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Home My WebLink AboutSDP200800013 Calculations8-- -----`--`.---------___ -----`---____ -^``.` If I.., -_''_--____-----__----- -___ A ----_____________ Dunlora Pond Narrative.txt BIO- FILTER DETENTION BASIN NARRATIVE. 1. THE REQUIRED REMOVAL RATE FOR THE DEVELOPMENT AREA (5.12 ACRES) IS 5.41 POUNDS OF PHOSPHOUROUS PER DAY. THE LOADING FOR THE DETAINED AREA (4.11 ACRES) IS 10.96 LBS OF PHOSPHOUROUS PER DAY. THE REQUIRED PERCENT REMOVAL RATE EQUALS THE REMOVAL RATE DIVIDED BY THE LOADING OR 49 %. BASED ON THIS A BIO- FILTER DETENTION BASIN AND SEDIMENT FOREBAY WITH A PERCENT REMOVAL RATE OF 50% HAS BEEN SELECTED. 1. SEDIMENT FOREBAY HAS BEEN SIZED TO HOLD 1/4" PER IMPERVIOUS AREA OF THE DEVELOPMENT AREA. THE VOLUME OF THE FOREBAY IS 1,812 CUBIC FEET, WITH A WEIR ELEVATION OF 386.98. A CONCRETE WALL WITH A RECTANGULAR WEIR AND BMP ORIFICE WILL SEPERATE THE FOREBAY FROM THE REST OF THE POND. 2. THE WATER QUALITY VOLUME (WQV) IS EQUIVALENT TO 112" PER IMPERVIOUS AREA OF THE DEVELOPMENT AREA OR 3,634 CUBIC FEET. THE WATER SURFACE ELEVATION FOR THE WQV IS 385.19. THIS LEAVES 1.19' BETWEEN THE BOTTOM OF POND AND THE DETENTION ORIFICE. 3. 385.19E DETENTION ORIFICE IS DIAMETER HOLE INVERT OF STORM BELOW THE ALLOWABLE DISCHARGE. 4. BASED ON BASINFLOW ROUTING THE 2, 10, AND 100 YEAR WATER SURFACE ELEVATIONS ARE 386.57, 387.29, AND 388.16 RESPECTIVELY. S. THE TOP WIDTH OF THE DAMN IS 8 FEET IN ACCORDANCE WITH THE VIRGINIA STORMWATER MANAGEMENT HANDBOOK SECTION 3.01. 6. THE EMERGENCY SPILLWAY IS 5 FEET IN LENGTH WITH AN ELEVATION OF 388. Page 1 Short Version BMP Computations For Worksheets 2 - 6 Albemarle County Water Protection Ordinance: Modified Simple Method Plan: Dunlora Gates Water Resources Area: Development Area Preparer: TED Date: 27- Mar -08 Project Drainage Area Designation Total Development Area L storm pollutant export in pounds, L = [P(Pj)Rv /12 ] [ C(A)2.72 ] Rv mean runoff coefficient, Rv = 0.05 + 0.009(1) Pj small storm correction factor, 0.9 1 percent imperviousness P annual precipitation, 43" in Albemarle A project area in acres in subject drainage area, A =5.12 C pollutant concentration, mg/( or ppm target phosphorus f factor applied to RR V 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 Length Width subtotal Length Width subtotal 0 0 0 673 26 17498 0 0 0 0 0 0 0 0 0 0 0 0 0 17498 Driveways Length Width no. subtotal Length Width no.subtotal and walks 0 0 0 0 40 18 18 12960 0 0 0 0 628 5 1 3140 0 0 0 16100 Parking Lots 1 2 3 4 1 2 3 4 0 0 0 0 0 Gravel areas Area Area 2468.67 x 0.70 = 0 x 0.70 =1728.07 Structures Area no. subtotal Area no.subtotal 0 0 0 6691 4 26764 0 0 0 8934 1 B934 0 0 0 0 4479 1 4479 40177 Actively - grazed pasture & Area Area yards and cultivated turf x 0.08 = 0 1467B3.53 x 0.08 =11742.7 Active crop land Area Area x0.25= 0 x0.25=0 Other Impervious Areas 0 0 Impervious Cover 0%39% I(pre)I(post) Rv(post) V 0.40 134.6 New Development (For Development Areas, existing impervious cover 20 %) C f 1 (pre)' Rv(pre) L(pre) L(post) RR RR Area Type 0.70 1.00 20% 0.23 7.23 12.64 5.41 43% Development Area 0,35 1.00 0% 0.05 0.79 6.32 5.53 88% Drinking Water Watersheds 0,40 1.00 1% 0.06 1.06 7.22 6.16 85% Other Rural Land min. values Redevelopment (For Development Areas, existing impervious cover > 20 %) C f I (pre) ` Rv(pre) L(pre) L(post) RR RR Area Type 0.70 0.90 20% 