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HomeMy WebLinkAboutWPO201700037 Calculations 2022-06-22BROOKHILL STORMWATER MANAGEMENT CALCULATIONS PACKET- VSMP AMENDMENT #4 SCI TT R. LINS7 J Lic. No. 035791 -6r� s/17/22 ZONAL (Village Center) Date of Calculations MAY 17, 2022 I:1C1W_1RAIII -yil 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 Unit Name Percent of Symbol AOI 27B Elioak loam, 2 35.5% to 7 percent slopes 27C Elioak loam, 7 27.0% to 15 percent slopes 27D Elioak loam, 2.6% 15 to 25 percent slopes 34C Glenelg loam, 24.9% 7 to 15 percent slopes 34D Glenelg loam, 9.9% 15 to 25 percent slopes Totals for Area of 100.00/0 Interest Note: The soils' properties within this development are predominantly composed of hydrologic group type B. Pre -Construction Aerial Photography This aerial photograph was taken prior to recent disturbances (shown on the previous page). This photograph is used soley for the purposes of establishing the pre -construction land covers (i.e. for the pre -development CN values) for Subareas A & B. Subareas A & B are approximated to be 25% woods and 75% grass in the pre -development state for this section of Brookhill. This is a more conservative approach/land cover when compared to using the exposed earth shown on the previous page. Precipitation Data (Source: NOAA's National Weather Service, Hydrometerorological Design Studies Center, Atlas 14 Point Precipitation Frequency Estimates) Map e El Terrain a e �u POINT PRECIPITATION FREQUENCY (PF) ESTIMATES WITH 90%CONFIDENCE INTERVALS AND SUPPLEMENTARY INFORMATION NOAA Atlas 14, Volume 2, Version 3 PDS-based precipitation frequency estimates with 90% confidence intervals (in inches)' Average rewn Interval yeah) Duration 1 z s 1a zs so Igo zoo soo logo F o0000-0 z4.M 3.00 3.62 4.63 (2.69-3.37) (3.254.08) (4.145.20) 5.4-1 6.72 7.79 (4.88b.14) (5.9&7.51) (686-8.69) 8.97 10.3 122 133 p.839.99) (8.87-11.4) (10.413.6) (tt615.4) Q O N N Z � � � N N n m v v Z❑ v c 0 Y z x v �0 ¢ 0 u v v 0 ° a v c v E o `w Y w n m c u ai c - ❑ j O q u u N 0 ° — L Z m > U w S E z c o o c o a 0 V1 a N � U II \ a u v m t a m o m o m o a 3 — o Z 0 u z u o v a O m m O a N 0 o M o N O o m O N y o (I1 O j r ry m N m m m a 0 a d v o "m' o ro o o a m e n o Q a ONj a N N O O vl .--i m O O Z m .-I vt w v) N v) N v) o `v m n c o � e o c � o v n '0 E c o_ o a n o °1 � > c a o c 0 v u v u c 00 c 0 0 c 0 0 c 0 0 E c c a o a o a c a v o c o o uu u o a° n mma; a a o a ¢m o o o o o o o a• U, U Uo Uc U Uocuuuu U00 Uc 'O� 0 0 0 c 0 c c c c c v 0 v 0 v 0 v 0 E 3 E 3 E 3 3 E 3 3 m 0 4 ox a o L a a n c n m m m m °1 n E � O � N v c ❑ ❑ ❑ v ❑ c v a ❑ 0 m U.S. Department of Agriculture Natural Resources Conservation Service TR 55 Worksheet 3: Time of Concentration (T,) or Travel Time (TJ Project: Brookhill Location: Village Center Designed By: FGM, PE Checked By: SRC, PE FL-ENG-21 06/C Date: 5/17/2022 «� 5/17/2022 Check One: Present X Developed X Check One: Tc X T, Through subarea ' T I DA A DAA DA B DA B Segment ID� (Pre-Dev.) (Post-Dev.) (Pre-Dev.) (Post-Dev.) Sheet Flow: (Applicable to T, only) Dense Dense Dense Dense 1 Surface description (Table 