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HomeMy WebLinkAboutWPO201900050 Calculations 2021-05-24BERKMAR OVERLOOK STORMWATER MANAGEMENT CALCULATIONS PACKET UP No. 35791 5/20/21 10NAI, Amendment #2 Date of Amendment #2 Calculations May 20, 2021 PREPARED BY: 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 Revisions: The Berkmar Overlook VSMP Amendment #2 plans and calculations were generated for only one reason, the retaining walls behind lots 22-29 and 35-42 were removed and replaced with tie back grading. The following updates to the approved stormwater management report were made: 1. The tie -back grading behind lots 24-28 introduced some additional pervious runoff to 'DA B & C' in the pre -development state and 'DA B' in the post -development state. As a result the SCS TR-55 Worksheets 2 & 4, the facility's post -development routing calculations, the 'Watershed Summary' table shown below Worksheet 4 and the 9VAC 25-870-66 channel protection sheet were revised. 2. The tie -back grading expanded the limits of disturbance slightly. This increase in disturbances is reflected in the revised VRRM calculations. Please note, a nutrient credit purchase of 1.24 Ibs/yr of phosphorous was made with the original VSMP approval, which envisioned a greater impervious area footprint. When the VSMP Amendment #1 calculations were approved reflecting the current final site plan, the impervious area footprint was decreased and yielded a smaller required phosphorous removal rate of 0.79 Ibs/yr. The phosphorous removal rate required with VSMP Amendment #2 (0.81 Ibs/yr) increased with the limits of disturbance increase, but like VSMP Amendment #1 it remains below the originally provided 1.24 bs/yr purchased. Therefore, water quality requirements continue to be met with this project and Amendment #2. SCS TR-55 Calculations Soils Map (Source: NRCS Web Soil Survey Online Database) Soils Table (Source: NRCS Web Soil Survey Online Database) Albemarle County, Virginia (VA003) Albemarle County, Virginia (VA003) 0 Map Unit Map Unit Name Acres Percent of Symbol in A0I A0I 27C Elioak loam, 7 3.5 39.7% to 15 percent slopes 34D Glenelg loam, 0.7 8.5% 15 to 25 percent slopes 55B Pacolet sandy 4.6 51.8% loam, 2 to 7 percent slopes Totals for Area of 8.8 100.0% Interest Note: The NRCS web soil survey has catorgized this development's soils as predominately consisting of hydrologic soil group type B. Precipitation Data (Source: NOAA's National Weather Service, Hydrometerorological Design Studies Center, Atlas 14 Point Precipitation Frequency Estimates) ©Ten h Fork I' �a IW of°°7� Fivanna River �e Reservoir r I O� s� �F 04 ;nor 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 900/6 confidence intervals (in inches)" aacra9e rcwneze inbrvai (yeah) DuraM1on o000000000 1 2 6 10 25 50 100 200 500 1090 x4ar 3.01 3.64 0.65 550 6.75 7.a3 9.01 103 123 10.0 r2.70-338) (3.274.09) 11 (4.165.72) 11 (4.91-6.15) 11 (5.99-7.53) 11 (6.898.71) 11 (7.97-10.0) 11 (8.91-11.4) (10.4136) (11.7-15.5) U.S. Department of Agriculture Natural Resources Conservation Service Project Berkman Overlook Location: Berkman Drive Check One: Present % 1. Runoff curve Number (CN) TR 55 Worksheet 2: Runoff Curve Number and Runoff Developed % Designed By: FGM,RE Checked By: SRC, PE FL-ENG-21A 06/04 Date 5/20/20 2 Date: 5/20/2021 Soil name and Cover description CN(weighted)= Dmr- hydrologic group (Cover type, treatment, one hydrologic a- Area Product of CN a ma, area Calculated De .Acres) Area '6'Value IAmendlxA) imperNou,u �ected/co�neactl hv- total area Imperm_ OA2 41A L Grave _ L05 88.8 00 79.4 2.59 Condition 44A 11 1- 34A 4reas 2.S5 216.6 DAB & C Prenp " 070 57.5 