HomeMy WebLinkAboutWPO201700042 Calculations WPO VSMP 2017-10-31 EMERSON COMMONS STORMWATER MANAGEMENT
CALCULATIONS PACKET
Date of Calculations
MAY 18, 2017
Revised on
AUGUST 4, 2017
(Revisions are highlighted in yellow)
PREPARED BY:
COLLINS
200 GARRETT STREET, SUITE K
CHARLOTTESVILLE, VA 22902
434.293.3719 PH
434.293.2813 FX
www.collins-engineering.com
DEQ Virginia Runoff Reduction Method
Water Quality Calculations
(COMPLIANCE IS MET THROUGH THE BEST MANAGEMENT PRACTICES OF PROTECTING
FOREST & OPEN SPACES AND THROUGH THE PURCHASING OF NUTRIENT CREDITS)
In accordance with an August 3rd phone conversation with Mr. Frank Pohl of County Engineering the following water quality
calculations were updated.The VRRM spreadsheet for ReDevelopments was utilized to calculate the phosphorous removal
requirement for the construction of this project,and the VRRM spreadsheet for New Developments was used to compute the
nutrient credit associated with the proposed preserved open space easements outside the limits of disturbance.
DEQ Virginia Runoff Reduction Method Re-Development Compliance Spreadsheet-Version 3.0
02011 BMP Standards and Specifications 02013 Draft BMP Standards and Specifications
__ Project Name: Emerson Commons VSMP Amendment-RR Requirement data input cells
Date: 8/3/2017 =II constant values •
Linear Development Project? No _ .calculation cells
Site Information
_. final results
Post-Development Project(Treatment Volume and Loads) •
•
Enter Total Disturbed Area(acres) 1 6.08 ( Check:•
BMP Design Specifications List: 2011 Stds&Specs
Maximum reduction required: r Linear project? No —
—
The site's net increase in impervious cover/acres)is Land cover areas entered correctly? I
• Post-Development iP Load Reduction for Site(lb/yr) Total disturbed area entered? 1
1
Pre-ReDevelopment Land Cover(acres) j
A soils B Soils C Soils D Soils Totals
Forest/Open Space(acres)–undisturbed, --'
protected forest/open space or reforested land 102 1.02
Managed Turf Nasal--disturbed,graded for
4.48 `; —� –
yards or other turf to be mowed/managed 4.48 I
Impervious Cover(acres) 0.58 0.58
6.08 I
P
Post-Development Land Cover(acres) I
A Soils B Soils C Soils D Soils Totals i
Forest/Open Space(acres)–undisturbed, —7-
protected forest/open space or reforested land 0.00 0.00
Managed Turf(acres)--disturbed,graded for
–
yards or other turf to be mowed/managed 4.14
4.14
Impervious Cover(acres) t
n 1.94 _..
1.94 ••
Area Check OK. OK. OK. OK. 6.08
__---_` .
Constants • !Runoff Coefficients(Rv)
Annual Rainfall(inches) 43 ASoils BSoils CSoils 11 Soils
Target Rainfall Event(inches) 1.00 Forest/Open Space 0.02 0.03 0.04 0.05
Total Phosphorus(TP)EMC(mg/l) 0.26 managed Turf 0.15 020 0.22 0.25
Total Nitrogen(TN)EMC(mg/L) 1.86 _ Impervious Cover 0.95 0.95 0.95 0.95
Target TP Load(lb/acre/yr) 0.41 --�^ — --'--'j--
Fl(unitlesscorrection factor) 0.90
LAND COVER SUMMARY-- PRE-REDEVELOPMENT LAND COVER SUMMARY-POST DEVELOPMENT
Land Cover Summary-Pre Land Cover Summary-Post(Final) Land Cover Summary-Post Land Cover Summary-Past
Pre-ReDevNopment LI ted Adjusted' Post ReDev.&New Impervious post-ReDevelopment Ppt.00wleament New bnpesvleue
Forest/Open 5 Cover Forest/Open Space Cover Forest/Open Space
pen pace (acres) 1.02 0.00 0.00 Cover(acres) 0.00
acres
• - '®' o.DD 0.00 _ _.Weighted Rv(forest) 0.00
11111131=ME= Ellei==.11:11M %Forest 0% III
_- _
Managed Turf Cover(acres) 4.48 4.14 liagail 4.14 .Managed Turf Cover
(acres) 4.14 __ III
Weighted Rv(turf) 0.20 0.20 Weighted Rv(turf) 0.20 Weighted Rv(turf) 0.20 .