3-1) Grass Grass Grass Grass 2 Manning's roughness coeff., n (Table 3-1 0.24 0.24 0.24 0.24 3 Flow length, L (total L < 100) (ft) 100 100 100 30 4 Two-year 24-hour rainfall, P,(in.) 3.62 3.62 3.62 3.62 5 Land slope, s (ft/ft) 0.040 0.22 0.040 0.03 6 Compute T, = [0.007(n*L)° a) / Pz s s° ° 0.17 0.09 0.17 0.07 Shallow Concentrated Flow: 7 Surface description (paved or unpaved) Unpaved Unpaved paved 8 Flow Length, L (ft) 320 400 60 9 Watercourse slope, s (ft/ft) 0.016 0.045 0.050 10 Average velocity, V (Figure 3-1) 2.1 3.4 3.4 11 T,=L/3600*V 0.04 0.03 0.00 Channel Flow 12 Cross sectional flow area, a (ft) c c 0 jE o ^ 13 Wetted perimeter, P,, (ft) LL o 14 Hydraulic radius, r = a/Pw (ft) w w .2 15 Channel Slope, s (ft/ft) A + ,c 16 Manning's Roughness Coeff, n `o `o u $ 17 V= [ 1.49r111s°s ) / n a y o w 18 Flow length, L (ft) a 19 T,= L/3600*V 0.08 L 0.00 20 Watershed or subarea T, or T, (Add T, in steps 6, 11 and 19) 0.10 0.27 0.10 Notes: 1. The shoretest SCS Tc is 0.10 hrs The pre -development time of concentration data shown above reflects current conditions and the contours shown in this VSMP Amendment #4. However, to avoid skewing the calculations for the pre -development conditions the originally approved (longer) pre -development times of concentration are listed in line #20 ensuring the following Worksheet 4 calculations accurately reflect pre -construction conditions. In other words, the previously approved times of concentration are utilized for the pre - construction subareas A & B. Also, the post -development subarea A & B times of concentration remain the same at 0.10 hrs. U.S. Department of Agriculture Natural Resources Conservation Service Project: Brookhill Location: Village Center Check One: Present X DevelopedX TR 55 Worksheet 4: Graphical Peak Discharge Method Designed By: FGM, PE Checked By: SRC, PE Date: 5/17/2022 Date: 5/17/2022 1. Data Drainage Area Description Drainage Area Description Drainage Area Description Drainage Area Description DAA (Pre-Dev.) DAA (Post-Dev.) DAB (Pre-Dev.) DAB (Post-Dev.) Drainage Area( Am) in miles' = 0.0254 0.0253 0.0113 0.0113 Runoff curve number CN= 60 85 60 94 Time of concentration (Tc)= 0.41 0.10 0.27 0.10 Rainfall distribution type= II II II 11 Pond and swamp areas spread throughout the watershed= 0 0 0 0 2. Frequency -years 1 2 10 1 2 10 1 2 10 1 2 10 3. Rainfall, P(24 hour -inches 3.00 3.62 5.47 3.00 3.62 5.47 3.00 3.62 5.47 3.00 3.62 5.47 4. Initial Abstraction, la -inches 1.36 1.36 1.36 0.36 0.36 0.36 1.36 1.36 1.36 0.13 0.13 0.13 S. Compute la/P 0.45 0.38 0.25 0.12 0.10 0.07 OAS 0.38 0.25 0.04 0.03 0.02 6. Unit peak discharge, Qu-csm/in 350 425 525 975 1000 1000 425 525 650 1000 1000 1000 7. Runoff, Qfrom Worksheet 2-inches 0.32 0.56 1.55 1.57 2.10 3.78 0.32 0.56 I.55 2.36 2.96 4.78 8. Pond and Swamp adjustment factor, FIR 1 1 1 1 1 1 1 1 1 1 1 1 9. Peak Discharge, Qp- cfs where Qp=Qu Am Q Fp 2.83 6.07 20.62 Routed Through The SWM Facility 1.52 3.32 11.31 Routed Through The SWM Facility WATERSHED SUMMARY FOR SUBAREA Area, CN C. DAA (Pre-Dev.) 60 16.25 DA A(Post-Dev.) 85 16.22 WATERSHED SUMMARY FOR SUBAREA B Area, CN C. DA