
Community Development Department
Date 10/8/20
File WP0201 R00054
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 Table (Source: NRCS Web Soil Survey Online Database)
Albemarle County, Virginia (VA003)
Map
Unit
Map Unit Name
percent of
percent
Symbol
16
Codorus silt
0.2%
loam, 0 to 2
percent slopes,
occasionally
flooded
47D
Philomont sandy
55.1%
loam, 15 to 25
percent slopes
47E
Philomont sandy
44.7%
loam, 25 to 45
percent slopes
Totals for Area of
100.0%
Interest
The NRCS web soil survey defines this development's soils as predominantly having a hydrologic group type A.
As a result of this, the preserved open spaces are depicted in this report's VRRM calculations as having a type
A soil group. Please note however, this site requires signficant compacted fill for the disturbed areas
(subareas A & B). As a result of this, and the overwhelming presence of type B fill soils in Albmarle County, the
stormwater quantity calculations were conservatively analyzed as having a hydrologic group type B.
Precipitation Data
(Source: NOAA's National Weather Service, Hydrometerorological Design Studies Center, Atlas 14
Point Precipitation Frequency Estimates)
Terrain
Z
v
6
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b
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o
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1 Ci
rn yr
O i'4
« 0.
ro°"ey Pine Rr1Q O
m 8
3
n
200m
POINT PRECIPITATION FREQUENCY (PF) ESTIMATES
WITH 90'S CONFIDENCE INTERVALS AND SUPPLEMENTARY INFORMATION
NOAA Atlas 14, Volume 2, Version 3
PDS-based precipitation frequency estimates With 90% confidence intervals (in inches)'
A.,age recunence ink iil ears)
P"'
t z s 10 zs so too zoo soo t000
DDO0000000
24hr �. ) (3.21d 03) (4.0&5.14) (481606) (5,876.42) (fi]",% (T 9386) (8.R- 1.3) (10.2-13.4) (11.4152)
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9.
U.S. Department of Agriculture
Natural Resources Conservation Service
TR 55 Worksheet 3: Time of Concentration (Tc) or Travel Time (T,)
Project: Briarwood Phase 6 VSMP Amendment #1
Location: Sunset Drive Extension
Check One: Present X Developed X
Check One: Tc X Tt Through subarea
Segment ID.
Sheet Flow: (Applicable to T, only)
Designed By: FGM, PE
Checked By: SRC, PE
n/a
FL-ENG-21A
06/04
Date: 9/3/2020
Date: 9/3/2020
DAA&B
DAA
DAB
DA C
(Pre-Dev.)
(Post-Dev.)
(Post-Dev.)
(Offsite-Routed
Thru the Site)
1 Surface description (Table 3-1)
Dense Grass
Dense Grass
Dense Grass
Dense Grass
2 Manning's roughness coeff., n (Table 3-1
0.24
0.24
0.24
0.24
3 Flow length, L (total L < 100) (ft)
100
40
100
100
4 Two-year 24-hour rainfall, Px(in.)
3.58
3.58
3.58
3.6
5 Land slope, s (ft/ft)
0.12
0.15
0.28
0.06
6 Compute T, _ [0.007(n*L)°8] / Pi .s 5 .4
0.11
0.05
0.08
0.14
Shallow Concentrated Flow:
7 Surface description (paved or unpaved)
Unpaved
Unpaved
Unpaved
8 Flow Length, L (ft)
350
75
260
9 Watercourse slope, s (ft/ft)
0.189
0.533
0.073
10 Average velocity, V (Figure 3-1) (ft/s)
7.0
11.5
4.4
11 T,=L/3600*V
0.01
0.00
0.02
Channel Flow:
12 Cross sectional flow area, a (ft)
3 c
13 Wetted perimeter, P. (ft)
w m
14 Hydraulic radius, r=a/Pw(ft)
c E o
15 Channel Slope, s (ft/ft)
m a �
u >
16 Manning's Roughness Coeff, n
o o.
xy os
18 Flow length, L (ft)
19 T,= L / 3600*V
F 0.02
20 Watershed or subarea T.or Tt
(Add T, in steps 6, 11 and 19)
0.12 0.10 0.10 0.16
U.S. Department of Agriculture
Natural Resources Conservation Service
Project: Briarwood Phase 6 VSMP Amendment#1
Location: Sunset Drive Extension
Check One: Present X Developed X
TR 55 Worksheet4: Graphical Peak Discharge Method
Designed By: FGM, PE Date: 9/3/2020
Checked By: SRC, PE Date: 9/3/2020
1. Data
Drainage Area Description
Drainage Area
Description
Drainage Area Description
Drainage Area
Description
DAA&B
(Pre-Dev.)
