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Home My WebLink AboutWPO201600071 Calculations WPO VSMP 2019-05-08 BELVEDERE PHASE IIB STORMWATER MANAGEMENT CALCULATIONS PACKET Date of Approved Calculations JANUARY 31, 2017 Amended on JANUARY 24, 2019 Revised on MARCH 21, 2019 Revised on MAY 8, 2019 PREPARED BY: COLLINS 200 GARRETT STREET, SUITE K Q.tiy��T flay CHARLOTTESVILLE,VA 22902 ,�► jst_ 434.293.3719 PH ! ROLLI NS FX � Lic. No.036791 www.collins-engineering.com T"°io � 4 REVISIONS TO THIS CALCULATIONS PACKET, RELATIVELY TO THE PREVIOUSLY REVIEWED/LARGELY ACCEPTED MARCH 215t CALCULATIONS ARE LIMITED TO THE FOLLOWING: 1. The cover sheet's revision date was updated &this summary sheet has been inserted. 2. The 'Belvedere Phase IIA Basin-Amended' routing calculations were revised to show an emergency spillway consistent with the approved WPO 201200096 plans. The previously submitted plans and calculations modified the emergency spillway to ensure the 10-year design storm was not activated to avoid armoring the spillway with riprap.This was found to be unacceptable, so the attached May 8th plans and calculations now reflect the approved WPO 201200096 emergency spillway, with it now being armored with VDOT Class I riprap. A small portion of the 10-year design storm now flows through the spillway with a peak design depth of 1".This design is not problematic and is not erosive on the newly proposed VDOT Class I riprap. The approved WPO 201200096 plan, as shown on its sheet 5, depicts the contributing drainage area (DA#1A)for Belvedere Phase IIA's stormwater management facility. In the impervious cover computations, it accounts for 8.92 acres (239,220 sf+ 149,500 sf) of road and lot coverage.The design conservatively assumed an additional 0.76 acres (33,162 sf) of impervious cover for a factor of safety. The approved WPO 201600071 plans utilized 11,200 sf of this 33,162 sf factor of safety. The proposed amendment plans will utilize an additional 14,700 sf of this factor of safety. Following this addition, the basin continues to provide a factor of safety. Additionally, this amendment proposes to make minor modifications to the riser structure, barrel and spillway to ensure post- development outflows associated with this amendment continue to be below the originally approved flows with WPO 201200096. Belvedere Phase IIA Basin- Amended BasinFlow printout INPUT: Basin: Belvedere Phase IIA Basin- AMENDMENT 9 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 405.00 8574.00 0.0 406.00 9545.00 335.4 408.00 11582.00 1116.6 410.00 13741.00 2053.4 411.00 16149.00 2606.3 412.00 17158.00 3223.0 414.00 19573.00 4582.4 416.00 22135.00 6126.2 418.00 23148.00 7803.2 Start_Elevation(ft) 411.00 Vol. (cy) 2606.31 5 Outlet Structures Outlet structure 0 Orifice name: Low-Flow Orifices area (sf) 0.087 diameter or depth (in) 4.000 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 411.000 multiple 4 discharge into riser Outlet structure 1 Orifice name: Mid-Flow Orifices area (sf) 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 414.000 multiple 3 discharge into riser Page 1 Belvedere Phase IIA