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HomeMy WebLinkAboutWPO202100007 Stormwater Management Maintenance Agree. 2021-10-16BROOKHILL STORMWATER MANAGEMENT CALCULATIONS PACKET (WPO 20210007- Blocks 16-17) Date of Calculations JANUARY 21, 2021 Revised On JUNE 91 2021 Revised On July 30, 2021 Revised On October 14, 2021 PREPARED BY: COLLINS ""'RING 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: 10/14/2021 Date: 10/14/2021 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 20 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 0.30 0.135 1 0.10 0.10 0.17 0.135 0.135 0.12 0.02 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 f•PIPIIf• ]� § !! $) ) !® 99 Channel & Flood Protection Calculations 9 VAC 25-870-66 (Point of Analysis 'A'- For Proposed Watershed) 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 < 0.11 cfs Section B.4 Limits of Analysis 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) 0.10 cfs (For Proposed Watershed) 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 < 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) 0.10 cfs (For Full Buildout) 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 = 371.36 ft (For Proposed Plan) 10-yr 24-hr SCS Design Storm's Peak WSE = 371.49 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 of Analysis'B'- For Proposed Plan) 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 < 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) 0.18 cfs (For Proposed Watershed) 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 < 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) 0.27 cfs (For Full Buildout Watershed) 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 UGD system] 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 = 361.64 ft (For Proposed Plan) 10-yr 24-hr SCS Design Storm's Peak WSE = 362.49 ft (For Final Buildout) SWM Facility Top of Dam Elevation = 364.00 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 UGD system'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) Brookhill Blocks 16-17- Subarea A Proposed Development BasinFlow printout INPUT: Basin: Brookhill Blocks 16-17- Subarea A Proposed Development 21 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 363.00 304.00 0.0 363.50 2168.00 20.3 364.00 2324.00 61.9 364.50 2504.00 106.5 365.00 2705.00 154.8 365.50 2926.00 206.9 366.00 3167.00 263.3 366.50 3429.00 324.4 367.00 5259.00 404.2 367.50 5424.00 503.1 368.00 5558.00 604.8 368.50 5695.00 709.0 369.00 5822.00 815.6 369.50 5934.00 924.5 370.00 6022.00 1035.2 370.50 6065.00 1147.1 371.00 5979.00 1258.6 371.50 5871.00 1368.3 372.00 6136.00 1479.5 372.50 6434.00 1595.9 373.00 5017.00 1701.6 Start_Elevation(ft) 363.00 Vol.(cy) 0.00 5 Outlet Structures Outlet structure 0 Orifice name: Low -Flow Orifice area (sf) 0.009 diameter or depth (in) 1.313 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 363.000 multiple 1 discharge into riser Page 1 Brookhill Blocks 16-17- Subarea A Proposed Development 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 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 Page 2 Brookhill Blocks 16-17- Subarea A Proposed Development 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 Hydrograph 2 SCS name: 10-yr 24-hr SCS Design Storm Area (acres) 