HomeMy WebLinkAboutWPO201700052 Calculations 2021-09-14DUNLORA OVERLOOK STORMWATER
MANAGEMENT CALCULATIONS PACKET
(WPO 201700052; VSMP Amendment #4 to WPO 201600058)
Date of Amendment #4 Calculations
G
SCOTT/R. CD•LL1NS7'
Lic. No. 035791 a
8/25/21 `44�
ZONAL
August 25, 2021
PREPARED BY:
COLLINS e=NGINEERING
200 GARRETT STREET, SUITE K
CHARLOTTESVILLE, VA 22902
434.293.3719 PH
434.293.2813 FX
www.collins-engineering.com
Amendment #4 Water Quality Calculations
Summary:
1. The proposed VSMP amendment #4 reflects an updated Phase II approved under SDP
202100023. This updated Phase II is less impervious and requires an overall (phases I and 11)
phosphorous removal rate of 5.11 Ibs/yr. This can be viewed in the following 'AMENDED
Dunlora Park- VSMP Amendment No. 4' VRRM calculations dated 8/25/21.
2. The previously reviewed and approved WPO 201700052 Amendment #3 accounted for a
more impervious overall (phases I and 11) development. The approved WPO 201700052
Amendment #3 provided a 5.52 Ibs/yr nutrient credit, and this exceeds the required removal
rate associated with this WPO Amendment #4. Please see the following 'APPROVED Dunlora
Park- VSMP Amendment No. 3' VRRM calculations dated 2/13/18.
APPROVED Impervious Areas for Water Quality
(Total Onsite Impervious Areas)
Phase I Roadway
29,600 sf
Phase II Roadway
9,534 sf
Phase I Sidewalks & Driveways
21,188 sf
Phase 11 Sidewalks & Driveways
5,964 sf
Phase I Buildings
63,700 sf
Phase 11 Buildings
10,995 sf
140,981 sf
AMENDED Impervious Areas for Water Quality
(Total Onsite Impervious Areas)
Phase I Roadway
29,600 sf
Phase II Roadway
6,974 sf
Phase I Sidewalks & Driveways
21,188 sf
Phase II Sidewalks & Driveways
1,267 sf
Phase I Buildings
63,700 sf
Phase II Buildings
7,955 sf
Total = 130,684 sf
NOTE: This Amendment #4 proposes less impervious areas than the
previously approved plans. Therefore the prior nutrient credit purchase
and SWM Preservation Easements remain
sufficient to meet the
minimum water quality requirements.
OEO Virginia Runoff Reduction Method New Development Cnmpflanca Spreadsheet - VlrsiOn 3.0
❑ 2011 BMP St a adarda a nd Specifications
02013 Draft LIMP Standards and Spedficadions
Project Name:
data input calls
AMENDED Dunlora Park- VSMP Amendment No. 4
Date:
8/25/2021
canstantvzlues
LIMP Design Specifications List: 2013 Draft Stds & Specs
calculation cells
Site Information
Project (Treatment Volume and Loads)
(Aver
protected foresuo en space or reforested land 1.09 1.09
Managed to. (.) disturbed, graded for
rds or other turf to be auneedlatarused 4.53 4.53
ImpernasoCoverlarrez) 3.00 3.00
'Forest/Oven Spear cancer must In, yrotectedin acrordana with the Vlrglnla Rurml/ Reduction Method 8.62
COnstaMs Runoff Coefficients RV
Annual Rainfall(inches) 43 ASoils BSoils CSoils I DSoils
Target Rainfall Event (inmes) 1.00 Forert/Open Space 0.02 0.03 0.04 0.05
Total Phosphoros(TP) lZrn RJ 0.26 Managed Turf 0.15 0.20 0.22 0.25
Total Nitrogen (TN) EMCImgA) 1.86 Impewiouz [aver 0.95 0.95 0.95 0.95
argetW Load (lb/acre/yr) 0.41
Pjluni ..'ectionfamor) 0.90
Farest/Open Space Cover (acres)
1.09
Weighted Rv (forest)
0.03
%Forest
13%
Managed Tud Cover (acres)
4.53
Weighted Re (turf)
0.20
%Managed Tud
53%
Impervious Career (acres)
3.00
Re (impervious)
0.95
%Impervious
35%
Site Area(acres)
8.62
Site Rv
a."