Basin- Amended Outlet structure 2 Weir name: Emergency Spillway length (ft) 50.000 side angle 71.600 coefficient 3.300 invert (ft) 416.250 multiple 1 discharge through dam Outlet structure 3 Curve name: Anti-Vortex Device multiple 1 Invert (ft) 415.500 discharge into riser file: thirtysix.txt Outlet structure 4 Culvert name: AMENDMENT- Barrel with a Plate multiple 1 discharge out of riser D (in) 15.000 h (in) 0.000 Length (ft) 100.000 Slope 0.005 Manning's n 0.013 Inlet coeff. Ke 0.500 Equation constant set 3 Invert (ft) 404.500 Page 2 Belvedere Phase IIA Basin- Amended 3 Inflow Hydrographs Hydrograph 0 SCS name: 2-Year, 24-hr Peak SCS Storm Event Area (acres) 19.280 CN 79.600 Type 2 rainfall, P (in) 3.700 time of conc. (hrs) 0.1700 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 38.551 peak time (hrs) 11.961 volume (cy) 4575.603 Hydrograph 1 SCS name: 10-Year, 24-hr Peak SCS Storm Event Area (acres) 19.280 CN 79.600 Type 2 rainfall, P (in) 5.600 time of conc. (hrs) 0.1700 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 73.818 peak time (hrs) 11.961 volume (cy) 8761.434 Hydrograph 2 SCS name: 100-Year Storm Event- FREEBOARD CHECK Area (acres) 19.280 CN 79.600 Type 2 rainfall, P (in) 9.100 time of conc. (hrs) 0.1700 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 144.306 peak time (hrs) 11.961 volume (cy) 17127.562 Page 3 Belvedere Phase IIA Basin- Amended OUTPUT: Routing Method: storage-indication Hydrograph 0 Routing Summary of Peaks: 2-Year, 24-hr Peak SCS Storm Event inflow (cfs) 38.548 at 11.96 (hrs) discharge (cfs) 3.820 at 12.50 (hrs) water level (ft) 414.338 at 12.50 (hrs) storage (cy) 4830.317 Hydrograph 1 Routing Summary of Peaks: 10-Year, 24-hr Peak SCS Storm Event inflow (cfs) 73.812 at 11.96 (hrs) discharge (cfs) 22.580 at 12.14 (hrs) water level (ft) 416.337 at 12.14 (hrs) storage (cy) 6403.555 Hydrograph 2 Routing Summary of Peaks: 100-Year Storm Event- FREEBOARD CHECK inflow (cfs) 144.293 at 11.96 (hrs) discharge (cfs) 139.458 at 11.98 (hrs) water level (ft) 416.997 at 11.98 (hrs) storage (cy) 6953.125 Wed May 08 15:04:51 EDT 2019 Page 4 �J_-M 9oott6Eo Mhov.ICaYxoN5 �X, a4 �P( L TS G.AL �s„ 1q� �1. -ro CI. 07M mint. . 0 .?/fr.,"-/, !? 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To A 154 -$nPREc, Short Version BMP Computations For Worksheets 2-6 Albemarle County Water Protection Ordinance Modified Simple Method Plan SWM Facility#2 Water Resources Area Development Area Preparer: SRC,PE Date: Approved on 11/2/2012,Amended on 3/21/19 Project Drainage Area Designation SWM Facility#2(Proposed Type III Retention Basin,or facility w/equivalent removal rate) L storm pollutant export in pounds, L=[P(Pj)Rv/12 I[C(A)2.72] Rv mean runoff coefficient, Rv=0.05+0.009(1) Pj small storm correction factor,0.9 percent imperviousness P annual precipitation,43"in Albemarle A project area in acres in subject drainage area, A= 19.28 C pollutant concentration,mg/1 or ppm target phosphorus factor applied to RR ✓ required treatment volume in cy,0.5"over imperv.area= A(1)43560(0 5/12)/27 RR required removal, L(post)-f x L(pre) %RR removal efficiency, RR100/L(post) Impervious Cover