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HomeMy WebLinkAboutWPO201900003 Calculations 2020-11-06PLEASANT GREEN- PHASES II & III STORMWATER MANAGEMENT CALCULATIONS PACKET Date of Calculations NOVEMBER 31 2020 PREPARED BY: COLLINS El<! "'-"' 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 Acres Percent of Symbol in AOI AOI 713 Braddock loam, 7.6 21.7% 2 to 7 percent slopes 8C3 Braddock clay 6.7 19.1% loam, 7 to 15 percent slopes, severely eroded 17 Craigsville 4.4 12.6% loam, 0 to 2 percent slopes, frequently flooded 25B Dyke silt loam, 13.2 37.7% 2 to 7 percent slopes 36D Hayesville 2.7 7.8% loam, 15 to 25 percent slopes 37D3 Hayesville clay 0.4 1.1% loam, 15 to 25 percent slopes, severely eroded Totals for Area of 35.0 100.0% Interest The NRCS web soil survey defines this development's soils as predominantly having a hydrologic group type B. Precipitation Data (Source: NCAA's National Weather Service, Hydrometerorological Design Studies Center, Atlas 14 Point Precipitation Frequency Estimates) P,Aap v 71 Terrain t c 0 ooA 00 e{V 9laa a a° 4 0a ;p O4, Vanetdr Ocy: O' ae od' O °t clarrook or O � °a.! �a a Braeburn St Ir o J C/d 1 a' ? Laura L'o o E � R Ci��F J C 'PQ; o e OJO S/ VO � C ya aorOaq i �CCon� d J, yo` l V a y v Waylan� 0 n y` Crozet o V q G p Wmdmere Ln e O T < u Q Tabor St,i V f r tb rG _o "o TOP St p. 2 'Yoo a as/F Oo — 0.4kR1 0,. ao,.., hr �00, ° MYrne St cnudlus � .2 Cmxl Pak POINT PRECIPITATION FREQUENCY (PF) ESTIMATES WITH 90'. CONFIDENCE INTERVALS AND SUPPLEMENTARY INFORMATION NOAA Atlas 14. VOIIIITe 2, Version 3 SII PDS-based precipitation frequency estimates with 90% confidence intervals (in inches)' Ammwnence n al (yams) 309 24-Ilr (2.78a4el (3.�4.17) (4.275.31) (s.02�is) (e. a7B..w) n.00-8..a2t n. ;o.n (9.O1-11b) (%5- a.e) t1v°;sn p N N Z N � LL OLL LL � K � b b c m `v a � m m Z a a c � � m w U w L � O U C J K N W L Y 3 ubi x m r ma 0 u v � 2 N c w 0 Z � w - c U m N L E o w u m y U z W 3 Lu s v a E v Z 0 o o c z a y W � m in U 11 N � � 01 a 3 q O Z O " U x U v ad v ai vi v �ri a ai v 4 ai a5 ? �ri .: a 0 a N` eel m p m Of N W n O b ei m p e1 N V e9 1I1 p� n o N p n O e1 m p m V O QN a eel N 4 eel V T OO N N fV I!1 O T V1 OO N N OO Z m ei VI W eel Vl m eel VI m eel vl m V1 m V1 U Ol b i/1 m b Vl m b Vl O1 b Vl O1 b Vf O1 b Vf o c - w m w w6 C � O � o_ m C it O n u E C o o a p o a C. T C y L C m m 0 c `w m c c c c c c c c c c c c u vt; o° o o o o a E c a s m I M I M c a c a v c 0 c c 0 c c 0 c c 0 c c 0 c c 0 c c 0 c 0 m u u 0 m u u 0 m u u 0 m u u 0 m u u 0 m u u w w a 0 m o< a o a o< a o a o 4 0 0¢ o o¢ 0 0¢ 0 0¢ 0 0¢ 0 0 o 0 0 U' 0 0 U' 0 0 0 0 0 0 l"J O 0 0 O O 0 U' O o c 5 o c c o c c a c c a c c o c c 0 v a 2 a Z a Z a z 'e Z' v E E 3 o m 3 o a 3 o a 3 o E 3 o a 3 0 E 3 E 3 3 E 3 3 E 3 3 E 3 3 m 0 4 o x ry L y a n c m m m m m m an w n m� c 0 pp w 6 m w U w O w m 0 U O D o v aI? o v a9 v o o p v a a o a a a o a 0 a U.S. Department of Agriculture Natural Resources Conservation Service TR 55 Worksheet 3: Time of Concentration (T,) or Travel Time (T,) Project: Pleasant Green Phases II & III Location: West of McComb Street Check One: Present X Developed X Check One: T, X T, Segment ID: 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, P, (in.) 5 Land slope, s (ft/ft) 6 Compute T, = [0.007(n*L)"aj / Pi .5 0.4 Shallow Concentrated Flow Designed By: FGM, PE Checked By: SRC, PE Through subarea n/a FL-ENG-21A 06/04 Date: 11/3/2020 Date: 11/3/2020 DAB DA C DA D DAB