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HomeMy WebLinkAboutWPO202100007 Calculations 2021-06-24BROOKHILL STORMWATER MANAGEMENT CALCULATIONS PACKET 7 °� r� SCOTT R. ]NS'Z Lic. No. 035791 6/9/21 A��' SIONAL �� (Blocks 16-17) Date of Calculations JANUARY 21, 2021 Revised On JUNE 912021 I:1C1W-AW4III -i'il COLLINS ENGINEERING 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 Mapping (Source: NRCS Web Soil Survey Online Database) Soils Table (Source: NRCS Web Soil Survey Online Database) Albemarle County, Virginia (VA003) Map Unit Map Uni[ Name percent of Percent Symbol AOI 16 Codorus silt 2.6% loam, 0 to 2 percent slopes, occasionally flooded 27B Elioak loam, 2 24.7% to 7 percent slopes 27C Elioak loam, 7 20.5% to 15 percent slopes 28133 Elioak clay 0.2% loam, 15 to 25 percent slopes, severely eroded 34B Glenelg loam, 2 8.5% to 7 percent slopes 34C Glenelg loam, 7 9.2% to 15 percent slopes 34D Glenelg loam, 3.0% 15 to 25 percent slopes 39D Hazel loam, 15 25.9% to 25 percent slopes 39E Hazel loam, 25 3.5% to 45 percent slopes 95 Hatboro silt 2.0% loam, 0 to 2 percent slopes, occasionally flooded Totals for Area of 100.00% Interest Note: The soils' properties witin this development are predominantly (96%+/-) composed of hydrologic soil group type B soils. Precipitation Data (Source: NOAA's National Weather Service, Hydrometerorological Design Studies Center, Atlas 14 Point Precipitation Frequency Estimates) Pbr�rr O tpOaQ Plop � 2 Terrain Pshwoo°e /4 � a T15 O1n,le9e W°v ` a i330° o � �i1 m p c 0 Y 30 0 m 0 I c o e`N ��ao o`Qo1n c Polo Q` o°°°s Ra c 7�e95aw` 0 o at N 4 2, °c CO 000 0.4km d.2mi .v l:an ehrnnt POINT PRECIPITATION FREQUENCY (PF) ESTIMATES WITH 90 % CONFIDENCE INTERVALS AND SUPPLEMENTARY INFORMATION NOAA Atlas 14, VOlume 2, Version 3 IRDSbased precipitation frequency estimates with 90% confidence Intervals (In inches)' Avaa9e nxen Fleml (yeas) �� oo© 10 25 60 100 200 ® 10W 244r 3.00 (2.693.38) 3.63 1 (3.26-4.09) 4,64 1 (4.155.21) 5A8 1 (4.896.15) 6.74 1 (5.97-7.53) 7.81 (6.87-9.71) 8.99 (7.85-10.0) 10.3 (8.89-11.4) 12.3 (10.4-13.6) 135 1 (113-15.5) O c N a° _ � x °� 00 o i E00 o m 6 o c = c -t2 z nl q O C V J Z m m V E z0 `o 0 o j U ao �o ry ry ao ry ry m ry v G A o m m m m d p q m m r m m o �serr V w o $ eo m mo g n ommo roNao of �,'� 0 6 e e8e�8 rry 80't .m.8 umimmon.o+ ¢` QI mni o v nio o vi C; cio 000.l oo � m em m e moo mm�o vmi moo vmi m Qom r O u p d O j C O_ C V O E `o c V m 1. V V m V V E C V a V a a D a s a s a D W O r in un O V O V in in O V O m V O m V O m V V G C G yN�j V a V 00 p 3 p 3 a V ° °o V a c 00 V ¢ c °o Z o A� a m N m N m .mi y v m a` N m» m m m a i m i LL '^ '" > m i LL Z'" N E a°<-Ep3�p3<waw��3��3��3��� m o ¢ o . n c A m m m m m m m m E 0 E 'o O m Q d Q Q 0 Q c m m m e V V Q pp U 6 an G m G i _ G a ao d 60 — de U.S. Department of Agriculture Natural Resources Conservation Service TR 55 Worksheet 3: Time of Concentration (T,) or Travel Time (Tj Project: Brookhill Location: Blocks 16-17 Check One: Present X Developed X Check One: T, X T, Segment ID Sheet Flow: (Applicable to T, only) 1 Surface description (Table 3-I) 2 Manning's roughness coeff., o (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)0a) / Pi 3 sa Shallow Concentrated Flow: ] Surface description (paved or unpaved) 8 Flow Length, L (ft) 9 Watercourse slope, s(R/ft) 10 Average velocity, V (Figure 3-I) (ft/s) 11 T,=L/3600•V Channel Flow: 12 Cress 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 17 V= (1.49ru3s°'' ] / n 18 Flow length, L (R) 19 T� L/3600•V 20 Watershed or subarea T, or T, (Add T, in steps 6, 31 and 19) Note: The shoretest SCS Tc is 0.10 hm. Designed By: FGM, PE Checked By: SRC, PE Through subarea n/a FL-ENG-21A 06/04 Date: 6/9/2021 Date: 6/9/2021 DAA DAA DAA DAB DAB DAB DAC CAC (Pre-Dev.) (Post-Dev.) (Final Buildout) (Pre-Dev.) (Port-Dev.) (Final Buildout) (Pre-Dev.) (Post-Dev.) Dense Dense Dense Dense Dense Dense Dense Dense Grass Grass Grass Grass Grass Grass Grass Grass 0.24 0.24 0.24 0.24 0.24 0.24 0.24 0.24 100 50 SO 100 100 100 100 70 3.63 3.63 3.0 3.63 3.63 3.63 3.63 3.63 0.07 0.04 0.04 0.04 0.07 0.07 0.09 0.29 0.135 1 0.10 0.10 0.17 0.135 0.135 0.12 0.06 Unpaved Unpaved Unpaved Unpaved Unpaved 650 I1S0 215 215 75 0.14 0.07 0.04 0.04 0.20 6.3 4.3 3.2 3.2 7.2 0.029 0.07 0.019 0.019 0.00 o v ca d o+ o+ q+ q+ w d+ a d+ a x a x x x E o c o E `o Z e1 `o `o o. n `o 0 6 u 0 6 u o. Le a ' a On ve Len .o`, n 0.16 1 0.12 1 0.12 1 0.24 1 0.17 1 0.17 1 0.13 1 0.10 \: ah— \UIT } SMIT \ 05210 � ,!!l.•� " !! � ._. ,!!!N T !,}!m ! A „T 11 !,!, 11 0._ . ,!!„ON|] ! NIT . jil , , __. ,!,! M ; {\} .i•,-= "! I N ! } A M S , ! 