6.370 CN 82.000 Type 2 rainfall, P (in) 5.480 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) 27.451 peak time (hrs) 11.922 volume (cy) 3004.907 Page 3 Brookhill Blocks 16-17- Subarea A Proposed Development 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) 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 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) 0.097 at 17.62 (hrs) 369.693 at 23.96 (hrs) 966.887 Peaks: 2-yr 24-hr SCS Design Storm 14.752 at 11.92 (hrs) 0.104 at 13.10 (hrs) 370.729 at 13.78 (hrs) 1198.314 Peaks: 10-yr 24-hr SCS Design Storm 27.403 at 11.92 (hrs) 24.719 at 11.98 (hrs) 371.359 at 11.98 (hrs) 1337.498 Peaks: 100-yr 24-hr 53.096 at 50.699 at 371.734 at 1419.717 Tue Oct 12 10:57:14 EDT 2021 SCS Design Storm 11.92 (hrs) 11.94 (hrs) 11.94 (hrs) Page 4 Dry Detention Basin 'A' & Underground Detention System 'A' Combined Routing Calculations (Subarea A- Final Buildout) BH Blocks 16-17 Subarea A- Full Buildout BasinFlow printout INPUT: Basin: Brookhill Blocks 16-17- Subarea A Full Buildout 21 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 363.00 304.00 0.0 363.50 2168.00 20.3 364.00 2324.00 61.9 364.50 2504.00 106.5 365.00 2705.00 154.8 365.50 2926.00 206.9 366.00 3167.00 263.3 366.50 3429.00 324.4 367.00 5259.00 404.2 367.50 5424.00 503.1 368.00 5558.00 604.8 368.50 5695.00 709.0 369.00 5822.00 815.6 369.50 5934.00 924.5 370.00 6022.00 1035.2 370.50 6065.00 1147.1 371.00 5979.00 1258.6 371.50 5871.00 1368.3 372.00 6136.00 1479.5 372.50 6434.00 1595.9 373.00 5017.00 1701.6 Start_Elevation(ft) 363.00 Vol.(cy) 5 Outlet Structures Outlet structure 0 Orifice name: Low -Flow Orifice area (sf) 0.009 diameter or depth (in) 1.313 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 363.000 multiple 1 discharge into riser Page 1 BH Blocks 16-17 Subarea A- Full Buildout 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 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 Page 2 BH Blocks 16-17 Subarea A- Full Buildout 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 Hydrograph 2 SCS name: 10-yr 24-hr SCS Design Storm Area (acres) 7.750 CN 83.000 Type 2 rainfall, P (in) 5.480 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) 34.347 peak time (hrs) 11.922 volume (cy) 3759.736 Page 3 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 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 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) 0.104 at 14.76 (hrs) 370.710 at 15.92 (hrs) 1194.008 Peaks: 2-yr 24-hr SCS Design Storm 18.688 at 11.92 (hrs) 4.208 at 12.12 (hrs) 370.934 at 12.12 (hrs) 1244.047 Peaks: 10-yr 24-hr SCS Design Storm 34.287 at 11.92 (hrs) 33.027 at 11.94 (hrs) 371.489 at 11.94 (hrs) 1365.891 Peaks: 100-yr 24-hr 65.763 at 64.043 at 371.877 at 1451.664 Tue Oct 12 10:57:56 EDT 2021 SCS Design Storm 11.92 (hrs) 11.94 (hrs) 11.94 (hrs) Page 4 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= 18,286 cf Q,= 0.65 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 ,= 5.9 cfs NOTES: q P, 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 Min. LS'B' Design Length= 93 ft (94' Proposed) Note: Max. LS Length = 130 ft Min. LS Length = 13 ft Underground Detention System 'B' Routing Calculations (Subarea B- Proposed Development) Note: The