DAA
(Post-Dev.)
DAB
(Past-Dev.)
DAC
(Offsite- Routed Thru the Site)
Drainage Area (Am) in miles
0.0073
0.0060
0.0013
&0074
Runoff curve number CN=
59.0
72.7
62.1
78.8
Time of concentration (Tc)=
0.12
0.10
0.10
0.16
Rainfall distribution type=
II
II
11
II
Pond and swamp areas spread
throughout the watershed=
0
0
0
0
2. Frequency -years
1
2
10
1
2
10
1
2
30
1
2
30
3. Rainfall, P (24 hour)- inches
2.95
3.58
5.39
2.95
3.58
5.39
2.95
3.58
5.39
2.95
3.58
5.39
4. Initial Abstraction, la -inches
1.39
1.39
1.39
0.75
0.75
0.75
1.22
1.22
1.22
0.54
0.54
0.54
5. Compute la/P
0.47
0.39
0.26
0.25
0.21
0.14
0.41
0.34
0.23
0.18
0.15
0.10
6. Unit peak discharge, Qu-rsm/in
575
775
925
975
975
100
825
900
975
825
850
875
7. Runoff, Qfrom Worksheet 2- inches
0.29
0.53
1.46
0.81
1.21
2.56
0.38
0.66
1.69
1.14
1.61
3.12
B. Pond and Swamp adjustment factor, Fp
1
1
1
1
1
1
1
1
1
1
1
1
9. Peak Discharge, Qp-cisRouted
where Qp=Qu Am QFp
1.21
2.99
9.93
Thru SWM
Facility'A'
Routed Thru SWM Facility
'B'
7.00
10.21
2031
WATERSHED SUMMARY
(For Onsite Areas Only & the Energy Balance Computations)
Pre-Develooment
DAA&B
WATERSHED SUMMARY
(For Basin Capacity / 100
DAA+DAC
Past-Develooment
Area, 1-year 2-year 10-year
Area,
1-year
2-year
10-year
CN
ac. Flow, cis Flow, cfs Flow, cfs
CN
ac.
Flow, cfs
Flow, cfs
Flow, cfs
59.0 4.70 1.21 2.99 9.93
72.7 3.86
0.10
0.18
11.53
62.1 0.94
0.10
0.12
0.17
70.8 4.70 0.20 0.30 11.70
Freeboard Check only)
CN Area, 100-year Backwater Freeboard During
ac. Elev., ft 100-yr Design Storm
72.7 3.86 424.97 1.03'
78.8 4.76
76.1 8.62
Channel and Flood Protection Computations
9 VAC 25-870-66 (Channel Protection Analysis for 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 Developed !� I•F• * (Q Pre -Developed * RV Pre -Developed ) / 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
< 0.38 cfs
Section B.4 Limits of Analysis
1.21 cfs
(From Worksheet 4 Watershed Summary)
59.0
(From Worksheet 4 Watershed Summary)
6.95
(From SCS TR-55 eq. 2-4)
0.29 in.
(From SCS TR-55 eq. 2-3)
70.8
(From Worksheet 4 Watershed Summary)
4.12
(From SCS TR-55 eq. 2-4)
0.72 in.
(From SCS TR-55 eq. 2-3)
0.80
(Factor of Safety)
0.20 cfs
The analysis terminates at the onsite Subareas A & B since subsection B.3.a is met.
9 VAC 25-870-66 (Flood Protection Analysis for 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:
"Concentrated stormwater flow to stormwater conveyance systems that currently experience
localized flooding during the 10-year 24-hour storm event" shall meet either a. or b.