Computation(values in feet&square feet) Item pre-development Area post-development Area Roads&Sidewalks subtotal Roads&Sidewalks Length Wdth subtotal Approved WPO 201200096 239.220 Impervious Road&Sidewalk Areas 239,220 Residences no. Sq.Ft subtotal Residences no. Sq.Ft subtotal Approved Lots under WPO 201200096 149,500 Approved 7 TH lots overage under WPO 201600071 11,200 Proposed VSMP Amendment#3 lot overage 14,700 175,400 Parking Lots Parking Lots Gravel areas Gravel areas x0.70= x0.70= Structures Area no. subtotal Structures Area no. subtotal Actively-grazed pasture& Actively-grazed pasture& yards and cultivated turf 839,837 x 0.08= 67,187 yards and cultivated turf 417,955 x 0.08= 33,436 Active crop land Area Active crop land Area x0.25= 0 x0.25= Other Impervious Areas Other Impervious Areas 0 Factor of Safety Impervious Areas 7262 Impervious Cover 80 54% I(pre) I(post) Rv(post) V 0.54 702.7 New Development(For Development Areas,existing impervious cover<=20%; C f I(pre)* Rv(pre) L(pre) L(post) RR %RR Area Type 0.60 1.00 20% 0.23 23.33 54.57 31.24 57% Development Area *min.values sN/ewaa (ww)awil mold (sd1)R1poI0A'/\ (sp)N!Oede3 mold Ilnd (U0/alawela a w 10 <o o a 0 x adols's m 0 n o 0 0 gaol s,6uluuew'u .76 a ))4160a11e091'1 U O c+> (4)lno uanul 0 o m u n v v v c CS. (g)w panul mop 1e(o1'l0 (sja)IOUI!MOII'0 /0lsowl Ile;um/'I (ulw)'9u00;o awn'ol palelnwnoue wns HO 011 lUawa1Uw vo O Lis' E 'fl o o n x gaoO leuonm'0 (sane) ewe aOeulwa'y U o wnlomis 6ulpu3 p p > w w Vj a/nlongs 6upgs ry W • a � Q w U w a 5d d d a a 1=11111111 C, 0 cz o ci) tri (-,i 6 ob 0 r- ct (.7 ,71- -71- cr O• [J• a > ' tu3 N N <1- <t 1+4 3 t — 6 tri -• - - -0 77 .4 a.) a N N .rr 4 x N kr) 1-, -. 6 v I... — ..---1 z z z ,-.. o N -- t-• - - 00 ,.co a, -. 0 0 ,..., 00 N N <Ch ,I7 ,- N •</- p kel N WI ... 6 6 ir w 00 co N 0 .-, . 0' ,.(7 C•1 0 Z CZ) N << 0 .... 0 6 6 6 .ti P ..c.j 4 N .1- co V tt = .4 C..) ›, kr) 00 ,- ,.to • er; a.) 6 Z 7:2 ..... — cc ,--4 0 oo s... o) "Cs = U CS k. C., ; i: a 6 6 = co C.) 75 s... = . . . .7) N <-i . >1 Z t' 6 6 6 "ts . ,.....?\ . r- cps 00 00 4:; 71: c.) a) N .tt N •-, 00 00 0 0 C•1 6 6 1 4:> N c> cv o c:, (7) o o c5 6 6 1--. (-,) c> VI .-. 6 6 6 . . 6 vo vo ,.o co 6 co 3 6 ,-. ..... w o) a ,.,, co co co y. 3 w C7, cr• Cr., I- L, tr; rn 6 c w a .tlo g ‘zi- kr.) ,.c.) m• 71- -1- EA 6 6 a.) 0 v, . (21 . 214 2 6 6 a ›-, >-. ›. l � pg BELVEDERE PHASE IIB STORMWATER MANAGEMENT CALCULATIONS PACKET Date of Calculations JANUARY 31, 2017 PREPARED BY: al COLLINS ar 4 hcti 200 GARRETT STREET, SUITE K CHARLOTTESVILLE, VA 22902 434.293.3719 PH 434.293.2813 FX www.collins-engineering.com The approved WPO 201600096 plan, as shown on its sheet 5, depicts the contributing drainage area (DA#1A)for Belvedere Phase IIA's stormwater management facility. In the impervious cover computation it accounts for 8.92 acres (239,220 sf+ 149,500 sf) of road and lot coverage. The design conservatively assumed an additional 0.76 acres (33,162 sf) of impervious cover for a factor of safety. The proposed plan uses a portion of this 0.76 acres with its addition of 7 townhouse units (11,200 sf). This has been accepted by