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Home My WebLink AboutSDP200800098 Calculations 2008-09-29Montessori Communitv School Stotmwater Management Narrative Albemarle County, Virginia September 29, 2008 McKEECARSON LANDSCAPE ARCHITECTS - CONSULTING ENGINEERS - LAND PLANNERS 301 East High Street, Charlottesville, VA -22902 Tel: 434 - 979 -7522 Fax: 434 -977 -1 194 www.mckeecarson.com FABLE CAF CONITN'1S Introduction Project Description Methodology Summary Pre- Developinent Drainage Divides Map and runoff calculations Post - Development Drainage Divides Map and runoff calculations Cistern Drainage Area Drainage Area Map Removal Rate spreadsheet Runoff contribution calculation Cistern Routing results Post - development Drainage Divide Without Cistern Drainage Area Drainage Area Map Removal Rate spreadsheet Runoff contribution calculation Proposed Swale Capacity Analysis Storm Drain Calculations and Inlet Drainage Area Maps IN'T'RODUCTION Tlie Montessori Cornimmity school is proposing the construction of one school building during Phase 1 of a four phase construction process. Currently the school is served t-)} 7 terriporary modular type buildings and the new building would produce a more permanent teaching environment. The stormwater management plan utilizes a cistern to capture all of the runoff from the new building and surrounding collonades. The Modified Rational Method recommended by both the state of Virginia and Albemarle Countv) is used for hydrologic analvsis. PROJECT DESCRIPTION Montessori is located in Albemarle Countv_ at TMP 78 -12A. The proposed development is located in a CO Commercial office zoning region and has obtained a special use permit (SP- 2006- 00038, Approval date: 03/14/07) for the current use. The site includes a total of 6.71 acres. It is located along Rt. 250 on Pantops. Other Adjacent properties include `IMP 78 -11, 78-11A, 78- 11 B, 78 -12. METHODOLOGY Pre- De „, elopment Hydrology The school site is located on the top of a hill and the drains roughly in 4 different directions providing 4 separate drainage divides. Phase 1 corresponds to one of these drainage divides, which accounts for 1.76 ares. The site is currently developed. The primary objective of phase 1 is to build one building to teach children that are currently attending classes in a temporary structure. A cistern is being proposed to capture all of the runoff from this building rooftop. This water will be re -used inside the building to flush toilets and to irrigate the surrounding landscape. The drainage divides are analvzed using the Modified Rational Method. The 2 -tier and 10 -vr runoff in the pre - development condition are 3.66 cfs and 4.74 cfs respectively. Post - Development Hydrology Cistern Drainage Area The proposed cistern receives a total drainage area of approximately 0.106 acres. This is all rooftop rainwater and will be routed directly to the cistern. No surface water will be directed to the cistern. It is proposed that 6000 gallons be a permanent storage supple Vyitliiri the cistern that will hold rainwater and will be replenished with tap water, when the supply is Iow, via a float. An air gap system will prevent any contamination or backflow of the cistern water. Above the 6000 gallon mark, a 1” orifice will provide an outlet for the remaining 6000 gallons, provided for the purpose of detention. An overflow would exist at the top of the cistern for emergency release. Tlie total sire of the cistern is 12,000 gallons. Please see the Rainwater Harvesting Cistern System layout sheet "14 for details. For the purposes of this analysis, the cistern was assumed to be already 40% full at the beginning of both storms. The cistern begins to release from the orifice and act as a detention no longer retention) facility once it reaches the 6000 gallon mark. This condition seemed realistic and was therefore modeled as under this scenario. The cistern