DA C DA D (Pre-Dev.) (Pre-Dev.) (Pre-Dev.) (Post-Dev.) (Post-Dev.) (Post-Dev.) Dense Dense Woods -Light Dense Dense Dense Grass Grass Underbrush Grass Grass Grass 0.24 0.24 0.4 0.24 0.24 0.24 100 100 100 100 100 100 3.73 3.73 3.73 3.73 3.73 3.73 0.06 0.05 0.04 0.06 0.05 0.03 0.14 0.15 0.25 0.14 0.15 0.19 7 Surface description (paved or unpaved) Unpaved Unpaved Unpaved Unpaved Unpaved Unpaved 8 Flow Length, L (ft) 535 150 400 300 150 225 9 Watercourse slope, s (ft/ft) 0.07 0.03 0.08 0.07 0.03 0.05 10 Average velocity, V (Figure 3-1) (ft/s) 4.2 2.8 4.5 4.2 2.8 3.6 11 T,=L/3600*V 0.04 0.01 0.02 0.02 0.01 0.02 Channel Flow: 12 Cross sectional flow area, a (ft) 13 Wetted perimeter, P. (ft) 14 Hydraulic radius, r= a/P* (ft) 15 Channel Slope, s (ft/ft) 16 Manning's Roughness Coeff, n 17V=[1.49r2"s"sj/n 18 Flow length, L (ft) 19 T,= L / 3600*V 20 Watershed or subarea T, or T, (Add T, in steps 6, 11 and 19) Note: The shoretest SCS Tc is 0.10 hrs. c c > c 0 > 30 l0 Y- 0 N O N LT d O tf E 11:W d i> 1.1 LL LL c y l0 ai Q> 6 7 X W Q �. 6 q C r m E C UI o'i m C=- u a x c U! oy m -6r o u y x u 0 v Y x ut " Q o o O m o 0 No 0 Q in LD a in p n o t7 n 1- c N Q D N W 1 O Q n 0.06 0.02 0.03 0.02 0.18 0.22 0.27 1 0.18 1 0.19 0.23 E e!§;!CR / 2 \!/!E \\\\\\}}\ \®!! \«!! )! / j�5 �£ �-§-§ �2 . f 6 :, / ! §L § 2 2 ;; KK Channel and Flood Protection Computations 9 VAC 25-870-66 (Concentrated Outfall from Subarea 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 Allowed !� I.F. * (Q Pre -Developed * IRV Pre -Developed I ) / (RV Developed J 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 < 2.21 cfs Section B.4 Limits of Analysis 4.74 cfs (From Worksheet 4 Watershed Summary) 65 (From Worksheet 4 Watershed Summary) 5.45 (From SCS TR-55 eq. 2-4) 0.54 in. (From SCS TR-55 eq. 2-3) 73 (From Worksheet 4 Watershed Summary) 3.68 (From SCS TR-55 eq. 2-4) 0.92 in. (From SCS TR-55 eq. 2-3) 0.80 (Factor of Safety) 1.78 cfs The analysis terminates at the SWM facility since subsection B.3.a is met. 9 VAC 25-870-66 (Concentrated Outfall from Subarea 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.b.: The point of discharge (the SWM facility outfall), "releases a postdevelopment 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." 10-Year, 24-Hour Pre -Development Subarea 1 Peak Flow Rate = 21.48 cfs 10-Year, 24-Hour Post -Development Subarea 1 Peak Flow Rate = 17.33 cfs Section C.3 Limits of Analysis The flood protection analysis terminates at the SWM facility since subsection C.2.b is met. 9 VAC 25-870-66 (Concentrated Cutfall from Subarea Q 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 Allowed !� I.F. * (Q Pre -Developed * IRV Pre -Developed I ) / (RV Developed J 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 < 1.48 cfs Section B.4 Limits of Analysis 3.79 cfs (From Worksheet 4 Watershed Summary) 63 (From Worksheet 4 Watershed Summary) 5.85 (From SCS TR-55 eq. 2-4) 0.48 in. (From SCS TR-55 eq. 2-3) 74 (From Worksheet 4 Watershed Summary) 3.49 (From SCS TR-55 eq. 2-4) 0.97 in. (From SCS TR-55 eq. 2-3) 0.80 (Factor of Safety) 1.29 cfs The analysis terminates at the SWM facility since subsection B.3.a is met. 9 VAC 25-870-66 (Concentrated Outfall from Subarea C) 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.a.: The point of discharge (the SWM facility outfall), "Confines the postdevelopment peak flow rate from the 10-year 24-hour storm event within the stormwater conveyance system [the SWM facility & outfall pipe] to avoid the localized flooding. Detention of stormwater or downstream improvements may be incorporated into the approved land -disturbing activity to meet this criterion...." 