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HomeMy WebLinkAboutWPO202000048 Calculations 2020-12-07MILL CREEK INDUSTRIAL PARK- LOT 11 STORMWATER MANAGEMENT CALCULATIONS PACKET Date of Calculations SEPTEMBER 21, 2020 Revised on DECEMBER 4, 2020 Lip N It. c'b� L4 0357g1 ti 12/4120 aJ x ENS .. o COLLINS ENGINEiL:-kiNG 200 GARRETT STREET, SUITE K CHARLOTTESVILLE, VA 22902 434.293.3719 PH 434.293.2813 FX www.collins-engineering.com Summary of this Report's Dec. W1 Changes 1. Sheet 1 was revised to include a revision date and a professional engineer's seal. 2. This sheet has been added to the report. 3. SCS TR-55 Worksheets 2 & 4 and the routing calculations were revised to account for additional areas captured and detained in the vicinity of the proposed entrance. 4. The section entitled Structure EX-1's Peak Surface Ponding Depth Analysis was added to this report. FYI: After the updates to item #3 above were completed, the impacts to the post -development peak outflows were minimal. The change in peak outflows for the 1-, 2- and 10-year design storms were +0.01 cfs, -0.15 cfs and +0.50 cfs, accordingly. Minimum channel and flood protection requirements continue to be met. SCS TR-55 Calculations Soils Mapping (Source: NRCS Web Soil Survey Online Database) M-,mk r � s X h� r► Soils Table (Source: NRCS Web Soil Survey Online Database) Albemarle County, Virginia (VA003) Map Unit Symbol Map Unit Name percent of AOL 13D Catoctin silt 89.6% loam, 15 to 25 percent slopes, very stony 39D Hazel loam, 15 8.0% to 25 percent slopes 59C Myersville very 2.5% stony silt loam, 7 to 15 percent slopes Totals for Area of 100.0% Interest The NRCS web soil survey defines this development's soils as predominantly having a hydrologic group type B. Precipitation Data (Source: NCAA's National Weather Service, Hydrometerorological Design Studies Center, Atlas 14 Point Precipitation Frequency Estimates) n Map ❑� Tertain n A a rBOorY Cq I I�a 9 y'oo a � Rd Ooo� c a° e o� S� 4i+ ye o� P 100m Oft °ro Fka,Y o L 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)' Avpage rsanaw:e Marva' (rears) FFRIIJ 3.04 (2. 33A1) 3.69 4.71 5.57 (3.311.13) (a.22d27) (d.97b.22) 6.81 7.9d 9.15 10.5 12.5 14.2 (6.W-7.6n (6.998.82) (/.99-10.1) (9.05-11.fi) (10.6-13.8) (11.9-15.7) v a v v C Y 0o w 01 u a v 0 0 a v N 2 0 � n m N U II � a G m v � v a 3 m o z a u x U o y m In u1 o m m m m ry Y, a � a 0 u` �^ o m In o m m to r to z u ao vl ry In .. m m m n to m In In .. m m In to o u n a 2 a � { o c .2 a o. v E c .2 o a S v w L c a v � c o c Y U v E c m c o c � m o > j a 0 c a a v E m v o c a n n o>a L m v C ^ a O j m n ¢ o < o c o 0 a 0 K R U.S. Department of Agriculture Natural Resources Conservation Service TR 55 Worksheet 3: Time of Concentration (TJ or Travel Time (TJ Project: Mill Creek Industrial Park Designed By: FGM, PE Location: Lot 11 on Stoney Ridge Rd. Checked By: SRC, PE Check One: Present X Developed X Check One: T, X Tt Through subarea n/a Segment ID: Sheet Flow: (Applicable to Tc only) 1 Surface description (Table 3-1) 2 Manning's roughness coeff., n (Table 3-1 3 Flow length, L (total L < 100) (ft) 4 Two-year 24-hour rainfall, Pz (in.) 5 Land slope, s (ft/ft) 6 Compute Tt = ]0.007(n*L)0e] / PZ .s s°a Shallow Concentrated Flow: 7 Surface description (paved or unpaved) 8 Flow Length, L (ft) 9 Watercourse slope, s (ft/ft) 10 Average velocity, V (Figure 3-1) (ft/s) 11 T,= L/3600*V Channel Flow: 12 Cross sectional flow area, a (ft) 13 Wetted perimeter, P. (ft) 14 Hydraulic radius, r = a/Pw (ft) 15 Channel Slope, s (ft/ft) 