HomeMy WebLinkAboutWPO201800092 Calculations 2020-09-01FOOTHILL CROSSING, PHASE V
STORMWATER MANAGEMENT CALCULATIONS PACKET
Date of Calculations
DECEMBER 71 2018
Revised on
APRIL 2, 2019
Revised on
JANUARY 28, 2020
3WI? R. COLUM
Uc. No. 035791
PREPARED BY:
COLLINS ENGINEERING
200 GARRETT STREET, SUITE K
CHARLOTTESVILLE, VA 22902
434.293.3719 PH
434.293.2813 FX APPROVED
www.collins-engineering.com by the Albemarle County
Commu4it DElelppmg�ntDepartment
Date U 9/ U Z lJ 0
RIP WPO
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)
Albemarle County, Virginia
(VA003)
Map
Unit Map Unit Name
Per[ of
Percent
symbol
58 Belvoir loam, 2
4.5%
to 7 percent
slopes
7B Braddock loam,
42.4%
2 to 7 percent
slopes
37C3 Hayesvllle clay
43.1%
loam, 7 to 15
percent slopes,
severely eroded
95 Wehadkee silt
10.0%
loam
Totals for Area of
100.00/b
Interest
Note: Soils' properties witin this property are predominantly composed of hydrologic type B soils.
Precipitation Data
(Source: NOAA's National Weather Service, Hydrometerorological Design Studies Center, Atlas 14
Point Precipitation Frequency Estimates)
Map
l� Terrain
a
0
S
a
a
0
o a
Claude s
Croat Park V
n
c
c
N Cro,,, Pa,y
0
i�
J!Git
POINT PRECIPITATION FREQUENCY (PF) ESTIMATES
WITH 90% CONFIDENCE INTERVALS AND SUPPLEMENTARY INFORMATION
NOAA Atlas 14, Volume 2, Version 3
PDS-based precipitabon frequency
estimates with 90% confidence intervals in inches)'
Average recurtence Irftwal (years)
ourd6on
Ol�l� 10
25 I� 100 200 ® 1000
2M
(2.75142) (33i4.14) (L235.28) (4.98-6.21)
(6.0559) (695A.77) (Z92-00.1) (8.95-5) (104137) (11745.5)
U.S. Department of Agriculture
Natural Resources Conservation Service
TR 55 Worksheet 2: Runoff Curve Number and Runoff
FL-ENG-21A
06/04
Project: Foothill Crossing -Phase V Designed By: FGM, PE Dale: 4/2/2019
Location: East of Indigo Road Checked By: SRC, PE Date: 4/2/2019
Check One: Present X Developed
1. Runoff curve Number (CN)
Soil name and
Cover description
CN (weighted) =
Drainage Area
Area
Product of CN
[alcualued'S'
hydrologic group
(Cover type, treatment, and hydrologic condition; percent
CN
total product/
Description
(Appendix A)
impervious;unconnected/ connected impervious area ratio)
(Acres)
zArea
totalarea
Value
Impervious Areas
98
0.00
0.0
DAA
B
Woods in Good Condition
55
3.33
183.2
55.0
8.18
(Pre -De,)
Lawns in Good Condition
61
0.00
0.0
Impervious Areas
98
0.00
0.0
DAB
(Pre-Dev.)
B
Woods in Good Condition
55
0.69
38.0
55.0
8.18
Lawns in Good Condition
61
0.00
0.0
Impervious Areas (Includes Offsite Ex. Imp. Areas)
98
2.52
247.0
DAA&B
B
Woods in Good Condition
55
0.00
0.0
84.2
1.88
(Post-Dev.)
Lawns in Good Condition (75%+Groundcover)
61
1.50
91.5
Used onlyfor EmergencyAccess Energy BalanceCola.:
Sheet Flow
(Port-Dev.)
B
Impervious Areas
98
0.03
2.9
72.1
3.87
Lawns in Good Condition (75%+Groundermr)
61
0.07
4.3
Used onlyfor Emergency Access Energy Balance Calcs.:
Sheet Flow
B
Access Road Impervious Areas
98
0.08
7.8
82.1
2.17
(Post- ev.)
Lawns in Good Condition (75%+Groundoeor)
61
0.06
3.7
Used onlyfor Emergency Access Energy Balance Calla.:
Sheet Flow
(Port-Dev.)
B
Impervious Areas
98
0.08
7.8
79.5
2.58
Lawns in Good Condition (75%+Groundermr)
1 61
1 0.08
1 4.9
Used only for Emergency Access Energy Balance Cala.:
Sheet FlowF
(Port-Dev.)