critical duration 2 -yr and 10 -yr routed discharge are 0.09 cfs and 0.16 cfs respectively. See attached summary and calculations. Please contact Alex Foraste at Mckee Carson if you have any questions regarding this system. Remaining Drainage Divide, excluding Cistern D.A. The total post - development drainage divide is approximately 1.86 acres. However, with the subtraction of the drainage area that the cistern captures, the remaining drainage area is 1.75 acres. Due to the fact that the cistern is capturing a large part of the added impervious area, the impervious area in the remaining drainage divide (31 %) is less than under the pre - development condition (49 %) and therefore requires a -34% removal rate or 0 %. Likewise, no detention is proposed for this drainage divide. The 2 -yr and 10 -yr discharges from this drainage area are 3.58 cfs and 4.64 cfs respectively. SUMMARY In summary, adding the routed discharge from the cistern to the remaining drainage divide yields a 2 and 10 year peak discharge of 3.61 cfs and 4.73 cfs, respectively. These runoffs are maintained to the pre - development 2 and 10 year discharges of 3.66 cfs and 4.74 cfs. See below for a summary of the discharges. Drainage Area /Divide Pre - development Area 2 Acre) Yr Runoff 10 -Yr Runoff Pre - deviopment DA 1.76 3766 4.74 Post - development Cistern 1 0.106 0.03 0.09 D.A. excluding cistern 1.74 3.58 4.64 Total 1 1.846 3.61 4.73 Table: Summary of Peak Discharges for site Montessori Community School Pre - Development Drainage Divide Map and runoff calculations V. Vf P y pp Ali ideas, designs, sections, dediis, andal— arethepropeilvMcKeeCarsonandFielaSpLrt oncepix, Lld erve iher common low cnpYngnt and other property nghfs in these drowtngs. - - - - - - - - - - - t. Carson and F 'd Soot Pn.. planr ore IT. 1 to be reproduced, charged, or coped,, any form d, -e, -soe- nor are they to be assigned to any Mirdt arts 0 b q0 HO abtap g the e .. ss peanissron and --1 of either McKee Corson as c6fil »y' 'McKe :Czars Designer:: Alexcrast; : ;: .: Source: Seelye Chart as modified in VDOT Drainage Manual (Appendix 6D -1) Notes: 1) Length of overland flow always <= 300 ft 2) Calculate a separate Tc for each ground cover condition along the flow path 260.00 Enter Length of Strip (ft) 12.80 Enter Slope (% or ft/100ft) 0.47 Enter Rational Method "C" value 1) For small drainage basins. t--or concrete cnann 0.00 5.00 5.00 0.80 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Tc(TOTAL) = Is, use 0.2"Tc Bottom Width (ft) Side Slope (Left) Side Slope (Right) Height (ft) Cross Sectional Area (ft ^2) Wetted Perimeter (ft) Hydraulic Radius (ft) Channel Slope Manning 'n' Velocity (ft/sec) Length of Flow (ft) Result Tc (minutes) 7.31 mins Caripariy;.:McKe:Crsiir :: >: i7i? Igti r : f Cez ..... `ti . ... TYPE Duration TOTAL PERCENT C CA Buildings in /hr) 2121.00 5.0 0.90 1908.90 Grass & Dirt 5.0 62796.60 0.35 21978.81 Concrete 971.00 5.0 0.90 873.90 Pervious Parking 5.0 0 0.40 0.00 Asphalt 10777.00 0.90 9699.30 Total 76665.60 100%34460.91 sqft)of total land area sqft) Average C =0.45 Tc(TOTAL) =7.30 mins Return Enter Intensity Period Duration Intensity (b /(d +Tc) ^e 2 5.0 4.6 in /hr) 5 5.0 cfs 10 5.0 6.0 25 5.0 50 5.0 100 5.0 7.8 C = 0.45 A = 1.76 acres 2 -Yr Peak Discharge Q =1 3.66 cfs 10 -Yr Peak Discharge Q =1 4.74 cfs 100 -Yr Peak Discharge Q100=1 6.19 cfs Montessori Community School Post - Development Drainage Divide Map and runoff calculations McKEECARSON nor e J gf LLI N E AR EARAI' 0Ll LUACRES 047 zXLu 02 4. C > X10 Fs Lu Z Z :E CL Z 0 < LU CL J HC 'M AkSLE I t o1111111 v A'; 5 t eb o CHECKED IE s e IRCJECT d sc.. eaw .w.. rr C' pt'. 0, Cop mcrnw — pi", ga, oprcdad c han g ed , K.. Carson arrd fl 1d S - - - - - - - - - - - - - orti ariy:_ NfcKe ...arson : - - iesig er tex c r ....... : te< :'920/41080 ...:.a Source: Seelye Chart as modified in VDOT Drainage Manual (Appendix 6D -1) Notes: 1) Length of overland flow always <= 300 ft 2) Calculate a separate Tc for each ground cover condition along the flow path 260.00 Enter Length of Strip (ft) 12.80 Enter Slope (% or ft/100ft) 0.47 Enter Rational Method "C" value 1) For small drainage basins. 