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Home My WebLink AboutWPO201700052 Calculations 2021-06-29DUNLORA PARK STORMWATER MANAGEMENT CALCULATIONS PACKET (VSMP Amendment #3) Date of Amendment #3 Calculations February 13, 2018 Revised on August 22, 2019 .41$ALyi, cp, ' w�LIIVJ o No.gq35791 S13/12-f PREPARED BY: COLLINS NGINEERING 200 GARRETT STREET, SUITE K CHARLOTTESVILLE, VA 22902 434.293.3719 PH 434.293.2813 FX www.collins-engineering.com DUNLORA PARK STORMWATER MANAGEMENT CALCULATIONS PACKET (VSMP Amendment #3) Date of Amendment #3 Calculations February 13, 2018 Revised on August 22, 2019 PREPARED BY: COLLINS =NGINEERING 200 GARRETT STREET, SUITE K CHARLOTTESVILLE, VA 22902 434.293.3719 PH 434.293.2813 FX www.collins-engineering.com Amendment #3 Water Quality Calculations Summary: The proposed amendment does not modify the development's impervious and managed turf areas, but does include updated water quality calculations to reflect the removal of the approved bioretention basin. The calculations otherwise have not been modified, and therefore the prior approved site phosphorous removal rate of 5.52 Ibs/yr is still applicable. As a result, this amendment now requires a nutrient credit purchase of 5.52 Ibs/yr for water quality compliance to mitigate the removal of the bioretention basin. DEQ VIMInb Runoff Reduction Method New DevelopmenfC M 2011 BMP Standards and 5peci0catlons 132013 Draft BMP Standards and Specifiadons Project Name: Dunlora Park- VSMP Amendment No.3 Date: 2/13/2018 OMP Design Specifscdtnmd List: 2011 Stds & Specs Site Information Project (Treatment Volume and Loads) Cover mtectetl brest/o en rrebresadland 1.09 1.09 Manages Tudlaaes) 5 aiztumed, gaden t yardsoratherturf wbemowed/managed 429 429 Imperviousl000(acrea) 324 3.24 ForesVOpen Span areas most Le proteehdin accordance with the Vi Rinia RurwH Reduction Method 8.62 rnnelaMe Annual Reirrall(inchez) 43 Target Rainfall Event (inches) 1.00 iaal Phozphoruz(TP) EMC(.110 0.26 Taal Nitrogen (TN) EMC Img(L) 1.86 target W Lood(Iblave/yr) 0.41 Pj(unide,corecgonfactn) 0.90 Forest/Open Space Cover (at.) 1.09 Weighted Rv (forest) 0.03 %Forest 13% Managed Turf Cover (acres) 4.29 Weighted Rv(turf) 0.20 %Managed Turf 50% Impervious Cover (acres) 3.24 Rv(impervicus) 0.95 Impervious 138%Area(acres) Site Rv Rnnnff rnoFRAenle tool ASoils BSolis 1 CSails DSolis Forent/open Space 0.02 i 0.03 i 0.04 0.05 Managed Turf 0.15 1 0.20 1 0.22 0.25 Imperviouz [aver 0.95 1 0.95 1 0.95 0.95 Treatment Volume and Nutrient Loads Treatment Volume (acre-ft) 0.3307 Treatment Volume (cubic eet) 14,406 TP Load(lb/yr) 9.05 ,. TN Load (Ib/yr) _ 64.75 date input.16 constant values salcolxion cells Amendment #3 Water Quantity Calculations (Subarea A Routings) Summary: This proposed amendment does not modify the development's impervious areas and the subarea (CN, Tc, Area, etc.). The amendment's changes are limited to the modification of Subarea A's detention basin's contour areas and to the riser so that it may be consistent with asbuilt conditions and be compliant with minimum requirements (orifice with a diameter>3"). The changes to the riser and the basin's grading are the only revisions to the following calculations which were previously reviewed and accepted. Please note that the updated areas are larger than the previously reviewed/accepted areas, which allow for additional storage. The amendment's 1-, 2- and 10-year flows exiting the basin are less than or equal to the corresponding approved flows, hereby maintaining this development's compliance with water quantity requirements. Channel protection compliance was previously met via 9VAC25-870-66, subsection B.3.a. Flood protection compliance was previously met via 9VAC25-870-66, subsection C.2.b. Both the channel and flood protection requirements continue to be met with this amendment. Basin A- AMENDMENT No. 3 BasinFlow printout INPUT: Basin: Basin A Routing 6 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 402.00 409.00 0.0 404.00 1204.00 57.2 406.00 2151.00 179.7 408.00 3239.00 378.0 410.00 4539.00 664.7 412.00 5971.00 1052.8 Start—Elevation(ft) 5 Outlet Structures Outlet structure 0 Orifice 402.00 Vol.