following routing calculations are for the upper Underground Detention Basin 'B'. These routing calculations were used to generate outflow hydrographs, which were later used for the inflow hydrographs in the lower Dry Detention Basin 'B' routings. Brookhill Blocks 16-17- Upper UGD System Routing of Subarea B Proposed Development BasinFlow printout INPUT: Basin: Brookhill Blocks 16-17- Upper UGD System Routing of Subarea B Proposed Development 13 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 369.00 1584.00 0.0 369.50 1584.00 29.3 370.50 2689.00 107.6 371.50 3058.00 213.9 372.50 3272.00 331.1 373.50 3389.00 454.5 374.50 3426.00 580.7 375.50 3389.00 706.9 376.50 3272.00 830.2 377.50 3058.00 947.4 378.50 2689.00 1053.8 379.50 1584.00 1132.0 380.50 1584.00 1190.7 Start-Elevation(ft) 369.00 Vol.(cy) 0.00 5 Outlet Structures Outlet structure 0 Orifice name: Low -Flow Orifice area (sf) 0.545 diameter or depth (in) 10.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 369.500 multiple 2 discharge into riser Outlet structure 1 Orifice name: Emergency Overflow in Weir Plate area (sf) 2.461 diameter or depth (in) 21.240 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 377.670 multiple 1 discharge into riser Page 1 Brookhill Blocks 16-17- Upper UGD System Routing of Subarea B Proposed Development Outlet structure 2 Weir name: Access Manhole for Inspection diameter (in) 30.000 side angle 0.000 coefficient 3.000 invert (ft) 389.000 multiple 1 discharge into riser Outlet structure 3 Orifice name: Outfall to Dry Basin area (sf) 4.909 diameter or depth (in) 30.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 369.470 multiple 1 discharge out of riser Outlet structure 4 Orifice name: Emergency Bypass Outfall to Dry Basin area (sf) 4.909 diameter or depth (in) 30.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 377.000 multiple 2 discharge through dam Page 2 Brookhill Blocks 16-17- Upper UGD System Routing of Subarea B Proposed Development 3 Inflow Hydrographs Hydrograph 0 SCS name: 1-yr 24-hr SCS Design Storm Area (acres) 12.160 CN 82.000 Type 2 rainfall, P (in) 3.000 time of conc. (hrs) 0.1700 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 18.977 peak time (hrs) 11.961 volume (cy) 2252.353 Hydrograph 1 SCS name: 2-yr 24-hr SCS Design Storm Area (acres) 12.160 CN 82.000 Type 2 rainfall, P (in) 3.630 time of conc. (hrs) 0.1700 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 26.016 peak time (hrs) 11.961 volume (cy) 3087.808 Hydrograph 2 SCS name: 10-yr 24-hr SCS Design Storm Area (acres) 12.160 CN 82.000 Type 2 rainfall, P (in) 5.480 time of conc. (hrs) 0.1700 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 48.327 peak time (hrs) 11.961 volume (cy) 5735.928 Page 3 Brookhill Blocks 16-17- Upper UGD System Routing of Subarea B Proposed Development OUTPUT: Routing Method: storage -indication Hydrograph 0 Routing Summary of Peaks: 1-yr 24-hr SCS Design Storm inflow (cfs) 18.975 at 11.96 (hrs) discharge (cfs) 8.669 at 12.12 (hrs) water level (ft) 373.842 at 12.12 (hrs) storage (cy) 497.501 Hydrograph 1 Routing Summary of Peaks: 2-yr 24-hr SCS Design Storm inflow (cfs) 26.014 at 11.96 (hrs) discharge (cfs) 10.516 at 12.14 (hrs) water level (ft) 375.693 at 12.14 (hrs) storage (cy) 731.046 