12PT m.i.,:.. !! ! �,i MIT MT ; ,1silo , M R-11 \] bb $» $q Gm § § § Channel & Flood Protection Calculations 9 VAC 25-870-66 (Point ofAnalysis'A') 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: Under no condition shall Q Developed be greater than Q Pre -Developed nor shall Q Developed be required to be less than that calculated in the equation: Q Allowed < (Q Forest * RV Forest ) I RV Developed Q Allowed Where: QForest = CN Forest= S Forest RV Forest= CN Post -Developed= S Post -Developed = RV Post -Developed = Q Developed Q Developed < 0.11 cfs 0.82 cfs (From Worksheet 4 Watershed Summary) 55 (From Worksheet 4 Watershed Summary) 8.18 (From SCS TR-55 eq. 2-4) 0.19 in. (From SCS TR-55 eq. 2-3) 82 (From Worksheet 4 Watershed Summary) 2.14 (From SCS TR-55 eq. 2-4) 1.40 in. (From SCS TR-55 eq. 2-3) = 0.44 cfs (For Proposed Watershed with 3" Orifice) 0.10 cfs (With a 1.33" Orifice) Section B.4 Limits of Analysis The analysis terminates at the onsite subarea since subsection B.3.a is met. 9 VAC 25-870-66 (Point of Analysis'A'- For Final Buildout) 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: Under no condition shall Q Developed be greater than Q Pre -Developed nor shall Q Developed be required to be less than that calculated in the equation: Q Allowed < (Q Forest * RV Forest ) I RV Developed Q Allowed Where: QForest = CN Forest= S Forest RV Forest= CN Post -Developed= S Post -Developed = RV Post -Developed = Q Developed Q Developed < 0.13 cfs Section B.4 Limits of Analysis 1.00 cfs (From Worksheet 4 Watershed Summary) 55 (From Worksheet 4 Watershed Summary) 8.18 (From SCS TR-55 eq. 2-4) 0.19 in. (From SCS TR-55 eq. 2-3) 83 (From Worksheet 4 Watershed Summary) 2.02 (From SCS TR-55 eq. 2-4) 1.46 in. (From SCS TR-55 eq. 2-3) 0.49 cfs (For Full Buildout Watershed with 3" Orifice) 0.11 cfs (With a 1.33" Orifice) The analysis terminates at the onsite subarea since subsection B.3.a is met. 9 VAC 25-870-66 (Point of Analysis W) 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.: Concentrated stormwater flow to stormwater conveyance systems that currently experience localized flooding [i.e. the floodplain] during the 10-year 24-hour storm event shall confine the postdevelopment peak flow rate from the 10-year 24-hour storm event within the stormwater conveyance system [i.e. the detention facilities] to avoid the localized flooding. Detention of stormwater or downstream improvements may be incorporated into the approved land -disturbing activity to meet this criterion." Please note, the detention facilities contain the 10-yr design storm, herby satisifying this critei 10-yr 24-hr SCS Design Storm's Peak WSE = 371.29 ft (For Proposed Plan) 10-yr 24-hr SCS Design Storm's Peak WSE = 371.47 ft (For Final Buildout) SWM Facility Top of Dam Elevation = 373.00 ft Section C.3 Limits of Analysis "...Stormwater conveyance systems shall be analyzed for compliance with flood protection criteria to a point where" either a, b or c are met. Section C.3.c.: "The stormwater conveyance system [i.e. the facilities' outfall] enters a mapped floodplain or other flood -prone area, adopted by ordinance, of any locality." 9 VAC 25-870-66 (Point ofAnalysis'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: Under no condition shall Q Developed be greater than Q Pre -Developed nor shall Q Developed be required to be less than that calculated in the equation: Q Allowed < (Q Forest * RV Forest ) I RV Developed Q Allowed Where: QForest = CN Forest= S Forest RV Forest= CN Post -Developed= S Post -Developed = RV Post -Developed = Q Developed Q Developed < 0.18 cfs Section B.4 Limits of Analysis 1.30 cfs (From Worksheet 4 Watershed Summary) 55 (From Worksheet 4 Watershed Summary) 8.18 (From SCS TR-55 eq. 2-4) 0.19 in. (From SCS TR-55 eq. 2-3) 82 (From Worksheet 4 Watershed Summary) 2.21 (From SCS TR-55 eq. 2-4) 1.37 in. (From SCS TR-55 eq. 2-3) 0.41 cfs (For Proposed Watershed with 3" Orifice) 0.15 cfs (With a 1.75" Orifice) The analysis terminates at the onsite subarea since subsection B.3.a is met. 9 VAC 25-870-66 (Point of Analysis'B'- For Final Buildout) 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: Under no condition shall Q Developed be greater than Q Pre -Developed nor shall Q Developed be required to be less than that calculated in the equation: Q Allowed < (Q Forest * RV Forest ) I RV Developed Q Allowed Where: QForest = CN Forest= S Forest RV Forest= CN Post -Developed= S Post -Developed = RV Post -Developed = Q Developed Q Developed < 0.29 cfs Section B.4 Limits of Analysis 2.14 cfs (From Worksheet 4 Watershed Summary) 55 (From Worksheet 4 Watershed Summary) 8.18 (From SCS TR-55 eq. 2-4) 0.19 in. (From SCS TR-55 eq. 2-3) 83 (From Worksheet 4 Watershed Summary) 2.06 (From SCS TR-55 eq. 2-4) 1.44 in. (From SCS TR-55 eq. 2-3) 1.09 cfs (For Full Buildout Watershed with 3" Orifice) 0.20 cfs (With a 1.75" Orifice) The analysis terminates at the onsite subarea since subsection B.3.a is met. 9 VAC 25-870-66 (Point of Analysis W) 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.: Concentrated stormwater flow to stormwater conveyance systems that currently experience localized flooding [i.e. the floodplain] during the 10-year 24-hour storm event shall confine the postdevelopment peak flow rate from the 10-year 24-hour storm event within the stormwater conveyance system [i.e. the wet pond] to avoid the localized flooding. Detention of stormwater or downstream improvements may be incorporated into the approved land - disturbing activity to meet this criterion." Please note, UGD System W contains the 10-yr design storm, herby satisifying this criterion. 