14.8 337 Woods mGood CondiOpn ss 0.26 1a.3 Condition 2.46 150.2 J B Gravu cos 81.9 2.21 _ Lawns it . Condition 1� Impervi 2_ B Woad, bait"") c_ 69.0 450 (Dm. Lawns u Imperv1_ _ 3EU. B Wood, 0.19 10.5 39.2 2.63 (Deed Lawns a 3.56 217.1 Imperei_ 012 115 0 Woods 0.0 65.8 SAA (Dca Lawns m 47J Imperm_ 0.0 B Woad, 0.0 61.0 6.39 (Dc.. Lawns m 1.9 2. Runoff n Drainage Area Description Frequency -years 1 2 1D n/a Rainfal' - 3.01 3.64 5.50 n/a Runo'1 1.22 1.71 3.28 DA A(Present) Runo'1 096 1.39 2.8C DA B & C(Presentj Bono 1.38 1.89 3.S2 DA D(Present) Runo'1 0.67 1.04 2.32 DA A (Developed) Runo'I 121 1.0 3.25 DA B(Developed) Runo'1 0.54 0.82 2.06 DA C(Developed) Bunch _ 0.37 0.64 - 68 DA 0 (Developed) U.S. Department of Agriculture Natural Resources Conservation Service TR 55 Worksheet 3: Time of Concentration (T,) or Travel Time (Tr) Project: Berkmar Overlook Location: Berkmar Drive Check One: Present X Developed X Check One: T, X Tr Designed By: KKW, PE Checked By: SRC, PE Through subarea n/a FL-ENG-21A 06/04 Date: 3/15/2021 Date: 3/15/2021 DAA DAB&C DAD DAA DAB DAC DAD J (Present) (Present) (Present) (Developed) (Developed) (Developed) (Developed) Segment Segment I Sheet Flow: (Applicable to T, only) 1 Surface description (Table 3-1) Dense Grass Dense Grass Dense Grass Dense Grass Dense Grass Dense Grass Dense Grass 2 Manning's roughness coeff., n (Table 3-1 0.24 0.24 0.24 0.24 0.24 0.24 0.24 3 Flow length, L(total L< 100)(ft) 40 20 40 40 60 100 30 4 Two-year 24-hour rainfall, P,(in.) 3.6 3.6 3.6 3.6 3.6 3.6 3.6 5 Land slope, s (ft/ft) 0.075 0.10 0.025 0.075 0.05 0.18 0.2 6 Compute Tt= [0.007(n'L)"I / Pis S14 0.06 0.03 0.10 0.06 0.10 0.09 0.03 Shallow Concentrated Flow: 7 Surface description (paved or unpaved) Unpaved Unpaved 8 Flow Length, L (ft) 75 110 9 Watercourse slope, s (ft/ft) 0.013 0.082 10 Average velocity, V (Figure 3-1) (ft/s) 1.8 4.6 11 Tr= L / 3600'V 0.01 0.01 Channel Flow: 12 Cross sectional flow area, a (ft) m m o 3 o x 13 Wetted perimeter, P. (ft) o m " x o LL X 0 LL x w w« o w v« o 14 Hydraulic radius, r=a/P„(ft) 2 n c n v a m v o w 15 Channel Slope, s(ft/ft) t m s on on a E c a« u E 16 Manning's Roughness Coeff, n u u u m o 'o o.- o 17V=[1.49r�s05]/n o' r o « u 18 Flow length, L (ft) - �n o Q`u m Q 19 Tr= L / 3600'V 0.03 0.02 0.00 20 Watershed or subarea T,or Tt (Add Tr in steps 6, 11 and 19) .l.11 0.10 0.12 0.10 0.10 Time of Concentration Notes: 1. The most conservative post -development SCS Tc is 0.10 hrs. 2. The Post -Development DA B Tc is slightly longer because the ReDevelopment scenario has less impervious areas and promotes infiltration with more grass D o m m y1 m O ti Vf N N n b 'I N ry N N m O tQO ,y O '{ O N Q ypj m '{ O m �+ O n o O 0 v0i o m n C .y m 'i vi'4 omri N Q Q m o N m ry b 0 N b b ri ri o m C � m .+ ri rl o m o 0 O O T O p Y1 ti Vf b M y1 N ,y c O '« Q Tao m � d d m y v i O ri b G G N m N ei m m n m m ti vio om ry rri ry� m ry n o ,y vmi nio om.. ,1 o m m 10/1 .N ti o nio om6 0 O C m 0 O N m m cN 'i vi op o o N ti ry Q n b ,4 Ifl rio om.+ ri o ti o m Q n m m nio om'i O 1011 b d O O y b M W � 1n O p N N N Q b n b N M of 01 T � ri G G m .ti eNi ,y O b ry y1 b 01 T b 1+id dmd o6 O N ci O ,y m ry vrvi ti n e .y n ri o ci v rl N � V1 N O O n N rio c om,y o w LL o c C w E W L W t � c v E O G 1 ° a 'o o y 0 3 aCf o' N m a Y, a `L' E E m Q Cf m n o. o 0 d a «_ E .c a v ._ U J u' o a w t ri 1+i 6 of w n w of 3 3 0 w m b N N O m O 0 N 3 0 m m m a T rl T O O b o <w Flo a m a n o 0 0 O m m w N Q C T O 3 0 � Lb yJ H M N � N N 3 o n T Z O W mlb u m e w n n m n Channel and Flood Protection Calculations & Summaries 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 < I.F. * (Q Pre -Developed » r u-Developed) / RV Developed Q Allowed = 11.79 cfs Where the followinr re used: CN Pre-ReDev.