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HomeMy WebLinkAboutWPO201200062 Calculations 2022-12-02SHIMP ENGINEERING, P.C. Design Focused Engineering December 21, 2022 John Anderson County of Albemarle Department of Community Development 401 McIntire Road, Room 227 Charlottesville, Virginia 22902-4596 RE: Old Trail Creekside III Phase III - WP0201200062 As -Built SWM Calculation Narrative Dear John, Please accept this narrative and attached HydroCAD analysis as verification that the as -built conditions of the stormwater detention pond at Old Trail Creekside III Phase III is in compliance with the stormwater regulations in place at the time of plan approval and meets the intent of the approved stormwater management plan. The primary spillway orifices were installed with a vertical separation of 3.21-ft between inverts, whereas the approved plan proposed a separation of 4.0-11. To ensure that the runoff is being released at rates in compliance with regulations, an as -built HydroCAD analysis has been completed showing that the release rates and velocities are in compliance. Specifically, the 10-yr runoff is released to the POA at a rate less than predevelopment and the 2-yr runoff is released at rates that do not cause erosion of the system. Please see the Watershed Summary table in Figure 1 below, taken from the approved plans, which identifies the pre and post development flowrates to the POA and provides a breakdown of the detained and undetained flows. Note that DA # 1 B, shown in Figure 2 below, is the undetained area from the total drainage area to the POA. The 10-yr flow from DA #113 is added to the As -Built flow being released by the new detention pond. Please see below for a summary of this analysis. 10-yr Pre-Dev POA Total 26.85 Proposed POA Total 25.84 As -Built POA Total 26.55 AS -BUILT POA Post-Dev DA #3A Post-Dev DA #1B cfs cfs 2-yr 2.68 5.85 10-yr 8.49 18.06 2-yr 10-yr POA Total F 8.53 26.55 Creekside Asbuilt Modified LD-268 Roadside and Median Ditch Desi n Form Side Slope Min. I Max Ditch A Tc Q2 Q10 — :1 L — :1 R Slope Ft/Ft Slope Allowable Velocity 2-yr 10-yr Lining Ft/Ft *Velocity '*Depth Outfall 9.62 15.0 2.68 8.49 3 3 0.07 0.09 50.0 2.63 0.94 Rip -Rap F'2-yr Velocity calculations performed using steepest slope & 2-yr flow " 10-yr Depth calculation performed using shallowest slope & 10-yr flow Regards, Michael Chandler Shimp Engineering, P.C. SHIMP ENGINEERING, P.C. Design Focused Engineering PRE -DEVELOPMENT SCALE: 1' 120 r ,• 1 �.' '' 1I!! /)Z' ��� ��..I III � � � — � . ,\1� TO, Ili'llllii'I 1l �kf / rl 1' 11l if1��1'���1 /Ir;/(l rillfr�l 'gl'(II� chi '\\_\ . / III\\, \\\�'\ ,11\ ,{� 11 ,\ ,,•„ I I /, ! \ , `\_ ')rl .nl 1,11`I I II} If rim 1 Iy1\\1111II 11 (// I I o.e..xe ow xu�o...w n✓r s.0 x.n vn 11111111(I/ f n,o n.n w a..es e.a n.�o.wo n.x xufx a u Y`\I`11111,M; 11 I iA) x46 L+ 7..9 xSM Figure 1: WPO201200062 SWMPIan (Sheet SWM 1) - Predev DA Map SHIMP ENGINEERING, P.C. Design Focused Engineering Figure 2: WPO201200062 SWMP(an (Sheet SWM I) - Postdev DA Map Old Trail Creekside III Phase III As -Built HydroCAD Report 10-yr 3S A#1A 44P AS UILT POND 2L POA Subcat Reach On Link Drainage Diagram for Creekside As -Built HydroCAD Prepared by {enter your company name here}, Printed 10120/2022 HydroCAD® 9.10 s/n 07054 0 2010 HydroCAD Software Solutions LLC AS -BUILT POA ANALYSIS 10-YR Creekside As -Built HydroCAD Type/1 24-hr 10-YR Rainfall=5.61" Prepared by {enter your company name here} Printed 10/20/2022 HydroCAD® 9.10 s/n 07054 © 2010 HydroCAD Software Solutions LLC Page 2 Time