2) For concrete channel s, 0.00 B 5.00 Si 5.00 Si 0.80 H 0.00 C 0.00 W 0.00 H 0.00 C 0.00 M 0.00 V 0.00 L 0.00 R Tc(TOTAL) - use 0.2*Tc ottom Width (ft) de Slope (Left) de Slope (Right) eight (ft) Cr Sectional Area (ft ^2) etted Perimeter (ft) ydraulic Radius (ft) hannel Slope anning 'n' elocity (ft/sec) ength of Flow (ft) suit Tc (minutes) 7.31 minsTc(TOTAL) - use 0.2*Tc ottom Width (ft) de Slope (Left) de Slope (Right) eight (ft) Cr Sectional Area (ft ^2) etted Perimeter (ft) ydraulic Radius (ft) hannel Slope anning 'n' elocity (ft/sec) ength of Flow (ft) suit Tc (minutes) 7.31 mins Coi ,pariy: :A?tcKee: ars.... 1641 : =: <:: os or os TYPE AREA (A)PERCENTI C CA Buildings 5134.00 6%0.90 4620.60 Grass & Dirt 62848.60 78%0.35 21997.01 Concrete 2262.00 3%0.90 2035.80 Asphalt 10777.00 13%0.90 9699.30 Bio- filter 0.00 0.0%0.20 0.00 Total 81021.60 100%1 38352.71 sgft) f total land area (sqft) Average C = 0.47 Open Space 62848.60 78% i"% .D ' kY 'N rv S' a ' & ee^F nf Tc(TOTAL) 7.30 ® Return Period Enter Duration Calculated sity (b /(d+ (in /hr) 2 5.0 4.6 cfs 5 5.0 6.88 cfs 10 5.0 6.0 25 5.0 50 5.0 100 5.0 7.8 Acceptable values for duration are between 5 and 60 minutes. Use values outside this range at risk. Peak Discharge Q = CIA 000 C = 0.47 A = 1.86 jacres 2 -Yr Peak Discharge Q =4.07 Tcfs 10 -Yr Peak Discharge Q =5.27 cfs 100 -Yr Peak Discharge Q100 6.88 cfs C = 0.47 A = 1.86 jacres 2 -Yr Peak Discharge Q =4.07 Tcfs 10 -Yr Peak Discharge Q =5.27 cfs 100 -Yr Peak Discharge Q100 6.88 cfs Montessori Community School Cistern Drainage Area Map and calculations Al CISTERN DRAINAGE AREA ;; A 0. 1055 ACRES C = 0. 90 CFS020, 49 010 = 0, 63 CFS and Ud lh-l- 1 — ony McKEECARSON tr as ix LU 44 > LU z z 00 u Ix W z4A LU E Z LU U s— c--pt, nA rtlaps besgns, sacirons, Batons. Pr tl Dlpns are •he DrppBtlY of McKee Carson ono riaitl Sport ConceDfs. o P q nsen o, archer McKee Carson w Freld Sport Short Version BMP Computations For Worksheets 1 -1 Modified Simple Method Plan: Montessori School Water Resources Area: Development Area Preparer: Alex Date:10- Jun -08 Project Drainage Area Designation D.A. consisting of rooftop,colonnades and porch to Cistern 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 =0.11 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 0 0 0 0 0 0 0 0 0 Driveways Length Width no. subtotal Length Width no. subtotal 0 0 and walks - - - 0 0 0 0 0 0 0 0 0 0 0 0 Parking Lots Area 1 Area 2 Area 3 Area 4 Area 1 Area 2 Area 3 Area 4 0 0 0 0 0 0 0 Gravel areas Area 1 Area 2 subtotal 0 0 0 Area 1 Area 2 subtotal 0 0 0 0 0 x 0.70 = 0 0 0 0 x 0.70 =0StructuresAreano. subtotal Area no. subtotal 0 1 0 4487 1 0 1 0 2 0 1 0 Actively - grazed pasture & Area 0 1 0 0 Area 1 0 4595 yards and cultivated turf 4487 x 0.08 = 358.96 0 x 0.08 =0ActivecroplandAreaArea 0x025= 0 Other Impervious Areas Area 1 Area 2 Area 3 0x0.25= Area 1 Area 2 Area 3 0 0 0 0 0 0 0 0 0ImperviousCover8%100% Rv(post) V l(pre) e)post) 095 7.1 New Development (For Development Areas, existing impervious cover 20 %) C f I (pre)' Rv(pre) L(pre) L(post) RR RR Area Type0.70 1.00 20% 023 015 062 047 76% Development Area0.35 1.00 0% 0.12 004 0.31 0.27 87% Drinking Water Watersheds0.40 1.00 1 % 0.12 005 0.35 031 87% 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 Type0.70 0.90 20% 0.23 0.15 062 048 78% Development Area0350.85 0% 0.12 004 031 027 89% Drinking Water Watersheds0.40 0.85 1 % 0.12 0.05 0.35 031 89% Other Rural Land rev_ 30 May 2002 Cartipariy; = Mci`$e:Crsvi TYPE AREA(A)PERCENTI C CA Buildings 4595.00 100%0.90 4135.50 Grass & Dirt 0.00 0%0.35 0.00 Concrete 0.00 0%0.90 0.00 Asphalt 0.00 0%0.90 0.00 Bio- filter 0.00 0.0%0.20 0.00 Total 4595.00 100%4135.50 sqft) f