B(Pre -Dnc. 60 7.20 DA B (Post -Dew) 94 7.23 Overall Acreage Summary Pre -Development Post -Development Area, ac. Area,ac. DAA 16.25 1622 DAB 7.20 7.23 23.45 23.45 1-year Flow, 2-year Flow, 10-year Flow, cfs cfs cfs 2.83 6.07 20.62 0.33 1.13 41.38 1-year Flow, 2-year Flow, 10-year Flow, cfs cfs cfs Channel & Flood Protection Compliance Verifications 9 VAC 25-870-66 (Point of Interest #1) 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: Q Allowed < I.F. * [ (Q Pre -Developed * RV Pre -Developed) / RV Developed ] Q Allowed < 0.46 cfs Where: QPre-Developed= 2.83cfs (From Worksheet 4 Watershed Summary) CN Pre -Developed- 60 (From Worksheet 4 Watershed Summary) 5 Pre -Developed = 6.81 (From SCS TR-55 eq. 2-4) RV Pre -Developed= 0.32 in. (From SCS TR-55 eq. 2-3) CN Post -Developed= 85 (From Worksheet 4 Watershed Summary) S Post -Developed = 1.80 (From SCS TR-55 eq. 2-4) RV Post -Developed = 1.57 in. (From SCS TR-55 eq. 2-3) I.F. = 0.80 (Factor of Safety) Q Developed = 0.33 Cf5 Section B.4 Limits of Analysis The analysis terminates at Approved 'Wet Pond 4' since subsection B.3.a is met. 9 VAC 25-870-66 (Point of Interest #1) 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: "Concentrated stormwater flow to stormwater conveyance systems that currently experience localized flooding [downstream floodplain] during the 10-year 24-hour storm event: The point of discharge either" must meet a. or b. Section C.2.a: "Confines the postdevelopment peak flow rate from the 10-year 24-hour storm event within the stormwater conveyance system [Wet Pond] to avoid the localized flooding. Detention of stormwater..." was incorporated. Section C.3: Limits of Analysis "Stormwater conveyance systems shall be analyzed for compliance with flood protection criteria to a point where:" a., b. or c. is met. Section C.3.c The limits of analysis ends when "The stormwater conveyance system enters a mapped floodplain or other flood -prone area..." 9 VAC 25-870-66 (Point of Interest #2) 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: Q Allowed < I.F. * [ (Q Pre -Developed * RV Pre -Developed) / RV Developed ] Q Allowed < 0.16 cfs Where: QPre-Developed= 1.52cfs (From Worksheet 4 Watershed Summary) CN Pre -Developed- 60 (From Worksheet 4 Watershed Summary) 5 Pre -Developed = 6.81 (From SCS TR-55 eq. 2-4) RV Pre -Developed- 0.32 in. (From SCS TR-55 eq. 2-3) CN Post -Developed= 94 (From Worksheet 4 Watershed Summary) S Post -Developed = 0.63 (From SCS TR-55 eq. 2-4) RV Post -Developed = 2.36 in. (From SCS TR-55 eq. 2-3) I.F. = 0.80 (Factor of Safety) Q Developed = 0.16 Cf5 Section B.4 Limits of Analysis The analysis terminates at UGD System B's outfall since subsection B.3.a is met. 9 VAC 25-870-66 (Point of Interest #2) 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: "Concentrated stormwater flow to stormwater conveyance systems that currently experience localized flooding [downstream floodplain] during the 10-year 24-hour storm event: The point of discharge either" must meet a. or b. Section C.2.a: "Confines the postdevelopment peak flow rate from the 10-year 24-hour storm event within the stormwater conveyance system [UGD system & storm pipes] to avoid the localized flooding. Detention of stormwater..." was incorporated. Section C.3: Limits of Analysis "Stormwater conveyance systems shall be analyzed for compliance with flood protection criteria to a point where:" a., b. or c. is met. Section C.3.c The limits of analysis ends when "The stormwater conveyance system enters a mapped floodplain or other flood -prone area..." 