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HomeMy WebLinkAboutWPO202000037 Stormwater Management Maintenance Agree. 2022-04-29GALAXIE FARM STORMWATER MANAGEMENT CALCULATIONS PACKET (Amendment #2 to WPO 202000037) Date of Calculations APRIL 29, 2022 SCOTT/R. 05-iLINSZ Lic. No. 035791.11 4/29/22 tona1, G COLLINS ENGINEERING 200 GARRETT STREET, SUITE K CHARLOTTESVILLE, VA 22902 434.293.3719 PH 434.293.2813 FX www.collins-engineering.com Summary of VSMP Amendment #2 Proposed Changes: 1. A new riser (str. A14) and its outfall system have been added. This addition releases runoff into a new Level Spreader'A', which then directs sheet flow runoff into a new vegetated filter strip. This improvement provides stormwater quality treatment onsite for the subarea entitled Onsite DA W. The riser str. A14 includes a low -flow orifice that restricts the 1-year flow to channel protection threshold requirements for compliance. The str. A14 riser's outfall system contains 12" pipes, which were sized to handle the treatment volume. 2. Detention Basin 'A' and its riser, str. A6, have been modified with this amendment. These modifications, as detailed in the plans, account for additional runoff that is directed to the basin with this amendment. These modifications also further restrict the peak outflows, allowing this amendment to meet channel and flood protection requirements for both subareas A and B, not just subarea A. Additionally, this report now includes an analysis of the Cow Branch stream's watershed. An exhibit has been provided with this report, and calculations have been included that show the peak post -development flows in the Cow Branch stream are now less than the peak pre -development flows immediately downstream from the development. 3. The VRRM calculations have been updated and are in accordance with revisions #1 & 2 above. SCS TR-55 Calculations (Worksheets 2, 3 & 4 and Summaries of the Subareas) z 0 e � � u N ry N v � � 3 � V Z U r Z U p O O p OZ O p O O N O m Q °°Qem uNi $ e mvNi om+�vNi��ao umi v°�umi�v°�umi�v°� uNi�v°�vNi 2 E ae O j u a C o A u `'�O� N`o$ u u u 12 o Q Q a A o a o a p d g d g o o Z° Nam t� H mm N Via° N g m oo E g3 g p g 33E m3E m63lb 36 m3623 W p3 m o ¢ o a �a m m m m m m m m E � =_ m N o o o o= o 0 U.S. Department of Agriculture Natural Resources Conservation Service TR 55 Worksheet 3: Time of Concentration (T j or Travel Time (T j Project: Galaxie Farm Location: Galaxie Farm Ln. Check One: Present X Developed X Check One: T� X Tt Segment ID: Designed By: FGM, PE Checked By: SRC, PE Through subarea n/a FL-ENG-21A O6/04 Date: 4/29/222 Date: 4/29/222 Onsite DA A Onsite DA A Onsite DAB Onsite DAB Offsite DA C Offsite DA C Offsite DA D Offsite DA D (Pre-Dev.) (Post-Dev.) (Pre-Dev.) (Past-Dev.) (Pre-Dev.) (Past-Dev.) (Pre-Dev.) (Past-Dev.) Sheet Flow: (Applicable to T, only) Short Short Short Short Woods (Light Woods (Light Woods (Light Woods (Light 1 Surface description (Table 3-1) Grass Grass Grass Grass Underbrush) Underbrush) Underbrush) Underbrush) 2 Manning's roughness coeff., in (Table 3-1 0.15 0.15 0.15 0.15 0.4 0.4 0.4 0.4 3 Flow length, L(total L< 100)(ft) 100 1DO 100 45 100 100 LOD LOD 4 Two-year 24-hour rainfall, P'(in.) 3.67 3.67 3.67 3.67 3.67 3.67 3.67 3.67 5 Land