0.23 7.23 12.64 6.13 49% Development Area 0.35 0.85 0% 0.05 0.79 6.32 5.65 89% Drinking Water Watersheds 0.40 0.85 1% 0.06 1.06 7.22 6.32 88% Other Rural Land Interim Manual, Page 70 rev. 16 Feb 1998 GEB Short Version BMP Computations For Worksheets 2 - 6 Albemarle County Water Protection Ordinance: Modified Simple Method Plan: Dunlora Gates Water Resources Area: Development Area Preparer: TED Date: 27- Mar -08 Project Drainage Area Designation Detained Area L storm pollutant export in pounds, L = [P(Pj)Rv /12 J [ C(A)2.72 JRvmeanrunoffcoefficient, Rv = 0.05 + 0.009(1) Pi small storm correction factor, 0.9 1 percent imperviousness P annual precipitation, 43" in Albemarle A project area in acres in subject drainage area, A = 4.11 C pollutant concentration, mg /I or ppm target phosphorus f factor applied to RR V required treatment volume in cy, 0.5" over imperv. area = A(1)43560(0.5/12)/27RRrequiredremoval , 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 AreaRoadsLengthWidthsubtotalLengthWidthsubtotal 0 0 0 673 26 17498 0 0 0 0 0 0 0 0 0 0 Driveways Length Width no. subtotal 0 0 Length Width no. subtotal 0 17498 and walks 0 0 0 0 40 18 18 12960 0 0 0 0 628 5 1 3140 0 0 Parking Lots 1 2 3 4 1 0 16100 0 2 3 4 Gravel areas Area 0 Area 2468.67 0 0 x 0.70 = 0 Structures Area no. subtotal x 0.70 =1728.069 Area no. subtotal 0 0 0 4050 3 12150 0 0 0 8934 1 8934 Actively - grazed pasture & Area 0 0 0 0 11170 1 11170 32254 yards and cultivated turf 0 x 0.08 = 0 Area 110710.93 x 0.08 =8856.8744ActivecroplandAreaArea Other Impervious Areas x0.25= 0 x0.25 =0 0 Impervious Cover o0 /0 0 43% Rv(post) V I(pre)I(post) 0.43 118.0 New Development (For Development Areas, existing impervious cover <= 20 %) C f 1 (pre) * Rv(pre) L(pre) L(post) RR % RR Area Type0.70 1.00 20% 0.23 5.80 10.96 5.15 47%Development Area0351.00 0% 0.05 0.63 5.48 4.85 88%Drinking Water Watersheds0-40 1.00 1% 0.06 0.85 6.26 5.41 86%Other Rural Landmin. values Redevelopment (For Development Areas, existing impervious cover > 20 %) C f 1 (pre) * Rv(pre) L(pre) L(post) RR % RR Area Type0.70 0.90 20% 0.23 5.80 10.96 5.73 52%Development Area0.35 0.85 0% 0.05 0.63 5.48 4.94 90%Drinking Water Watersheds0.40 0.85 1% 0.06 0.85 6.26 5.54 88%Other Rural Land Interim Manual, Page 70 rev. 16 Feb 1998 GEB RR required based on entire DA drainage of 5.12 Ac L(post) loading based on area draining to pond of 4.11Ac: Required %RR = [(RRpre /L(post)] * 100: based on VSWMH Equation 5 -22) 5.41 lb/day 10.96 lb/day 49% < -- choose an Bio- filter detention pond with a removal rate of 50% Pre Development Type A C AC Woodlands 5.12 0.25 1.28 Total 5.12 0.9 1.28 comp. C 0.44 0.25 0.396 Post Development Type A C AC 50' ROW 0.8 0.9 0.72 Houses 0.92 0.9 0.828 DW and Walk 0.44 0.9 0.396 Yards 2.51 0.3 0.753 Woodlands 0.45 0.25 0.1125 Total 5.12 12205.31 2.8095 comp. C 1 0.55 Un Detained Type A C AC 50' ROW 0 0.9 0 Houses 0.18 0.9 0.162 DW and Walk 0 0.9 0 Yards 0.83 0.3 0.249 Woodlands 1 01 0.251 0 Total 1 1.011 12205.31 1 0.411 comp. C 1 1 0.41 Graphical Modified Detained Tc: 10 mins Rational Rational Method Method Type Cf I Q 2yr 1.00 3 3.84 10yr 1.00 4 5.12 1 00 r 1.25 5.3 8.48 Graphical Modified Detained Tc: 10 mins Rational Rational Method Method Type Cf I Q 2yr 1.00 4.1 11.52 10yr 1.00 5.4 15.17 1 00y 1.25 7 24.58 Graphical Modified Detained Tc: 10 mins Rational Rational Method Method Type Cf I Q 2yr 1.00 