'Wet Pond 4' Routing Calculations This amendment #4 only proposes one change to the originally approved 'Wet Pond 4'. This plan proposes to modify the approved riser structure's two SWM orifices, which are currently plugged for erosion & sediment control / construction purposes. The originally approved 6" low -flow and 9" mid -flow orifice are being replaced with a new 3" low -flow orifice (as approved in amendment #2) and six 4" mid -flow orifices (as proposed with this amendment #4). The following routing calculations otherwise simply show the approved 'Wet Pond 4' can adequately handle the proposed Village Center and Salamander Street. Village Center Wet Pond 4 Routings BasinFlow printout INPUT: Basin: Village Center Wet Pond 4 Routings 9 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 358.00 3690.00 0.0 360.00 5715.00 345.6 362.00 8315.00 862.2 364.00 11483.00 1592.3 365.00 14332.00 2069.4 366.00 18294.00 2672.1 368.00 29044.00 4410.1 370.00 33823.00 6736.3 372.00 38733.00 9421.5 Start—Elevation(ft) 5 Outlet Structures Outlet structure 0 Orifice 366.00 Vol.(cy) 2672.13 name: APPROVED Barrel area (sf) 19.635 diameter or depth (in) 60.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 363.300 multiple 1 discharge out of riser Outlet structure 1 Orifice name: APPROVED Low -Flow Orifice area (sf) 0.049 diameter or depth (in) 3.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 366.000 multiple 1 discharge into riser Page 1 Outlet structure 2 Orifice Town Center Wet Pond 4 Routings name: AMENDED Mid -Flow Orifice area (sf) 0.087 diameter or depth (in) 4.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 369.150 multiple 6 discharge into riser Outlet structure 3 Weir name: APPROVED Riser diameter (in) 72.000 side angle 0.000 coefficient 3.300 invert (ft) 369.800 multiple 1 discharge into riser transition at (ft) 1.823 orifice coef. 0.500 orifice area (sf) 28.274 Outlet structure 4 Weir name: APPROVED Emergency Spillway length (ft) 30.000 side angle 0.000 coefficient 3.300 invert (ft) 370.200 multiple 1 discharge through dam Page 2 Town Center Wet Pond 4 Routings 4 Inflow Hydrographs Hydrograph 0 SCS name: AMENDED 1-yr 24-hr TR-55 SCS Design Storm Area (acres) 16.220 CN 85.000 Type 2 rainfall, P (in) 3.000 time of conc. (hrs) 0.1000 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 32.689 peak time (hrs) 11.917 volume (cy) 3460.757 Hydrograph 1 SCS name: AMENDED 2-yr 24-hr TR-55 SCS Design Storm Area (acres) 16.220 CN 85.000 Type 2 rainfall, P (in) 3.620 time of conc. (hrs) 0.1000 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 43.659 peak time (hrs) 11.917 volume (cy) 4622.217 Hydrograph 2 SCS name: AMENDED 10-yr 24-hr TR-55 SCS Design