slope, s (ft/ft) 0.11 0.06 0.06 0.18 0.20 0.20 0.20 0.20 6 Compute Tt=)0.00](n•L)"]/P,"s" 0.08 0.10 0.10 0.03 0.13 0.13 0.13 0.13 Shallow Concentrated Flow: Surface description(paved or unpaved) Unpaved Unpaved Unpaved Unpaved Unpaved Unpaved Unpaved 8 Flow Length, L (ft) 250 225 425 1875 1875 2225 2225 9 Watercourse slope, s (ft/ft) 0.14 0.06 0.09 0.42 0.42 0.40 0.40 10 Average velocity, V (Figure 3-1) (ft/s) 6.0 3.9 4.8 9.2 9.2 9.1 9.1 11 Tt= L / 3600'V 0.01 0.02 0.02 0.06 0.06 0.07 0.07 Channel Flow: 12 Cross sectional flow area, a(ft) o o o o o Ze o un m u. 13 Wetted perimeter, P. (ft) w m w m w x °' 14 Hydraulic radius, r=a/P„.(ft) c E c c E un m u' + m u' + 15 Channel Slope, s(ft/ft) tj q u A u ti u ti u ry u ry 16 Manning's Roughness Coeff, in `o Nn t 0 ` wo Not o o a o a 0 0 0 0 17 V= I lA9r'As" ]/ n n o 4 E °a o. p o n E o. 5 a a > v > v 18 Flow length, L (ft) '^ = iO F. t a v o 4 0 ri 0 19 Tt= L/36W*V F 0.03 0.00 0.00 0.2] 0.2] 0.14 0.14 20 Watershed or subarea T.or Tt (Add Tt in steps 6, 11 and 19) 0.12 0.12 0.12 0.10 0.46 0.46 0.34 0.34 Note: The shoretest SCS Tc is 0.10 hrs. CC CC EE Y .Y % � 33 � z y S ;'z �� �8 �� Es �� o g. �.Q �� a� OVERALL FLOWS TO COW BRANCH STREAM (AT POA) (USED TO CONFIRM POST -DEVELOPMENT FLOWS ARE LESS THAN PRE -DEVELOPMENT FLOWS DOWNSTREAM FROM THE GALAXIE FARM DEVELOPMENT) Area, 1-year Flow, 2-year Flow, 10-year Flow, Pre -Development CN ac. cfs cfs cfs Onsite DA A (Pre-Dev.) 64 6.40 4.45 7.29 18.02 Onsite DA B (Pre-Dev.) 64 3.00 2.10 3.43 8.46 Offsite DA C (Pre-Dev.) 58 226 13.58 27.35 68.39 Offsite DA D (Pre-Dev.) 57 146 7.95 17.72 47.84 Avinity I Routings 61.9 35.37 3.41 4.74 37.20 Avinity 11 Routings 80.5 7.95 1.04 2.09 8.50 Spring Hill Village DA'A' Routings 74.4 19.10 2.34 12.60 39.91 Spring Hill Village DA 'B' 1 72 12.25 7.41 11.54 25.66 59 456.07 42.28 86.75 253.99 Post -Development CN Area, ac. 1-year Flow, cfs 2-year Flow, cfs 10-year Flow, cfs Onsite DA A (Post-Dev.) 78 8.67 0.35 6.69 19.95 Onsite DA B (Post-Dev.) 83 0.73 1.73 2.34 4.28 Offsite DA C (Post-Dev.) 58 226 13.69 27.48 68.47 Offsite DA D (Post-Dev.) 57 146 7.95 17.72 47.84 Avinity I Routings 61.9 35.37 3.41 4.74 37.20 Avinity II Routings 80.5 7.95 1.04 2.09 8.50 Spring Hill Village DA'A' Routings 74.4 19.10 2.34 12.60 39.91 Spring Hill Village DA 'B' 1 72 12.25 7.41 11.54 25.66 60 456.07 37.92 85.21 251.81 ONSITE SUBAREAS A & B (USED IN THE CHANNEL & FLOOD PROTECTION CALCULATIONS) Area, 1-year Flow, 2-year Flow, 10-year Flow, Pre -Development CN ac. cfs cfs cfs Onsite DA A (Pre-Dev.) 64 6.40 4.45 7.29 18.02 Onsite DA B (Pre-Dev.) 64 3.00 2.10 3.43 8.46 64 9.40 6.55 10.72 26.49 Post -Development CN Area, 1-year Flow, 2-year Flow, 10-year Flow, Onsite DA A (Post-Dev.) Onsite DA B (Post-Dev.) 78 83 8.67 0.73 0.35 1.73 6.69 2.34 19.95 4.28 79 9.40 2.08 9.03 24.23 Channel and Flood Protection Calculations 9 VAC 25-870-66 (Onsite DA A + 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: Q Allowed (Calcuated( 15 1.F• * (Q Pre -Developed * RV Pre -Developed) I RV Developed Q Allowed (Calcuated( < Where: Q Pre -Developed = CN Pre -Developed= 5 Pre -Developed — RV Pre -Developed = CN Post -Developed — 5 Post -Developed — RV Post-Developed— I.F. _ Q Developed 2.10 cfs 6.55 cfs (From Worksheet 4 Watershed Summary) 64 (From Worksheet 4 Watershed Summary) 5.69 (From SCS TR-55 eq. 2-4) 0.48 in. (From SCS TR-55 eq. 2-3) 79 (From Worksheet 4 Watershed Summary) 2.73 (From SCS TR-55 eq. 2-4) 1.19 in. (From SCS TR-55 eq. 2-3) 0.80 (Factor of Safety) 2.08 cfs Section B.4 Limits of Analysis The analysis terminates at the detention faclity since "...subsection 3 of this subsection is utilized to show compliance with the channel protection criteria..." 