Section C.2.a.:
The proposed stormwater conveyance systems, i.e. the basins, confine "...the postdevelopment
peak flow rate from the 10-year 24-hour storm event within the stormwater conveyance system
[the basins] to avoid the localized flooding."
Section C.3.c Limits of Analysis
The flood protection analysis terminates at the basins' outfalls since "The stormwater conveyance
system [i.e. the basins' outfalls] enters a mapped floodplain or other flood -prone area..."
Post -Development Routing Calculations of Subarea A
(Used to represent Onsite Areas for Channel Protection Compliance & Calculations)
Briarwood SWM Facility A- Onsite Phase 6
BasinFlow printout
INPUT:
Basin: Briarwood SWM Facility A- Onsite Phase 6
(Used for Energy Balance Computations)
5 Contour Areas
Elevation(ft)
Area(sf)
Computed Vol.(cy)
418.00
616.00
0.0
420.00
1437.00
73.9
422.00
2558.00
219.9
424.00
3810.00
454.2
426.00
5191.00
786.3
Start—Elevation(ft)
4 Outlet Structures
Outlet structure 0
Orifice
418.00 Vol.(cy) 0.00
name: APPROVED Barrel
area (sf)
1.227
diameter or depth (in)
15.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
416.000
multiple
1
discharge out
of riser
Outlet structure 1
Orifice
name: APPROVED Low -Flow Orifice
area (sf)
0.012
diameter or depth (in)
1.500
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
418.000
multiple
1
discharge into
riser
Page 1
Briarwood SWM Facility A- Onsite Phase 6
Outlet structure 2
Weir
name:
APPROVED Riser Standpipe
diameter (in)
30.000
side angle
0.000
coefficient
3.300
invert (ft)
423.500
multiple
1
discharge
into riser
transition at (ft)
0.760
orifice coef.
0.500
orifice area (sf)
4.909
Outlet structure 3
Weir
name: AMENDED Emergency Spillway
length (ft)
40.000
side angle
75.960
coefficient
3.300
invert (ft)
424.500
multiple
1
discharge
through dam
4 Inflow Hydrographs
Hydrograph 0
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)
1-yr SCS Peak 24-hr Design Storm
3.860
72.700
2
2.950
0.1000
0.0200
30.000
1.00
true
3.978
11.917
421.113
Page 2
Briarwood SWM Facility A- Onsite Phase 6
Hydrograph 1
SCS
name: 2-yr
SCS Peak 24-hr Design Storm
Area (acres)
3.860
CN
72.700
Type 2
rainfall, P (in)
3.580
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)
5.954
peak time (hrs)
11.917
volume (cy)
630.295
Hydrograph 2
SCS
name: 10-yr
SCS Peak 24-hr Design Storm
Area (acres)
3.860
CN
72.700
Type 2
rainfall, P (in)
5.390
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)
12.557
peak time (hrs)
11.917
volume (cy)
1329.414
Hydrograph 3
SCS
name: 100-yr
SCS Peak 24-hr Design Storm
Area (acres)
3.860
CN
72.700
Type 2
rainfall, P (in)
8.830
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)
27.014
peak time (hrs)
11.917
volume (cy)
2860.003
Page 3
Briarwood SWM Facility A- Onsite Phase 6
OUTPUT:
Routing Method: storage -indication
Hydrograph 0
Routing Summary
of
Peaks:
1-yr SCS Peak
24-hr Design Storm
inflow
(cfs)
3.959
at
11.92
(hrs)
discharge
(cfs)
0.103
at
13.28
(hrs)
water level
(ft)
422.447
at
14.70
(hrs)
storage
(cy)
264.375
Hydrograph 1
Routing Summary
of
Peaks:
2-yr SCS Peak 24-hr Design Storm
inflow
(cfs)
5.926
at
11.92
(hrs)
discharge
(cfs)
0.180
at
13.14
(hrs)
water level
(ft)
423.519
at
13.16
(hrs)
storage
(cy)
389.188
Hydrograph 2
Routing Summary
of
Peaks:
10-yr SCS
Peak 24-hr Design Storm
inflow
(cfs)
12.498
at
11.92
(hrs)
discharge
(cfs)
11.527
at
11.96
(hrs)
water level
(ft)
424.079
at
11.96
(hrs)
storage
(cy)
465.380
Hydrograph 3
Routing Summary
of
Peaks:
100-yr
SCS
Peak 24-hr Design Storm
inflow
(cfs)
26.888
at
11.92
(hrs)
discharge
(cfs)
25.814
at
11.92
(hrs)
water level
(ft)
424.698
at
11.92
(hrs)
storage
(cy)
558.604
Tue Sep 01 15:50:44 EDT 2020
Page 4
Post -Development Routing Calculations of Subarea A
(Used to confirm 1' of freeboard is achieved during the 100-year storm for the total watershed.)