10-Year, 24-Hour Design Storm Peak Water Surface Elevation = 726.53 ft SWM Facility'C' Top of Dam Elevation = 728.00 ft Section C.3 Limits of Analysis ....stormwater conveyance systems shall be analyzed for compliance with flood protection criteria to a point where:" a, b or c are met. 9 VAC 25-870-66 (Concentrated Outfall from Subarea D) 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 Allowed !� I.F. * (Q Pre -Developed * IRV Pre -Developed I ) / (RV Developed J 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 < 0.32 cfs Section B.4 Limits of Analysis 1.29 cfs (From Worksheet 4 Watershed Summary) 61 (From Worksheet 4 Watershed Summary) 6.53 (From SCS TR-55 eq. 2-4) 0.38 in. (From SCS TR-55 eq. 2-3) 79 (From Worksheet 4 Watershed Summary) 2.68 (From SCS TR-55 eq. 2-4) 1.25 in. (From SCS TR-55 eq. 2-3) 0.80 (Factor of Safety) 0.30 cfs The analysis terminates at the SWM facility since subsection B.3.a is met. 9 VAC 25-870-66 (Concentrated Outfall from Subarea D) 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.a.: The point of discharge (the SWM facility outfall), "Confines the postdevelopment peak flow rate from the 10-year 24-hour storm event within the stormwater conveyance system [the SWM facility & outfall pipe] to avoid the localized flooding. Detention of stormwater or downstream improvements may be incorporated into the approved land -disturbing activity to meet this criterion...." 10-Year, 24-Hour Design Storm Peak Water Surface Elevation = 700.28 ft SWM Facility'D' Top of Dam Elevation = 702.00 ft Section C.3 Limits of Analysis ....stormwater conveyance systems shall be analyzed for compliance with flood protection criteria to a point where:" a, b or c are met. SWM Facility'B' Routing Calculations (Confirms the 1-year outflow is compliant with energy balance computations and the 10-year outfall is less than the pre -development 10-year flow.) Pleasant Green- Phases II and III SWM Facility B BasinFlow printout INPUT: Basin: Pleasant Green- Phases II and III SWM Facility B 7 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 739.75 1.00 0.0 740.00 320.00 1.0 742.00 1188.00 53.5 744.00 2573.00 189.5 746.00 4442.00 446.2 748.00 6755.00 857.9 750.00 9497.00 1457.0 Start—Elevation(ft) 5 Outlet Structures Outlet structure 0 Orifice 739.75 Vol.(cy) 0.00 name: Barrel area (sf) 1.767 diameter or depth (in) 18.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 739.480 multiple 1 discharge out of riser Outlet structure 1 Orifice name: Low -Flow Orifice area (sf) 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 739.750 multiple 1 discharge into riser Page 1 Pleasant Green- Phases II and III SWM Facility B Outlet structure 2 Orifice name: Mid -Flow Orifices area (sf) 0.785 diameter or depth (in) 12.