16 Manning's Roughness Coeff, n 17 V= [ 1.49r2/3s05 ] / n 18 Flow length, L (ft) 19 Tt= L / 3600*V 20 Watershed or subarea Tcor Tt (Add Tt in steps 6, 11 and 19) FL-ENG-21A 06/04 Date: 9/21/2020 Date: 9/21/2020 DA A DA A (Pre-Dev.) (Post-Dev.) Woods- Light Woods- Light Underbrush Underbrush 0.4 0.4 100 100 3.68 3.68 0.280 0.280 0.12 0.12 Unpaved Unpaved 250 105 0.120 0.276 5.6 8.5 0.01 0.00 0.13 1 0.12 U.S. Department of Agriculture Natural Resources Conservation Service Project: Location: Check One: TR 55 Worksheet 4: Graphical Peak Discharge Method Mill Creek Industrial Park Lot 11 on Stoney Ridge Rd. Present X Developed X Designed By: FGM, PE Date: 12/4/2020 Checked By: SRC, PE Date: 12/4/2020 1. Data Drainage Area Description Drainage Area Description Drainage Area Description DA A (Pre-Dev.) DA A (Post-Dev.) Drainage Area (Am) in milesz = 0.0021 0.0021 Runoff curve number CN= 64.1 73.8 Time of concentration (Tc)= 0.13 0.12 Rainfall distribution type= II II Pond and swamp areas spread throughout the watershed= 0 0 2. Frequency -years 1 2 10 1 2 30 3. Rainfall, P (24 hour)- inches 3.04 3.68 5.57 3.04 3.68 5.57 4. Initial Abstraction, la- inches 1.12 1.12 1.12 0.71 0.71 0.71 5. Compute la/P 0.37 0.30 0.20 0.23 0.19 0.13 6. Unit peak discharge, Qu- csm/in 800 875 900 900 925 950 7. Runoff, Qfrom Worksheet 2- inches 0.49 0.80 1.97 0.92 1.35 2.81 8. Pond and Swamp adjustment factor, Fp 1 1 1 1 1 1 9. Peak Discharge, Qp- cfs where Qp=Qu Am Q Fp See SCS Unit Hydrographs See Dry Detention Basin Routings WATERSHED SUMMARY FOR ENERGY BALANCE CALCULATIONS CN ac. cfs cfs cfs FDA A (Pre-Dev.) 64.1 1.37 0.98 1.61 4.26 DA A (Post-Dev.) 73.8 1.37 0.41 0.87 3.60 Channel and Flood Protection Computations 9 VAC 25-870-66 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 Developed !� I•F• * (Q Pre -Developed * RV Pre -Developed ) / RV Developed Q Allowed Where: Q Pre -Developed = CN Pre -Developed = S Pre -Developed = RV Pre -Developed= CN Post -Developed= S Post -Developed = RV Post -Developed — I.F. = Q Developed < 0.42 cfs Section B.4 Limits of Analysis 0.98 cfs (From Worksheet 4 Watershed Summary) 64.1 (From Worksheet 4 Watershed Summary) 5.61 (From SCS TR-55 eq. 2-4) 0.49 in. (From SCS TR-55 eq. 2-3) 73.8 (From Worksheet 4 Watershed Summary) 3.55 (From SCS TR-55 eq. 2-4) 0.92 in. (From SCS TR-55 eq. 2-3) 0.80 (Factor of Safety) 0.41 cfs The channel protection analysis ends here since subsection B.3.a is met. 9 VAC 25-870-66 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." Section C.3 Limits of Analysis The flood protection analysis ends here since subsection C.2.b is met. Pre -Development Inflow Hydrographs (Pre-Dev. Subarea 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 (mil -in) DAA (Pre-Dev.) 0.13 0.13 0.37 0.0010 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 0.16 12.0 0.60 12.1 0.98 12.2 0.55 12.3 0.23 12.4 0.18 12.5 0.16 12.6 0.13 12.7 0.11 12.8 0.10 13.0 0.09 13.2 0.08 13.4 0.07 13.6 0.07 13.8 0.06 14.0 0.06 14.3 0.05 14.6 0.05 15.0 0.04 15.5 0.04 16.0 0.04 16.5 0.03 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 1.20 1.00 0.80 0.60 0.40 0.20 0.00 -0.20 SCS TR-55 Pre -Development Unit Hydrograph (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 (mi'-in) DAA (Pre-Dev.) 0.13 0.13 0.30 0.0017 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 0.26 12.0 0.98 12.1 2.61 12.2 0.90 12.3 0.37 12.4 0.30 12.5 0.26 12.6 0.22 12.7 0.18 12.8 0.17 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 0.08 15.0 0.07 15.5 0.07 16.0 0.06 16.5 0.05 17.0 0.05 17.5 0.05 