Hydrograph 2 Routing Summary of Peaks: 10-yr 24-hr SCS Design Storm inflow (cfs) 48.323 at 11.96 (hrs) discharge (cfs) 44.202 at 12.02 (hrs) water level (ft) 378.512 at 12.02 (hrs) storage (cy) 1054.987 Thu Oct 07 13:51:11 EDT 2021 Page 4 Underground Detention System 'B' Outflow Hydrographs, Proposed Development (Used for the Lower Dry Detention Basin 'B' inflow hydrographs) Post -Development DA B (Proposed Development) Outflow Hydrograph From UGD B's Routings Time (hrs) Outflow rN) 0.0 0.00 0.5 0.00 1.0 0.00 1.5 0.00 2.0 0.00 2.5 0.00 3.0 0.00 3.5 0.00 4.0 0.00 4.5 0.00 5.0 0.00 5.5 0.00 6.0 0.00 6.5 0.00 7.0 0.00 7.5 0.00 8.0 0.00 8.5 0.00 9.0 0.06 9.5 0.14 10.0 0.22 10.5 0.44 11.0 0.82 11.5 1.64 11.9 6.21 12.0 8.15 12.1 &67 12.2 8.45 12.3 7.89 12.4 7.22 12.5 6.48 13.0 2.46 13.5 0.77 14.0 0.40 14.5 0.23 15.0 0.16 15.5 0.09 16.0 0.04 16.5 0.01 17.0 0.00 17.5 0.00 18.0 0.00 18.5 0.00 19.0 0.00 19.5 0.00 20.0 0.00 20.5 0.00 21.0 0.00 21.5 0.00 22.0 0.00 22.5 0.00 23.0 0.00 23.5 0.00 24.0 0.00 24.5 0.00 25.0 0.00 25.5 0.00 26.0 0.00 26.5 0.00 27.0 0.00 27.5 0.00 28.0 0.00 28.5 0.00 29.0 0.00 29.5 0.00 30.0 0.00 30.5 0.00 31.0 0.00 to.w 9.m am Ton 6.00 5m 4m 3.00 2.00 t.00 000 10000 Outflow Hydrograph (1-yr Design Storm) 5.0 10.0 15.0 20.0 25.0 30.0 35.0 Post -Development DA B (Proposed Development) Outflow Hydrograph From UGD B's Routings Time (hrs) Outflow rN) 0.0 0.00 0.5 0.00 1.0 0.00 1.5 0.00 2.0 0.00 2.5 0.00 3.0 0.00 3.5 0.00 4.0 0.00 4.5 0.00 5.0 0.00 5.5 0.00 6.0 0.00 6.5 0.02 7.0 0.04 7.5 0.07 8.0 0.09 8.5 0.17 9.0 0.31 9.5 0.40 10.0 0.53 10.5 0.85 11.0 1.39 11.5 2.17 11.9 7.47 12.0 9.78 12.1 10.52 12.2 10.36 12.3 9.86 12.4 9.24 12.5 8.56 13.0 4.74 13.5 1.69 14.0 0.78 14.5 0.53 15.0 0.42 15.5 0.30 16.0 0.21 16.5 0.14 17.0 0.10 17.5 0.07 18.0 0.04 18.5 0.02 19.0 0.00 19.5 0.00 20.0 0.00 20.5 0.00 21.0 0.00 21.5 0.00 22.0 0.00 22.5 0.00 23.0 0.00 23.5 0.00 24.0 0.00 24.5 0.00 25.0 0.00 25.5 0.00 26.0 0.00 26.5 0.00 27.0 0.00 27.5 0.00 28.0 0.00 28.5 0.00 29.0 0.00 29.5 0.00 30.0 0.00 30.5 0.00 31.0 0.00 12.00 10.00 am 6.00 4.00 2.00 0.00 -2.00 Outflow Hydrograph (2-yr Design Storm) Post -Development DA B (Proposed Development) Outflow Hydrograph From UGD B's Routings Time (hrs) Outflow rN) 0.0 0.00 0.5 0.00 1.0 0.00 1.5 0.04 2.0 0.10 2.5 0.13 3.0 0.16 3.5 0.18 4.0 0.21 4.5 0.26 5.0 0.30 5.5 0.37 6.0 0.42 6.5 0.49 7.0 0.54 7.5 0.61 8.0 0.66 8.5 0.85 9.0 1.15 9.5 1.31 10.0 1.56 10.5 2.02 11.0 2.66 11.5 3.83 11.9 10.65 12.0 44.20 12.1 39.45 12.2 15.93 12.3 11.66 12.4 11.22 12.5 10.69 13.0 7.59 13.5 4.39 14.0 2.17 14.5 1.56 15.0 1.32 15.5 1.09 16.0 0.88 16.5 0.73 17.0 0.65 17.5 0.57 18.0 0.50 18.5 0.43 19.0 0.35 19.5 0.29 20.0 0.22 20.5 0.18 21.0 0.16 21.5 0.14 22.0 0.13 22.5 0.12 23.0 0.11 23.5 0.10 24.0 0.08 24.5 0.00 25.0 0.02 25.5 0.00 26.0 0.00 26.5 0.00 27.0 0.00 27.5 0.00 28.0 0.00 28.5 0.00 29.0 0.00 29.5 0.00 30.0 0.00 30.5 0.00 31.0 0.00 50.00 40.00 30.00 20.00 10.00 0.00 -10.00 Outflow Hydrograph (10-yr Design Storm) Underground Detention System 'B' Routing Calculations (Subarea B- Full Buildout) Note: The following routing calculations are for the upper Underground Detention Basin 'B'. These routing calculations were used to generate outflow hydrographs, which were later used for the inflow hydrographs