= 76.6 CN Post-ReDev.= 76.7 S Pre-ReDev.= 3.05 S Post-ReDev.= 3.05 RV Pre-ReDev.= 1.06 RV Post-ReDev.= 1.06 Q Pre -Developed - 14.77 I.F. = 0.8 Notes: CN represents the weighted Curve Numbers (Shown in the Watershed Summary Table at the bottom of Worksheet 4.) S was calculated by using the SCS TR-55 manual's equation 2-4. (The Weighted CN values described above were used.) RV was calculated by using the SCS TR-55 manual's equation 2-3. (The Weighted CN values & RV values described above were used.) Q Developed = 5.33 cfs Section B.4 Limits of Analysis The analysis terminates at subareas A, B, C & D 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 site "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 terminates at subareas A, B, C & D since subsection C.2.b is met. Amendment #2 Routing Calculations Berkmar Drive- SWM Facility Routings -Amendment No. 2 BasinFlow printout INPUT: Basin: Berkmar Drive- SWM Facility Routings Amendment No. 2 5 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 518.00 37.00 0.0 520.00 964.00 29.4 522.00 2647.00 158.0 524.00 4867.60 432.2 526.00 7535.00 887.9 Start—Elevation(ft) 5 Outlet Structures Outlet structure 0 Orifice 518.00 Vol.(cy) 0.00 name: Barrel area (sf) 1.767 diameter or depth (in) 18.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 516.000 multiple 1 discharge out of riser Outlet structure 1 Orifice name: Low -Flow Orifice area (sf) 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 518.000 multiple 1 discharge into riser Page 1 Berkmar Drive- SWM Facility Routings -Amendment No. 2 Outlet structure 2 Orifice name: Mid -Flow Orifice area (sf) 0.442 diameter or depth (in) 9.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 522.000 multiple 1 discharge into riser Outlet structure 3 Weir name: Riser diameter (in) 48.000 side angle 0.000 coefficient 3.300 invert (ft) 523.500 multiple 1 discharge into riser transition at (ft) 1.215 orifice coef. 0.500 orifice area (sf) 12.566 Outlet structure 4 Weir name: Emergency Spillway length (ft) 25.000 side angle 75.960 coefficient 3.300 invert (ft) 524.500 multiple 1 discharge through dam 4 Inflow Hydrographs Hydrograph 0 SCS name: 1-yr 24-hr. SCS Design Storm Area (acres) 7.430 CN 79.200 Type 2 rainfall, P (in) 3.010 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) 11.014 peak time (hrs) 11.922 volume (cy) 1205.625 Page 2 Berkmar Drive- SWM Facility Routings -Amendment No. 2 Hydrograph 1 SCS name: 2-yr 24-hr. SCS Design Storm Area (acres) 7.430 CN 79.200 Type 2 rainfall, P (in) 3.640 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) 15.408 peak time (hrs) 11.922 volume (cy) 1686.611 Hydrograph 2 SCS name: 10-yr 24-hr. SCS Design Storm Area (acres) 7.430 CN 79.200 Type 2 rainfall, P (in) 5.500 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) 29.686 peak time (hrs) 11.922 volume (cy) 3249.579 Hydrograph 3 SCS name: 100-yr 24-hr. SCS Design Storm Area (acres) 7.430 CN 79.200 Type 2 rainfall, P (in) 9.010 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) 59.076 peak time (hrs) 11.922 volume (cy) 6466.679 Page 3 Berkmar Drive- SWM Facility Routings -Amendment No. 2 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 10.995 at 3.602 at 523.420 at 335.281 Peaks: 2-yr 24-hr 15.381 at 11.552 at 523.822 at 400.738 Peaks: 10-yr 24-hr 29.635 at 24.441 at 524.643 at 557.355 SCS Design Storm 11.92 (hrs) 12.10 (hrs) 12.10 (hrs) SCS Design Storm 11.92 (hrs) 12.02 (hrs) 12.02 (hrs) SCS Design Storm 11.92 (hrs) 12.00 (hrs) 12.00 (hrs) Peaks: 100-yr 24-hr. 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