County Engineering, and a factor of safety/overage still remains with the facility's design. The proposed plan also has a slight increase in acreage. These two factors cause a slight shift in the hydraulics for the existing basin. The following two analyses apply the calculated area and CN values to the approved basin, with the proposed analysis having a smaller outflow. Imwe 'Sup, Approved Belvedere Phase IIA Basin Inputs BasinFlow printout INPUT: Basin: Approved Belvedere Phase IIA Basin Inputs 9 Contour Areas Elevation(ft) Area(sf) Computed Vol. (cy) 405.00 8574.00 0.0 406.00 9545.00 335.4 408.00 11582.00 1116.6 410.00 13741.00 2053.4 411.00 16149.00 2606.3 412.00 17158.00 3223.0 414.00 19573.00 4582.4 416.00 22135.00 6126.2 418.00 23148.00 7803.2 Start_Elevation(ft) 411.00 Vol.(cy) 2606.31 5 Outlet Structures Outlet structure 0 Orifice name: Low-Flow Orifices area (sf) 0.087 diameter or depth (in) 4.000 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 411.000 multiple 4 discharge into riser Outlet structure 1 Orifice name: Mid-Flow Orifices area (sf) 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 414.000 multiple 4 discharge into riser Outlet structure 2 Weir name: Emergency Spillway length (ft) 50.000 side angle 71.570 coefficient 3.300 invert (ft) 416.250 multiple 1 discharge through dam Page 1 Approved Belvedere Phase IIA Basin Inputs Outlet structure 3 Curve name: Anti-Vortex Device multiple 1 Invert (ft) 415.500 discharge into riser file: thirtysix.txt Outlet structure 4 Culvert name: Barrel multiple 1 discharge out of riser D (in) 24.000 h (in) 0.000 Length (ft) 100.000 Slope 0.005 Manning's n 0.013 Inlet coeff. Ke 0.500 Equation constant set 3 Invert (ft) 404.500 3 Inflow Hydrographs Hydrograph 0 SCS name: 2-Year, 24-hr Peak SCS Storm Event Area (acres) 18.440 CN 80.400 Type 2 rainfall, P (in) 3.700 time of conc. (hrs) 0.1667 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 38.380 peak time (hrs) 11.950 volume (cy) 4523.873 Hydrograph 1 SCS name: 10-Year, 24-hr Peak SCS Storm Event Area (acres) 18.440 CN 80.400 Type 2 rainfall, P (in) 5.600 time of conc. (hrs) 0.1667 time increment (hrs) 0.0200 Page 2 Approved Belvedere Phase IIA Basin Inputs time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 72.744 peak time (hrs) 11.950 volume (cy) 8574.390 Hydrograph 2 SCS name: 100-Year Storm Event- FREEBOARD CHECK Area (acres) 18.440 CN 80.400 Type 2 rainfall, P (in) 9.100 time of conc. (hrs) 0.1667 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 141.053 peak time (hrs) 11.950 volume (cy) 16626.031 OUTPUT: Routing Method: storage-indication Hydrograph 0 Routing Summary of Peaks: 2-Year, 24-hr Peak SCS Storm Event inflow (cfs) 38.153 at 11.96 (hrs) discharge (cfs) 3.923 at 12.48 (hrs) water level (ft) 414.296 at 12.48 (hrs) storage (cy) 4799.300 Hydrograph 1 Routing Summary of Peaks: 10-Year, 24-hr Peak SCS Storm Event inflow (cfs) 72.315 at 11.96 (hrs) discharge (cfs) 22.788 at 12.14 (hrs) water level (ft) 416.246 at 12.14 (hrs) storage (cy) 6328.792 Hydrograph 2 Routing Summary of Peaks: 100-Year Storm Event- FREEBOARD CHECK inflow (cfs) 140.221 at 11.96 (hrs) discharge (cfs) 134.492 at 11.98 (hrs) water level (ft) 416.916 at 11.98 (hrs) storage (cy) 6885.310 Tue Jan 31 11:08:30 EST 2017 Page 3 • Approved Belvedere Phase IIA Basin WITH 7 TOWNHOMES BasinFlow printout INPUT: Basin: Approved Belvedere Phase IIA Basin WITH 7 TOWNHOMES 9 Contour Areas Elevation(ft) Area(sf) Computed Vol. (cy) 405.00 8574.00 0.0 406.00 9545.00 335.4 408.00 11582.00 1116.6 410.00 13741.00 2053.4 411.00 16149.00 2606.3 412.00 17158.00 3223.0 414.00 19573.00 4582.4 416.00 22135.00 6126.2 418.00 23148.00 7803.2 Start_Elevation(ft) 411.00 Vol. (cy) 2606.31 5 Outlet Structures Outlet structure 0 Orifice name: Low-Flow Orifices area (sf) 0.087 diameter or depth (in) 4.000 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 411.000 multiple 4 discharge into riser Outlet structure 1 Orifice name: Mid-Flow Orifices area (sf) 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.600 invert (ft) 414.000 multiple 4 discharge into riser Outlet structure 2 Weir name: Emergency Spillway length (ft) 50.000 side angle 71.570 coefficient 3.300 invert (ft) 416.250 multiple 1 discharge through dam Page 1 Approved Belvedere Phase IIA Basin WITH 7 TOWNHOMES Outlet structure 3 Curve name: Anti-Vortex Device multiple 1 Invert (ft) 415.500 discharge into riser file: thirtysix.txt Outlet structure 4 Culvert name: Barrel multiple 1 discharge out of riser D (in) 24.000 h (in) 0.000 Length (ft) 100.000 Slope 0.005 Manning's n 0.013 Inlet coeff. Ke 0.500 Equation constant set 3 Invert (ft) 404.500 3 Inflow Hydrographs Hydrograph 0 SCS name: 2-Year, 24-hr Peak SCS Storm Event Area (acres) 18.940 CN 79.400 Type 2 rainfall, P (in) 3.700 time of conc. (hrs) 0.1667 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 37.817 peak time (hrs) 11.950 volume (cy) 4457.476 Hydrograph 1 SCS name: 10-Year, 24-hr Peak SCS Storm Event Area (acres) 18.940 CN 79.400 Type 2 rainfall, P (in) 5.600 time of conc. (hrs) 0.1667 time increment (hrs) 0.0200 Page 2 Nome Num, Approved Belvedere Phase IIA Basin WITH 7 TOWNHOMES time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 72.599 peak time (hrs) 11.950 volume (cy) 8557.243 Hydrograph 2 SCS name: 100-Year Storm Event- FREEBOARD CHECK Area (acres) 18.940 CN 79.400 Type 2 rainfall, P (in) 9.100 time of conc. (hrs) 0.1667 time increment (hrs) 0.0200 time limit (hrs) 30.000 fudge factor 1.00 routed true peak flow (cfs) 142.213 peak time (hrs) 11.950 volume (cy) 16762.709 OUTPUT: Routing Method: storage-indication Hydrograph 0 Routing Summary of Peaks: 2-Year, 24-hr Peak SCS Storm Event inflow (cfs) 37.594 at 11.96 (hrs) discharge (cfs) 3.746 at 12.50 (hrs) water level (ft) 414.259 at 12.50 (hrs) storage (cy) 4771.944 Hydrograph 1 Routing Summary of Peaks: 10-Year, 24-hr Peak SCS Storm Event inflow (cfs) 72.170 at 11.96 (hrs) discharge (cfs) 22.681 at 12.14 (hrs) water level (ft) 416.242 at 12.14 (hrs) storage (cy) 6325.478 Hydrograph 2 Routing Summary of Peaks: 100-Year Storm Event- FREEBOARD CHECK inflow (cfs) 141.374 at 11.96 (hrs) discharge (cfs) 135.748 at 11.98 (hrs) water level (ft) 416.921 at 11.98 (hrs) storage (cy) 6889.435 Tue Jan 31 11:08:14 EST 2017 Page 3