in the lower Dry Detention Basin 'B' routings. Brookhill Blocks 16-17- Upper UGD System Routing of Subarea B Full Buildout BasinFlow printout INPUT: Basin: Brookhill Blocks 16-17- Upper UGD System Routing of Subarea B Full Buildout 13 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 369.00 1584.00 0.0 369.50 1584.00 29.3 370.50 2689.00 107.6 371.50 3058.00 213.9 372.50 3272.00 331.1 373.50 3389.00 454.5 374.50 3426.00 580.7 375.50 3389.00 706.9 376.50 3272.00 830.2 377.50 3058.00 947.4 378.50 2689.00 1053.8 379.50 1584.00 1132.0 380.50 1584.00 1190.7 Start-Elevation(ft) 369.00 Vol.(cy) 0.00 5 Outlet Structures Outlet structure 0 Orifice name: Low -Flow Orifice area (sf) 0.545 diameter or depth (in) 10.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 369.500 multiple 2 discharge into riser Outlet structure 1 Orifice name: Emergency Overflow in Weir Plate area (sf) 2.461 diameter or depth (in) 21.240 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 377.670 multiple 1 discharge into riser Page 1 Brookhill Blocks 16-17- Upper UGD System Routing of Subarea B Full Buildout Outlet structure 2 Weir name: Access Manhole for Inspection diameter (in) 30.000 side angle 0.000 coefficient 3.000 invert (ft) 389.000 multiple 1 discharge into riser Outlet structure 3 Orifice name: Outfall to Dry Basin area (sf) 4.909 diameter or depth (in) 30.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 369.470 multiple 1 discharge out of riser Outlet structure 4 Orifice name: Emergency Bypass Outfall to Dry Basin area (sf) 4.909 diameter or depth (in) 30.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 377.000 multiple 2 discharge through dam Page 2 Brookhill Blocks 16-17- Upper UGD System Routing of Subarea B Full Buildout 3 Inflow Hydrographs Hydrograph 0 SCS name: 1-yr 24-hr SCS Design Storm Area (acres) 20.080 CN 83.000 Type 2 rainfall, P (in) 3.000 time of conc. (hrs) 0.1700 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 32.873 peak time (hrs) 11.961 volume (cy) 3901.652 Hydrograph 1 SCS name: 2-yr 24-hr SCS Design Storm Area (acres) 20.080 CN 83.000 Type 2 rainfall, P (in) 3.630 time of conc. (hrs) 0.1700 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 44.732 peak time (hrs) 11.961 volume (cy) 5309.184 Hydrograph 2 SCS name: 10-yr 24-hr SCS Design Storm Area (acres) 20.080 CN 83.000 Type 2 rainfall, P (in) 5.480 time of conc. (hrs) 0.1700 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 82.070 peak time (hrs) 11.961 volume (cy) 9740.817 Page 3 Brookhill Blocks 16-17- Upper UGD System Routing of Subarea B Full Buildout OUTPUT: Routing Method: storage -indication Hydrograph 0 Routing Summary of Peaks: 1-yr 24-hr SCS Design Storm inflow (cfs) 32.870 at 11.96 (hrs) discharge (cfs) 15.647 at 12.12 (hrs) water level (ft) 377.460 at 12.12 (hrs) storage (cy) 942.910 Hydrograph 1 Routing Summary of Peaks: 2-yr 24-hr SCS Design Storm inflow (cfs) 44.728 at 11.96 (hrs) discharge (cfs) 39.192 at 12.02 (hrs) water level (ft) 378.354 at 12.02 (hrs) storage (cy) 1039.107 Hydrograph 2 Routing Summary of Peaks: 10-yr 24-hr SCS Design Storm inflow (cfs) 82.062 at 11.96 (hrs) discharge (cfs) 81.203 at 11.98 (hrs) water level (ft) 380.175 at 11.98 (hrs) storage (cy) 1171.587 Thu Oct 07 13:46:26 EDT 