B
Impervious Areas
98
0.03
2.9
73.1
4.0]
Lawns in Good Condition (75%+Groundcover)
61
0.08
4.9
2. Runoff
1-year Storm
I 2-year Storm
10-year Storm
Drainage Area Description
Frequencyyears1
2
10
n/a
Rainfall, P(24 hour) -inches
3.06
3.70
5.57
n/a
Runoff, Q-inches
0.21
0.42
1.28
DAA(Pre-Dev.)
Runoff, Q- inches
0.21
0.42
1.28
DA B (Prc- e )
Runoff, Q-inches
1.58
2.13
3.82
DAA&B(Part -Dev.)
Runoff, Q- inches
0.85
1.26
2.65
Sheet Flow C (Port -De,)
Runoff, Q- inches
1.44
1.96
3.61
Sheet Flow D (PostUDev.)
Runoff, Q- inches
1.26
1.76
3.35
Sheet Flow E past D )
Runoff, Q- inches
0.80
1.20
2.56
Sheet Flow F (Post-Dev.)
U.S. Department of Agriculture
Natural Resources Conservation Service
TR 55 Worksheet 3: Time of Concentration (TJ or Travel Time (T,)
Project: Foothill Crossing- Phase V
Location: East of Indigo Road
Check One: Present X Developed X
Check One: T, X Tt
Segment ID:
Sheet Flow: (Applicable to Tc 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)a.e) / Pz .5 s.4
Shallow Concentrated Flow
Designed By: FGM, PE
Checked By: SRC, PE
Through subarea n/a
FL-ENG-21A
06/04
Date: 4/2/2019
Date: 4/2/2019
DAA
DAB
DAA&B
(Pre-Dev.)
(Pre-Dev.)
(Post-Dev.)
Woods- Light
Woods- Light
Underbrush
Underbrush
Dense Grass
0.4
0.4
0.24
100
100
67
3.70
3.70
3.70
0.05
0.04
0.02
0.23
0.25
0.15
7 Surface description (paved or unpaved)
Unpaved
Unpaved
8 Flow Length, L (ft)
150
125
9 Watercourse slope, s (ft/ft)
0.160
0.072
10 Average velocity, V (Figure 3-1) (ft/s)
6.7
3.3
11 T,= L / 3600*V
0.01
0.01
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= [ 1.49r2/3s°.5 ) / n
18 Flow length, L (ft)
19 Ti= L/3600*V
20 Watershed or subarea T, or Tt
(Add T, in steps 6, 11 and 19)
0.24 1 0.26 1 0.16
NOTE: Sheet flow subareas C, D, E & F are assumed to have the most conservative SCS Tc of 0.10 hrs.
U.S. Department of Agriculture FL-ENG-21C
Natural Resources Conservation Service 06/04
TR 55 Worksheet 4: Graphical Peak Discharge Method
Project: Foothill Crossing -Phase V Designed By: KKW, PE Date: 1/28/2020
Location: East of Indigo Road Checked By: SRC, PE Date: 1/28/2020
Check One: Present X Developed X
1. Data
Drainage Area
Description
Drainage Area
Description
Drainage Area Description
Drainage Area
Description
DAA
(Pre-Dev.)
DAB
(Pre-Dev.)
DAAB,B
(Post-Dev.)
Drainage Area (Am) in miles'
0.0052
0.0011
0.0063
Runoff curve number CN=
55.0
55.0
84.2
Time of concentration(Tc)=
0.24
0.26
0.16
Rainfall distribution type-
II
II
II
Pond and swamp areas spread
throughout the watershed-
0
0
0
2. Frequency -years
1
2
10
1
2
10
1
2
10
3. Rainfall, P(24 hour) -inches
3.06
3.70
5.57
3.06
3.70
5.57
3.06
3.70
5.57
4. Initial Abstraction, la -inches
1.64
LM
1.64
1.64
1.64
LM
0.38
0.38
0.38
5. Compute Ia/P
0.53
0."