9 VAC 25-870-66 (Onsite DA A + B) 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.b.: The point of discharge "releases a postdevelopment peak flow rate for the 10-year 24-hour storm event that is less than the predevelopment peak flow rate from the 10-year 24-hour storm event. Downstream stormwater conveyance systems do not require any additional analysis to show compliance with flood protection criteria if this option is utilized." 10-Year, 24-Hour Pre -Development Peak Flow Rate = 26.49 cfs 10-Year, 24-Hour Post -Development Peak Flow Rate = 24.23 cfs Section C.3 Limits of Analysis The flood protection analysis terminates at the SWM facility since subsection C.2.b is met. Hydrographs Worksheet 5b (Modified for Clarity): Basic watershed Data 1-yr. 24-hr. SCS TR-55 Method Unit Hydrograph for the la/P & Tc Listed Below Subarea Tc (hrs.) Tt (hr.) la / P Am;Q (mi2-in) Onsite DA A (Pre-Dev.) 0.12 0.12 0.37 0.0048 Hydrograph Time 1-yr Discharges (hrs.) (cfs) 0.0 0.00 11.0 0.00 11.3 0.00 11.6 0.00 11.9 0.73 12.0 2.70 12.1 4.45 12.2 2.49 12.3 1.03 12.4 0.82 12.5 0.71 12.6 0.60 12.7 0.51 12.8 0.46 13.0 0.41 13.2 0.36 13.4 0.33 13.6 0.30 13.8 0.28 14.0 0.25 14.3 0.23 14.6 0.22 15.0 0.20 15.5 0.18 16.0 0.16 16.5 0.15 17.0 0.14 17.5 0.13 18.0 0.13 19.0 0.11 20.0 0.10 22.0 0.09 26.0 0.00 5.00 4.50 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 -0.50 SCS TR-55 Pre -Development Unit Hydrograph (Onsite DA A) 30.0 Worksheet 5b (Modified for Clarity): Basic watershed Data 2-yr. 24-hr. SCS TR-55 Method Unit Hydrograph for the la/P & Tc Listed Below Subarea Tc (hrs.) Tt (hr.) la / P Am;Q (mi2-in) Onsite DA A (Pre-Dev.) 0.12 0.12 0.31 0.0078 Hydrograph Time 2-yr Discharges (hrs.) (cfs) 0.0 0.00 11.0 0.00 11.3 0.00 11.6 0.00 11.9 1.20 12.0 4.42 12.1 7.29 12.2 4.08 12.3 1.69 12.4 1.34 12.5 1.16 12.6 0.98 12.7 0.83 12.8 0.76 13.0 0.67 13.2 0.59 13.4 0.54 13.6 0.49 13.8 0.45 14.0 0.41 14.3 0.37 14.6 0.36 15.0 0.33 15.5 0.30 16.0 0.26 16.5 0.24 17.0 0.23 17.5 0.22 18.0 0.21 19.0 0.19 20.0 0.16 22.0 0.15 26.0 0.00 SCS TR-55 Pre -Development Unit Hydrograph (Onsite DA A) 8.00 7.00 6.00 5.00 4.00 3.00 2.00 1.00 0.00 0.0 5.0 10.0 15.0 20.0 25.0 30.0 Worksheet 51b (Modified for Clarity): Basic watershed Data 10-yr. 24-hr. SCS TR-55 Method Unit Hydrograph for the la/P & Tc Listed Belov Subarea Tc (hrs.) Tt (hr.) la / P Am`Q (mil -in) Onsite DA A (Pre-Dev.) 0.12 0.12 0.21 0.0193 Hydrograph Time 10-yr Discharges (hrs.) (cfs) 0.0 0.00 11.0 0.00 11.3 0.00 11.6 0.00 11.9 2.97 12.0 10.94 12.1 18.02 12.2 10.09 12.3 4.18 12.4 3.31 12.5 2.87 12.6 2.43 12.7 2.06 12.8 1.87 13.0 1.66 13.2 1.46 13.4 1.33 13.6 1.21 13.8 1.12 14.0 1.02 14.3 0.92 14.6 0.89 15.0 0.81 15.5 0.73 16.0 0.65 16.5 0.60 17.0 0.58 17.5 0.54 18.0 0.52 19.0 0.46 20.0 0.39 22.0 0.37 26.0 0.00 LU.UU 18.00 16.00 