=MN=TM __-•=1.1® %Managed Turf 88% -
po,vous New lm 1 Cover
Impervious Cover(acres) 0.58 0.58 Impervious Cover(acres) 1.94 ReDev.I'(acres) 0'58 M e°)
Cover ac 1361r-
�� �� _. Rvtmpervous) 0.95 i Rv(ImP rvous) 095
%Impervious 10% A2 °%Impervious 32% ... %Impervious 12% , .,-,r.-
Total Site Area(acres) 6.08 4.72 Final Site Arae T t R D .Site Area
(acres) 6.08 4.72
1 es)
Site Rv 0.24 0.29 Final Port Dee see Co 0.44 ReDev Site Rv 0.29
•
Treatment Volume and Nutrient Load I Treatment Volume and Nutrient Load
Pre-Pe Development Treatment Volume Final Post-Development Post.ReDevelopm nt Post-Development
(acre-n) 0.1231 0.1149 Treatment Volume 0.2226 Treatment Volume 0.1149 Treatment Volume 0.1077
(acro-ft) (acre-ft) (acre-ft)
PregeDevelo Treatment Volume Final Post-Development post-ReDevelopment Post-Development
pment 5,364 5,006 Treatment Volume 9,696 Treatment Volume 5,006 Treatment Volume(cubic 4,690
(cubic het) cubic feet
( l (cubic feet) feet)
Pre-ReDevelopment TP Load
Final past- Post-ReDevelopment Post-Development TP Load
(Ib/yr) 3.37 3.15 Development TP Load 6.09 Load(TP) 3.15 (Ib/yr) 2.95
(Ib/yr) (Ib/yr)'
Pre-R,100elopmen[TP Load per acreFinal Post.Development TP Post ReDevelopment IP
(lb/acre/WI0.55 0.67 Load per acre 1.00 Load per acre 0.67
(lb/acre/yr) (Ib/acre/yr)
Baseline TP Load(lb/yr) ',i Max.Reduction Required
(0.41 lbs/acre/yr applied to pre-redevelopment area excluding pervious 194 ' (Below Pre. 20%
land proposed for new impervious cover) '� ReDevelopment Load)
TP Load Reduction
r Adjusted Land Cover Summary: TP Load Reduction
Pre ReDevelopment land cover minus pervious land cover(forest/open space or managed Required for
turf)ncreage proposed for new impervious cover. Redeveloped Area 0.63 Required for New 2.39
(Ib/yr) Impervious Area(Ib/yr)
•Adjusted tocol acreage is consistent with Post-ReDevelopment acreage(minus acreage of
new impervious cover).
Column I shows load reduction revilement for new impervious cover(based on new
development load limit,0.41 lbs/acre/year).