Storm Area (acres) 16.220 CN 85.000 Type 2 rainfall, P (in) 5.470 time of conc. (hrs) 0.1000 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 78.311 peak time (hrs) 11.917 volume (cy) 8290.812 Page 3 Hydrograph 3 SCS Town Center Wet Pond 4 Routings name: AMENDED 100-yr 24-hr TR-55 SCS Design Storm Area (acres) 16.220 CN 85.000 Type 2 rainfall, P (in) 8.970 time of conc. (hrs) 0.1000 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 147.207 peak time (hrs) 11.917 volume (cy) 15584.872 OUTPUT: Routing Method: storage -indication Hydrograph 0 Routing Summary of 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 AMENDED 1-yr 32.535 at 0.331 at 368.951 at 5471.994 Peaks: AMENDED 2-yr 43.454 at 1.125 at 369.450 at 6060.596 24-hr TR-55 SCS Design Storm 11.92 (hrs) 18.74 (hrs) 20.34 (hrs) 24-hr TR-55 SCS Design Storm 11.92 (hrs) 14.70 (hrs) 14.70 (hrs) Peaks: AMENDED 10-yr 24-hr TR-55 SCS Design Storm 77.944 at 11.92 (hrs) 41.382 at 12.04 (hrs) 370.407 at 12.04 (hrs) 7253.810 Peaks: AMENDED 100-yr 24-hr TR-55 SCS Design Storm 146.517 at 11.92 (hrs) 133.680 at 11.96 (hrs) 370.904 at 11.96 (hrs) 7904.818 Tue May 17 11:33:47 EDT 2022 Page 4 Underground Detention System 'B' Routing and Riprap Lined Outlet Protection Calculations AMENDMENT No. 4- BH VC UGD System B Routings2 BasinFlow printout INPUT: Basin: AMENDMENT No. 4- BH VC UGD System B Routings 13 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 431.50 2997.00 0.0 432.00 2997.00 55.5 433.00 5063.00 203.1 434.00 5752.00 403.2 435.00 6153.00 623.7 436.00 6371.00 855.6 437.00 6441.00 1092.8 438.00 6371.00 1330.1 439.00 6153.00 1562.0 440.00 5752.00 1782.4 441.00 5063.00 1982.6 442.00 2997.00 2130.2 442.50 2997.00 2185.7 Start-Elevation(ft) 431.50 Vol.(cy) 5 Outlet Structures Outlet structure 0 Orifice name: Low -Flow Orifice area (sf) 0.014 diameter or depth (in) 1.625 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 432.000 multiple 1 discharge into riser Outlet structure 1 Orifice name: High Flow Orifice area (sf) 3.142 diameter or depth (in) 24.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 440.000 multiple 1 discharge into riser Page 1 AMENDMENT No. 4- BH VC UGD System B Routings2 Outlet structure 2 Weir name: Access Manhole for Inspection diameter (in) 30.000 side angle 0.000 coefficient 3.000 invert (ft) 446.000 multiple 1 discharge into riser Outlet structure 3 Orifice name: Lower Primary Outlet area (sf) 3.142 diameter or depth (in) 24.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 431.970 multiple 1 discharge out of riser Outlet structure 4 Orifice name: Elevated Bypass Outlet area (sf) 4.909 diameter or depth (in) 30.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 440.000 multiple 1 discharge through dam 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) 1-yr 24-hr SCS Design Storm 7.230 94.000 2 3.000 0.1000 0.0200 30.000 1.00 true 21.560 11.917 2282.602 Page 2 AMENDMENT No. 4- BH VC UGD System B Routings2 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) 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) 2-yr 24-hr SCS Design Storm 7.230 94.000 2 3.620 0.1000 0.0200 30.000 