14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 0 SCS TR-55 Pre -Development Unit Hydrograph (Onsite DA A) Worksheet 5b (Modified for Clarity): Basic watershed Data 1-yr. 24-hr. SCS TR-55 Method Unit Hydrograph for the la/P & Tc Listed Below Subarea Tc (hrs.) Tt (hr.) la / P Am;Q (mi2-in) Onsite DA B (Pre-Dev.) 0.12 0.12 0.37 0.0022 Hydrograph Time 1-yr Discharges (hrs.) (cfs) 0.0 0.00 11.0 0.00 11.3 0.00 11.6 0.00 11.9 0.34 12.0 1.27 12.1 2.10 12.2 1.17 12.3 0.49 12.4 0.38 12.5 0.33 12.6 0.28 12.7 0.24 12.8 0.22 13.0 0.19 13.2 0.17 13.4 0.15 13.6 0.14 13.8 0.13 14.0 0.12 14.3 0.11 14.6 0.10 15.0 0.09 15.5 0.09 16.0 0.08 16.5 0.07 17.0 0.07 17.5 0.06 18.0 0.06 19.0 0.05 20.0 0.04 22.0 0.04 26.0 0.00 2.50 2.00 1.50 1.00 0.50 0.00 0.0 -0.50 SCS TR-55 Pre -Development Unit Hydrograph (Onsite DA B) 5.0 10.0 15.0 20.0 25.0 30.0 Worksheet 5b (Modified for Clarity): Basic watershed Data 2-yr. 24-hr. SCS TR-55 Method Unit Hydrograph for the la/P & Tc Listed Below Subarea Tc (hrs.) Tt (hr.) la / P Am;Q (mi2-in) Onsite DA B (Pre-Dev.) 0.12 0.12 0.31 0.0037 Hydrograph Time 2-yr Discharges (hrs.) (cfs) 0.0 0.00 11.0 0.00 11.3 0.00 11.6 0.00 11.9 0.56 12.0 2.08 12.1 3.43 12.2 1.92 12.3 0.79 12.4 0.63 12.5 0.55 12.6 0.46 12.7 0.39 12.8 0.36 13.0 0.31 13.2 0.28 13.4 0.25 13.6 0.23 13.8 0.21 14.0 0.19 14.3 0.18 14.6 0.17 15.0 0.15 15.5 0.14 16.0 0.12 16.5 0.11 17.0 0.11 17.5 0.10 18.0 0.10 19.0 0.09 20.0 0.07 22.0 0.07 26.0 0.00 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 -0.50 SCS TR-55 Pre -Development Unit Hydrograph (Onsite DA B) 30.0 Worksheet 5b (Modified for Clarity): Basic watershed Data 10-yr. 24-hr. SCS TR-55 Method Unit Hydrograph for the la/P & Tc Listed Below Subarea Tc (hrs.) Tt (hr.) la / P Am•Q (mil -in) Onsite DA B (Pre-Dev.) 0.12 0.12 0.20 0.0090 Hydrograph Time 10-yr Discharges (hrs.) (cfs) 0.0 0.00 11.0 0.00 11.3 0.00 11.6 0.00 11.9 1.39 9.00 12.0 5.14 8.00 12.1 8.46 7.00 12.2 4.74 6.00 12.3 1.96 5.00 12.4 1.56 4.00 12.5 1.35 3.00 12.6 1.14 2.00 12.7 0.97 1.00 12.8 0.88 0.00 13.0 0.78 -1.00 13.2 0.69 13.4 0.62 13.6 0.57 13.8 0.52 14.0 0.48 14.3 0.43 14.6 0.42 15.0 0.38 15.5 0.34 16.0 0.31 16.5 0.28 17.0 0.27 17.5 0.25 18.0 0.24 19.0 0.22 20.0 0.18 22.0 0.17 26.0 0.00 SCS TR-55 Pre -Development Unit Hydrograph (Onsite DA B) 30.0 Worksheet 51b (Modified for Clarity): Basic watershed Data 1-yr. 24-hr. SCS TR-55 Method Unit Hydrograph for the la/P & Tc Listed Belov Subarea Tc (hrs.) Tt (hr.) la / P Am*Q (m1 2-in) Onsite DA B (Post-Dev.) 0.10 0.10 0.13 0.0017 Hydrograph Time 1-yr Discharges (hrs.) (cfs) 0.0 0.00 11.0 0.04 11.3 0.06 11.6 0.09 11.9 0.57 12.0 1.11 12.1 1.73 12.2 1.07 12.3 0.37 12.4 0.25 12.5 0.21 12.6 0.18 12.7 0.15 12.8 0.13 13.0 0.11 13.2 0.10 13.4 0.09 13.6 0.08 13.8 0.07 14.0 0.07 14.3 0.06 14.6 0.05 15.0 0.05 15.5 0.04 16.0 0.04 16.5 0.04 17.0 0.03 17.5 0.03 18.0 0.03 19.0 0.03 20.0 0.02 22.0 0.02 26.0 0.00 2.00 1.80 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 SCS TR-55 Post -Development Unit Hydrograph (Onsite DA B) 0.0 5.0 10.0 15.0 20.0 2s.0 30.0 Worksheet 51b (Modified for Clarity): Basic watershed Data 2-yr. 24-hr. SCS TR-55 Method Unit Hydrograph for the la/P & Tc Listed Belov Subarea Tc (hrs.) Tt (hr.) la / P Am*Q (mi2-in) Onsite DA B (Post-Dev.) 0.10 0.10 0.11 0.0023 Hydrograph