Briarwood SWM Facility A- Freeboard Check with Total Watershed
BasinFlow printout
INPUT:
Basin: Briarwood SWM Facility A- Freeboard Check with Total Watershed
5 Contour Areas
Elevation(ft)
Area(sf)
Computed Vol.(cy)
418.00
616.00
0.0
420.00
1437.00
73.9
422.00
2558.00
219.9
424.00
3810.00
454.2
426.00
5191.00
786.3
Start—Elevation(ft)
4 Outlet Structures
Outlet structure 0
Orifice
418.00 Vol.(cy) 0.00
name: APPROVED Barrel
area (sf)
1.227
diameter or depth (in)
15.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
416.000
multiple
1
discharge out
of riser
Outlet structure 1
Orifice
name: APPROVED Low -Flow Orifice
area (sf) 0.012
diameter or depth (in) 1.500
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 418.000
multiple 1
discharge into riser
Page 1
Briarwood SWM Facility A- Freeboard Check with Total Watershed
Outlet structure 2
Weir
name:
APPROVED Riser Standpipe
diameter (in)
30.000
side angle
0.000
coefficient
3.300
invert (ft)
423.500
multiple
1
discharge
into riser
transition at (ft)
0.760
orifice coef.
0.500
orifice area (sf)
4.909
Outlet structure 3
Weir
name: AMENDED Emergency Spillway
length (ft)
40.000
side angle
75.960
coefficient
3.300
invert (ft)
424.500
multiple
1
discharge
through
dam
1 Inflow Hydrographs
Hydrograph 0
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 SCS Peak 24-hr Design Storm CHECK
8.620
76.100
2
8.830
0.1600
0.0200
30.000
1.00
true
58.730
11.938
6866.658
Page 2
Briarwood SWM Facility A- Freeboard Check with Total Watershed
OUTPUT:
Routing Method: storage -indication
Hydrograph 0
Routing Summary of
Peaks: 100-yr
SCS
Peak 24-hr Design Storm CHECK
inflow (cfs)
58.703
at
11.94 (hrs)
discharge (cfs)
58.128
at
11.96 (hrs)
water level (ft)
424.969
at
11.96 (hrs)
storage (cy)
602.250
Tue Sep 01 15:50:03 EDT 2020
Page 3
Post -Development Routing Calculations of Subarea B
Briarwood SWM Facility B- Onsite Phase 6
BasinFlow printout
INPUT:
Basin: Briarwood SWM Facility B- Phase 6
4 Contour Areas
Elevation(ft)
404.00
406.00
408.00
410.00
Start—Elevation(ft)
4 Outlet Structures
Outlet structure 0
Orifice
Area(sf) Computed Vol.(cy)
107.00
0.0
567.00
22.7
1145.00
84.9
1826.00
194.0
404.00 Vol.(cy) 0.00
name: APPROVED Barrel
area (sf)
1.227
diameter or depth (in)
15.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
403.500
multiple
1
discharge out
of riser
Outlet structure 1
Orifice
name: APPROVED Low -Flow Orifice
area (sf)
0.022
diameter or depth (in)
2.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
404.000
multiple
1
discharge into
riser
Page 1
Briarwood SWM Facility B- Onsite Phase 6
Outlet structure 2
Weir
name:
APPROVED Riser Standpipe
diameter (in)
30.000
side angle
0.000
coefficient
3.300
invert (ft)
407.900
multiple
1
discharge
into riser
transition at (ft)
0.760
orifice coef.