0.29
0.53
0.44
0.29
0.12
0.10
0.07
6. Unit peak discharge, Qu-csm/m
325
475
650
325
475
650
950
950
950
]. Runoff, Q from Worksheet 2-inches
0.21
0.42
1.28
0.21
0.42
1.28
1.58
2.13
3.82
B. Pond and Swamp adjustment factor, Fp
1
1
1
1
1
1
1
1
1
9. Peak Discharge, Qp- cfs
where Qp=Qu Am QFp
0.36
1.03
4.32
0.07
0.21
0.90
Routed Through The SW M
Facility
WATERSHED SUMMARY
Pre -Development
1-year Flow, 2-year Flow,
10-year Flow,
CN
Area, ac cts cts
cfs
DAA
55.0
3.33 0.36 1.03
4.32
DA 8
55.0
0.69 0.07 0.21
0.90
55.0
4.02 0.43 1.24
5.21
Stormwater Management Facility Routing Calculations
Foothill- Phase V-rev 200128
BasinFlow printout
INPUT:
Basin: Foothill Crossing- Phase VI
6 Contour Areas
Elevation(ft)
Area(sf)
Computed Vol.(cy)
646.00
983.00
0.0
648.00
1983.00
107.7
650.00
3265.00
300.1
652.00
4683.00
592.9
654.00
6159.00
993.2
656.00
7722.00
1506.2
Start—Elevation(ft)
646.00 Vol.(cy)
5 Outlet Structures
Outlet structure 0
Orifice
name: Barrel
area (sf) 1.227
diameter or depth (in) 15.000
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 644.000
multiple 1
discharge out of riser
Outlet structure 1
Orifice
name: Low -Flow Orifice
area (sf) 0.005
diameter or depth (in) 1.000
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 646.000
multiple 1
discharge into riser
Outlet structure 2
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)
652.580
Page 1
cVMM
Foothill- Phase V-rev 200128
multiple 4
discharge into riser
Outlet structure 3
Weir
name: Riser
diameter (in)
10.000
side angle
0.000
coefficient
3.300
invert (ft)
653.750
multiple
1
discharge
into riser
transition at (ft)
0.253
orifice coef.
0.500
orifice area (sf)
0.545
Outlet structure 4
Weir
name: Emergency Spillway
length (ft)
65.000
side angle
75.960
coefficient
3.300
invert (ft)
654.750
multiple
1
discharge
through dam
4 Inflow Hydrographs
Hydrograph 0
SCS
name: 1-yr,
24-hr. SCS Design Storm
Area (acres)
4.020
CN
84.200
Type 2
rainfall, P (in)
3.060
time of conc. (hrs)
0.1300
time increment (hrs)
0.0200
time limit (hrs)
30.000
fudge factor
1.00
routed
true
peak flow (cfs)
7.661
peak time (hrs)
11.930
volume (cy)
853.394
Hydrograph 1
SCS
name: 2-yr, 24-hr. SCS Design Storm
Area (acres) 4.020
CN 84.200
Page 2
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 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)
OUTPUT:
`a
Foothill- Phase V-rev 200128
3.700
0.1300
0.0200
30.000
1.00
true
10.303
11.930
1147.738
10-yr, 24-hr. SCS Design Storm
4.020
84.200
2
5.570
0.1300
0.0200
30.000
1.00
true
18.502
11.930
2060.926
100-yr, 24-hr. SCS
4.020
84.200
2
9.060
0.1300
0.0200
30.000
1.00
true
34.624
11.930
3856.908
Routing Method: storage -indication
Hydrograph 0
Design Storm
Page 3
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)
Foothill- Phase V-rev 200128
Peaks: 1-yr, 24-hr. SCS Design Storm
7.620 at 11.92 (hrs)
0.056 at 15.52 (hrs)
652.543 at 17.92 (hrs)
690.848
Peaks: 2-yr, 24-hr
10.249 at
0.813 at
652.892 at
757.982
Peaks: 10-yr, 24-hr
18.403 at
5.196 at
654.687 at
1156.399
SCS Design Storm
11.92 (hrs)
12.54 (hrs)
12.56 (hrs)
. SCS Design Storm
11.92 (hrs)
12.10 (hrs)
12.10 (hrs)
Peaks: 100-yr, 24-hr. SCS Design Storm
34.440 at
11.92 (hrs)
32.670 at
11.94 (hrs)
654.999 at
11.94 (hrs)
1234.893
Tue ]an 28 11:05:32 EST 2020
Page 4
Channel and Flood Protection Calculations
9 VAC 25-870-66 (Analysis corresponding to DA A & B's 1" Low -Flow Orifice)
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.
"Under no condition shall Q Developed be greater than Q Pre -Developed nor shalIQ Developed be required to
be less than that calculated in the equation (Q Forest * RV Forest ) I RV Developed
Q Developed :S (Q Forest * RV Forest ) / RV Developed Where: Q Forest = 0.43
RV Forest = 0.21
RV Developed= 1.58
Q Developed < 0.06 cfs
Section B.4 Limits of Analysis
The analysis terminates at the proposed SWM facility's outfall since subsection B.3.a is met.
9 VAC 25-870-66 (Analysis corresponding to DA A & B's 1" Low -Flow Orifice)
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 "releases a post development 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 analysis terminates at the proposed SWIM facility's outfall since subsection C.2.b is met.