Time 2-yr Discharges (hrs.) (cfs) 0.0 0.00 11.0 0.06 11.3 0.08 11.6 0.12 11.9 0.77 12.0 1.50 12.1 2.34 12.2 1.44 12.3 0.50 12.4 0.34 12.5 0.29 12.6 0.24 12.7 0.20 12.8 0.18 13.0 0.15 13.2 0.13 13.4 0.12 13.6 0.11 13.8 0.10 14.0 0.09 14.3 0.08 14.6 0.07 15.0 0.07 15.5 0.06 16.0 0.05 16.5 0.05 17.0 0.05 17.5 0.04 18.0 0.04 19.0 0.03 20.0 0.03 22.0 0.03 26.0 0.00 2.so 2.00 1.50 1.00 0.50 0.00 SCS TR-55 Post -Development Unit Hydrograph (Onsite DA B) 0.0 5.0 10.0 15.0 20.0 25.0 30.0 Worksheet 51b (Modified for Clarity): Basic watershed Data 10-yr. 24-hr. SCS TR-55 Method Unit Hydrograph for the la/P & Tc Listed Belov Subarea Tc (hrs.) Tt (hr.) la / P Am*Q (mi2-in) Onsite DA B (Post-Dev.) 0.10 0.10 0.07 0.0042 Hydrograph Time 10-yr Discharges (hrs.) (cfs) 0.0 0.00 11.0 0.10 11.3 0.14 11.6 0.22 11.9 1.41 12.0 2.74 12.1 4.28 12.2 2.64 12.3 0.92 12.4 0.62 12.5 0.52 12.6 0.44 12.7 0.36 12.8 0.32 13.0 0.28 13.2 0.24 13.4 0.22 13.6 0.19 13.8 0.18 14.0 0.16 14.3 0.14 14.6 0.14 15.0 0.12 15.5 0.11 16.0 0.10 16.5 0.09 17.0 0.08 17.5 0.08 18.0 0.08 19.0 0.06 20.0 0.06 22.0 0.05 26.0 0.00 4.50 SCS TR-55 Post -Development Unit Hydrograph (Onsite DA B) Proposed Detention Basin 'A' Routing Calculations Galaxie Farm- Basin Routings BasinFlow printout INPUT: Basin: Galaxie Farm- Basin Routings 7 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 473.50 145.00 0.0 474.00 2349.00 19.0 476.00 3406.00 230.9 478.00 4524.00 523.7 480.00 5342.00 888.7 482.00 6230.00 1316.8 484.00 7582.00 1827.6 Start—Elevation(ft) 7 Outlet Structures Outlet structure 0 Orifice 474.00 Vol.(cy) 19.00 name: Barrel- Str. AS area (sf) 0.785 diameter or depth (in) 12.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 469.900 multiple 1 discharge out of riser Outlet structure 1 Orifice name: Low -Flow Orifice in Str. A14 area (sf) 0.036 diameter or depth (in) 2.563 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 473.500 multiple 1 discharge into riser Page 1 Galaxie Farm- Basin Routings Outlet structure 2 Orifice name: Barrel- Str. A13 area (sf) 0.785 diameter or depth (in) 12.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 473.500 multiple 1 discharge out of riser Outlet structure 3 Orifice name: Mid -Flow Orifices area (sf) 0.743 diameter or depth (in) 11.670 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 479.750 multiple 5 discharge into riser Outlet structure 4 Weir name: diameter (in) side angle coefficient invert (ft) multiple discharge transition at (ft) orifice coef. orifice area (sf) Outlet structure 5 Weir name: diameter (in) side angle coefficient invert (ft) multiple discharge transition at (ft) orifice coef. orifice area (sf) Riser- Str. A6 48.000 0.000 3.300 481.500 1 into riser 1.215 0.500 12.566 Riser- Str. A14 24.000 0.000 3.300 481.500 1 into riser 0.608 0.500 3.142 in Str. A6 Page 2 Galaxie Farm- Basin Routings Outlet structure 6 Weir name: Emergency Spillway length (ft) 40.000 side angle 75.960 coefficient 3.300 invert (ft) 482.500 multiple 1 discharge through dam 4 Inflow Hydrographs Hydrograph 0 SCS name: 1-yr 24-hr SCS TR-55 Design Storm Area (acres) 8.670 CN 78.000 Type 2 rainfall, P (in) 3.040 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) 12.314 peak time (hrs) 11.922 volume (cy) 1347.931 Hydrograph 