10-yr 24-hr SCS Design Storm's Peak WSE = 365.84 ft (For Proposed Plan) 10-yr 24-hr SCS Design Storm's Peak WSE = 368.07 ft (For Final Buildout) UGD System 'B' Top Elevation at Outlet = 368.50 ft Section C.3 Limits of Analysis "...Stormwater conveyance systems shall be analyzed for compliance with flood protection criteria to a point where" either a, b or c are met. Section C.3.c.: "The stormwater conveyance system [i.e. the wet pond's outfall] enters a mapped floodplain or other flood -prone area, adopted by ordinance, of any locality." 9 VAC 25-870-66 (Point ofAnalysis'C') 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.a. "The manmade stormwater conveyance system shall convey the postdevelopment peak flow rate from the two-year 24-hour storm event without causing erosion of the system. " Note, the 2-yr 24-hour design storm's peak flow rate is not erosive in the concrete storm sewer and the rirpap lined stream's bed below the double 60"x60" concrete box culverts. This conclusion is logical since the storm sewer and manmade riprap were designed for storms in excess of the 2-year design storm and were recently approved with adequacies. Section B.4.a. Limits of Analysis Stormwater conveyance systems shall be analyzed for compliance with channel protection criteria to a point where, based on land area, the site's contributing drainage area is less than or equal to 1.0% of the total watershed area. Note, post -development subarea C's contributing drainage area is less than 1% of the overall watershed draining to the point of analysis (i.e. the riprap lined stream's bed below the double 60"x60" concrete box culverts). 9 VAC 25-870-66 (Point of Analysis'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.: Concentrated stormwater flow to stormwater conveyance systems that currently experience localized flooding [i.e. the floodplain] during the 10-year 24-hour storm event shall confine the postdevelopment peak flow rate from the 10-year 24-hour storm event within the stormwater conveyance system [i.e. the storm sewer] to avoid the localized flooding." Please note, the storm sewer's hydraulic grade line calculations have been provided showing the peak back water elevations do not overtop the inlets, herby satisifying this requirement. This is consistent with logic since storm sewer systems are designed for the 10-year storm events. Section C.3 Limits of Analysis "...Stormwater conveyance systems shall be analyzed for compliance with flood protection criteria to a point where" either a, b or c are met. Section C.3.c.: "The stormwater conveyance system [i.e. the downstream storm sewer] enters a mapped floodplain or other flood -prone area, adopted by ordinance, of any locality." Dry Detention Basin 'A' & Underground Detention System 'A' Combined Routing Calculations (Subarea A- Proposed Development) Note: Routing calculations have been provided using a low -flow orifice diameter of 3", matching the proposed design. Routing calculations have also been provided using a low -flow orifice diameter less than 3", meeting minimum energy balance requirements. BH Blocks 16-17 Subarea A- Proposed Plan NOTE: The following routing calculations use a low -flow orifice diameter of 3". This analysis corresponds to the proposed Subarea A watershed. BasinFlow printout INPUT: Basin: Brookhill Blocks 16-17- Subarea A Proposed Plan 21 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 363.00 1557.00 0.0 363.50 1695.00 30.1 364.00 2410.00 67.9 364.50 2798.00 116.1 365.00 3141.00 171.1 365.50 3468.00 232.2 366.00 3787.00 299.4 366.50 4106.00 372.4 367.00 4428.00 451.4 367.50 4661.00 535.6 368.00 4864.00 623.8 368.50 5074.00 715.8 369.00 5278.00 811.6 369.50 5476.00 911.2 370.00 5666.00 1014.4 370.50 5845.00 1120.9 371.00 6006.00 1230.7 371.50 6142.00 1343.2 372.00 6222.00 1457.6 372.50 5961.00 1570.4 373.00 6270.00 1683.7 Start_Elevation(ft) 363.00 Vol.(cy) Page 1 BH Blocks 16-17 Subarea A- Proposed Plan 5 Outlet Structures Outlet structure 0 Orifice name: Low -Flow Orifice area (sf) 0.049 diameter or depth (in) 3.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 363.000 multiple 1 discharge into riser Outlet structure 1 Orifice name: Lower Treatment Volume Outfall area (sf) 1.227 diameter or depth (in) 15.