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HomeMy WebLinkAboutWPO202100034 Correspondence 2021-06-16M COLLINS ENGINEERING June 7, 2021 Department of Community Development 401 McIntire Road Charlottesville, VA 22902 RE: Victorin Heights VSMP (WP02021XXXXX) Energy Balance Exception Letter 200 GARRETT ST, SUITE K CHARLOTTESVILLE VA 22902 (�434.293.3719 PH 434.293.2813 FX www.coll ns-eng ineeri ng.com The purpose of this letter is to request an exception to allow a greater discharge than what the energy balance equation requires of this site. In order to meet energy balance requirements, the proposed underground detention system must release runoff at or below a peak flow rate of 0.07 cfs for the 1-year 24-hour storm. The proposed system utilizes a 3-inch orifice and releases the 1-year 24-hour storm at 0.53 cfs, which is still less than the exiting peak discharge. In order to meet energy balance requirements the system would need a 1-inch orifice as shown in the calculations provided with this letter. However, the smallest orifice size allowed in Albemarle County is 3 inches to avoid clogging. As the site does not allow for a large enough underground detention system to meet energy balance and utilize a 3-inch orifice, it is requested that a higher release rate than what is required by the energy balance equation is allowed. Should you have any questions, please feel free to contact Scott Collins at scott@collins-engineering.com. Sincerely, Scott Collins, PE Inflow Hydrographs (Post -Development Subarea A) Post -Development DA A Inflow Hydrograph 0.0 0.00 0.5 0.08 1.0 0.09 1.5 0.09 2.0 0.10 2.5 0.10 3.0 0.10 3.5 0.11 4.0 0.11 4.5 0.12 5.0 0.13 5.5 0.13 6.0 0.14 6.5 0.15 7.0 0.16 7.5 0.17 8.0 0.17 8.5 0.21 9.0 0.25 9.5 0.25 10.0 0.31 10.5 0.40 11.0 0.55 11.3 0.75 11.6 1.75 11.9 9.94 12.0 7.95 12.1 2.65 12.2 1.44 12.3 1.18 12.4 1.00 12.5 0.81 12.6 0.69 12.7 0.64 12.8 0.60 13.0 0.51 13.2 0.45 13.4 0.42 13.6 0.37 13.8 0.34 14.0 0.30 14.3 0.28 14.6 0.26 15.0 0.24 15.5 0.21 16.0 0.18 16.5 0.18 17.0 0.17 17.5 0.16 18.0 0.14 19.0 0.12 20.0 0.10 22.0 0.10 26.0 0.00 26.5 0.00 27.0 0.00 27.5 0.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 -2.00 Inflow Hydrograph (1-yr Design Storm) 1.