1 SCS name: 2-yr 24-hr SCS TR-55 Design Storm Area (acres) 8.670 CN 78.000 Type 2 rainfall, P (in) 3.670 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) 17.318 peak time (hrs) 11.922 volume (cy) 1895.682 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) Galaxie Farm- Basin Routings 10-yr 24-hr SCS 8.670 78.000 2 5.550 0.1200 0.0200 30.000 1.00 true 33.881 11.922 3708.707 TR-55 Design Storm 100-yr 24-hr SCS TR-55 Design Storm 8.670 78.000 2 9.130 0.1200 0.0200 30.000 1.00 true 68.561 11.922 7504.912 Page 4 Galaxie Farm- Basin Routings OUTPUT: Routing Method: storage -indication Hydrograph 0 Routing Summary of Peaks: 1-yr 24-hr SCS TR-55 Design Storm inflow (cfs) 12.293 at 11.92 (hrs) discharge (cfs) 0.354 at 13.38 (hrs) water level (ft) 479.681 at 13.82 (hrs) storage (cy) 826.300 Hydrograph 1 Routing Summary of Peaks: 2-yr 24-hr SCS TR-55 Design Storm inflow (cfs) 17.288 at 11.92 (hrs) discharge (cfs) 6.692 at 12.10 (hrs) water level (ft) 480.330 at 12.10 (hrs) storage (cy) 954.762 Hydrograph 2 Routing Summary of Peaks: 10-yr 24-hr SCS TR-55 Design Storm inflow (cfs) 33.822 at 11.92 (hrs) discharge (cfs) 19.946 at 12.06 (hrs) water level (ft) 482.304 at 12.06 (hrs) storage (cy) 1388.006 Hydrograph 3 Routing Summary of Peaks: 100-yr 24-hr SCS TR-55 Design Storm inflow (cfs) 68.441 at 11.92 (hrs) discharge (cfs) 67.583 at 11.94 (hrs) water level (ft) 482.989 at 11.94 (hrs) storage (cy) 1556.968 Thu Apr 28 14:39:59 EDT 2022 Page 5 Water Quality Calculations Using the DEQ Virginia Runoff Reduction Method for New Developments (Phosphorous removal rate requirements are met through the use of a level spreader discharging flows to a vegetated filter strip, and a prior nutrient credit purchase.) MQVirg bBuwffR C M&M New Qeuebpmem Cmpl Spreatl -Verdian 34 02 BMP Standards and Specifications M 2013 Draft BMP Standards and Specifications; Pro)ect Nam: Go ande Fard, VSMp Amendment N2 m�sme y®Ils Datee: 4/29/2022 ® hies enfe0esrynspecihsmions.n. 2013Drah Stdsa Spea elaWtian Mls (Site Information Project (Treatment Volume and Loads) Impamin.scontentions) Constants all pnNerl c3 ngmmes) 1.00 vrurallPI EMC(ini 0.26 gen IT NI EMC Img/N I.B6 ao IIL/eve1M 0A1 Pj Wu ancd 090 F.ren/OPan SWcaC—,(sored 000 se-Onai Re("ans") 000 is F.rert M ManapdTic Cnearlares) ].66 WeyMad Rv Muhl O.M %Managed lit 60M Im—.... Case, (sores) s.00 Rv to mon) 0.95 M lmpvNwe CO% Site Rmz (so—) sM RumlFUeffidents lRvl T 'N VDdume Os2]0 o.anaee ae.a as wa 4. Rcm�rmeoeeen,em x c. Ilxx-x�wr xeaernorp "�"`"°"�"1 eip1vj'a•• o�ini e^ai .�awAi naa.�M w .. mt a ziM r�m�w naa�..iq �aoa ^a'aoa om m..—�..����s — �;_ ,o.H�i.m w �o��00000momm� �o��00000momm� o��00000momm� o��0000©momm� u w.�.a .,.,.o�� �o��00000momm� o��000000000� �o��0000©momm� o��00000momm� o��00000momm� �o��00000momm� �o��000000000� �o,.� ro....�m,�,�.�.�o,.� a� a �..,..�.o.�.E.�,.,.a�.Eo o. � o� .�w� .....�.o.�.E.w..�.�......�.�... �o..o�m�,.,. o_. �d„� u W x a m w m m ry O n 0 00 0 00 O.6 F G W OC Q Q O O O O O Y Q O O O O O '888SSY � g g $ c 0000co QQ,oS chi a 00000x QoS o S S a E m p 0 o 0 0 0 0 0 0 o 0 0 m 0 m O o O 0 O 0 O 0 m 7 W � Q Soo ry ry in of Q m m °° a ry O ^. ry arvaom .� G � O ao 3 d G v e W v pox v O Q;o �o vi a ui m vmvi ai ni mna om� _ p D N N Y � .K Z. T T � W W W Q D a L a YI S_] Y� w UQAL U W� �� 0000 •..