span=5.00-20.00 hrs, dt=0.05 hrs, 301 points Runoff by SCS TR-20 method, UH=SCS Reach routing by Stor-Ind+Trans method - Pond routing by Stor-Ind method Subcatchment 3S: DA # 1A Runoff Area=9.620 ac 43.00% Impervious Runoff Depth>2.53" Tc=15.0 min CN=73 Runoff=34.07 cfs 2.029 of Pond 4P: AS -BUILT POND Peak Elev=741.44' Storage=38,760 cf Inflow=34.07 cfs 2.029 of Outflow=8.49 cfs 1.847 of Link 2L: POA Inflow=8.49 cfs 1.847 of Primary=8.49 cfs 1.847 of Total Runoff Area = 9.620 ac Runoff Volume = 2.029 of Average Runoff Depth = 2.53" 57.00% Pervious = 5.483 ac 43.00% Impervious = 4.137 ac AS -BUILT POA ANALYSIS 10-YR Creekside As -Built HydroCAD Type/1 24-hr 10-YR Rainfall=5.61" Prepared by {enter your company name here} Printed 10/20/2022 HydroCAD® 9.10 s/n 07054 © 2010 HydroCAD Software Solutions LLC Page 3 Summary for Subcatchment 3S: DA # 1A Runoff = 34.07 cfs @ 12.07 hrs, Volume= 2.029 af, Depth> 2.53" Runoff by SCS TR-20 method, UH=SCS, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Type II 24-hr 10-YR Rainfall=5.61" a (ac) CN Description 4.137 98 Impervious 9.620 73 Weighted Average 5.483 57.00% Pervious Area 4.137 43.00% Impervious Area Tc Length Slope Velocity Capacity Description (min) (feet) (ft/ft) (ft/sec) (cfs) 15.0 Direct Entry, Summary for Pond 4P: AS -BUILT POND Inflow Area = 9.620 ac, 43.00% Impervious, Inflow Depth > 2.53" for 10-YR event Inflow = 34.07 cfs @ 12.07 hrs, Volume= 2.029 of Outflow = 8.49 cfs @ 12.40 hrs, Volume= 1.847 af, Atten= 75%, Lag= 19.5 min Primary = 8.49 cfs @ 12.40 hrs, Volume= 1.847 of Routing by Stor-Ind method, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs Peak Elev= 741.44' @ 12.40 hrs Surf.Area= 8,409 sf Storage= 38,760 cf Plug -Flow detention time= 107.1 min calculated for 1.841 of (91 % of inflow) Center -of -Mass det. time= 76.8 min ( 872.5 - 795.7 ) Volume Invert Avail.Storage Storage Description #1 734.86' 68,208 cf Custom Stage Data (Prismatic) Listed below (Recalc) Elevation Surf.Area Inc.Store Cum.Store (feet) (sq-ft) (cubic -feet) (cubic -feet) 734.86 3,622 0 0 736.00 4,212 4,465 4,465 738.00 5,762 9,974 14,439 740.00 7,277 13,039 27,478 742.00 8,851 16,128 43,606 744.00 10,432 19,283 62,889 744.50 10,843 5,319 68,208 Device Routing Invert Outlet Devices #1 Primary 732.68' 24.0" Round Culvert L= 117.0' CPP, mitered to conform to fill, Ke= 0.700 Inlet / Outlet Invert= 732.68' / 731.56' S= 0.0096 '/' Cc= 0.900 n= 0.012 #2 Device 1 734.86' 4.0" Vert. Orifice/Grate X 2.00 C= 0.600 AS -BUILT POA ANALYSIS 10-YR Creekside As -Built HydroCAD Type/1 24-hr 10-YR Rainfall=5.61" Prepared by {enter your company name here} Printed 10/20/2022 HydroCAD® 9.10 s/n 07054 © 2010 HydroCAD Software Solutions LLC Page 4 #3 Device 1 738.07' 6.0" Vert. Orifice/Grate X 2.00 C= 0.600 #4 Device 1 741.20' 30.0" Horiz. Orifice/Grate C= 0.600 Limited to weir flow at low heads #5 Primary 743.00' 50.0' long Sharp -Crested Rectangular Weir 2 End Contraction(s) Primary OutFlow Max=8.45 cfs @ 12.40 hrs HW=741.44' (Free Discharge) 1=Culvert (Passes 8.45 cfs of 37.18 cfs potential flow) 2=Orifice/Grate (Orifice Controls 2.13 cfs @ 12.19 fps) 3=Orifice/Grate (Orifice Controls 3.34 cfs @ 8.50 fps) =Orifice/Grate (Weir Controls 2.98 cfs @ 1.59 fps) S=Sharp-Crested Rectangular Weir ( Controls 0.00 cfs) Summary for Link 2L: POA Inflow Area = 9.620 ac, 43.00% Impervious, Inflow Depth > 2.30" for 10-YR event Inflow = 8.49 cfs @ 12.40 hrs, Volume= 1.847 of Primary = 8.49 cfs @ 12.40 hrs, Volume= 1.847 af, Atten= 0%, Lag= 0.0 min Primary outflow = Inflow, Time Span= 5.00-20.00 hrs, dt= 0.05 hrs