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.3 11.6 11.9 12.0 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 13.0 13.2 13.4 13.6 13.8 14.0 14.3 14.6 15.0 15.5 16.0 16.5 17.0 17.5 18.0 19.0 20.0 22.0 26.0 26.5 27.0 27.5 Post -Development DA A Inflow Hydrograph 0.11 0.12 0.12 0.13 0.13 0.14 0.14 0.15 0.16 0.17 0.17 0.19 0.19 0.21 0.22 0.23 0.27 0.33 0.33 0.41 0.53 0.72 0.98 2.29 13.06 10.44 3.48 1.89 1.54 1.31 1.07 0.91 0.84 0.78 0.67 0.59 0.55 0.48 0.44 0.40 0.37 0.35 0.32 0.28 0.24 0.23 0.22 0.21 0.19 0.16 0.14 0.13 0.00 0.00 0.00 0.00 16.00 14.00 12.00 10.00 8.00 s.m 4.00 zoo 0.00 0 -2.00 Inflow Hydrograph (2-yr Design Storm) 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.3 11.6 11.9 12.0 12.1 12.2 12.3 12.4 12.5 12.6 12.7 12.8 13.0 13.2 13.4 13.6 13.8 14.0 14.3 14.6 15.0 15.5 16.0 16.5 17.0 17.5 18.0 19.0 20.0 22.0 26.0 26.5 27.0 27.5 Post -Development DA A Inflow Hydrograph 0.18 0.20 0.20 0.22 0.22 0.23 0.24 0.25 0.27 0.29 0.29 0.32 0.33 0.36 0.38 0.40 0.47 0.57 0.58 0.70 0.91 1.25 1.70 3.96 22.57 20.31 6.02 3.27 2.67 2.27 1.85 1.56 1.46 1.35 1.17 1.02 0.94 0.84 0.76 0.69 0.63 0.60 0.54 0.49 0.42 0.40 0.38 0.36 0.33 0.27 0.24 0.22 0.00 0.00 0.00 0.00 25.00 20.00 15.m io.W 5.00 o.W -5.00 Inflow Hydrograph (10-yr Design Storm) MO Underground Detention Design (Proposed 3-Inch Orifice) BasinFlow printout INPUT: Basin: 0 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 3 Storage Pipes Storage pipe 0 name: name diameter (in) 120.000 length (ft) 122.330 invert (ft) 512.500 angle 0.287 volume (cy) 355.844 Storage pipe 1 name: name diameter (in) 120.000 length (ft) 122.330 invert (ft) 512.500 angle 0.287 volume (cy) 355.844 Storage pipe 2 name: name diameter (in) 120.000 length (ft) 122.330 invert (ft) 512.500 angle 0.287 volume (cy) 355.844 Start—Elevation(ft) 512.50 Vol.(cy) 6 Outlet Structures Outlet structure 0 Orifice name: BARREL area (sf) 1.227 diameter or depth (in) 15.000 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 512.400 multiple 1 discharge out of riser Outlet structure 1 Orifice name: LOW-RISE ORIFICE area (sf) 0.049 diameter or depth (in) 3.000 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 512.500 multiple 1 discharge into riser Outlet structure 2 Orifice name: MID -RISE ORIFICE area (sf) 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 517.700 multiple 1 discharge into riser Outlet structure 3 Orifice name: HIGH RISE ORIFICE area (sf) 0.417 diameter or depth (in) 6.000 width for rect. (in) 10.000 coefficient 0.600 invert (ft) 518.700 multiple 1 discharge into riser Outlet structure 4 Orifice name: EMERGENCY BYPASS PIPE area (sf) 1.227 diameter or depth (in) 15.000 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 521.250 multiple 1 discharge out of riser Outlet structure 5 Orifice name: OVERFLOW WEIR area (sf) 0.520 diameter or depth (in) 9.760 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 522.250 multiple discharge into riser Underground Detention