Post-Development Requirement for Site Area
TP Load Reduction Required(Ib/yr) 3.02 •
Nitrogen Loads(Informational Purposes Only)
•
Final Post-Development TN Load
Pre-ReDevelopment TN Load(Ib/yr) 24.11 (Post-ReDevelopment&New Impervious) 43.58
(Ib/yr)
DEQ Virginia Runoff Reduction Method New Development Compliance Spreadsheet-Version 3.0
F.2011 BMP Standards and Specifications 02013 Draft BMP Standards and Specifications
Project Name: Emerson Commons VSMP Amendment-Nutrient Credit Calc. I data input cells
Date: 8/3/2017 constant values
BMP Design Specifications List:2011 Stds&Specs calculation cells
Site Information final results
Post-Development Project(Treatment Volume and Loads)
Land Cover(acres)
A Soils B Soils C Soils D Soils Totals
Forest/Open Space(acres)--undisturbed, 1.83
protected forest/open space or reforested land 1.83
Managed Turf(acres)--disturbed,graded for 0.00
yards or other turf to be mowed/managed
Impervious Cover(acres) 0.00
'Forest/Open Space areas must be protected in accordance weth the Virginio Runoff Reduction Method 1.83
Constants Runoff Coefficients(Rv)
Annual Rainfall(inches) 43 A Soils B Soils C Soils D Soils
Target Rainfall Event(inches) 1.00 Forest/Open Space 0.02 0.03 0.04 0.05
Total Phosphorus(TP(EMC(mg/L) 0.26 Managed Turf 0.15 0.20 0.22 0.25
Total Nitrogen(TN)EMC(mg/Li 1.86 Impervious Cover 0.95 0.95 0.95 0.95
Target TP Load(lb/acre/yr) 0.41
PI(unitless correction factor) 0.90
Post-Development Requirement for Site Area
TP Load Reduction Required(lb/yr) -0.63 ** TP LOAD REDUCTION NOT REQUIRED
LAND COVER SUMMARY-POST DEVELOPMENT
Land Cover Summary Treatment Volume and Nutrient Loads
Forest/Open Space Cover(acres) 1.83 Treatment Volume
(acre-ft) 0.0046
Weighted Rv(forest) 0.03 Treatment Volume(cubic feet) 199
%Forest 100% TP Load(Ib/yr) 0.13
TN Load(Ib/yr)
Managed Turf Cover(acres) 0.00 (Informational Purposes Only)
0.90
Weighted Rv(turf) 0.00
%Managed Turf 0%
Impervious Cover(acres) 0.00
Rv(impervious) 0.95
%Impervious 0%
Site Area(acres) 1.83
Site Rv 0.03
SCS TR-55 Calculations
Soils Mapping (Source: NRCS Web Soil Survey Online Database)
Soils Table (Source: NRCS Web Soil Survey Online Database)
Map Unit Legend
Albemarle County, Virginia (VA003)
Map Map Unit Name Percent
Unit of AOI
Symbol
4D Ashe loam, 15 tc 25 percent 57 2°3
slopes
37B3 Hayesville clay loam, 2 to 7 42.8%
percent slopes, severely
eroded
Totals for Area of Interest 100.0%
Note:The soils' properties for this development are predominantly composed of hydrologic type B.
01,
6
r
favor Creeka Park
c f ,
ir
arm a meg
No t ildOct
ti
POINT PRECIPITATION FREQUENCY (PF) ESTIMATES
WITH 9O''io CONFIDENCE INTERVALS AND SUPPLEMENTARY INFORMATION
NOAA Atlas 14,Volume 2,Version 3
PDS-based precipitation frequency estimates with 90%confidence intervals(in inches)1
Duration I Average recurrence interval(years)
1 t II 2 I I 5 II 10 II 25 II 50 Il 100 11 200 II 5g0 II 1000 I
24-hr 3.06 1 3.70 4.72 5.57 6.82 7.89 9.07 10.4 12.3 14.0
'2 75-3 42. 1 i3 33.4 14: (4.23-5.27) ,"4.98-6.21', 16 05-7 591 (696-8.77) 17.93-10.1) i8 96-11 5) 10 5-'3.7) 110 7.15-61
U.S.Department of Agriculture FL-ENG-21A
Natural Resources Conservation Service 06/04
TR 55 Worksheet 2:Runoff Curve Number and Runoff
Project: Emerson Commons Designed By:FGM,PE Date: 8/1/2017