2021 Page 4 Underground Detention System 'B' Outflow Hydrographs, Full Buildout (Used for the Lower Dry Detention Basin 'B' inflow hydrographs) Post -Development DA B (Full Buildout( Outflow Hydrograph From UGD B's Routings Time (h.) Outflow (ds) 0.0 0.00 0.5 0.00 1.0 0.00 1.5 0.00 2.0 0.00 2.5 0.00 3.0 0.00 3.5 0.00 4.0 0.01 4.5 0.02 5.0 0.04 5.5 0.08 6.0 0.11 6.5 0.15 7.0 0.18 7.5 0.22 8.0 0.25 8.5 0.37 9.0 0.56 9.5 0.68 10.0 0.85 10.5 1.26 11.0 1.86 11.5 2.78 11.9 8.54 12.0 10.87 12.1 15.65 12.2 12.10 12.3 11.27 12.4 10.67 12.5 10.02 13.0 6.40 13.5 2.77 14.0 1.23 14.5 0.84 15.0 0.69 15.5 0.53 16.0 0.40 16.5 0.31 17.0 0.26 17.5 0.20 18.0 0.17 18.5 0.12 19.0 0.08 19.5 0.05 20.0 0.02 20.5 0.01 21.0 0.00 21.5 0.00 22.0 0.00 22.5 0.00 23.0 0.00 23.5 0.00 24.0 0.00 24.5 0.00 25.0 0.00 25.5 0.00 26.0 0.00 26.5 0.00 27.0 0.00 27.5 0.00 28.0 0.00 28.5 0.00 29.0 0.00 29.5 0.00 30.0 0.00 30.5 0.00 31.0 0.00 Outflow Hydrograph (1-yr Design Storm) 18.00 16.00 14.00 12.00 10.00 am 6.00 4.00 2.00 0.00 200 00 5.0 10.0 ls.o 20.0 25.0 30.0 3.o Post -Development DA B (Full Buildout( Outflow Hydrograph From UGD B's Routings Tim.(h.) Outflow(cN) 0.0 0.00 0.5 0.00 1.0 0.00 1.5 0.01 2.0 0.05 2.5 0.08 3.0 0.11 3.5 0.13 4.0 0.15 4.5 0.20 5.0 0.24 5.5 0.29 6.0 0.34 6.5 0.40 7.0 0.45 7.5 0.52 8.0 0.56 8.5 0.73 9.0 1.01 9.5 1.16 10.0 1.40 10.5 1.88 11.0 2.45 11.5 3.61 11.9 10.18 12.0 39.19 12.1 28.74 12.2 15.07 12.3 11.59 12.4 11.12 12.5 10.56 13.0 7.35 13.5 4.05 14.0 2.03 14.5 1.39 15.0 1.17 15.5 0.95 16.0 0.77 16.5 0.63 17.0 0.56 17.5 0.48 18.0 0.42 18.5 0.35 19.0 0.29 19.5 0.23 20.0 0.16 20.5 0.13 21.0 0.11 21.5 0.10 22.0 0.09 22.5 0.08 23.0 0.07 23.5 0.06 24.0 0.05 24.5 0.00 25.0 0.00 25.5 0.00 26.0 0.00 26.5 0.00 27.0 0.00 27.5 0.00 28.0 0.00 28.5 0.00 29.0 0.00 29.5 0.00 30.0 0.00 30.5 0.00 31.0 0.00 Outflow Hydrograph (2-yr Design Storm) 45.00 40.00 35.00 30.00 25.00 20.00 15.00 10.00 5.00 0.00 5000'0 5.0 10.0 15.0 20.0 25.0 30.0 3.o Post -Development DA B (Full Buildout) Outflow Hydrograph From UGD B's Routings Tim.(h.) Outflow(cN) 0.0 0.00 0.5 0.00 1.0 0.39 1.5 0.52 2.0 0.57 2.5 0.62 3.0 0.68 3.5 0.73 4.0 0.78 4.5 0.88 5.0 0.96 5.5 1.08 6.0 1.17 6.5 1.29 7.0 1.38 7.5 1.51 8.0 1.58 8.5 1.84 9.0 2.15 9.5 2.37 10.0 2.68 10.5 3.30 11.0 4.20 11.5 5.83 11.9 63.62 12.0 81.20 12.1 52.80 12.2 24.24 12.3 15.64 12.4 12.24 12.5 11.53 13.0 9.36 13.5 6.89 14.0 4.52 14.5 2.95 15.0 2.43 15.5 2.16 16.0 1.92 16.5 1.69 17.0 1.56 17.5 1.42 18.0 1.31 18.5 1.18 19.0 1.04 19.5 0.92 20.0 0.78 20.5 0.70 21.0 0.66 21.5 0.64 22.0 0.61 22.5 0.59 23.0 0.56 23.5 0.54 24.0 0.51 24.5 0.00 25.0 0.62 25.5 0.36 26.0 0.00 26.5 0.00 27.0 0.00 27.5 0.00 28.0 0.00 28.5 0.00 29.0 0.00 29.5 0.00 30.0 0.00 30.5 0.00 31.0 0.00 90.00 80.00 70.00 60 00 50 00 40 00 30 00 2000 Mon 0 00 -mnn 0 Outflow Hydrograph (10-yr Design Storm) Dry Detention Basin 'B' Routing Calculations (Subarea B- Proposed Development) Brookhill Blocks 16-17- Lower Dry Basin Routing of Subarea B Proposed Development BasinFlow printout INPUT: Basin: Brookhill Blocks 16-17- Lower Dry Basin Routing of Subarea B Proposed Development 8 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 350.00 2672.00 0.0 352.00 3816.00 239.0 354.00 5127.00 569.1 356.00 6609.00 1002.6 358.00 8268.00 1552.4 360.00 10105.00 2231.8 362.00 12089.00 3052.7 364.00 14178.00 4024.5 Start—Elevation(ft) 4 Outlet Structures Outlet structure 0 Orifice 350.00 Vol.