0.500
orifice area (sf)
4.909
Outlet structure 3
Weir
name: APPROVED Emergency Spillway
length (ft)
25.000
side angle
75.960
coefficient
3.300
invert (ft)
408.900
multiple
1
discharge
through dam
4 Inflow Hydrographs
Hydrograph 0
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)
1-yr SCS Peak 24-hr Design Storm
0.840
62.100
2
2.950
0.1000
0.0200
30.000
1.00
true
0.407
11.917
43.085
Page 2
Briarwood SWM Facility B- Onsite Phase 6
Hydrograph 1
SCS
name: 2-yr
SCS Peak 24-hr Design Storm
Area (acres)
0.840
CN
62.100
Type 2
rainfall, P (in)
3.580
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)
0.701
peak time (hrs)
11.917
volume (cy)
74.231
Hydrograph 2
SCS
name: 10-yr
SCS Peak 24-hr Design Storm
Area (acres)
0.840
CN
62.100
Type 2
rainfall, P (in)
5.390
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)
1.804
peak time (hrs)
11.917
volume (cy)
190.967
Hydrograph 3
SCS
name: 100-yr
SCS Peak 24-hr Design Storm
Area (acres)
0.840
CN
62.100
Type 2
rainfall, P (in)
8.830
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)
4.501
peak time (hrs)
11.917
volume (cy)
476.506
Page 3
Briarwood SWM Facility B- Onsite Phase 6
OUTPUT:
Routing Method: storage -indication
Hydrograph 0
Routing Summary
of
Peaks:
1-yr SCS Peak
24-hr Design Storm
inflow
(cfs)
0.405
at
11.92
(hrs)
discharge
(cfs)
0.103
at
12.08
(hrs)
water level
(ft)
405.465
at
12.10
(hrs)
storage
(cy)
13.098
Hydrograph 1
Routing Summary
of
Peaks:
2-yr SCS Peak 24-hr Design Storm
inflow
(cfs)
0.698
at
11.92
(hrs)
discharge
(cfs)
0.124
at
12.08
(hrs)
water level
(ft)
406.105
at
12.14
(hrs)
storage
(cy)
24.969
Hydrograph 2
Routing Summary
of
Peaks:
10-yr SCS
Peak 24-hr Design Storm
inflow
(cfs)
1.795
at
11.92
(hrs)
discharge
(cfs)
0.169
at
12.32
(hrs)
water level
(ft)
407.801
at
12.44
(hrs)
storage
(cy)
76.691
Hydrograph 3
Routing Summary
of
Peaks:
100-yr
SCS
Peak 24-hr Design Storm
inflow
(cfs)
4.480
at
11.92
(hrs)
discharge
(cfs)
4.319
at
11.92
(hrs)
water level
(ft)
408.195
at
11.92
(hrs)
storage
(cy)
93.349
Tue Sep 01 16:11:17 EDT 2020
Page 4
Water Quality Calculations Using the DEQ
Virginia Runoff Reduction Method
(Used to Determine the Required Phosphorous Removal Rate)
THE VRRM BOUNDARY IS DEFINED BY THE PROPOSED ESC LIMITS OF DISTURBANCE. PLEASE NOTE, THERE ARE
EXTENSIVE AREAS LOCATED WITHIN THE PROPERTY THAT ARE OUTSIDE THE LIMITS OF DISTURBANCE. THESE
AREAS ARE NOT REPRESENTED IN THE FOLLOWING CALCULATIONS SINCE THEY ARE NOT DISTURBED.
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Water Quality Calculations Using the DEQ
Virginia Runoff Reduction Method
(Used to Determine the Phosphorous Removal Rate Credit for Preserving Open Spaces)
THIS PLAN's WATER QUALITY COMPLIANCE IS MET THROUGH THE BEST MANAGEMENT PRACTICE OF
PRESERVING OPEN SPACES. THE PREVIOUS VRRM REQUIRED PHOSPHOROUS REMOVAL RATE IS OFFSET WITH
THE EXTENSIVE LAND PRSERVATION PROPOSED AND ITS CORRESPONDING CREDIT, SHOWN IN THE
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