Routings (Proposed 3-Inch Orifice) Post -Development DA A Outflow Hydrograph (From UGD Routings) 0.0 0.00 0.5 0.11 1.0 0.12 1.5 0.13 2.0 0.14 2.5 0.14 3.0 0.15 3.5 0.15 4.0 0.16 4.5 0.16 5.0 0.17 5.5 0.17 6.0 0.17 6.5 0.18 7.0 0.18 7.5 0.19 8.0 0.19 8.5 0.20 9.0 0.21 9.5 0.22 10.0 0.23 10.5 0.24 11.0 0.26 11.3 0.28 11.6 0.31 11.9 0.43 12.0 0.49 12.1 0.51 12.2 0.52 12.3 0.52 12.4 0.53 12.5 0.53 12.6 0.53 12.7 0.53 12.8 0.53 13.0 0.53 13.2 0.53 13.4 0.53 13.6 0.53 13.8 0.53 14.0 0.52 14.3 0.52 14.6 0.52 15.0 0.51 15.5 0.50 16.0 0.49 16.5 0.49 17.0 0.48 17.5 0.47 18.0 0.46 19.0 0.44 20.0 0.42 22.0 0.37 26.0 0.23 26.5 0.21 27.0 0.19 27.5 0.16 Outflow Hydrograph (1-yr Design Storm) 0.60 0.50 0.40 0.30 0.20 0.10 0.00 0.0 5.0 10.0 15.0 20.0 25.0 30.0 Post -Development DA A Outflow Hydrograph (From UGD Routings) 0.0 0.00 0.5 0.12 1.0 0.14 1.5 0.15 2.0 0.16 2.5 0.16 3.0 0.17 3.5 0.18 4.0 0.18 4.5 0.18 5.0 0.19 5.5 0.19 6.0 0.20 6.5 0.20 7.0 0.21 7.5 0.22 8.0 0.22 8.5 0.23 9.0 0.24 9.5 0.25 10.0 0.26 10.5 0.27 11.0 0.30 11.3 0.32 11.6 0.35 11.9 0.48 12.0 0.73 12.1 1.29 12.2 1.36 22.3 1.38 12.4 1.38 12.5 1.37 12.6 1.34 12.7 1.31 12.8 1.28 13.0 1.21 13.2 1.13 13.4 1.03 13.6 0.93 13.8 0.82 14.0 0.74 14.3 0.64 14.6 0.57 15.0 0.53 15.5 0.53 16.0 0.52 16.5 0.51 17.0 0.50 17.5 0.50 18.0 0.49 19.0 0.47 20.0 0.45 22.0 0.41 26.0 0.30 26.5 0.27 27.0 0.25 27.5 0.23 1.60 1.40 1.20 1.00 0.80 0.60 o.4a 0.20 0.00 0.0 Outflow Hydrograph (2-yr Design Storm) 5.0 10.0 15.0 20.0 25.0 30.0 Post -Development DA A Outflow Hydrograph (From UGD Routings) 0.0 #NIP 0.5 0.15 1.0 0.17 1.5 0.19 2.0 0.20 2.5 0.21 3.0 0.22 3.5 0.22 4.0 0.23 4.5 0.23 5.0 0.24 5.5 0.25 6.0 0.25 6.5 0.26 7.0 0.27 7.5 0.28 8.0 0.28 8.5 0.29 9.0 0.31 9.5 0.32 10.0 0.34 10.5 0.36 11.0 0.39 11.3 0.42 11.6 0.45 11.9 3.19 12.0 5.54 12.1 6.34 12.2 5.99 12.3 5.59 12.4 5.21 12.5 4.82 12.6 4.43 12.7 4.04 12.8 3.66 13.0 2.91 13.2 2.05 13.4 1.64 13.6 1.41 13.8 1.34 14.0 1.26 14.3 1.14 14.6 1.02 15.0 0.84 15.5 0.70 16.0 0.60 16.5 0.54 17.0 0.53 17.5 0.53 18.0 0.52 19.0 0.51 20.0 0.50 22.0 0.47 26.0 0.38 26.5 0.36 27.0 0.34 27.5 0.32 7.00 6.00 5.00 4.00 3.00 2.00 1.00 Outflow Hydrograph (10-yr Design Storm) 5.0 10.0 15.0 20.0 25.0 30.0 Underground Detention Design (Meeting Energy Balance with 1-Inch Orifice) 4 Outlet Structures Outlet structure 0 Orifice name: BARREL area (sf) 1.227 diameter or depth (in) 15.000 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 512.400 multiple 1 discharge out of riser Outlet structure 1 Orifice name: LOW-RISE ORIFICE area (sf) 0.005 diameter or depth (in) 1.000 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 