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Home My WebLink AboutWPO202000053 Calculations 2022-08-30PLEASANT GREEN- PHASES II & III STORMWATER MANAGEMENT CALCULATIONS PACKET ADDENDUM #1 (WPO 202000053 Amendment #1) Date of Calculations JULY 29, 2022 Off, SCOTT7R. CID-LLINSZ No. 35791 a 7/29/22 7onAL PREPARED BY: COLLINS ENGINEERIN( 200 GARRETT STREET, SUITE K CHARLOTTESVILLE, VA 22902 434.293.3719 PH 434.293.2813 FX www.collins-engineering.com Summary of Addendum #1 1. This addendum is intended to accompany the previously approved Pleasant Green Phases II & III WPO 202000053 Stormwater Management (SWM) Calculations Packet originally dated November 30, 2020 and last revised on March 24, 2021. 2. The March 24, 2021 SWM calculations packet took into consideration the (then future) development of phase III. With the finalization of phase III under review at this time, this addendum was created to accompany the attached plan amendment (WPO 202000053 Amendment #1) reflecting the minor changes that have occurred since March of 2021. 3. The primary SWM change is associated with the open space located approximately at the intersection of Litchford Road and Rambling Brook Lane. This grass lawn open space revision impacted the time of concentration in the post -development subarea D. The previously approved post -development subarea D time of concentration of 0.23 hrs. has been reduced to 0.18 hrs. This changed prompted revisions to Worksheet 3, Worksheet 4, the routing calculations for SWM Facility'D' and the 9VAC25-870-66 flood protection calculations for SWM Facility'D'. Please note, no modifications to the designs of the three SWM facilities were required. U.S. Department of Agriculture Natural Resources Conservation Service TR SS Worksheet 3: Time of Concentration (TJ or Travel Time (Tt) Project: Pleasant Green Phases II & III Location: West of McComb Street Check One: Present X Developed X Check One: T, X Tt 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 Tt = [0.007(n* L)° a] / Pis SDA Designed By: FGM, PE Checked By: SRC, PE Through subarea n/a FL-ENG-21A 06/04 Date: 7/29/2022 Date: 7/29/2022 DA B DA C DA D DA B 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 0.05 0.14 0.15 0.25 0.14 0.15 0.15 Shallow Concentrated Flow: 7 Surface description (paved or unpaved) Unpaved Unpaved Unpaved Unpaved Unpaved Unpaved 8 Flow Length, L (ft) 535 150 400 300 150 130 9 Watercourse slope, s (ft/ft) 0.07 0.03 0.08 0.07 0.03 0.02 10 Average velocity, V (Figure 3-1) (ft/s) 4.2 2.8 4.5 4.2 2.8 2.3 11 Tt= 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, Pw (ft) 14 Hydraulic radius, r =a/Pw (ft) 15 Channel Slope, s (ft/ft) 16 Manning's Roughness Coeff, n 17 V= [ 1.49r2/3s05 ] / n 18 Flow length, L (ft) 19 Tt= L / 3600*V 20 Watershed or subarea T,or Tt (Add Tt in steps 6, 11 and 19) Note: The shoretest SCS Tc is 0.10 hrs. 3 1 Y o LL> O O O O O C ? - d rl a:Y E IY ry rl o L a W t m E V a« o7J io V 'x r w, E r¢ z E a•X N U O- O > m Q Y O > o O ¢ in 0 a in p CL o l7 a .y m ¢ ro d y ¢ 0.06 1 0.02 0.03 0.01 0.18 1 0.22 1 0.27 1 0.18 1 0.19 1 0.18 }\ -§!§■§-®® ® ® - cR IR cR Lq -§2!E§- ! EE E \\\\\\\\{ Pleasant Green- Phases II and III SWM Facility D- AMENDED Tc BasinFlow printout INPUT: Basin: Pleasant Green- Phases II and III SWM Facility D- AMENDED Tc 0 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 1 Storage Pipes Storage pipe 0 name: APPROVED UGD System 'D' diameter (in) 120.000 length (ft) 177.750 invert (ft) 695.500 angle 0.287 volume (cy) 517.054 Start—Elevation(ft) 695.50 Vol.(cy) 0.00 5 Outlet Structures Outlet structure 0 Orifice name: APPROVED Low -Flow Barrel area (sf) 1.227 diameter or depth (in) 15.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 694.470 multiple 1 discharge out of riser Outlet structure 1 Orifice name: APPROVED Low -Flow Orifice in Weir Plate area (sf) 0.028 diameter or depth (in) 2.250 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 695.500 multiple 1 discharge into riser Page 1 Pleasant Green- Phases II and III SWM Facility D- AMENDED Tc Outlet structure 2 Orifice name: APPROVED Overflow / Emergency Spillway in Weir Plate area (sf) 2.796 diameter or depth (in) 22.640 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 703.500 multiple 1 discharge into riser Outlet structure 3 Weir name: APPROVED Access MH Below System diameter (in) 48.000 side angle 0.000 coefficient 3.300 invert (ft) 707.000 multiple 1 discharge into riser transition at (ft) 1.215 orifice coef. 0.500 orifice area (sf) 12.566 Outlet structure 4 Orifice name: APPROVED High Flow Bypass Barrel area (sf) 1.767 diameter or depth (in) 18.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 703.500 multiple 1 discharge into riser Page 2 Pleasant Green- Phases II and III SWM Facility D- AMENDED Tc 3 Inflow Hydrographs Hydrograph 0 SCS name: AMENDED 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) 0.1800 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 6.705 peak time (hrs) 11.970 volume (cy) 807.238 Hydrograph 1 SCS name: AMENDED 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) 0.1800 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 9.334 peak time (hrs) 11.970 volume (cy) 1123.760 Hydrograph 2 SCS name: AMENDED 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) 0.1800 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 17.813 peak time (hrs) 11.970 volume (cy) 2144.699 Page 3 Pleasant Green- Phases II and III SWM Facility D- AMENDED Tc OUTPUT: Routing Method: storage -indication Hydrograph 0 Routing Summary of Peaks inflow (cfs) discharge (cfs) water level (ft) storage (cy) Hydrograph 1 Routing Summary of Peaks: inflow (cfs) discharge (cfs) water level (ft) storage (cy) Hydrograph 2 Routing Summary of Peaks inflow (cfs) discharge (cfs) water level (ft) storage (cy) AMENDED 1-yr 24-hr SCS Design Storm 6.705 at 11.96 (hrs) 0.311 at 12.66 (hrs) 703.478 at 12.84 (hrs) 417.567 AMENDED 2-yr 24-hr SCS Design Storm 9.334 at 11.96 (hrs) 6.217 at 12.08 (hrs) 704.204 at 12.08 (hrs) 456.340 AMENDED 10-yr 24-hr SCS Design Storm 17.813 at 11.96 (hrs) 16.081 at 12.02 (hrs) 705.770 at 12.02 (hrs) 513.258 Tue Aug 23 09:28:45 EDT 2022 Page 4 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 Back Water Surface 705.77 ft SWM Facility'D' Crown of Pipe Elevation at High Point = 705.87 ft SWM Facility'D' Rim Elevation at Lowest Access MH / Weir Plate 707.00 ft (No overtopping Occurs) 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. Section C.3.c.: "The stormwater conveyance system [the SWM Facility outfall] enters a mapped floodplain or other flood -prone area, adopted by ordinance, of any locality."