(cy) 0.00 name: Barrel area (sf) 7.069 diameter or depth (in) 36.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 346.500 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) 350.000 multiple 1 discharge into riser Page 1 Brookhill Blocks 16-17- Lower Dry Basin Routing of Subarea B Proposed Development Outlet structure 2 Weir name: Riser diameter (in) 60.000 side angle 0.000 coefficient 3.300 invert (ft) 361.330 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) 50.000 side angle 75.964 coefficient 3.300 invert (ft) 362.330 multiple 1 discharge through dam Page 2 Brookhill Blocks 16-17- Lower Dry Basin Routing of Subarea B Proposed Development 4 Inflow Hydrographs Hydrograph 0 Curve name: 1-yr SCS TR-55 24-hr Design Storm- Proposed Development time incr (minlhrs) 0.01 time limit (minlhrs) 720.00 file: lyr 24hr SCS TR-55 DA B Proposed Buildout Inflow Hydrograph.txt volume (cy) 1488.31 routed true time in hours true Hydrograph 1 Curve name: 2-yr SCS TR-55 24-hr Design Storm- Proposed Development time incr (minlhrs) 0.01 time limit (minlhrs) 720.00 file: 2yr 24hr SCS TR-55 DA B Proposed Buildout Inflow Hydrograph.txt volume (cy) 2239.91 routed true time in hours true Hydrograph 2 Curve name: time incr (minlhrs) time limit (minlhrs) file: Hydrograph.txt volume (cy) routed time in hours 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) 10-yr SCS TR-55 24-hr Design 0.01 720.00 Storm- Proposed Development 10yr 24hr SCS TR-55 DA B Proposed Buildout Inflow 5049.20 true true 100-yr SCS TR-55 24-hr Design Storm 12.160 82.000 2 8.990 0.1700 0.0200 30.000 1.00 true 93.637 11.961 11113.730 Page 3 Brookhill Blocks 16-17- Lower Dry Basin Routing of Subarea B Proposed Development 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 SCS 8.669 at 0.182 at 357.453 at 1389.752 Peaks: 2-yr SCS 10.516 at 0.206 at 359.566 at 2072.767 Peaks: 10-yr SCS 44.202 at 9.144 at 361.639 at 2893.660 TR-55 24-hr Design 12.12 (hrs) 13.95 (hrs) 14.84 (hrs) TR-55 24-hr Design 12.14 (hrs) 14.53 (hrs) 15.96 (hrs) Storm- Proposed Development Storm- Proposed Development TR-55 24-hr Design Storm- Proposed Development 12.02 (hrs) 12.62 (hrs) 12.62 (hrs) Peaks: 100-yr SCS TR-55 24-hr Design Storm 93.629 at 11.96 (hrs) 89.261 at 11.98 (hrs) 362.559 at 11.98 (hrs) 3308.932 Thu Oct 07 14:06:51 EDT 2021 Page 4 Dry Detention Basin 'B' Routing Calculations (Subarea B- Full Buildout) Brookhill Blocks 16-17- Lower Dry Basin Routing of Subarea B Full Buildout BasinFlow printout INPUT: Basin: Brookhill Blocks 16-17- Lower Dry Basin Routing of Subarea B Full Buildout 8 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 350.00 2672.00 0.0 352.00 3816.00 239.0 354.00 5127.00 569.1 356.00 6609.00 1002.6 358.00 8268.00 1552.4 360.00 10105.00 2231.8 362.00 12089.00 3052.7 364.00 14178.00 4024.5 Start—Elevation(ft) 4 Outlet Structures Outlet structure 0 Orifice 350.00 Vol.(cy) 0.00 name: Barrel area (sf) 7.069 diameter or depth (in) 36.