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 745.330 multiple 4 discharge into riser Outlet structure 3 Weir name: Riser diameter (in) 48.000 side angle 0.000 coefficient 3.300 invert (ft) 747.330 multiple 1 discharge into riser transition at (ft) 1.215 orifice coef. 0.500 orifice area (sf) 12.566 Outlet structure 4 Weir name: Emergency Spillway length (ft) 20.000 side angle 0.000 coefficient 3.300 invert (ft) 748.330 multiple 1 discharge through dam 4 Inflow Hydrographs Hydrograph 0 SCS name: 1-yr 24-hr SCS Design Storm Area (acres) 8.390 CN 73.000 Type 2 rainfall, P (in) 3.090 time of conc. (hrs) 0.1800 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 8.547 peak time (hrs) 11.970 volume (cy) 1029.077 Page 2 Pleasant Green- Phases II and III SWM Facility B Hydrograph 1 SCS name: 2-yr 24-hr SCS Design Storm Area (acres) 8.390 CN 73.000 Type 2 rainfall, P (in) 3.730 time of conc. (hrs) 0.1800 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 12.512 peak time (hrs) 11.970 volume (cy) 1506.470 Hydrograph 2 SCS name: 10-yr 24-hr SCS Design Storm Area (acres) 8.390 CN 73.000 Type 2 rainfall, P (in) 5.610 time of conc. (hrs) 0.1800 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 25.909 peak time (hrs) 11.970 volume (cy) 3119.453 Hydrograph 3 SCS name: 100-yr 24-hr SCS Design Storm Area (acres) 8.390 CN 73.000 Type 2 rainfall, P (in) 9.120 time of conc. (hrs) 0.1800 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 54.420 peak time (hrs) 11.970 volume (cy) 6552.155 Page 3 Pleasant Green- Phases II and III SWM Facility B OUTPUT: Routing Method: storage -indication Hydrograph 0 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) Peaks: 1-yr 24-hr SCS Design Storm 8.547 at 11.96 (hrs) 1.782 at 12.22 (hrs) 745.121 at 12.24 (hrs) 316.303 Peaks: 2-yr 24-hr SCS Design Storm 12.512 at 11.96 (hrs) 6.559 at 12.10 (hrs) 745.863 at 12.10 (hrs) 423.964 Peaks: 10-yr 24-hr SCS Design Storm 25.909 at 11.96 (hrs) 17.334 at 12.08 (hrs) 747.287 at 12.08 (hrs) 691.265 Peaks: 100-yr 24-hr 54.420 at 50.659 at 748.918 at 1107.782 Tue Nov 03 09:31:35 EST 2020 SCS Design Storm 11.96 (hrs) 12.02 (hrs) 12.02 (hrs) Page 4 SWM Facility'C' Routing Calculations (Confirms the 1-year outflow is compliant with energy balance computations and the 10-year outfall is confined within the SWM Facility.) Pleasant Green- Phases II and III SWM Facility C BasinFlow printout INPUT: Basin: Pleasant Green- Phases II and III SWM Facility C 7 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 718.75 1.00 0.0 719.00 447.00 1.4 720.00 927.00 26.4 722.00 2216.00 139.4 724.00 3595.00 352.5 726.00 5357.00 681.9 728.00 7369.00 1151.3 Start—Elevation(ft) 5 Outlet Structures Outlet structure 0 Orifice 718.75 Vol.(cy) 0.00 name: Str. C-101, Initial Barrel area (sf) 1.767 diameter or depth (in) 18.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 718.000 multiple 1 discharge out of riser Outlet structure 1 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) 718.750 multiple 1 discharge into riser Page 1 Pleasant Green- Phases II and III SWM Facility C Outlet structure 2 Orifice name: Mid -Flow Orifices area (sf) 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 724.500 multiple 4 discharge into riser Outlet structure 3 Weir name: Str. C-100, Riser diameter (in) 48.000 side angle 0.000 coefficient 3.300 invert (ft) 725.500 multiple 1 discharge into riser transition at (ft) 1.215 orifice coef. 0.500 orifice area (sf) 12.566 Outlet structure 4 Weir name: Emergency Spillway length (ft) 35.000 side angle 0.000 coefficient 3.300 invert (ft) 726.500 multiple 1 discharge through dam 4 Inflow Hydrographs Hydrograph 0 SCS name: 1-yr 24-hr SCS Design Storm Area (acres) 8.870 CN 74.000 Type 2 rainfall, P (in) 3.090 time of conc. (hrs) 0.1900 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 9.438 peak time (hrs) 11.953 volume (cy) 1150.598 Page 2 Pleasant Green- Phases II and III SWM Facility C Hydrograph 1 SCS name: 2-yr 24-hr