4.1 1.69 10yr 1.00 5.4 2.22 1 00y 1.25 7 3.60 Graphical Modified Detained Tc: 10 mins Rational Rational Method Method Type A C AC 50' ROW 0.8 0.9 0.72 Houses 0.74 0.9 0.666 DW and Walk 0.44 0.9 0.396 Yards 2.07 0.3 0.621 Woodlands 0.06 0.25 0.015 Total 4.11 12205.31 2.418 comp. C 0.59 Freq.Cf I Q Qall Vreq Vreq 2yr 1.00 4.1 9.91 2.15 5819.175 9157.032 10yr 1.00 5.4 13.06 2.90 7617.45 12957.834 1 00y 1.25 7 21.16 4.88 12205.31 17709.819 According to BMP worksheet: 3635cf needed for WQV Staae - Storaae Stage- Storage: Conic Approximation Method PROJECT: DUNLORA GATES ENGINEER /DATE: TED 3/31/200812:04 Elevation Area Incremental Total Volume nlume Sc. -Ft)feet)Acres)acre -feet)acre -feet)cubic -teet) 2,436 384.00 0.056 0.000 0.000 0 3,292 385.00 0.076 0.066 0.066 2,853 4,178 386.00 0.096 0.086 0.151 6,580 5,093 387.00 0.117 0.106 0.257 11,207 6,0371 388.001 0.1391 0128 0.3851 16,766 7,0101 389.001 0.1611 0.1501 0.535 23,283 Stage- Storage 3x0 51) — -- - 380 00 = - --- --- 388 50 388,00 3X7 50 -- - 1871)11 386.50 10, 00 385011 - - -- 384.511 -- 183.50 0.000 i 5000 000 i i 1 T I I i I I 7 ?00,1100 10000.000 12500.11110 15000.000 17500.000 201100.0110 22500000 251100.1100 3/31/200812:10 PM 1Stg- sto.xls BasinFlow printout INPUT: Basin: Dunlora Gates 6 contour Areas Elevation(ft) 384.00 385.00 386.00 387.00 388.00 389.00 Start—Elevation(ft) 4 outlet structures outlet structure 0 orifice Dunlora Gates Pond2.txt Area(sf) computed vol.(cy) 2436.00 0.0 3292.00 105.7 4178.00 243.7 5093.00 415.1 6037.00 621.0 7010.00 862.3 384.00 vol.(cy) name: riser outlet area (sf) 3.142 diameter or depth (in) 24.000 width for rect. (in) 0.000 coefficient 0.620 invert (ft) 384.000 multiple 1 discharge out of riser outlet structure 1 orifice name: riser inlet area (sf) 0.418 diameter or depth (in) 8.750 width for rect. (in) 0.000 coefficient 0.620 invert (ft) 385.190 multiple 1 discharge into riser Outlet structure 2 weir name:riser top diameter (in)48.000 side angle 0.000 coefficient 3.300 invert (ft)388.000 multiple 1 discharge into riser transition at (ft)1.215 orifice coef.0.500 orifice area (sf)12.566 outlet structure 3 weir 1 11 name:emergency spillway length (ft)5.000 side angle 0.000 coefficient 3.000 invert (ft)388.000 multiple 1 discharge through dam Page 1 Dunlora Gates Pond2.txt 3 Inflow Hydrographs Hydrograph 0modified—Ration 2 Yearname. C 0,590 Area (acres)4.110 IDF file: a2 10,00 time of concrecedingnfactor 1.000 time increment limit (min) 1.00 360.00time fudge ar - - - - - _fa 1:(3 -- cam .s orm: specified duration Vol ume .... cfs) 499.69 3:35-6-- -peak flowintensity (in /hr)1.384 time to peak (min)10.000 57.00durationofpeak (min)routed true Hydrograph 1modified—Rational 10 Yearname. C 0,590 Area (acres)4.110 IDF file: a10 10.00timeofconcerecedingfactor1.000 time increment time limit (min) 1.00 360.00` -- fudge factorstormsrx' specified 1.00 J peak flow (cfs) 7duration5718-volume (dv) peak flow (cfs) - --7.55 1,intensity (in /hr)961 10.000timetopeak (min) of peak (min)58.00 duration routed true Hydrograph 2modified—Rational name: 100 Year C 0.590 Area (acres) 4.110 IDF file: a100 8,00 time of concentration factor 1.000 time increment limit (min) 1.00 360.00time fudge factor 1.00 sto m: specified duration volume Ccy)1068.18 peak flow (cfs)7`174 2.959intensity (in /hr) to (min)8.000timepeak duration of peak (min)59.00 routed true OUTPUT: Routing