• W NNy uSW� x>p• 0W? O ScWQO Q) W ?F? Cij G N O 2 z :_ zCL O C 01J9 z U WO 0 l'1 Q. Y7C z W z1 z zz Y7 z o 0 FF a nFaF F �FaU Z O W W F O g 0 a O W O F p Wp, w C O W 6 Q H C O O O C O OC O L'� f OC O4 >> N Y.O J OJ Or OJ OJ �•60J W W Q a F z W a Q a u F y f C z W W C O z F OO z O z OD O O N O z Q Z z � z ' Q W o � W O C i Runoff Volume and Curve Number Calculations ................... Enter design storm rainfall depths(in): 1-yearrtorm 2-year storm 10-yearrtorm 3.00 3.fi) i55 Vse NVAT Atlas lO (help //M+cnus.rmw.Pu Mtl /Pfrt+/I --- "Notm (sale belowl: - -- -- -- -- [QThermarmmbersandmnaHvalumez computed in this special for each drainage area are limited in their applkabilM for determining and demannMinp compliance with water quanfity requirements. See VRRM Vzeh Guide and Documentation for ad]ibanal inhumation. [2] Runoff Volume [RV) for preandpost- development drainage areaz must be in valumebic units leg.,acmfeet or cubic het) when using the Erergy aalarce Equation. Runoff measured in wacershe6 inches and shown in the spreadsheet as RVlwatershedinch) ran only he used in the Energy balance Grumman when the pm- and pmtdevelopment drainage areas are equal. Other NVlsea. IinN) must be multiphad bythe drainage area. [3]Adryzted rm are based on mnaff reduction vdumezas calculated in D.A. tabs. An alternative CN adjustment calculation for Vegehted Ranh is included in EMP specification No. 5. --------- Drainage Area Curve Numbers and Runoff Depths Curve numbers (CN, Mobil and runoff depths (NV.A m) are computed with and without reduction practices. Drainage Area A ASoils B Soils CSails D Soils TOM Arm(acriah 8.67 ForesVDpen Space- undistuNed, recorded faresbropen spare or reforested land Area acres CN U.D] 3D Um 55 U.D] ID O.D] TF Run AN ReduRion Volume 1t 8,619 Manaretl Tud- disturbed, graded car yaNz w other but to be mowed/man ed 1 AmTrez) CN O.W 39 4b5 fil 70 NO impetuous corer Arealaaes) CN o.Db 98 qoz 9b O.Db 98 O.Db 98 CNIOA.N 1-year storm 2-year storm 10-year storm _ RVm_b,e (watershed -inch) with nor Runoff ReduRion" 1.1fi 1.fi3 3.1, RVormeP•e(waterched-Inchl with Runoff Retluation" b.ae 1.35 261 --_- Atljurtetl CN" T3 2a 3 Yee Notes shove Drainage Area B ASoils B Soils CSails D Soils TOM Arm(2s ): 0.00 Runoff ReduRlon Volumeh 0 read_ Forezt/Dpen Space- undistuNed, recorded hreshopen Tare, or reforested land I a(u,—) CN Rod 3D — 55 0-0 ID .W Tr Manaretl Tm(- to be mmed/manwrce bar yartlz vother turl to be mowetl/man ed Area ones CN Rod 39 Impervious Cover Area ac CN fe 0M 9e D.bD 9b 0M 98 O.Dp 98 CNlow el 1-year storm 2-year storm 10-year storm RVa }yy-(watershed-inch) with no Runoff ReductionD.Db D.bb b.Db NVorwOPeO (watershed -Inch) with Runoff RetluRion" b.Db b.bb Atljurtetl CN" o 1 b 1 o -_ "See Notes above -. -____-- Drainage Area C ASoils B Soils CSpils D Soils Total Arn(ames): 0.00 Forest/open Space- undedurbed, recorded brest/oldn Area (acres) O.W B.OB 0.. U.O] RunaHReduddon space or reforested! land Cal 30 55 ID ]] Volu 0 M anag.a tre-commedgropmenryarE car.m.rum Area(a[res) 0.00 0.00 0.00 0.O] to be mowetl/man H CN 39 fil 70 @] Impervious cover O+ea acres aSo 0.a0 aSo a.00 - CN 98 9B 98 98 ---- Chile, d 1-year storm 2-year storm 10-year storm