SCS Design Storm Area (acres) 8.870 CN 74.000 Type 2 rainfall, P (in) 3.730 time of conc. (hrs) 0.1900 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 13.692 peak time (hrs) 11.953 volume (cy) 1669.143 Hydrograph 2 SCS name: 10-yr 24-hr SCS Design Storm Area (acres) 8.870 CN 74.000 Type 2 rainfall, P (in) 5.610 time of conc. (hrs) 0.1900 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 27.945 peak time (hrs) 11.953 volume (cy) 3406.831 Hydrograph 3 SCS name: 100-yr 24-hr SCS Design Storm Area (acres) 8.870 CN 74.000 Type 2 rainfall, P (in) 9.120 time of conc. (hrs) 0.1900 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 58.031 peak time (hrs) 11.953 volume (cy) 7074.491 Page 3 Pleasant Green- Phases II and III SWM Facility C OUTPUT: Routing Method: storage -indication Hydrograph 0 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) Peaks: 1-yr 24-hr SCS Design Storm 9.434 at 11.96 (hrs) 1.285 at 12.32 (hrs) 724.482 at 12.36 (hrs) 420.153 Peaks: 2-yr 24-hr SCS Design Storm 13.686 at 11.96 (hrs) 3.991 at 12.20 (hrs) 725.431 at 12.20 (hrs) 574.711 Peaks: 10-yr 24-hr SCS Design Storm 27.934 at 11.96 (hrs) 20.440 at 12.06 (hrs) 726.532 at 12.06 (hrs) 792.441 Peaks: 100-yr 24-hr 58.008 at 57.332 at 726.968 at 890.561 Tue Nov 03 14:47:20 EST 2020 SCS Design Storm 11.96 (hrs) 11.98 (hrs) 11.98 (hrs) Page 4 SWM Facility'D' Routing Calculations (Confirms the 1-year outflow is compliant with energy balance computations and the 10-year outfall is confined within the SWM Facility.) Pleasant Green- Phases II and III SWM Facility D BasinFlow printout INPUT: Basin: Pleasant Green- Phases II and III SWM Facility D 5 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 695.00 1111.00 0.0 696.00 1600.00 49.9 698.00 2826.00 211.7 700.00 4376.00 476.4 702.00 6200.00 866.1 Start—Elevation(ft) 5 Outlet Structures Outlet structure 0 Orifice 695.00 Vol.(cy) 0.00 name: Str. D-101, A High -Flow Bypass Barrel area (sf) 3.142 diameter or depth (in) 24.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 696.240 multiple 1 discharge out of riser Outlet structure 1 Orifice name: Str. D-103, A Low -Flow Barrel area (sf) 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 694.840 multiple 1 discharge out of riser Page 1 Pleasant Green- Phases II and III SWM Facility D Outlet structure 2 Orifice name: Low -Flow Orifice area (sf) 0.034 diameter or depth (in) 2.500 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 695.000 multiple 1 discharge into riser Outlet structure 3 Weir name: Str. D100, A Riser diameter (in) 48.000 side angle 0.000 coefficient 3.300 invert (ft) 699.750 multiple 1 discharge into riser transition at (ft) 1.215 orifice coef. 0.500 orifice area (sf) 12.566 Outlet structure 4 Weir name: Emergency Spillway length (ft) 20.000 side angle 0.000 coefficient 3.300 invert (ft) 700.750 multiple 1 discharge through dam 4 Inflow Hydrographs Hydrograph 0 SCS name: 1-yr 24-hr SCS Design Storm Area (acres) 4.790 CN 79.000 Type 2 rainfall, P (in) 3.090 time of conc. (hrs) 0.2300 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 6.275 peak time (hrs) 11.991 volume (cy) 807.137 Page 2 Pleasant Green- Phases II and III SWM Facility D Hydrograph 1 SCS name: 2-yr 24-hr SCS Design Storm Area (acres) 4.790 CN 79.000 Type 2 rainfall, P (in) 3.730 time of conc. (hrs) 0.2300 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 8.735 peak time (hrs) 11.991 volume (cy) 1123.618 Hydrograph 2 SCS name: 10-yr 24-hr