Location: Parkview Drive Checked By:SRC,PE Date: 8/1/2017
Check One: Present X DevX
1.Runoff curve Number(CN)
Soil name and Cover description CN(weighted),'
Drainage Area Area Product of CN Calculated
hydrologic group (Cover type,treatment,and hydrologic condition;percent CN total product/
Description (Acres) x Area '5'Value
(Appendix A) impervious;unconnected/connected impervious area ratio) total area
DA A Impervious Areas 98 0.10 10.1
(Present)
B Woods in Good Condition 55 0.00 0.0 65.5 5.27
Lawns in Good Condition(75%+Groundcover) 61 0.75 45.6
DA B Impervious Areas 98 0.05 4.8
B Woods in Good Condition 55 0.26 14.3 61.2 6.33
(Present) Lawns in Good Condition(75%+Groundcover) 61 0.87 53.1
Impervious Areas(Includes Offsite Ex.Imp.Areas) 98 0.03 3.1
DA C
(Present) B Woods in Good Condition 55 0.00 0.0 62.5 5.99
Lawns in Good Condition(75%+Groundcover) 61 0.75 45.6
DA A Impervious Areas 98 0.36 34.9
(Developed) B Woods in Good Condition 55 0.08 4.4 75.6 3.23
Lawns in Good Condition(75%+Groundcover) 61 0.43 26.4
DA B Impervious Areas 98 0.42 40.8
(Developed) B Woods in Good Condition 55 0.03 1.7 75.1 3.31
Lawns in Good Condition(75%+Groundcover) 61 0.63 38.6
DA C Impervious Areas 98 0.51 50.2
(Developed) B Woods in Good Condition 55 0.02 1.1 75.8 3.19
Lawns in Good Condition(75%+Groundcover) 61 0.74 45.0
2.Runoff
1-Year Storm 2-Year Storm 10-Year Storm Drainage Area Description
Frequency-years 1 2 10 n/a
Rainfall,P(24 hour)-inches 3.06 3.70 5.57 n/a
Runoff,Q-inches 0.55 0.88 2.08 DA A(Present)
Runoff,Q-inches 0.40 0.68 1.74 DAB(Present)
Runoff,Q-inches 0.44 0.74 1.84 DA C(Present)
Runoff,Q-inches 1.03 1.49 2.98 DA A(Developed)
Runoff,Q-inches 1.01 1.45 2.93 DAB(Developed)
Runoff,Q-inches 1.05 1.50 3.00 DA C(Developed)
U.S.Department of Agriculture FL-ENG-21A
Natural Resources Conservation Service 06/04
TR 55 Worksheet 3:Time of Concentration(Ta)or Travel Time(Ti)
Project: Emerson Commons Designed By: FGM,PE Date: 5/18/2017
Location: Parkview Drive Checked By: SRC,PE Date: 5/18/2017
Check One: Present X Developed X
Check One: T, X Tt Through subarea n/a
DAA DAB DAC DAA DAB DA C
Segment ID: (Present) (Present) (Present) (Developed) (Developed) (Developed)
Sheet Flow:(Applicable to Tc only)
1 Surface description(Table 3-1) Dense Grass Dense Grass 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 0.24 0.24
3 Flow length,L(total L<100)(ft) 100 100 100 100 100 100
4 Two-year 24-hour rainfall,P2(in.) 3.7 3.7 3.7 3.7 3.7 3.7
5 Land slope,s(ft/ft) 0.09 0.08 0.1 0.09 0.1 0.09
6 Compute T5=[0.007(n*L)°8]/P2.5 s°4 0.12 0.13 0.12 0.12 0.12 0.12
Shallow Concentrated Flow:
7 Surface description(paved or unpaved) Unpaved Unpaved Unpaved
8 Flow Length,L(ft) 50 240 30
9 Watercourse slope,s(ft/ft) 0.180 0.208 0.200
10 Average velocity,V(Figure 3-1)(ft/s) 6.8 7.5 7.5
11 Tt=L/3600*V 0.00 0.01 0.00
Channel Flow:
12 Cross sectional flow area,a(ft2)
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.s]/n
18 Flow length,L(ft)
19 It=L/3600*V
20 Watershed or subarea Tcor T5
(Add Tt in steps 6,11 and 19) I 0.12 I 0.14 I 0.12 I 0.12 I 0.12 I 0.12 I
U.S.Department of Agriculture FL-ENG-21C
Natural Resources Conservation Service 06/04
TR 55 Worksheet 4:Graphical Peak Discharge Method