(cy) 0.00 name: ASBUILT Riser area (sf) 12.566 diameter or depth (in) 48.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 409.700 multiple 1 discharge into riser Outlet structure 1 Weir name: ASBUILT Emergency Spillway length (ft) 34.000 side angle 88.300 coefficient 3.300 invert (ft) 410.700 multiple 1 discharge through dam Page 1 Outlet structure 2 Orifice Basin A- AMENDMENT No. 3 name: AMENDMENT Low -Flow Orifice area (sf) 0.067 diameter or depth (in) 3.500 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 402.000 multiple 1 discharge into riser Outlet structure 3 Orifice name: AMENDMENT Mid -Flow Orifice area (sf) 0.785 diameter or depth (in) 12.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 408.250 multiple 4 discharge into riser Outlet structure 4 Orifice name: AMENDMENT Barrel area (sf) 0.613 diameter or depth (in) 10.600 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 400.900 multiple 1 discharge out of riser 4 Inflow Hydrographs Hydrograph 0 SCS name: Area (acres) CN Type rainfall, P (in) time of conc. (hrs) time increment (hrs) time limit (hrs) fudge factor routed peak flow (cfs) peak time (hrs) volume (cy) 1 yr 24hr SCS Method Storm Event 4.170 80.000 2 3.500 0.1800 0.0200 30.000 1.00 true 7.612 11.970 916.526 Page 2 Basin A- AMENDMENT No. 3 Hydrograph 1 SCS name: 2 yr 24hr SCS Method Storm Event Area (acres) 4.170 CN 80.000 Type 2 rainfall, P (in) 3.700 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) 8.357 peak time (hrs) 11.970 volume (cy) 1006.233 Hydrograph 2 SCS name: 10 yr 24hr SCS Method Storm Event Area (acres) 4.170 CN 80.000 Type 2 rainfall, P (in) 5.600 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) 15.921 peak time (hrs) 11.970 volume (cy) 1916.892 Hydrograph 3 SCS name: 100 yr 24hr SCS Method Storm Event Area (acres) 4.170 CN 80.000 Type 2 rainfall, P (in) 9.100 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) 30.997 peak time (hrs) 11.970 volume (cy) 3732.026 Page 3 Basin A- AMENDMENT No. 3 OUTPUT: Routing Method: storage -indication Hydrograph 0 Routing Summary of Peaks: 1 yr 24hr SCS Method Storm Event inflow (cfs) 7.612 at 11.96 (hrs) discharge (cfs) 0.656 at 12.42 (hrs) water level (ft) 408.145 at 12.56 (hrs) storage (cy) 395.625 Hydrograph 1 Routing Summary of Peaks: 2 yr 24hr SCS Method Storm Event inflow (cfs) 8.357 at 11.96 (hrs) discharge (cfs) 1.116 at 12.36 (hrs) water level (ft) 408.392 at 12.36 (hrs) storage (cy) 426.677 Hydrograph 2 Routing Summary of Peaks: 10 yr 24hr SCS Method Storm Event inflow (cfs) 15.921 at 11.96 (hrs) discharge (cfs) 7.173 at 12.12 (hrs) water level (ft) 409.859 at 12.12 (hrs) storage (cy) 641.202 Hydrograph 3 Routing Summary of Peaks: 100 yr 24hr SCS Method Storm Event inflow (cfs) 30.997 at 11.96 (hrs) discharge (cfs) 30.459 at 11.98 (hrs) water level (ft) 411.000 at 11.98 (hrs) storage (cy) 845.476 Fri Sep 13 14:54:43 EDT 2019 Page 4 %'�� Gwartn��. G�uarraTxw,=s (ZEN !tCes tFS¢a with qczat IAA PC'ptnvuy Z'19 -cn t• G"LJ= I1.13 CFSa Sa<' =' I'!. E�1"awi ¢..A¢4 M1NL7 4d�t��1 �5 ToP o.F- ygrt tr lEW�T:acl+¢, }"�'� 4YC q{ �r Ci�1' <cY �cupur0 F,4 ; 9rkk7 VLw CL Out�(�n fa°- CA/�PtCCK1tC**1 CotSFRVACI4 6% V-2VIR. OF WAVk 5uaznt�lils= 6' la" x i.0b 6 (OR t.0s) NaK3:0ow AZT kwe Starr 5zca xs 9ROV004D fig, TIC C aISM�T&Nw4r a tvaV66.Stc�Lews�, INV W IKE ricpt sral 53�� cua�eo sW�fuj�xt.� Gr �s•,x�. +4 oar- THA¢;l 6' ANO is lav "CI c Proposed Riprap Swale- 2yr. Complex Channnel Input: Slope 0.070 Flow (given) 8.370 line x y 0 0.000 1.000 1 3.000 0.000 2 6.000 1.000 Output: Depth 0.817 (Y) ( 0.817) Channel bed segment output: line Q V 1 4.19 4.18 2 4.19 4.18 Mon Sep 16 11:25:05 EDT 2019 n 0.050 (first n value not used) 0.050 0.050 A P 1.00 2.58 1.00 2.58 Page 1 Proposed Riprap Swale- 10yr. Complex Channnel Input: Slope 0.070 Flow (given) 11.130 line x y 0 0.000 1.000 1 3.000 0.000 2 6.000 1.000 Output: Depth 0.909 (y) ( 0.909) Channel bed segment output: line Q V 1 5.57 4.49 2 5.57 4.49 Mon Sep 16 11:25:30 EDT 2019 n 0.050 (first n value not used) 0.050 0.050 A P 1.24 2.88 1.24 2.88 Page 1