SCS Design Storm Area (acres) 4.790 CN 79.000 Type 2 rainfall, P (in) 5.610 time of conc. (hrs) 0.2300 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 16.671 peak time (hrs) 11.991 volume (cy) 2144.431 Hydrograph 3 SCS name: 100-yr 24-hr SCS Design Storm Area (acres) 4.790 CN 79.000 Type 2 rainfall, P (in) 9.120 time of conc. (hrs) 0.2300 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 32.800 peak time (hrs) 11.991 volume (cy) 4219.055 Page 3 Pleasant Green- Phases II and III SWM Facility D OUTPUT: Routing Method: storage -indication Hydrograph 0 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) Peaks: 1-yr 24-hr SCS Design Storm 6.244 at 11.98 (hrs) 0.295 at 12.86 (hrs) 699.744 at 12.86 (hrs) 435.895 Peaks: 2-yr 24-hr SCS Design Storm 8.692 at 11.98 (hrs) 5.231 at 12.12 (hrs) 699.992 at 12.12 (hrs) 475.034 Peaks: 10-yr 24-hr SCS Design Storm 16.589 at 11.98 (hrs) 16.354 at 12.02 (hrs) 700.281 at 12.02 (hrs) 523.132 Peaks: 100-yr 24-hr 32.637 at 30.113 at 700.907 at 636.333 Mon Nov 02 14:54:17 EST 2020 SCS Design Storm 11.98 (hrs) 12.04 (hrs) 12.04 (hrs) Page 4 Riprap Lined Level Spreader Calculations LEVEL SPREADER W DESIGN Per VSMH Chpt. 11, Section 11.5.3: Step 1 Area = 8.39 ac. (Eqn 11.11) Tv= 13578 cf Q,= 0.45 watershed inches Adjusted CN = 84 Step 2 Tc = 0.18 hrs Step 3 la = 0.38 la/P= 0.38 q = 625 csm/in A = 0.013 m i Z q PTV= 3.7 cfs Note, q pTv is the Treatment Volume peak discharge, or the peak flow rate for the 1-inch rainfall event. Per the VA DEQ Specification #2, page 14, the Level Spreader should be designed per the following: "Where the Conserved Open Space is a mix of grass and forest (or re -forested), establish the Level Spreader length by computing a weighted average of the lengths required for each vegetation type." 75%+/- Native Grasses near floodplain at a LS length of 13 ft per 1 cfs 25%+/- Forest at a LS length of 40 ft per 1 cfs Site Specific / Weighted Average LS length = 19.8 ft LS W Design Length= 72 ft Note: Max. LS Length = 130 ft Min. LS Length = 13 ft LEVEL SPREADER 'C' DESIGN Per VSMH Chpt. 11, Section 11.5.3: Step 1 Area = 8.87 ac. (Eqn 11.11) Tv= 15015 cf Q,= 0.47 watershed inches Adjusted CN = 86 Step 2 Tc = 0.19 hrs Step 3 la = 0.33 la/P= 0.33 q = 675 csm/in A = 0.014 m i Z q PTV= 4.4 cfs Note, q pTvis the Treatment Volume peak discharge, or the peak flow rate for the 1-inch rainfall event. Per the VA DEQ Specification #2, page 14, the Level Spreader should be designed per the following: "Where the Conserved Open Space is a mix of grass and forest (or re -forested), establish the Level Spreader length by computing a weighted average of the lengths required for each vegetation type." 75%+/- Native Grasses near floodplain at a LS length of 13 ft per 1 cfs 25%+/- Forest at a LS length of 40 ft per 1 cfs Site Specific / Weighted Average LS length = 19.8 ft LS'C' Design Length= 86 ft Note: Max. LS Length = 130 ft Min. LS Length = 13 ft LEVEL SPREADER 'D' DESIGN Per VSMH Chpt. 11, Section 11.5.3: Step 1 Area = 4.79 ac. (Eqn 11.11) Tv= 9767 cf Q,= 0.56 watershed inches Adjusted CN = 96 Step 2 Tc = 0.23 hrs Step 3 la = 0.09 la/P= 0.09 q = 775 csm/in A= 0.007 mi' q PTV= 3.3 cfs Note, q pTvis the Treatment Volume peak discharge, or the peak flow rate for the 1-inch rainfall event. Per the VA DEQ Specification #2, page 14, the Level Spreader should be designed per the following: "Where the Conserved Open Space is a mix of grass and forest (or re -forested), establish