Project: Emerson Commons Designed By: FGM,PE Date: 8/1/2017
Location: Parkview Drive Checked By:SRC,PE Date: 8/1/2017
Check One: Present X Developed X
Drainage Area Drainage Area Drainage Area Drainage Area Drainage Area Drainage Area
1.Data Description Description Description Description Description Description
DAA DAB DAC DAA DAB DA C
(Present) (Present) (Present) (Developed) (Developed) (Developed)
Drainage Area(Am)in miles2= 0.0013 0.0018 0.0012 0.0014 0.0017 0.0020
Runoff curve number CN= 65.5 61.2 62.5 75.6 75.1 75.8
Time of concentration(Tc)= 0.12 0.14 0.12 0.12 0.12 0.12
Rainfall distribution type= II II II II II II
Pond and swamp areas spread 0 0 0 0 0 0
throughout the watershed=
2.Frequency-years 1 2 10 1 2 10 1 2 10 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 3.7 5.57 3.06 3.70 5.57 3.06 3.7 5.57 3.06 3.70 5.57
4.Initial Abstraction,la-inches 1.05 1.05 1.05 1.27 1.27 1.27 1.20 1.20 1.20 0.65 0.65 0.65 0.66 0.66 0.66 0.64 0.64 0.64
5.Compute la/P 0.34 0.29 0.19 0.41 0.34 0.23 0.39 0.32 0.22 0.21 0.17 0.12 0.22 0.18 0.12 0.21 0.17 0.11
6.Unit peak discharge,Qu-csm/in 850 900 925 675 775 850 825 875 925 925 925 950 925 925 950 925 925 950
7.Runoff,Q from Worksheet 2-inches 0.55 0.88 2.08 0.40 0.68 1.74 0.44 0.74 1.84 1.03 1.49 2.98 1.01 1.45 2.93 1.05 1.50 3.00
8.Pond and Swamp adjustment factor,Fp 1 1 1 1 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 0.62 1.06 2.56 0.49 0.97 2.73 0.44 0.79 2.08 1.30 1.87 3.84 1.57 2.27 4.70 1.92 2.75 5.65
WATERSHED SUMMARY
Pre-Development Post-Development
1-year Flow, 2-year Flow, 10-year 1-year Flow, 2-year Flow, 10-year
CN Area,ac. cfs cfs Flow,cfs CN Area,ac. cfs cfs Flow,cfs
DA'A' 65.5 0.85 0.62 1.06 2.56 75.6 0.87 1.30 1.87 3.84
DA'B' 61.2 1.18 0.49 0.97 2.73 75.1 1.08 1.57 2.27 4.70
DA'C' 62.5 0.78 0.44 0.79 2.08 75.8 1.27 1.92 2.75 5.65
Outlet Protection & Adequate Channel Analysis to Parrott Branch
(The following computer-generated Manning's calculations and the handwritten calculations
were updated to account for the minor increased acreages associated with the overland flows
to subareas A, B & C's outfall points. These minimal increased acreages are 0.09 ac., 0.11 ac.
and 0.06 ac. for subareas A, B & C, respectfully. These calculations were also updated to no
longer propose disturbances to the stream per Engineering's direction.)
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Subarea C Trapizoidal Channel Analysis- 2 Year
Complex Channnel-
Subarea C's Proposed Adequate Channel
Input:
Slope 0.250
Flow (given) 2.750
line x y n
0 0.000 1.000 0.050 (first n value not used)
1 3.000 0.000 0.050
2 6.000 0.000 0.050
3 9.000 1.000 0.050
Output:
Depth 0.177
(Y) ( 0.177)
Channel bed segment output:
line Q V A P
1 0.13 2.85 0.05 0.56
2 2.48 4.68 0.53 3.00
3 0.13 2.85 0.05 0.56
Tue Aug 01 09:00:54 EDT 2017
Page 1
Subarea C Trapizoidal Channel Analysis- 10 Year
Complex Channnel-
Subarea C's Proposed Adequate Channel
Input:
Slope 0.250
Flow (given) 5.650
line x y n
0 0.000 1.000 0.050 (first n value not used)
1 3.000 0.000 0.050
2 6.000 0.000 0.050
3 9.000 1.000 0.050
Output:
Depth 0.265
(y) ( 0.265)
Channel bed segment output:
line Q V A P
1 0.39 3.73 0.11 0.84
2 4.87 6.13 0.79 3.00
3 0.39 3.73 0.11 0.84
Tue Aug 01 09:07:40 EDT 2017
Page 1