18.0 0.05 19.0 0.04 20.0 0.03 22.0 0.03 26.0 0.00 1.80 1.60 1.40 1.20 1.00 0.80 0.60 0.40 0.20 0.00 SCS TR-55 Pre -Development Unit Hydrograph (DA A) 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 Below Subarea Tc(hrs.) Tt(hr.) la/P Am*Q (mi2-in) DAA (Pre-Dev.) 0.13 0.13 0.20 0.0042 Hydrograph Time 10-yr Discharges (hrs.) (cfs) 0.0 0.00 11.0 0.00 11.3 0.14 11.6 0.22 11.9 1.41 12.0 2.73 12.1 4.26 12.2 2.63 12.3 0.91 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 0.21 13.6 0.19 13.8 0.18 14.0 0.16 14.3 0.14 14.6 0.13 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 0.05 22.0 0.05 26.0 0.00 4.50 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50 0.00 0 SCS TR-55 Pre -Development Unit Hydrograph (DA A) Post -Development Routing Calculations (Post-Dev. Subarea A) Mill Creek Lot 11- Dry Basin Routings BasinFlow printout INPUT: Basin: Mill Creek Lot 11- Dry Detention Basin Routings 7 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 451.00 45.00 0.0 452.00 528.00 9.0 453.00 1501.00 45.0 454.00 2886.00 124.9 454.50 4014.00 188.5 455.00 2234.00 245.5 455.67 2725.00 307.0 Start—Elevation(ft) 6 Outlet Structures Outlet structure 0 Orifice 451.00 Vol.(cy) 0.00 name: Structure 3- Barrel area (sf) 3.142 diameter or depth (in) 24.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 446.670 multiple 1 discharge out of riser Outlet structure 1 Orifice name: Str. 4- Low Flow Orifice area (sf) 0.039 diameter or depth (in) 2.688 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 451.000 multiple 1 discharge into riser Page 1 Mill Creek Lot 11- Dry Basin Routings Outlet structure 2 Orifice name: Str. 8- Low Flow Orifice area (sf) 0.039 diameter or depth (in) 2.688 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 451.000 multiple 1 discharge into riser Outlet structure 3 Weir name: Structure 8- Riser Inlet diameter (in) 48.000 side angle 0.000 coefficient 3.300 invert (ft) 452.800 multiple 1 discharge into riser transition at (ft) 1.215 orifice coef. 0.500 orifice area (sf) 12.566 Outlet structure 4 Weir name: Structure 4- Riser Inlet diameter (in) 48.000 side angle 0.000 coefficient 3.300 invert (ft) 452.800 multiple 1 discharge into riser transition at (ft) 1.215 orifice coef. 0.500 orifice area (sf) 12.566 Outlet structure 5 Weir name: Basin A Emergency Spillway length (ft) 15.000 side angle 75.960 coefficient 3.300 invert (ft) 453.800 multiple 1 discharge into riser transition at (ft) 1.018 orifice coef. 0.500 orifice area (sf) 12.566 Page 2 Mill Creek Lot 11- Dry Basin Routings 4 Inflow Hydrographs Hydrograph 0 SCS name: 1-yr 24-hr SCS TR-55 Design Storm Area (acres) 1.370 CN 73.800 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) 1.552 peak time (hrs) 11.922 volume (cy) 169.911 Hydrograph 1 SCS name: 2-yr 24-hr SCS TR-55 Design Storm Area (acres) 1.370 CN 73.800 Type 2 rainfall, P (in) 3.680 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) 2.274 peak time (hrs) 11.922 volume (cy) 248.964 Hydrograph 2 SCS name: 10-yr 24-hr SCS TR-55 Design Storm Area (acres) 1.370 CN 73.800 Type 2 rainfall, P (in) 5.570 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) 4.722 peak time (hrs) 11.922 volume (cy) 516.841 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) OUTPUT: Mill Creek Lot 11- Dry Basin Routings 100-yr 24-hr SCS TR-55 Design Storm 1.370 73.800 2 9.150 0.1200 0.0200 30.000 1.00 true 9.988 11.922 1093.324 Routing Method: storage -indication Hydrograph 0 Routing Summary of Peaks: 1-yr 24-hr SCS TR-55 Design Storm inflow (cfs) 1.549 at 11.92 (hrs) discharge (cfs) 0.407 at 12.00 (hrs) water level (ft) 452.773 at 12.02 (hrs) storage (cy) 33.536 Hydrograph 1 Routing Summary of Peaks: 2-yr 24-hr SCS TR-55 Design Storm inflow (cfs) 2.270 at 11.92 (hrs) discharge (cfs) 0.868 at 11.96 (hrs) water level (ft) 452.831 at 11.96 (hrs) storage (cy) 36.255 Hydrograph 2 Routing Summary of Peaks: 10-yr 24-hr SCS TR-55 Design Storm inflow (cfs) 4.713 at 11.92 (hrs) discharge (cfs) 3.595 at 11.94 (hrs) water level (ft) 452.914 at 11.94 (hrs) storage (cy) 40.375 Hydrograph 3 Routing Summary of Peaks: 100-yr 24-hr SCS TR-55 Design Storm inflow (cfs) 9.971 at 11.92 (hrs) discharge (cfs) 9.836 at 11.94 (hrs) water level (ft) 453.034 at 11.94 (hrs) storage (cy) 46.941 Wed Dec 02 13:43:16 EST 2020 Page 4 Structure EX-1's Peak Surface Ponding Depth Analysis (Analysis was completed to confirm the 10-yr design storm will not pond at the entrance.) Str. EX-1: surface Noodling Analysis Time(hrs) 10-Year Surface Inflow(cfs) Ponding Depth(ft) 0.00 0.00 448.85 0.15 0.06 448.86 0.30 0.13 448.87 0.45 0.19 448.89 0.60 0.26 448.90 0.75 0.32 448.91 0.90 0.39 448.92 1.05 0.45 448.93 1.20 0.52 448.94 1.35 0.58 448.9S 1.50 0.65 448.96 1.65 0.71 448.97 1.80 0.78 448.97 1.95 0." 448.98 2.10 0.91 448.99 2.25 0.97 449.00 2.40 1.04 449.00 2.55 1.10 449.01 2.70 1.17 449.02 2.85 1.23 449.02 3.00 1.30 449.03 3.15 1.36 449.04 3.30 1.43 449.04 3.45 1.49 449.05 3.60 1.56 449.06 3.75 1.62 449.06 3.90 1.69 449.07 4.05 1.75 449.07 4.20 1.81 449.08 4.35 1.88 449.09 4.50 1.94 449.09 4.65 2.00 449.10 4.80 2.07 449.10 4.95 2.13 449.11 5.00 2.16 449.11 5.25 2.06 449.11 5.40 1.99 449.10 5.55 1.93 449.10 5.70 1.86 449.09 5.85 1.80 449.09 6.00 1.73 449.08 6.15 1.67 449.07 6.30 1.60 449.07 6.45 1.54 449.06 6.60 1.47 449.06 6.75 1.41 449.05 6.90 1.34 449.04 7.05 1.28 449.04 7.20 1.21 449.03 7.35 1.1S 449.02 7.50 1.09 449.02 7.65 1.02 449.01 7.80 0.95 449.00 7.95 0.89 449.00 8.10 0.82 448.99 8.25 0.76 448.98 8.40 0.69 448.97 8.55 0.63 448.97 8.70 0.56 448.96 8.85 0.50 448.95 9.00 0.43 448.94 9.15 0.37 448.93 9.30 0.30 448.92 9.45 0.24 448.91 9.60 0.17 448.90 9.75 0.11 448.89 9.90 a." 448.88 10.05 0.00 448.86 10.20 0.00 448.86 10.35 0.00 448.85 10.50 1 0.00 1 448.85 Str. EX-1 Surface Pending Depths (10-yr Design Storm) 609.15 1 "Hao 0.00 <- Area= 0.54 ac. Cw=0.6 Tc = 5 min. I so= 6.7 in/hr 2.00 4.00 600 am 10.00 12r) Notes: The peak backwater elevation of 449.11' (0.26') in this analysis does not encroach into the proposed entrance (low, point elevation of=453.00'i. This analysis is based on the elevations and areas located at existing Str. EX-1. An additional analysis, shown in the following sheet, has been provided confirming this analysis' accuracy. This second analysis is based an VWT Drainage Manual's Chapter 9, Appendix 9C-12. The two analyses are within 1" of each other. Chapter 9 — Storm Drains Appendix 9C-12 Grate Inlet Capacity in Sump Conditions t3r�ct`.wr+Tca Gwx7rar OrgrK LLCU' e e a 4 3 0.2 0,8 0.9 06 0 0.55 ■{ �� t� 1 ONE F T 3 4 3 6 6 10 20 30 40 30 60 w log 0,0 MCNAS6SE G IFf3/9) Grate inlet Capacity in Sump Condtions - English Units Note: See nomographs qc-13 thru 9c-16 for VDOT St'd. grate inlets Lz V '? - atw' t;� 8 y. Source: HEC No. 22 FHWA 1 of 1 VDOT Drainage Manual Water Quality Calculations Using the DEQ Virginia Runoff Reduction Method for New Developments (COMPLIANCE IS MET THROUGH THE BEST MANAGEMENT PRACTICE OF PURCHASING NUTRIENT CREDITS. THE VRRM BOUNDARY IS DEFINED BY THE PROPOSED ESC LIMITS OF DISTURBANCE.) 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