RVpe*dye (warenhed-inch) witM1 no Runpff Reductiono.00 oao a.00 RVoc,alcd(warershad-Inch) witM1 Runpff Red uRion" B.Bd P.aP O.Bd Adjusted CM 0 R o __.. "See Notes above . Drainage Area D ASoils B Soils CSpils D Soils Total Arn(aoes): 0.00 rarest/Gran Space- undisturbed, recorded bresomoon space or reforested! land I yea(anes) [N 0.. m 0.00 55 0.O] ID 0.O] ]] RunoR ReduRlon Volume Z. 0 Managetl Iun-...men, gmtled for yards v other run to be mowetl/man Areala[res) CN 0.00 39 0.00 fil 0.O] ]0 0.00 &I Impervious cover Arealanes) CN O.Bg 98 P.OP 9B O.Bd 98 98 - '.. CNInk of 10 1 year storm 2-year storm 10-year storm RVoe,ro (Vatershedrdch)wth no Runpff ReductionBBd P.aP O.Bd RVorrelead(watmfiadlnch) With Runpff ReduRlon" 0.. P.aP O.Bd Adjusted Me 0 R o Kee Notes above DminW Area E B Soils CSpils D Soils TotalA"(anes): 0.00 ForesV0pen Space - untlbturbetlepmte[tetl brest/open spaceor reforested! Apra (a[res)055 Cal55 0.O] ID 0Tr ]] Runoff ReduRbn Volume 0 Managed Tm(- d,graddflandr yards tobernamed/ graded bryaNs vother turf to be mowetl/manages Prea (anes)hi Calfil 70 Impervious Crier Area(anes)a00 CN9R 98 aSo . aSo 98 ;�rm r storm 10-year storm 0 wth no Punpif ReduRion:amRVor..Iead(watershed-Inch) withRunpffReduRion"Adjusted CN"0 0 *See Notes above -.-____-- , \ ! § 5 ! ! | ! | k \ ) ) k § t \ ! 7 E ) k k )d'I 411 G I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I� L Ir 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$$ O O o$$ O O o?$ O O o A O A i mO O p o O p o O O p o O p o G O p R O p R O O q LA O V p� CL 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 'A' Calculations (Designed for the Treatment Volume Flow from the Onsite DA'A' subarea) LEVEL SPREADER'A' DESIGN Per VSMH Chpt. 11, Section 11.5.3 Step 1 Area = 8.67 ac. (Eqn 11.11) Tv= 17,239 cf Q,= 0.55 watershed inches (From Worksheet 2) CN = 78 Step 2 Tc = 0.12 hrs Step 3 la = 0.56 la/P= 0.56 q"= 450 csm/in A = 0.014 m i z q PTA= 3.3 cfs NOTES: q PT.is the Treatment Volume peak discharge, or the peak flow rate for the 1-inch rainfall event. The level spreader receives the initial/flash Treatment Volume runoff via a new 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 below Galaxie Farm's Level Spreader'A' 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'A'. Site Specific LS length -to -Flow Ratio = 13 ft per 1 cfs Min. Level Spreader'A' Design Length= 43 ft Notes: Max. LS Length = 130 ft Min. LS Length = 13 ft Proposed LS Length = 44 ft STP, A 13 oE�N X- ER'Dea. VAMt OrNSsrL SAS IMA-VAC-SC Qmumc-- (71j�aF 111d3`1 cF. * DER Tits --w". 1�I seei-z.ow �\.5"3; T r-Tj FLOVJ = 3.3cFs 5Tk,A15/?x?c QEs�cA� TO HANDLE _rkCATMG+? VOL✓ Mt E Fr64, O F- H/s4ila iE \AS-4-WG Tt+r- MpNNSN6'S FOAM\ALA F"oA STA, A\3 (S>O.s % i n= Q.0\'�,�A p1"a."' QiP C. FLotis MALL ontK a,l :PS, A OAS" ?IRV 'FwNs FALL At y"g cFS, fA \V''�9AAC7" s5 PN �'!, MK-rCN Ta'TKE T� FLau" A 1S" O1AM;7LR zS A I�kq'/ V\A7fK 'b THE T'j fW`J, TrE \a" D:%WtX6A IS t- n CN. AOOV. ZVWALL,81 THE rQo X-l7A' ASS ES FW1 FLa�v, PNi ovERA6E5 xN Flaw FMN 1ttE "C of A G�' O1AxE�"ER VA-LL $C OTAL),IEW V-MlK%X3 iisE HASIN B �oCE =z uL�xMATELY Sj QELERS�� fftRotc '�KC (2 Pz4E, FoR l'ht'S �EkSot$t A \a"AME7C0..S SELECIp To coNVE7 'f E T� LnW fv 114r- t-EuCL SCR RtlEk C F1EGCiAzZ-O FxL'TE2 4,7NZ4,