1.00 true 27.088 11.917 2867.868 10-yr 24-hr SCS Design Storm 7.230 94.000 2 5.470 0.1000 0.0200 30.000 1.00 true 43.781 11.917 4635.108 Page 3 AMENDMENT No. 4- BH VC UGD System B Routings2 OUTPUT: Routing Method: storage -indication Hydrograph 0 Routing Summary of Peaks: 1-yr 24-hr SCS Design Storm inflow (cfs) 21.459 at 11.92 (hrs) discharge (cfs) 0.163 at 14.90 (hrs) water level (ft) 439.991 at 15.30 (hrs) storage (cy) 1780.488 Hydrograph 1 Routing Summary of Peaks: 2-yr 24-hr SCS Design Storm inflow (cfs) 26.962 at 11.92 (hrs) discharge (cfs) 2.892 at 12.34 (hrs) water level (ft) 440.410 at 12.34 (hrs) storage (cy) 1867.500 Hydrograph 2 Routing Summary of Peaks: 10-yr 24-hr SCS Design Storm inflow (cfs) 43.576 at 11.92 (hrs) discharge (cfs) 35.276 at 12.00 (hrs) water level (ft) 442.337 at 12.00 (hrs) storage (cy) 2167.527 Wed May 25 13:45:26 EDT 2022 Page 4 Quo - 3 SAS cFs ( FMM Kowl zNst) ((tcv ccreo TO LC�<L SPRFAdER � FiL?E0. ai3tP> /� a5.y$ uCQ NTALL CF5 ooesle.o = 36 cFS x Pee 2LP«e 3,�-1 'at rK ar &�CNC, d = 1,5 x S we Q1AnnETGft, awT JcR Lre55 j HAGS die L,,q - IT (i+o+�cr�uL VxLLaaE c�,.n e2 VsMa 'L4 F�M,PE 5 � 1s(aa � O.-a 1 ` R,2' C� d REcvnne+,oEo ; 6 6 So MY rta�C- v 'S cL, AI STcNE dso' O AO CTiLC? �P EL�aav To 3E iLAT(rt O'/, 6A" V.wzT CL PSS F:6Y£e� FRgP l �GEY60 ir+TO C- tx75c«NG iSRcvNj f �2N p�- �' AT T11� 6 a ALOW6 _NC- PERiMElER, ~fJ6ES Cr-. Fu*T.0 , FA,CS"C e"LL QV ERLk4 A eQN, OF J" VooT GL, AL (xP*p (ka" Tk-k;(uCsS� ANo CL,-Z VZCT4ER.'Fi�%AZC. 1992 3.12 M LL _ 3 O O O •p ; �• s o 0 Gp �' i i- �_ a. Al o n LL ke $ O O g � u � oc 3 mQ i C O ~ G O E L E czo ++ yyy H W N E Km O O 6 O Source: USDA-SCS III - 165 yaa; •aziS deadia 05p N .y N s �E11� Plate 3.18-4 Virginia Runoff Reduction Method Water Quality Calculations DEq Viryirdo RumajjReduttion Merhotl New Development Compliarrrr Spreadshett - Version 3.0 02011 BMP Standards and Specifications M2013 Draft BMPStandardz and Specifications Project Name: Brookhill Section 1 Mass Grading. VSMP Plan Amendment N4 datalnputcallz Data: 5/17/2022 ronstantvalues BMP Design SpeclPcorlans List. 2013 Draft Stds & Specs calculation calls ISite Information Project (Treatment Volume and Loads) Impervious Ouss(aun) Constants Annual Rainfall (inches) 63 Target Rainfall Event (inches) 1.00 Total Phosphorus(TP) EMC Ing/LI 0.26 Total Name- (FN)EMC (ng/LI 1.W Target TP Load lVacre/yr) 0.61 P)luoitless mrrenion factor) 0.90 Runoff Cuseffidents Mal ASnilz BSollz 1 CSolls DSallz Olson Space 0.02 0.03 0.0a 0.05 Managetl Turt 0.15 0.20 0.22 015 Impervious Cover 0.95 1 0.95 1 0.95 0.95 Land Cover Summary Treatment Volume and Nutrient Loads Trent(ae'[-„'lame 1.5723 Treatment Volume (cubic feat) 611,491 TP Load(lb/,) 43.03 Iwo Y< 302.65 Forest/Open Space C er(acres) 0.W Weighed Rv(forest) am %Forest 0% Managed Turf Cover(acres) 13.02 Weighted Bv(turn 0.20 %Managetl Turf 43% Impervious Cmer(acres) 17.12 to(impervious) 0.95 %Impervious M Site A.(aae3) 30.14 o.anaee ae.a as wa Rcm�rmeoeeen,em x c. 