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 362.850 multiple 1 discharge out of riser Outlet structure 2 Orifice name: Upper Secondary Outfall area (sf) 4.909 diameter or depth (in) 30.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 365.250 multiple 1 discharge out of riser Outlet structure 3 Weir name: Riser diameter (in) 60.000 side angle 0.000 coefficient 3.300 invert (ft) 370.750 multiple 1 discharge into riser transition at (ft) 1.519 orifice coef. 0.500 orifice area (sf) 19.635 Page 2 BH Blocks 16-17 Subarea A- Proposed Plan Outlet structure 4 Weir name: Emergency Spillway length (ft) 30.000 side angle 75.964 coefficient 3.000 invert (ft) 371.750 multiple 1 discharge through dam 4 Inflow Hydrographs Hydrograph 0 SCS name: 1-yr 24-hr SCS Design Storm Area (acres) 6.370 CN 82.000 Type 2 rainfall, P (in) 3.000 time of conc. (hrs) 0.1200 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 10.780 peak time (hrs) 11.922 volume (cy) 1179.963 Hydrograph 1 SCS name: 2-yr 24-hr SCS Design Storm Area (acres) 6.370 CN 82.000 Type 2 rainfall, P (in) 3.630 time of conc. (hrs) 0.1200 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 14.778 peak time (hrs) 11.922 volume (cy) 1617.630 Page 3 Hydrograph 2 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) Hydrograph 3 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) BH Blocks 16-17 Subarea A- Proposed Plan 10-yr 24-hr SCS Design Storm 6.370 82.000 2 5.480 0.1200 0.0200 30.000 1.00 true 27.451 11.922 3004.907 100-yr 24-hr SCS Design Storm 6.370 82.000 2 8.990 0.1200 0.0200 30.000 1.00 true 53.189 11.922 5822.219 Page 4 BH Blocks 16-17 Subarea A- Proposed Plan 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 10.761 at 11.92 (hrs) 0.442 at 13.06 (hrs) 368.170 at 13.46 (hrs) 654.683 Peaks: 2-yr 24-hr SCS Design Storm 14.752 at 11.92 (hrs) 0.503 at 13.44 (hrs) 369.654 at 13.86 (hrs) 942.617 Peaks: 10-yr 24-hr SCS Design Storm 27.403 at 11.92 (hrs) 20.822 at 12.02 (hrs) 371.285 at 12.02 (hrs) 1294.379 Peaks: 100-yr 24-hr 53.096 at 51.091 at 371.733 at 1396.296 Mon ]un 21 10:41:51 EDT 2021 SCS Design Storm 11.92 (hrs) 11.94 (hrs) 11.94 (hrs) Page 5 BH Blocks 16-17 Subarea A- Proposed Plan EB NOTE: The following routing calculations show the minimum low -flow orifice diameter required (1.33") to meet the energy balance threshold. This analysis corresponds to the proposed Subarea A watershed. BasinFlow printout INPUT: Basin: Brookhill Blocks 16-17- Subarea A Proposed Plan Energy Balance 21 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 363.00 1557.00 0.0 363.50 1695.00 30.1 364.00 2410.00 67.9 364.50 2798.00 116.1 365.00 3141.00 171.1 365.50 3468.00 232.2 366.00 3787.00 299.4 366.50 4106.00 372.4 367.00 4428.00 451.4 367.50 4661.00 535.6 368.00 4864.00 623.8 368.50 5074.00 715.8 369.00 5278.00 811.6 369.50 5476.00 911.2 370.00 5666.00 1014.4 370.50 5845.00 1120.9 371.00 6006.00 1230.7 371.50 6142.00 1343.2 372.00 6222.00 1457.6 372.50 5961.00 1570.4 373.00 6270.00 1683.7 Start_Elevation(ft) 363.00 Vol.(cy) 0.00 Page 1 BH Blocks 16-17 Subarea A- Proposed Plan EB 4 Outlet Structures Outlet structure 0 Orifice name: Low -Flow Orifice area (sf) 0.010 diameter or depth (in) 1.330 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 363.000 multiple 1 discharge into riser Outlet structure 1 Orifice name: Low -Flow Treatment Volume Outfall area (sf) 1.227 diameter or depth (in) 15.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 362.850 multiple 1 discharge out of riser Outlet structure 2 Weir name: Riser diameter (in) 60.000 side angle 0.000 coefficient 3.300 invert (ft) 370.750 multiple 1 discharge into riser transition at (ft) 1.519 orifice coef. 0.500 orifice area (sf) 19.635 Outlet structure 3 Weir name: Emergency Spillway length (ft) 30.000 side angle 75.964 coefficient 3.000 invert (ft) 371.750 multiple 1 discharge through dam Page 2 I 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) OUTPUT: BH Blocks 16-17 Subarea A- Proposed Plan EB 1-yr 24-hr SCS Design Storm 6.370 82.000 2 3.000 0.1200 0.0200 30.000 1.00 true 10.780 11.922 1179.963 Routing Method: storage -indication Hydrograph 0 Routing Summary of inflow (cfs) discharge (cfs) water level (ft) storage (cy) Peaks: 1-yr 24-hr SCS Design Storm 10.761 at 11.92 (hrs) 0.100 at 17.52 (hrs) 369.756 at 23.66 (hrs) 963.597 Thu Jun 17 21:35:41 EDT 2021 Page 3 Dry Detention Basin 'A' & Underground Detention System 'A' Combined Routing Calculations (Subarea A- Final Buildout) Note: Routing calculations have been provided using a low -flow orifice diameter of 3", matching the proposed design. Routing calculations have also been provided using a low -flow orifice diameter less than 3", meeting minimum energy balance requirements. BH Blocks 16-17 Subarea A- Full Buildout NOTE: The following routing calculations use a low -flow orifice diameter of 3". This analysis corresponds to the final buildout of Subarea A's watershed. BasinFlow printout INPUT: Basin: Brookhill Blocks 16-17- Subarea A Full Buildout 21 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 363.00 1557.00 0.0 363.50 1695.00 30.1 364.00 2410.00 67.9 364.50 2798.00 116.1 365.00 3141.00 171.1 365.50 3468.00 232.2 366.00 3787.00 299.4 366.50 4106.00 372.4 367.00 4428.00 451.4 367.50 4661.00 535.6 368.00 4864.00 623.8 368.50 5074.00 715.8 369.00 5278.00 811.6 369.50 5476.00 911.2 370.00 5666.00 1014.4 370.50 5845.00 1120.9 371.00 6006.00 1230.7 371.50 6142.00 1343.2 372.00 6222.00 1457.6 372.50 5961.00 1570.4 373.00 6270.00 1683.7 Start_Elevation(ft) 363.00 Vol.