Treatment Volume and Nutrient
Loathe
Treatment Volume
(acre-ft)
0.3157
Treatment Volume (cubic feel)
13,753
TP Load (lb/,)
8.64
... TN Load Rb/yr)
61.82
DEQ ViMinia RunefJRedu knMa&dNew DevelopmeMComptlanm Spreadsheet-Version3.0
112011 BMP Standards and SpeCiRcatlons O 2013 Draft BMP Standards and Specifications
Project Name:
data input calls
APPROVED Dunlora Park- VSMP Amendment No. 3
Date:
2/13/2018
canstantvzlues
BMP Design Specifications List, 2013 Draft Stds & Specs
miculation cells
Site Information
Project (Treatment Volume and Loads)
Cover
romedforest/open en s r reforested land 1.09 1.09
re
Managed Tud(aaes) didurbed, gaded for
rdz or other turf m be mowed man ed 1 1 4.29 4.29
Impervious Cover lee.) 3.24 3.24
'Fpreso%hem Space ande must be pmeeteal accordance with the Virginia RurwH Reduction Method 8.62
Constants Runoff Coefficients Rv
Annual Rainfall(inchez) 43 ASails BSoils CSoils I DSoils
Target Rainfall Event (inmes) 1.00 Foreal/Open Space 0.02 0.03 0.04 0.05
Torzl Plhospbores JP) EMC(mg/L) 0.26 Managed Tud 0.15 0.20 0.22 0.25
Total NioNen(TN) EMCImgA) 1.86 Impewiouz [aver 0.95 0.95 0.95 0.95
argetW Lead(lb/acre/yr) 0.41
Pj (unidezz coneclionfector) 0.90
Farest/Open Space Cover (acres)
1.09
Weighted Re (forest)
0.03
%Forest
13%
Managed Tud Cover (acres)
4.29
Weighted Re (turf)
0.20
%Managed Tud
509%
Impervious Cover(acres)
3.24
Re (impervious)
0.95
1
%Impervious
138%Site
Area(acres)
Site Rv
Treatment Volume and Nutrient Loads
Treatment Volume
(acre-ft) 0.3307
Treatment Volume (cubic feet) 14,406
TV Load Rb/yr) 9.05
... TN Load(Ib/yr) fi435
Amendment #4 Water Quantity Calculations
(SCS Worksheets 2-4 & Routings for DA'B' )
Summary:
This proposed WPO amendment #4 accounts for a less impervious post -development DA'B'
and replaces the previously approved amendment #2 underground detention system with a
new dry detention basin (SWM Facility V). Please note, this amendment #4's peak post -
development outflows exiting the basin are less than the corresponding approved flows, hereby
maintaining this development's compliance with water quantity requirements.
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U.S. Department of Agriculture
Natural Resources Conservation Service
TR 55 Worksheet 3: Time of Concentration (T.) or Travel Time (Tt)
Project: Dunlora Overlook VSMP Amendment #4
Location: Dunlora Overlook Subdivision
Check One: Present Developed X
Check One: T, X T,
Segment
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)08) / Pi _5 s°"*
Shallow Concentrated Flow:
7 Surface description (paved or unpaved)
8 Flow Length, L (ft)
9 watercourse slope, s (ft/ft)
10 Average velocity, V (Figure 3-1) (ft/s)
11 T,= L / 3600*V
Channel Flow:
12 Cross sectional flow area, a (ft)
13 wetted perimeter, P. (ft)
14 Hydraulic radius, r = a/Pw (ft)
15 Channel Slope, s (ft/ft)
16 Manning's Roughness Coeff, n
17 V= [ 1.49r2S")/n
18 Flow length, L (ft)
19 T,= L/ 3600*V
20 Watershed or subarea Tor Tr
(Add T, in steps 6, 11 and 19)
Note: The shoretest SCS Tc is 0.10 hrs.
Designed By: FGM, PE Date: 8/25/2021
Checked By: SRC, PE Date: 8/25/2021
Through subarea n/a
AMENDED DA B
D: (Post-Dev.)
Dense
Grass
0.24
50
3.66
0.08
0.07
Paved
275
0.08
5.7
0.01
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Dunlora Overvlook VSMP Amendment No. 4- DA B
NOTE: This VSMP Amendment #4's peak outflows are less
than the previously approved peak outflows.