QtATocT ?P(nc *xcbJ 17ts�6N
Qvo,swti = y,04 cps
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* pcA ?4ar;. 3 ,18 -% I c�ccm+ of Su,,�,G SSE Oz*4tTCAj bur WT WSS TMAN 6"
st 11CA LA . lWa-M, a�
�NYN.'A'ESSdN> —� JVJj-D,,SQA(LA) p,Nt cos at"
Mat-
3.7s
row, VOA cLAsS AZ STeNE
HAS A LO-
away gw%rCfhoN 14 Bf FLAT/AT 0% 6W,'C IWCVR GLASS r E3lTEk. t4U4C Kelgo 3j,", THE
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'Feat a FAXgr_ StAALL Ova'µ LPR A tilt-. M ra"
14 FINAL. Ussr.N
wI Vt)PT c\. Ai iL%OPA CIS" -RAZC►CAISSSS)
;-- At"� CL� FSLlER F'AQ,0.'1t
Channel and Flood Protection Calculations
(Calculations associated with sheet flow from Subareas C, D, E &F)
9 VAC 25-870-66 Sheet Flow Subareas C, D, E & F
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.a.
"The manmade stormwater conveyance system [sheet flow and culvert] shall convey the post
development peak flow rate from the two-year 24-hour storm event without causing erosion of
the system. Detention of stormwater or downstream improvements may be incorporated into
the approved land -disturbing activity to meet this criterion..."
Section B.4.a Limits of Analysis
The sheet flow runoff for subareas C, D, E & F drain to the existing defined channel. The limits of
analysis ends at the subareas C, D, E & F since "the site's contributing drainage area is less than
or equal to 1.0% of the total watershed area."
NOTE:
Please see sheet 5A of the final site plan. Sheet 5A depicts the overall watershed to the proposed
culvert, which is located at the confluence of subareas C, D, E & F. The overall watershed is 53.4
acres.
Individual Overall Percent of Overall
Subarea, ac. Watershed, ac. Watershed
Subarea C
0.10
53.4
0.2%
Subarea D
0.14
53.4
0.3%
Subarea E
0.16
53.4
0.3%
Subarea F
0.11
53.4
0.2%
9 VAC 25-870-66 Sheet Flow Subareas C, D, E & F
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 CA:
"The point of discharge [sheet flow] releases stormwater into a stormwater conveyance
system that, following the land -disturbing activity, confines the postdevelopment peak flow
rate from the 10-year 24-hour storm event within the stormwater conveyance system
[culvert]. Detention of stormwater or downstream improvements may be incorporated into
the approved land -disturbing activity to meet this criterion, at the discretion of the VSMP
authority"
Section C.3.a Limits of Analysis
"...stormwater conveyance systems shall be analyzed for compliance with flood protection
criteria to a point where ... The site's contributing drainage area is less than or equal to 1.0% of
the total watershed area draining to a point of analysis [culvert] in the downstream
stormwater conveyance system."
Virginia Runoff Reduction Method Water Quality
Calculations
D QViryiniu Runoff Reduction Method NewD .iopmentcompli .spreadsheet - Version 3.0
02011 BMP Standards and Specifications M2013 Draft BMP Standards and Specification
Project Name: Foothill Crousing.PhaseV
Data: 4/2/2019
BMP De9go Speclfirodons List 2013 Draft Stds & Specs
Site Information
Post -Development Project (Treatment Volume and Loads)
Land cover facresl
A Solis
BSailz
cSe is
DSolls
Totals
forest/Open Spam(axs)- undisturbed
foresto m or ref casein land
015
0.25rotected
Managed Turf (aura) - disturbed, graded for
yards or other turf to be mowM/maned
3.01
3'08
Impervious Cover (ac esl
295
2.95
'F cUpen Seem acres must be proteRedin ocmrdonce with
the oinindo Aunoffireduction
Method
6.28
Constants
Annual rainfall(inchod 63
Target Rainfall Emnt(inrhea)1.00
Total Phosphorus (To FMC(mp/L) 0.26
Total trustee (MI EM[ (mg/LI 1.86
Target TP Load(m/acreNq 0.41
Pl unineu correction factor) 0.90
Runoff coefficients Rod
ASmis
I BSolis
1 [Sells
1 DSolls
Foost/Open Space 1 0.02
i 0.03
i 0.04
i 0.05
Managed Turt 1 0.15
1 0.20
1 0.22
1 0.25
Impervious Ca r 1 0.95
1 0.95
1 0.95
1 0.95
Land Cover Summary
Traatment Volume and Nutrient Loads
TreM(aere-ft)ume 0.2855
Treatment Volume (cubic feet) 12,436
TP Load (lb/yr) 7.81
Tn Loa yr 55.90
Forect/Open Spare Cmer(acres)
0.25
Weighted or(forest)
0.03
%Forest
<%
Managed Tudf rner(acres)
3.08
Weighted or(turf(
0.20
%Managed Turf
99%
Impervious Cover (acres)
2.95
or(imperviaus(
0.95
%Impervious
9]%
Site Area (acres)
6.28
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