(cy) Page 1 BH Blocks 16-17 Subarea A- Full Buildout 5 Outlet Structures Outlet structure 0 Orifice name: Low -Flow Orifice area (sf) 0.049 diameter or depth (in) 3.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 363.000 multiple 1 discharge into riser Outlet structure 1 Orifice name: Lower Treatment Volume Outfall area (sf) 1.227 diameter or depth (in) 15.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 362.850 multiple 1 discharge out of riser Outlet structure 2 Orifice name: Upper Secondary Outfall area (sf) 4.909 diameter or depth (in) 30.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 365.250 multiple 1 discharge out of riser Outlet structure 3 Weir name: Riser diameter (in) 60.000 side angle 0.000 coefficient 3.300 invert (ft) 370.750 multiple 1 discharge into riser transition at (ft) 1.519 orifice coef. 0.500 orifice area (sf) 19.635 Page 2 BH Blocks 16-17 Subarea A- Full Buildout Outlet structure 4 Weir name: Emergency Spillway length (ft) 30.000 side angle 75.964 coefficient 3.000 invert (ft) 371.750 multiple 1 discharge through dam 4 Inflow Hydrographs Hydrograph 0 SCS name: 1-yr 24-hr SCS Design Storm Area (acres) 7.750 CN 83.000 Type 2 rainfall, P (in) 3.000 time of conc. (hrs) 0.1200 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 13.758 peak time (hrs) 11.922 volume (cy) 1505.929 Hydrograph 1 SCS name: 2-yr 24-hr SCS Design Storm Area (acres) 7.750 CN 83.000 Type 2 rainfall, P (in) 3.630 time of conc. (hrs) 0.1200 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 18.721 peak time (hrs) 11.922 volume (cy) 2049.202 Page 3 Hydrograph 2 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) Hydrograph 3 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) BH Blocks 16-17 Subarea A- Full Buildout 10-yr 24-hr SCS Design Storm 7.750 83.000 2 5.480 0.1200 0.0200 30.000 1.00 true 34.347 11.922 3759.736 100-yr 24-hr SCS Design Storm 7.750 83.000 2 8.990 0.1200 0.0200 30.000 1.00 true 65.878 11.922 7211.198 Page 4 BH Blocks 16-17 Subarea A- Full Buildout 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 13.733 at 11.92 (hrs) 0.489 at 13.60 (hrs) 369.284 at 13.76 (hrs) 867.658 Peaks: 2-yr 24-hr SCS Design Storm 18.688 at 11.92 (hrs) 0.918 at 12.80 (hrs) 370.787 at 12.80 (hrs) 1183.623 Peaks: 10-yr 24-hr SCS Design Storm 34.287 at 11.92 (hrs) 31.952 at 11.96 (hrs) 371.466 at 11.96 (hrs) 1335.349 Peaks: 100-yr 24-hr 65.763 at 64.224 at 371.880 at 1430.065 Mon Jun 21 10:41:24 EDT 2021 SCS Design Storm 11.92 (hrs) 11.94 (hrs) 11.94 (hrs) Page 5 BH Blocks 16-17 Subarea A- Final Buildout EB NOTE: The following routing calculations show the minimum low -flow orifice diameter required (1.33") to meet the energy balance threshold. This analysis corresponds to the final buildout of Subarea A's watershed. BasinFlow printout INPUT: Basin: Brookhill Blocks 16-17- Subarea A Final Buildout Energy Balance 21 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 363.00 1557.00 0.0 363.50 1695.00 30.1 364.00 2410.00 67.9 364.50 2798.00 116.1 365.00 3141.00 171.1 365.50 3468.00 232.2 366.00 3787.00 299.4 366.50 4106.00 372.4 367.00 4428.00 451.4 367.50 4661.00 535.6 368.00 4864.00 623.8 368.50 5074.00 715.8 369.00 5278.00 811.6 369.50 5476.00 911.2 370.00 5666.00 1014.4 370.50 5845.00 1120.9 371.00 6006.00 1230.7 371.50 6142.00 1343.2 372.00 6222.00 1457.6 372.50 5961.00 1570.4 373.00 6270.00 1683.7 Start_Elevation(ft) 363.00 Vol.(cy) 0.00 Page 1 BH Blocks 16-17 Subarea A- Final Buildout EB 4 Outlet Structures Outlet structure 0 Orifice name: Low -Flow Orifice area (sf) 0.010 diameter or depth (in) 1.330 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 363.000 multiple 1 discharge into riser Outlet structure 1 Orifice name: Low -Flow Treatment Volume Outfall area (sf) 1.227 diameter or depth (in) 15.