Method: storage - indication Hydrograph 0 page 2 Hydrograph 2 Routing summary of Peaks inflow (cfs) discharge (cfs) water level (ft) storage (cy) 100 Year 7.174 at 8.00 (min) 6.792 at 67.00 (min) 388.155 at 67.00 (min) 656.012 Mon Mar 31 17:20:59 EDT 2008 Page 3 Dunlora Gates Pond2.txt Routing summary of Peaks: 2 Year inflow cfs)3.356 at 10.00 min) discharge cfs)2.096 at 71.00 min) water level ft)386.573 at 71.00 min) storage cy)337.683 Hydrograph 1 Routing Summary of Peaks: 10 Year inflow cfs)4.755 at 10.00 min) discharge cfs)2.734 at 72.00 min) water level ft)387.288 at 72.00 min) storage cy)470.783 Hydrograph 2 Routing summary of Peaks inflow (cfs) discharge (cfs) water level (ft) storage (cy) 100 Year 7.174 at 8.00 (min) 6.792 at 67.00 (min) 388.155 at 67.00 (min) 656.012 Mon Mar 31 17:20:59 EDT 2008 Page 3 4ID 0 7 4 GIS -Web Geographic Data Services www. albemarle.org t'` (434) 296 -5832 v display purposes only • Aerial Imagery 2002 Gommonwealth of Virginia March 11, 2008 Short Version BMP Computations %W Worksheets 2 - 6 Albemarle County Water Protection Ordinance: Modified Simple Method Plan: Dunlora Gates Water Resources Area: Preparer: PBC Date: 15- Feb -08 Project Drainage Area Designation DA# 2 L storm pollutant export in pounds, L = [P(Pj)Rv /12 ] [ C(A)2.72 ] Rv mean runoff coefficient, Rv = 0.05 + 0.009(l) Pi small storm correction factor, 0.9 1 percent imperviousness y7Pannualprecipitation, 43" in Albemarle A project area in acres in subject drainage area, A C pollutant concentration, mg /I or ppm target phosphorus f factor applied to RR V 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) aRR removal efficiency, RR100 /L(post) Impervious Cover Computation (values in feet & square feet) Item r Development Area pre - development Area post - development Roads Length Width subtotal L th W'd Driveways and walks Parking Lots Gravel areas Structures 0 0 Length Width no. 0 0 0 0 1 2 0 Area Actively - grazed pasture & yards and cultivated turf Active crop land Other Impervious Areas Impervious Cover x 0.70 = Area no. 0 0 0 Area 196020 x 0.08 = Area x 0.25 = Area 0 95502 x 0.08 = Area 0 x 0.25 = 0 0% I(pre) subtotal 20 0 0 0 0 0 subtotal 0 0 0 0 0 3 4 0 subtotal 1 15000 0 0 0 Rv(post)V eng i th 0 0 600 0 0 0 0 0 New Development (For Development 0 0 C f subtotal Rv(pre) Length Width no. 0 0 0 0 0 0,0 0 7.15 _54P/o 0 0 0.35 1.00 3 4 1 2 5.95 90% 0 20000 1.00 1%0.06 Area 7.56 6.67 88% 0 min. values x 0.70 = subtotal Area no. 0 0 Redevelopment (For Development Areas, existing 15000 0 0 f 0 0 0 0' L(post)RR % RR Area 0 95502 x 0.08 = Area 0 x 0.25 = 0 0% I(pre) subtotal 20 0 0 0 0 0 subtotal 0 0 0 0 0 3 4 0 subtotal 1 15000 0 0 0 Rv(post)V 0.50 144.9 New Development (For Development tr "o1sAreas, existing impervious cover <= 20 ° /,) C f I (pre) *Rv(pre)L(pre)L(post)RR % RR Area Type 0.70 1.00 20%0.23 6.07 13.23 7.15 _54P/o Development Area 0.35 1.00 0%0.05 0.66 6.61 5.95 90%Drinking Water Watersheds 0.40 1.00 1%0.06 0.89 7.56 6.67 88%Other Rural Land min. values Redevelopment (For Development Areas, existing impervious cover > 20 %) C f I (pre) *Rv(pre)L(pre)L(post)RR % RR Area Type 0.70 0.90 20%0.23 6.07 13.23 7.76 59%Development Area 0.35 0.85 0%0.05 0.66 6.61 6.05 92%Drinking Water Watersheds 0.40 0.85 1 %!0.06 0.89 7.56 6.80 90%Other Rural Land Area 34750 19227 0 32254 7640.16 0 0 post) Interim Manual, Page 70 rev. 16 Feb 1998 GEB