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T T Qi Li 6 O Q Q Q Q W Z 0 L a W OW V� W a Q Q A N u 7 x> Q CL W L W? C f? C f C Q G N C • U Q OC W_ z_ W_ O cy W z_ ? a W J Q 4 O O S yY1 i F W Y. L C Y. Q CQ Q L d F F W 1' U Q Q W � C. C C z W U Q W N W uU l'1 Q Y C p` z yCj 1' z � W z z W z Y y z j � FFF nFaFz F � ra z O O u L W N F O g 0 a O W W O F p Wp, c C a O W 6 Q H C >> O O C D OC O L f WE O 4 OJ 0 W N C Y.O Q J OJOJ d 4 O „LL„60 W L W 6 F z W Q 6 F y f C z W C O z F OO z N z w O O N 1 F z Y F 6 C O z Q Z z to z u ' @ z_ Q W o � W Q C i Runoff Volume and Curve Number Calculations -_---__--_--__--_- _-_ ................... Enter design storm rainfall depths(in): 1-yearrtorm 2-year storm 30-ye 9mr, B.BB B.Bz s.m Vse NOM Atlas lO (http //M+cnus.rmw.9u Mtl /PfrNn "Notm (sale beluwl: - -- - -- -- [3]Thecurvenumbersandmnohwlumes computed in this special for each drainage area are limited in their applicabillN for determining and demanaratinp compliance with water quantity requirements. See VRRM User's Guide and Documentarian for additional inhumation. [2] Runoff Volume [RV) for preandpost- development drainage arem must be in walumetiic units le.g.,acmfeet or rabic het) when using the Emig, Balance Equation. Runoff measuretlin watershe6 inches and shown in the spreadshreat t as RVlwershedinch) ran only he used in the Energy Balance Grumman when the pre- and pmtdevelopment drainage areas are mull. Otherwise llVlsea. IinN) be multiphad bythe damage area. [3] Adjusted CNsare based on runoff reduction vdumesas calculated in O.A.tabs. An alternative CN adimmort[alculation for Vegetated Ranh is included in BMe specifation No. 5. --------- Drainage Area Curve Numbers and Runoff Depths Curve numbers (CN, Mobil and runoff depths (NV prsrygm) are computed with and without reduction practices. Drainage Area A ASoils B Soils CSails D Soils Total Arm(aoesh 16.22 ForesVnpen Space- untlktuNed, recorded faresbropen spare or reforested land Area acres CN B.a] 3B O.BB 55 B.a] ID O.a] T] Runoff ReduRion Volumeh 30,108 armored Turf - disturbed, graded for yards w other but to be mowed/man ed 1 AmTres) CN O.BB 39 SN. fil 70 No impetuous toner Arealaaes) CN o.BB 98 1B.42 9B O.BB 98 O.BB 98 Me A.a 1-year storm 2-year storm 10-year storm RVcevdro•d (watershed -inch) with no Runoff RetluRion" 1.59 u] B.BB RVor dw-I(wate[shed-Inch) wi[M1 Punaff RetluRion" 1.0e 1.81 8.19 Atljurtetl CN" n ]a n Yee Notes above Drainage Area B ASoils B soils CSails D Soils Total Area(aoes): 2.23 Forest/Open Space- untlktuNed, recorded fareshopen spsommireforested I Ama(a[res) CN Got BB — 55 O.a] m B.BB n Runoff ReduRion volume h 11,432 ebarW Manaretl Ter( -to be mr-rufmanaryaNs vother tun to be mowed/man prea a[ 39 Ogs Bhi B.a] 70 @] Impervious Cover Area a[ jw CNfe O.OB 98 4 6.0J 9B B.W 98 B.W 98 _-- CNloz el 1-year storm 2-year storm 10-year storm RVav}yy-(waershed-inch) with no Runoff RetluRion" 235 211 4. -,- RVorwelwd(watershed-inch) with Runoff RetluRion"O.BO Atljurtetl Chile as e9 an ' --- ge C Draina Area ''� ASoilz BSoilz CSails DSoilz Total Arealaneq: 0.00 - -____-- ForexVopen Space -untlmurbetl,recorded breSVopen sparace or reforested!or Area a [acre O.W 30 55 o55 Op"0- ]R RandallVolume Volume R': 0 Managetl Tor(-tobe cretl,gmtletl o sed! dother turf