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 362.850 multiple 1 discharge out of riser Outlet structure 2 Weir name: Riser diameter (in) 60.000 side angle 0.000 coefficient 3.300 invert (ft) 370.800 multiple 1 discharge into riser transition at (ft) 1.519 orifice coef. 0.500 orifice area (sf) 19.635 Outlet structure 3 Weir name: Emergency Spillway length (ft) 30.000 side angle 75.964 coefficient 3.000 invert (ft) 371.800 multiple 1 discharge through dam Page 2 BH Blocks 16-17 Subarea A- Final Buildout EB 1 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) OUTPUT: 1-yr 24-hr SCS Design Storm 7.750 83.000 2 3.000 0.1200 0.0200 30.000 1.00 true 13.758 11.922 1505.929 Routing Method: storage -indication Hydrograph 0 Routing Summary of inflow (cfs) discharge (cfs) water level (ft) storage (cy) Peaks: 1-yr 24-hr SCS Design Storm 13.733 at 11.92 (hrs) 0.108 at 15.22 (hrs) 370.779 at 15.52 (hrs) 1181.692 Thu Jun 17 21:34:58 EDT 2021 Page 3 Level Spreader 'A' Calculations LEVEL SPREADER 'A' DESIGN (Full Buildout) Per VSMH Chpt. 11, Section 11.5.3 Step 1 Area = 7.75 ac. (Eqn 11.11) Tv= 18,613 cf Q,= 0.66 watershed inches (From Worksheet 2) CN = 83 Step 2 Tc = 0.12 hrs Step 3 la = 0.40 la/P= 0.40 q "= 750 csm/in A= 0.012mi2 q PT ,= 6.0 cfs NOTES: q PTv is the Treatment Volume peak discharge, or the peak flow rate for the 1-inch rainfall event. The dry basin's lower / Treatment Volume outfall is set at the basin's floor elevation. 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." 90%+/- Native Grasses/groundcover below tree canopy at a LS length of 13 ft per 1 cfs 30%+/- Forest groundcover below tree canopy at a LS length of 40 ft per 1 cfs Site Specific / Weighted Average LS length = 15.7 ft per 1 cfs LS'B' Design Length= 94 ft Note: Max. LS Length = 130 ft Min. LS Length = 13 ft Underground Detention System '13' Routing Calculations (Subarea B- Proposed Development) Note: Routing calculations have been provided using a low -flow orifice diameter of 3", matching the proposed design. Routing calculations have also been provided using a low -flow orifice diameter less than 3", meeting minimum energy balance requirements. BH Blocks 16-17 UGD System B- Proposed Plan NOTE: The following routing calculations use a low -flow orifice diameter of 3". This analysis corresponds to the proposed Subarea B watershed. BasinFlow printout INPUT: Basin: BH Blocks 16-17 Detention System B- Proposed Plan 13 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 357.00 5220.00 0.0 357.50 5220.00 96.7 358.50 8741.00 352.4 359.50 9914.00 697.7 360.50 10598.00 1077.4 361.50 10969.00 1476.8 362.50 11088.00 1885.3 363.50 10969.00 2293.7 364.50 10598.00 2693.1 365.50 9914.00 3072.9 366.50 8741.00 3418.1 367.50 5220.00 3673.9 368.50 5220.00 3867.2 Start_Elevation(ft) 357.00 Vol.(cy) 0.00 Page 1 5 Outlet Structures Outlet structure 0 Orifice BH Blocks 16-17 UGD System B- Proposed Plan name: Lower Primary Outfall Pipe area (sf) 4.909 diameter or depth (in) 30.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 357.450 multiple 1 discharge out of riser Outlet structure 1 Orifice name: Low Flow Orifice area (sf) 0.049 diameter or depth (in) 3.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 357.500 multiple 1 discharge into riser Outlet structure 2 Orifice name: Emergency Overflow Orifice area (sf) 4.568 diameter or depth (in) 28.940 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 364.670 multiple 1 discharge into riser Outlet structure 3 Weir name: Access Riser diameter (in) 36.000 side angle 0.000 coefficient 3.300 invert (ft) 369.500 multiple 1 discharge into riser transition at (ft) 0.912 orifice coef. 0.500 orifice area (sf) 7.069 Page 2 BH Blocks 16-17 UGD System B- Proposed Plan Outlet structure 4 Culvert name: multiple discharge D (in) h (in) Length (ft) Slope Manning's n Inlet coeff. Ke Equation constant set Invert (ft) 3 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) Hydrograph 1 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) Upper Secondary Outfall Pipe 1 through dam 36.000 0.000 5.000 0.010 0.013 0.500 3 364.500 1-yr SCS TR-55 24-hr Design Storm 12.160 82.000 2 3.000 0.1700 0.0200 30.000 1.00 true 18.977 11.961 2252.353 2-yr SCS TR-55 24-hr Design Storm 12.160 82.000 2 3.630 0.1700 0.0200 30.000 1.00 true 26.016 11.961 3087.808 Page 3 Hydrograph 2 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) OUTPUT: BH Blocks 16-17 UGD System B- Proposed Plan 10-yr SCS TR-55 24-hr Design Storm 12.160 82.000 2 5.480 0.1700 0.0200 30.000 1.00 true 48.327 11.961 5735.928 Routing Method: storage -indication Hydrograph 0 Routing Summary of Peaks: 1-yr SCS TR-55 24-hr Design Storm inflow (cfs) 18.975 at 11.96 (hrs) discharge (cfs) 0.405 at 15.24 (hrs) water level (ft) 361.857 at 15.92 (hrs) storage (cy) 1622.130 Hydrograph 1 Routing Summary of Peaks: 2-yr SCS TR-55 24-hr Design Storm inflow (cfs) 26.014 at 11.96 (hrs) discharge (cfs) 0.476 at 16.16 (hrs) water level (ft) 363.467 at 17.04 (hrs) storage (cy) 2280.249 Hydrograph 2 Routing Summary of Peaks: 10-yr SCS TR-55 24-hr Design Storm inflow (cfs) 48.323 at 11.96 (hrs) discharge (cfs) 19.229 at 12.14 (hrs) water level (ft) 365.840 at 12.14 (hrs) storage (cy) 3195.260 Mon Jun 21 13:44:45 EDT 2021 Page 4 BH Blocks 16-17 UGD System B- Proposed Plan EB NOTE: The following routing calculations show the minimum low -flow orifice diameter required (1.75") to meet the energy balance threshold. This analysis corresponds to the proposed Subarea B watershed. BasinFlow printout INPUT: Basin: BH Blocks 16-17 Detention System B- Proposed Plan Energy Balance 13 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 357.00 5220.00 0.0 357.50 5220.00 96.7 358.50 8741.00 352.4 359.50 9914.00 697.7 360.50 10598.00 1077.4 361.50 10969.00 1476.8 362.50 11088.00 1885.3 363.50 10969.00 2293.7 364.50 10598.00 2693.1 365.50 9914.00 3072.9 366.50 8741.00 3418.1 367.50 5220.00 3673.9 368.50 5220.00 3867.2 Start_Elevation(ft) 357.00 Vol.