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 346.500 multiple 1 discharge out of riser Outlet structure 1 Orifice name: Low -Flow Orifice area (sf) 0.020 diameter or depth (in) 1.938 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 350.000 multiple 1 discharge into riser Page 1 Brookhill Blocks 16-17- Lower Dry Basin Routing of Subarea B Full Buildout Outlet structure 2 Weir name: Riser diameter (in) 60.000 side angle 0.000 coefficient 3.300 invert (ft) 361.330 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) 50.000 side angle 75.964 coefficient 3.300 invert (ft) 362.330 multiple 1 discharge through dam Page 2 Brookhill Blocks 16-17- Lower Dry Basin Routing of Subarea B Full Buildout 4 Inflow Hydrographs Hydrograph 0 Curve name: 1-yr SCS TR-55 24-hr Design Storm- Full Buildout time incr (minlhrs) 0.01 time limit (minlhrs) 720.00 file: 1yr 24hr SCS TR-55 DA B Full Buildout Inflow Hydrograph.txt volume (cy) 3036.95 routed true time in hours true Hydrograph 1 Curve name: 2-yr SCS TR-55 24-hr Design Storm- Full Buildout time incr (minlhrs) 0.01 time limit (minlhrs) 720.00 file: 2yr 24hr SCS TR-55 DA B Full Buildout Inflow Hydrograph.txt volume (cy) 4607.74 routed true time in hours true Hydrograph 2 Curve name: time incr (minlhrs) time limit (minlhrs) file: volume (cy) routed time in hours 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) 10-yr SCS TR-55 24-hr Design Storm- Full Buildout 0.01 720.00 10yr 24hr SCS TR-55 DA B Full Buildout Inflow Hydrograph.txt 10503.70 true true 100-yr SCS TR-55 24-hr Design Storm 20.080 83.000 2 8.990 0.1700 0.0200 30.000 1.00 true 157.411 11.961 18682.996 Page 3 Brookhill Blocks 16-17- Lower Dry Basin Routing of Subarea B 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 SCS 15.647 at 0.273 at 361.120 at 2673.314 Peaks: 2-yr SCS 39.192 at 7.752 at 361.605 at 2878.774 Peaks: 10-yr SCS 81.203 at 75.323 at 362.488 at 3275.557 TR-55 24-hr Design 12.12 (hrs) 14.41 (hrs) 15.16 (hrs) TR-55 24-hr Design 12.02 (hrs) 12.82 (hrs) 12.82 (hrs) Storm- Full Buildout Storm- Full Buildout TR-55 24-hr Design Storm- Full Buildout 11.98 (hrs) 11.99 (hrs) 11.99 (hrs) Peaks: 100-yr SCS TR-55 24-hr Design Storm 157.397 at 11.96 (hrs) 157.153 at 11.96 (hrs) 362.835 at 11.96 (hrs) 3439.686 Thu Oct 07 14:02:43 EDT 2021 Page 4 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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Rxe0.AP T4 3£ ''FLAT, \-�' LON(s NZ NA'�E v.Z.OTNS aF ct' Q'46, 0�trAu,, A44 �'7' &,T ENd aG CV't UC-C VfW tetZ ONJ). 1992 3.18 2C O K _ O IJi W W N N o c C; p o �n + i �i7.l EY O H n fir-;� 1 yp s 6� d t-I W 1- W f- O H C LL O Z = t Y H M Y N d y O 6 O Source: USDA-SCS III - 165 ;aa; 'aziS deadta OSp -I 2; �i �il 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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OW C 7 Q OS�O Q Q� L W? C F? C F C Q 7 N C x> U OC W_ z_ W_ O � W Q 0N OSi W CY. Q CyCj Q L FWQW U CC z Ud Q W WZ O Uj Q jFF z 1 z W z z C z z oW a nFaF F 0F0z FO W L W F O g O a O W O F p Wp, w C O W C C f C Y OJ Oj W f. N Cd Y.O Q J OJ d 4 O �•600 W L W 6 F z W Q 6 F y f C z W C O z F 00 z z D OD O O N C F z Y F 6 C O z Q Z z 0 � z ' Q W o � W � C 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. Please see the previous section entitled 'Virginia Runoff Reduction Method Water Quality Calculations (For the Proposed Plan)' for this plan's water quality compliance. � I } 1111 l 11111111111 IN 11111111 Ill m