512.500 multiple 1 discharge into riser Outlet structure 2 Orifice name: EMERGENCY BYPASS PIPE area (sf) 1.227 diameter or depth (in) 15.000 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 520.500 multiple 1 discharge out of riser Outlet structure 3 Orifice name: MID -RISE ORIFICE area (sf) 0.750 diameter or depth (in) 6.000 width for rect. (in) 18.000 coefficient 0.600 invert (ft) 518.800 multiple 1 discharge into riser Underground Detention Routings (Meeting Energy Balance with 1-Inch Orifice) Post -Development DA A Outflow Hydrograph (From ferground Detention Routir 0.0 0.00 2.0 0.02 4.0 0.03 6.0 0.03 8.0 0.03 10.0 0.04 12.0 0.06 14.0 0.06 14.5 0.06 15.0 0.06 15.5 0.07 16.0 0.07 16.5 0.07 17.0 0.07 17.5 0.07 18.6 0.07 20.0 0.07 22.5 0.07 25.0 0.07 27.5 0.06 30.0 0.06 32.5 0.06 35.0 0.06 37.5 0.06 40.0 0.06 42.5 0.06 45.0 0.06 47.5 0.06 50.0 0.06 52.5 0.06 55.0 0.06 57.5 0.05 60.0 0.05 62.5 0.05 65.0 0.05 67.5 0.05 70.0 0.05 72.5 0.05 75.0 0.05 77.5 0.05 80.0 0.05 82.5 0.05 85.0 0.05 87.5 0.05 90.0 0.04 92.5 0.04 95.0 0.04 97.5 0.04 100.0 0.04 102.5 0.04 105.0 0.04 107.5 0.04 110.0 0.04 112.5 0.04 115.0 0.04 117.5 0.04 120.0 0.03 Outflow Hydrograph (1-yr Design Storm) 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 0.0 0.00 0.5 0.02 1.0 0.02 1.5 0.02 2.0 0.02 2.5 0.03 3.0 0.03 3.5 0.03 4.0 0.03 4.5 0.03 5.0 0.03 5.5 0.03 6.0 0.03 6.5 0.03 7.0 0.03 7.5 0.04 8.0 0.04 8.5 0.04 9.0 0.04 9.5 0.04 10.0 0.04 10.5 0.04 11.0 0.04 11.3 0.05 11.6 0.05 11.9 0.06 12.0 0.06 12.1 0.07 12.2 0.28 12.3 0.53 12.4 0.72 12.5 0.83 12.6 0.86 12.7 0.86 12.8 0.85 13.0 0.79 13.2 0.72 13.4 0.64 13.6 0.57 13.8 0.50 14.0 0.44 14.3 0.37 14.6 0.32 15.0 0.27 15.5 0.22 16.0 0.18 16.5 0.15 17.0 0.13 17.5 0.11 18.0 0.10 19.0 0.08 20.0 0.07 22.0 0.07 26.0 0.07 26.5 0.07 27.0 0.07 27.5 0.07 Outflow Hydrograph (2-yr Design Storm) 1.00 0.90 0.80 0.70 - 0.60 - 0.50 0.40 0.30 0.20 0.10 0.00 0.0 5.0 10.0 15.0 20.0 25.0 30.0 0.0 0.00 0.5 0.02 1.0 0.02 1.5 0.03 2.0 0.03 2.5 0.03 3.0 0.03 3.5 0.03 4.0 0.03 4.5 0.03 5.0 0.04 5.5 0.04 6.0 0.04 6.5 0.04 7.0 0.04 7.5 0.04 8.0 0.04 8.5 0.04 9.0 0.05 9.5 0.05 10.0 0.05 10.5 0.05 11.0 0.05 11.3 0.06 11.6 0.06 11.9 3.49 12.0 5.58 12.1 6.32 12.2 6.06 12.3 5.69 12.4 5.32 12.5 4.95 12.6 4.57 12.7 4.20 12.8 3.84 13.0 3.16 13.2 2.52 13.4 1.97 13.6 1.54 13.8 1.03 14.0 0.89 14.3 0.76 14.6 0.66 15.0 0.57 15.5 0.48 16.0 0.40 16.5 0.34 17.0 0.30 17.5 0.27 18.0 0.25 19.0 0.20 20.0 0.15 22.0 0.11 26.0 0.07 26.5 0.07 27.0 0.07 27.5 0.07 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 -1.00 Outflow Hydrograph (10-yr Design Storm) 1.0