(cy) 0.00 Page 1 BH Blocks 16-17 UGD System B- Proposed Plan EB 5 Outlet Structures Outlet structure 0 Orifice name: Lower Primary Outfall Pipe area (sf) 4.909 diameter or depth (in) 30.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 357.450 multiple 1 discharge out of riser Outlet structure 1 Orifice name: Low Flow Orifice area (sf) 0.017 diameter or depth (in) 1.750 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 357.500 multiple 1 discharge into riser Outlet structure 2 Orifice name: Emergency Overflow Orifice area (sf) 1.416 diameter or depth (in) 16.110 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 366.250 multiple 1 discharge into riser Outlet structure 3 Weir name: Access Riser diameter (in) 36.000 side angle 0.000 coefficient 3.300 invert (ft) 369.500 multiple 1 discharge into riser transition at (ft) 0.912 orifice coef. 0.500 orifice area (sf) 7.069 Page 2 BH Blocks 16-17 UGD System B- Proposed Plan EB Outlet structure 4 Culvert name: multiple discharge D (in) h (in) Length (ft) Slope Manning's n Inlet coeff. Ke Equation constant set Invert (ft) 1 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) OUTPUT: Upper Secondary Outfall Pipe 1 through dam 36.000 0.000 5.000 0.010 0.013 0.500 3 366.250 1-yr SCS TR-55 24-hr Design Storm 12.160 82.000 2 3.000 0.1700 0.0200 30.000 1.00 true 18.977 11.961 2252.353 Routing Method: storage -indication Hydrograph 0 Routing Summary of inflow (cfs) discharge (cfs) water level (ft) storage (cy) Peaks: 1-yr SCS 18.975 at 0.151 at 362.677 at 1957.874 Thu Jun 17 21:32:42 EDT 2021 TR-55 24-hr Design Storm 11.96 (hrs) 20.52 (hrs) 24.04 (hrs) Page 3 Underground Detention System '13' Routing Calculations (Subarea B- Final Buildout Development) Note: Routing calculations have been provided using a low -flow orifice diameter of 3", matching the proposed design. Routing calculations have also been provided using a low -flow orifice diameter less than 3", meeting minimum energy balance requirements. BH Blocks 16-17 UGD System B- Final Buildout NOTE: The following routing calculations use a low -flow orifice diameter of 3". This analysis corresponds to the final buildout of Subarea B's watershed. BasinFlow printout INPUT: Basin: BH Blocks 16-17 Detention System B- Final Buildout 13 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 357.00 5220.00 0.0 357.50 5220.00 96.7 358.50 8741.00 352.4 359.50 9914.00 697.7 360.50 10598.00 1077.4 361.50 10969.00 1476.8 362.50 11088.00 1885.3 363.50 10969.00 2293.7 364.50 10598.00 2693.1 365.50 9914.00 3072.9 366.50 8741.00 3418.1 367.50 5220.00 3673.9 368.50 5220.00 3867.2 Start_Elevation(ft) 357.00 Vol.(cy) 0.00 Page 1 5 Outlet Structures Outlet structure 0 Orifice BH Blocks 16-17 UGD System B- Final Buildout name: Lower Primary Outfall Pipe area (sf) 4.909 diameter or depth (in) 30.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 357.450 multiple 1 discharge out of riser Outlet structure 1 Orifice name: Low Flow Orifice area (sf) 0.049 diameter or depth (in) 3.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 357.500 multiple 1 discharge into riser Outlet structure 2 Orifice name: Emergency Overflow Orifice area (sf) 4.568 diameter or depth (in) 28.940 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 364.670 multiple 1 discharge into riser Outlet structure 3 Weir name: Access Riser diameter (in) 36.000 side angle 0.000 coefficient 3.300 invert (ft) 369.500 multiple 1 discharge into riser transition at (ft) 0.912 orifice coef. 0.500 orifice area (sf) 7.069 Page 2 BH Blocks 16-17 UGD System B- Final Buildout Outlet structure 4 Culvert name: multiple discharge D (in) h (in) Length (ft) Slope Manning's n Inlet coeff. Ke Equation constant set Invert (ft) 3 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) Hydrograph 1 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) Upper Secondary Outfall Pipe 1 through dam 36.000 0.000 5.000 0.010 0.013 0.500 3 364.500 1-yr SCS TR-55 24-hr Design Storm 20.080 83.000 2 3.000 0.1700 0.0200 30.000 1.00 true 32.873 11.961 3901.652 2-yr SCS TR-55 24-hr Design Storm 20.080 83.000 2 3.630 0.1700 0.0200 30.000 1.00 true 44.732 11.961 5309.184 Page 3 BH Blocks 16-17 UGD System B- Final Buildout Hydrograph 2 