the Level Spreader length by computing a weighted average of the lengths required for each vegetation type." 75%+/- Native Grasses near floodplain at a LS length of 13 ft per 1 cfs 25%+/- Forest at a LS length of 40 ft per 1 cfs Site Specific / Weighted Average LS length = 19.8 ft LS'D' Design Length= 64 ft Note: Max. LS Length = 130 ft Min. LS Length = 13 ft Riprap Lined Outlet Protection Calculations 3.18 :m 1992 �I Y r y r J d�ayl 6 } I C G h 4 s w Q � a - - go '' N J iv 3 �e 5 2 ✓ s M v W NI s 7 7 (Y � yN i W V! 7 I J V N d J Z u I j a Jd w Yb — o s� m G � � L 3 S� N J 0 m c L w°i ♦+ a a� N E y6N ono Source: USDA-SCS ;aa; '9ZLS d2.adtd OSp m N ... IA Plate 3.18-3 III - 164 1992 IS §2 _ � a \ C Ail \� Source: u o*s & , _/©a +aa % /\ � 7 � r/ a )<1a � .. Q 44 \ }\� $¥\/) . P!<IIa Water Quality Calculations for Pleasant Green, Using the DEQ Virginia Runoff Reduction Method for New Developments (Includes Phases I, II, III and the Connector Road) NOTEWORTH WATER QUALITY ITEMS: 1. A water quality analysis for the entire Pleasant Green development was completed. The analysis included approved WPO 201900003's phase I, the proposed connector road under WPO 201900027 and this plan's phases II and III. The analysis also used the total ESC limits of disturbance boundary for the VRRM water quality boundary. 2. Please see the following sheet for a summary of the required minimum phosphorous removal rate, as well as a breakdown of how the Pleasant Green development meets this requirement. SUMMARY OF PHOSPHOROUS REMOVAL RATE REQUIRED Total Phosphorous Removal Rate Required for Proposed Pleasant Green Phases 1, II, III and the Connector Road 25.35 Ibs/yr Additional Phosphorous Removal Rate Required for Connector Road's Impacts to SWM Forest & Open Space Esmt. 0.37 Ibs/yr Total Phosphorous Removal Rate Required 25.72 Ibs/yr SUMMARY OF PHOSPHOROUS REMOVAL RATE PROVIDED Approved Phase I (WPO 201900003) Phosphorous Removal Rate Achieved from Nutrient Credits 6.97 Ibs/yr Connector Road (WPO 201900027) Phosphorous Removal Rate Achieved from Level II Bioretention Basin (Subarea A) 1.17 Ibs/yr Proposed Phosphorous Removal Rate Achieved from Phase II's Outfall to Conserved Open Spaces (Subarea B) 6.39 Ibs/yr Proposed Phosphorous Removal Rate Achieved from Phase II's Outfall to Conserved Open Spaces (Subarea C) 7.07 Ibs/yr Proposed Phosphorous Removal Rate Achieved from Phase II & III's Outfall to Conserved Open Spaces (Subarea D) 4.60 Ibs/yr Approved Phase I (WPO 201900003) Phosphorous Removal Rate Achieved from the Level II Bioretention Basin (Subarea E) 1.42 Ibs/yr Total Phosphorous Removal Rate Provided 27.62 Ibs/yr Total Phosphorous Removal Rate Provided Onsite 20.65 Ibs/yr Percentage of the Required Phosphorous Removal Rate Treated Onsite (20.65 / 25.72 x 100) 80 % ml m i § ,I IF .1 }� 1 m litAll � 1 B IIIIIIIII E I : I a11 a e e e e e e e e e e e e e e e e e e e e e e w e e eeeeeeeee ae eee ee ee ee ee � e e eeeeeeeee ae eee ee ee ee se � e e e e e e e e e e e a e e e e e e e e e e s e s v a e e v e v e e a e e e e e e e e e v e e v a a d ggg §cgjgiig$ {{ jF bi d 8 6 Ef § {� I ,� \� ,�}1 l t � 1111111111111 oil 1111111111111 oil lit li !� �� !I {I !1 )� !1 � { � IH !� � a 1111 oil M I NE 1111111111111111 M I NE 1111111111111111 M I NE 1111111111111 oil MINE 1111111111111111 § t � 1111 l lit li !� �� {� !� !� !� � { � �1 ! )�)� )� 1111 oil MINIMUM IIIIIII! 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