HomeMy WebLinkAboutWPO201700042 Calculations Minor Amendment 2018-05-07 EMERSON COMMONS STORMWATER MANAGEMENT
AMENDMENT CALCULATIONS PACKET
Date of Approved Calculations
AUGUST 4, 2017
Date of Amended Calculations
May 7, 2018
(Revisions are highlighted in yellow)
PREPARED BY:
COLLINS - c Ii I
200 GARRETT STREET, SUITE K
CHARLOTTESVILLE, VA 22902
434.293.3719 PH
434.293.2813 FX
www.collins-engineering.com
The following calculations are intended to update and supplement the previously
approved/provided stormwater management calculations. The applicant kindly requests
these calculations be added to the County's project folder and files.
SCS TR-55 Calculations
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: 5/7/2018
Location: Parkview Drive Checked By:SRC,PE Date: 5/7/2018
Check One: Present X DevX
1.Runoff curve Number(CN)
Draina a Area Soil name and Cover description CN(weighted),r--
H Area Product of CN Calculated'S'
hydrologic group (Cover type,treatment,and hydrologic condition;percent CN total product/
Description (Acres) x Area Value A) impervious;unconnected connected impervious area ratio)
total area
APPROVED 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
APPROVED DAB 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
APPROVED DA C Impervious Areas(Includes Offsite Ex.Imp.Areas) 98 0.03 3.1
(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
APPROVED 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
AMENDED DA 8&C Impervious Areas 98 0.93 91.0
(Developed) B Woods in Good Condition 55 0.05 2.8 75.6 3.22
Lawns in Good Condition(75%+Groundcover) 61 1.35 82.4
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 APPROVED DA A(Present)
Runoff,Q-inches 0.40 0.68 1.74 APPROVED DAB(Present)
Runoff,Q-inches 0.44 0.74 1.84 APPROVED DA C(Present)
Runoff,Q-inches 1.03 1.49 2.98 APPROVED DA A(Developed)
Runoff,Q-inches 1.03 1.49 2.98 AMENDED DA 8&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: 5/7/2018
Location: Parkview Drive Checked By:SRC,PE Date: 5/7/2018
Check One: Present X Developed X
Check One: T, X T, Through subarea n/a
APPROVED DAA APPROVED DAB APPROVED DAC APPROVED DAA APPROVED DA$&&CTc'
Segment ID: (Present) (Present) (Present) (Developed) (Developed)
Sheet Flow:(Applicable to T,only)
1 Surface description(Table 3-1) 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
3 Flow length,L(total L<100)(ft) 100 100 100 100 100
4 Two-year 24-hour rainfall,P2(in.) 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
6 Compute T,=[0.007(n*L)08)/p20.5 s.4 0.12 0.13 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,Ps,,(ft)
14 Hydraulic radius,r=a/Pw(ft)
15 Channel Slope,s(ft/ft)
16 Manning's Roughness Coeff,n
17 V=[1.49r213s'5]/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) 0.12 I 0.14 I 0.12 I 0.12 I 0.12
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: 5/7/2018
Location: Parkview Drive Checked By: SRC,PE Date: 5/7/2018
Check One: Present X Developed X
Drainage Area Drainage Area Drainage Area Drainage Area Drainage Area
1.Data Description Description Description Description Description
APPROVED DAA APPROVED DAB APPROVED DAC APPROVED DAA AMENDED DAB&C
(Present) (Present) (Present) (Developed) (Developed)
Drainage Area(Am)in miles2= 0.0013 0.0018 0.0012 0.0014 0.0036
Runoff curve number CN= 65.5 61.2 62.5 75.6 75.6
Time of concentration(Tc)= 0.12 0.14 0.12 0.12 0.12
Rainfall distribution type= II II II II II
Pond and swamp areas spread 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
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
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 0.64 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 0.21 0,17 0.12
6.Unit peak discharge,Qu-csm/in 850 900 925 675 775 850 825 875 925 925 925 950 925 925 950
7.Runoff,Qfrom 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 1.03 1.49 2.98
8.Pond and Swamp adjustment factor,Fp 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 3.48 5.01 10.29
WATERSHED SUMMARY
Pre-Development Post-Development
1-year Flow, 2-year Flow, 10-year 1-year Flow, 2-year Flow, 10-year Flow,
CN Area,ac. cfs cfs Flow,cfs CN Area,ac. cfs cfs 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 0.972.73 75.6 2.33 3.48 ' 5.01. "; 1029 !-
DA'C' 62.5 0.78 0.44 0.79 2.08
Outlet Protection
(The following updated calculations, where shown in red, reflect the revised VDOT Cl. Al riprap
lined outfall design for subareas B & C. To summarize the updates, subareas B & C's new outfall
design is required to be 14' long with an end width of 15'. The previously approved initial
width of 3' with a stone depth of 2.5' remain the same.)
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Underdrain Calculations
(The following updated calculations reflect the new underdrain network for subareas B & C.)
AMENDED YARD DRAINAGE DESCRIPTIONS
STR. NO. STR. NO. DIA. LENGTH' SLOPE INVERT INVERT MATERIAL RIM* COMMENTS
(in.) (ft.) (%) IN OUT ELEVATION
YD1 YD31 12 61.1 18.41% 636.60 625.35 HDPE
12"MIN.COVER&SOLID
YD31 630.50
COVER REQ'D
YD31 YD1A 12 19.0 28.22% 625.35 620.00 HDPE
YD12 YD29 12 173.7 0.75% 628.75 627.45 HDPE
12"MIN.COVER&SOLID
YD29 628.00 COVER REQ'D
YD29 YD30 12 65.8 2.69% 627.45 625.68 HDPE
12"MIN.COVER&SOLID
YD30 629.90 COVER REQ'D
YD30 YD31 12 3.8 1.00% 625.68 625.35 HDPE
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