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) OUTPUT: 10-yr SCS TR-55 24-hr Design Storm 20.080 83.000 2 5.480 0.1700 0.0200 30.000 1.00 true 82.070 11.961 9740.817 Routing Method: storage -indication Hydrograph 0 Routing Summary of Peaks: 1-yr SCS TR-55 24-hr Design Storm inflow (cfs) 32.870 at 11.96 (hrs) discharge (cfs) 1.092 at 13.84 (hrs) water level (ft) 364.649 at 13.86 (hrs) storage (cy) 2751.093 Hydrograph 1 Routing Summary of Peaks: 2-yr SCS TR-55 24-hr Design Storm inflow (cfs) 44.728 at 11.96 (hrs) discharge (cfs) 12.316 at 12.18 (hrs) water level (ft) 365.546 at 12.18 (hrs) storage (cy) 3089.680 Hydrograph 2 Routing Summary of Peaks: 10-yr SCS TR-55 24-hr Design Storm inflow (cfs) 82.062 at 11.96 (hrs) discharge (cfs) 72.411 at 12.02 (hrs) water level (ft) 368.068 at 12.02 (hrs) storage (cy) 3783.717 Mon Jun 21 13:44:10 EDT 2021 Page 4 BH Blocks 16-17 UGD System B- Final Buildout EB NOTE: The following routing calculations show the minimum low -flow orifice diameter required (1.75") to meet the energy balance threshold. This analysis corresponds to the final buildout of Subarea B's watershed. BasinFlow printout INPUT: Basin: BH Blocks 16-17 Detention System B- Final Buildout Energy Balance 13 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 357.00 5220.00 0.0 357.50 5220.00 96.7 358.50 8741.00 352.4 359.50 9914.00 697.7 360.50 10598.00 1077.4 361.50 10969.00 1476.8 362.50 11088.00 1885.3 363.50 10969.00 2293.7 364.50 10598.00 2693.1 365.50 9914.00 3072.9 366.50 8741.00 3418.1 367.50 5220.00 3673.9 368.50 5220.00 3867.2 Start_Elevation(ft) 357.00 Vol.(cy) 0.00 Page 1 BH Blocks 16-17 UGD System B- Final Buildout EB 5 Outlet Structures Outlet structure 0 Orifice name: Lower Primary Outfall Pipe area (sf) 4.909 diameter or depth (in) 30.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 357.450 multiple 1 discharge out of riser Outlet structure 1 Orifice name: Low Flow Orifice area (sf) 0.017 diameter or depth (in) 1.750 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 357.500 multiple 1 discharge into riser Outlet structure 2 Orifice name: Emergency Overflow Orifice area (sf) 1.416 diameter or depth (in) 16.110 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 366.250 multiple 1 discharge into riser Outlet structure 3 Weir name: Access Riser diameter (in) 36.000 side angle 0.000 coefficient 3.300 invert (ft) 369.500 multiple 1 discharge into riser transition at (ft) 0.912 orifice coef. 0.500 orifice area (sf) 7.069 Page 2 BH Blocks 16-17 UGD System B- Final Buildout EB Outlet structure 4 Culvert name: multiple discharge D (in) h (in) Length (ft) Slope Manning's n Inlet coeff. Ke Equation constant set Invert (ft) 1 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) OUTPUT: Upper Secondary Outfall Pipe 1 through dam 36.000 0.000 5.000 0.010 0.013 0.500 3 366.250 1-yr SCS TR-55 24-hr Design Storm 20.080 83.000 2 3.000 0.1700 0.0200 30.000 1.00 true 32.873 11.961 3901.652 Routing Method: storage -indication Hydrograph 0 Routing Summary of inflow (cfs) discharge (cfs) water level (ft) storage (cy) Peaks: 1-yr SCS 32.870 at 0.197 at 366.235 at 3330.869 Thu Jun 17 21:33:08 EDT 2021 TR-55 24-hr Design Storm 11.96 (hrs) 17.34 (hrs) 18.88 (hrs) Page 3 Subarea C's Analysis of the Storm Sewer System (For Subarea C's Channel & Flood Protection Compliance) This analysis shows the 10-year hydraulic grade lines do not overtop the inlets, and the pipes are adequately sized for their runoff, hereby confirming the 10-year design storm is confined within the stormwater conveyance system. 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Aro4 �71 (Pr ENO ov: avlLo pRo\ t-&z sU). 1992 Source: USDA-SCS III - 165 zaa; 'aziS deadly OSp 3.18 0 0 N Plate 3.18-4 Virginia Runoff Reduction Method Water Quality Calculations (For the Proposed Plan) This analysis is for the proposed plan and shows its water quality compliance. Please note, the level spreader was designed for the more stringent final buildout's higher treatment volume. The final buildout's higher treatment volume can be viewed in the next section entitled 'Virginia Runoff Reduction Method Water Quality Calculations (For the Final Buildout)'. 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O J OJ O J Oj O �• 6 0J W L W Q 6 F z W d Q 4 6 F y f C z W W C O z F OO z O z OD O O N O Z Q Z z 0 � z ' Q W o � W O C i Virginia Runoff Reduction Method Water Quality Calculations (For the Final Buildout) This analysis was used to obtain the more stringent/larger design treatment volume for the level spreader. Since the final buildout of Brookhill will likely introduce additional impervious areas & runoff directed to the level spreader with the construction of the future Block 18, the level spreader was overdesigned with this plan to meet the following final buildout's requirements. 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