BasinFlow printout
INPUT:
Basin: Dunlora Overvlook VSMP Amendment No. 4- DA B
4 Contour Areas
Elevation(ft)
434.00
436.00
438.00
440.00
Start_Elevation(ft)
4 Outlet Structures
Outlet structure 0
Orifice
Area(sf) Computed Vol.(cy)
45.00
0.0
495.00
17.0
1058.00
73.2
1726.00
175.3
434.00 Vol.(cy) 0.00
name: Barrel
area (sf) 0.196
diameter or depth (in) 6.000
width for rect. (in) 0.000
coefficient 0.500
invert (ft) 432.000
multiple 1
discharge out of riser
Page 1
Dunlora Overvlook VSMP Amendment No. 4- DA B
Outlet structure 1
Orifice
name:
Low -Flow Orifice
area (sf)
0.049
diameter or depth (in)
3.000
width for rect. (in)
0.000
coefficient
0.500
invert (ft)
434.000
multiple
1
discharge
into riser
Outlet structure 2
Weir
name: Riser
diameter (in)
24.000
side angle
0.000
coefficient
3.300
invert (ft)
437.500
multiple
1
discharge
into riser
transition at (ft)
0.608
orifice coef.
0.500
orifice area (sf)
3.142
Outlet structure 3
Weir
name: Emergency Spillway
length (ft)
15.000
side angle
75.960
coefficient
3.300
invert (ft)
438.500
multiple
1
discharge
through dam
4 Inflow Hydrographs
Hydrograph 0
SCS
name: 1-yr
24-hr SCS TR-55 Design Storm
Area (acres)
1.050
CN
73.000
Type 2
rainfall, P (in)
3.030
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)
1.167
peak time (hrs)
11.917
volume (cy)
123.533
Page 2
Dunlora Overvlook VSMP Amendment No. 4- DA B
Hydrograph 1
SCS
name: 2-yr
24-hr SCS TR-55 Design Storm
Area (acres)
1.050
CN
73.000
Type 2
rainfall, P (in)
3.660
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)
1.717
peak time (hrs)
11.917
volume (cy)
181.743
Hydrograph 2
SCS
name: 10-yr
24-hr SCS TR-55 Design Storm
Area (acres)
1.050
CN
73.000
Type 2
rainfall, P (in)
5.530
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)
3.602
peak time (hrs)
11.917
volume (cy)
381.298
Hydrograph 3
SCS
name: 100-yr
24-hr SCS TR-55 Design Storm
Area (acres)
1.050
CN
73.000
Type 2
rainfall, P (in)
9.090
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)
7.710
peak time (hrs)
11.917
volume (cy)
816.297
Page 3
Dunlora Overvlook VSMP Amendment No. 4- DA B
OUTPUT:
Routing Method: storage -indication
Hydrograph 0
Routing Summary
of
Peaks:
1-yr 24-hr
SCS TR-55 Design Storm
inflow
(cfs)
1.161
at
11.92
(hrs)
discharge
(cfs)
0.317
at
12.08
(hrs)
water level
(ft)
436.723
at
12.10
(hrs)
storage
(cy)
32.609
Hydrograph 1
Routing Summary
of
Peaks:
2-yr 24-hr
SCS TR-55 Design Storm
inflow
(cfs)
1.709
at
11.92
(hrs)
discharge
(cfs)
0.359
at
12.12
(hrs)
water level
(ft)
437.441
at
12.12
(hrs)
storage
(cy)
53.202
Hydrograph 2
Routing Summary
of
Peaks:
10-yr 24-hr
SCS TR-55 Design Storm
inflow
(cfs)
3.585
at
11.92
(hrs)
discharge
(cfs)
1.945
at
12.04
(hrs)
water level
(ft)
438.351
at
12.04
(hrs)
storage
(cy)
87.657
Hydrograph 3
Routing Summary
of
Peaks:
100-yr
24-hr
SCS TR-55 Design Storm
inflow
(cfs)
7.674
at
11.92
(hrs)
discharge
(cfs)
7.614
at
11.92
(hrs)
water level
(ft)
438.727
at
11.92
(hrs)
storage
(cy)
104.685
Thu Sep 09 09:47:30 EDT 2021
Page 4