to be mowetl/man H Nee (acres) Cis O.W 39 oat fil . O74 U.O] ]0 @] a.00 a.00 98 98 CNlozd -_ 16,ar storm ---___ r.or r.ao -- o --- Imperviom Corer Nealane:) CN a.00 98 T. T. _- RVp"�p� (wmrshesh) witM1 na Punaff RetluRion" Rv ...mp.d(wate[shed-lash) with Runoff ReduRion" Atljurtetl CN" 1,mar storm ''2ryear storm dodo rra tram r.or o a Kee Not. above Drainage Area D ASoilz B Soils CSails D Soils Total Ara lanes): 0.00 PoresVopen Spate - untllrtuNetl,land etlforest/open space r reiterated!arW Nee a [N Odp 30 o.W 55 O.O] oa a.O] ]] RunoR ReduRlon Volume(R': 0 Managed Tor(-tobe cretived/ma o cal nother turl to be mowetl/manages Nee lanes) CN O.W 39 odi fil O.O] ]a @] Impervious Cover Nealane:) CN otoo 98 o. 9B sad 98 atoo 98 CNryaol 0 1-year storm 2-year storm 10-year storm Maa•tleRpetl (wmrshed-lash) with na Runoff RetluRion" a.ar RVeg.e (watefshedJnsh) with Runoff RetluRion" a.ar rra road _ -- Adjusted Me o a o Kee Not. above I coinage Area E ASoilz B Soils CSails D Soils TotalAm(aoez): 0.00 finest/open Space— undlstorbed corrected forrojopen Twe or reforested land Nealanes) cx 0.00 3o 0.0o Ss a.0o ID a.0o n RumNReduNon Vnlume R : 0 Montreal Tuff— di:Whed graded for yard: or other ton to be mowee/manager prey anes Cis39 road oao fil a.0o ]a a.0o corm N a9M . od 9. sa d roada[fe toIngenuous CNluw9 O 1-year storm 2-year storm 10-year storm -- RV,_,,(watenhed-idsh) with no Runoff Redurtion" roar Ora r.. RVpMlapm (ware[shed-Insh/wi[M1 Runoff Redurtion" roar rra r.ao I Adjurted CN" o a o -_ "See Nofez above I a11� OlggS 0 o m M¢ . 0 6 a 9 d a ^ O pN O ry O � — c — c a s s c V O C O Q $ y > rc a A 6 IL H O O O V C V d G I'I� �- )�! ��i !! k\ ! ] } ƒ ƒ /\{ � , I |)` �|E ! 2||| / �\ Ea I V m O O p o p O O p o p o G O p R p R O q O O O O O 0 O O G p O p O p O p O p O m Li V m A o o$$ d o o$$ 0 0 o?$ 0 0 o N O A q Y C LA O O q LA C gg yL q yy�� a T K T K T V Q m 2 ry, L a 0 p L 0 0 c _ 6 V c N ¢ O � T Level Spreader'B' Calculations LEVEL SPREADER W DESIGN Per VSMH Chpt. 11, Section 11.5.3 Step 1 Area = 7.23 ac. (Eqn 11.11) Tv= 22,864 cf Q,= 0.87 watershed inches (From Worksheet 2) CN = 94 Step 2 Tc = 0.10 hrs Step 3 la = 0.13 la / P = 0.13 q"= 1000 csm/in A = 0.011 m i z q PTv= 9.8 cfs NOTES: q P, is the Treatment Volume peak discharge, or the peak flow rate for the 1-inch rainfall event. The level spreader receives this Treatment Volume runoff via a separate pipe outfall. Per the VA DEQ Specification #2, page 14, the Level Spreader should be designed per the following: "13 feet of Level Spreader length per every 1 cubic foot per second (cfs) of inflow for discharges to a Vegetated Filter Strip or Conserved Open Space consisting of native grasses or thick ground cover." The proposed vegetated filter strip located below subarea B's Underground Detention System 'B' will predominantly consist of native grasses and will have a thick ground cover. Therefore a 13' per 1 cfs design is applicable to Level Spreader'B'. Site Specific LS length -to -Flow Ratio = 13 ft per 1 cfs Min. Level Spreader'A' Design Length= 128 ft Notes: Max. LS Length = 130 ft Min. LS Length = 13 ft Proposed LS Length = 128 ft