total land area (sqft) Average C = 0.90 Open Space 1 0.00 1 0% Tc(TOTAL) =5.00 mins Return Period Enter Duration Calculated sity (b /(d+ (in /hr) 2 5.0 5.2 cfs 5 5.0 0.83 10 5.0 6.7 25 5.0 50 5.0 100 5.0 8.8 Acceptable values for duration are between 5 and 60 minutes. Use values outside this range at risk. Peak Discharge Q = CIA 000 C = 0.90 A = 0.11 1 acres 2 -Yr Peak Discharge Q =1 0.49 cfs 10 -Yr Peak Discharge Q =0.63 cfs 100 -Yr Peak Discharge Q100 0.83 cfs C = 0.90 A = 0.11 1 acres 2 -Yr Peak Discharge Q =1 0.49 cfs 10 -Yr Peak Discharge Q =0.63 cfs 100 -Yr Peak Discharge Q100 0.83 cfs Cistern routing results 0. 18 — 0. 16 2 year critical storm 10 year critical storm 0. 14 100 year critical storm 0. 12 m tM 0. 1 N 0. 08 7 0. 06 0. 04 0. 02 L 0 1 19 37 55 73 91 109 127 145 163 181 199 217 235 253 271 289 307 325 343 361 379 397 415 433 Time (min) l Cistern routing results 10 9 8 7 6 0 M m 5 w 4cc 3 2 1 0 1 18 35 52 69 86 103 120 137 154 171 188 205 222 239 256 273 290 307 324 341 358 375 392 409 426 Time (min) Cistern routing results 2 year critical duration storm BasinFlow printout INPUT: Basin: 7 Contour Areas Elevation(ft)Area(sf)Computed Vol.(cy) 0.00 0.00 0.0 1.00 186.00 2.3 2.00 241.00 10.2 3.00 270.00 19.6 3.75 276.00 27.2 5.80 235.00 46.6 7.60 0.00 51.8 3 Outlet Structures Outlet structure 0 Orifice name: 2yr area (so 0.035 diameter or depth (in) 1.000 width for rect. (in) 5.000 coefficient 0.500 invert (ft) 3.750 multiple 1 discharge into riser Outlet structure 1 Orifice name: emergency spillway area (sf) 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 5.800 multiple 1 discharge into riser Outlet structure 2 Culvert name: outlet multiple 1 discharge out of riser D (in)6.000 11 (111)0.000 Length (ft)47.000 Slope 0.023 MaIlning's n 0.013 Inlet coeff. Ke 0.500 Equation constant set 3 Invert (ft) 3.750 I hlflow Hydrographs Hydrograph 0 Modified Rational name: post dev captured C 0.900 Area (acres) 0.106 IDF file: a2 time of concentration (min) 5.00 receding factor 1.670 time increment 1.00 time limit (min) 400.00 fudge factor 1.00 storm: crit. duration (215 iterations) volume (cy)29.81 peak flow (cfs)0.035 intensity (in/hr)0.364 time to peak (min)5.000 duration of peak (min)380.04 routed true OUTPUT: Routing Method: storage- indication Hydro graph 0 Routing Summary of Peaks: post dev captured inflow (cfs) 0.035 at 5.00 (min) discharge (cfs) 0.026 at 384.00 (min) water level (ft) 3.828 at 386.00 (min) storage (cy) 28.015 2 year peak storm BasinFlow printout INPUT: M2 Basin: 7 Contour Areas multiple l Elevation(ft)Area(sf)Computed Vol.(cy) 0.00 0.00 0.0 1.00 186.00 2.3 2.00 241.00 10.2 3.00 270.00 19.6 3.75 276.00 27.2 5.80 235.00 46.6 7.60 0.00 51.8 3 Outlet Structures Outlet structure 0 Orifice name: 2yr area (sf) 0.035 diameter or depth (in) 1.000 width for rect. (in) 5.000 coefficient 0.500 invert (ft) 3.750 multiple 1 discharge into riser Outlet structure 1 Orifice name: emergency spillway area (sf) 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 5.800 multiple 1 discharge into riser Outlet structure 2 Culvert name: outlet multiple l discharge out of riser D (in)6.000 h (in)0.000 Length (ft)47.000 Slope 0.023 Manning's n 0.013 hllet coeff. Ke 0.500 Equation constant set 3 Invert (ft) 3.750 1 Inflow Hydrographs Hydrograph 0 Modified Rational name: post dev captured C 0.900 Area (acres) 0.106 IDF file: a2 time of concentration (min)5.00 receding factor 1.670 time increment 1.00 time limit (min)200.00 fudge factor 1.00 storm: peak intensity (zero duration) volume (cy)7.33 peak flow (cfs)0.494 intensity (in /hr)5.180 time to peak (min)5.000 duration of peak (min)0.00 routed true OUTPUT: Routing Method: storage- indication Hydrograph 0 Routing Summ, inflow (cfs) discharge (cfs; water level (ft) storage (cy) iry of Peaks: post dev captured 0.494 at 5.00 (min) 0.000 at 0.00 (min) 1.653 at 13.00 (min) 7.212 10 year critical duration storm BasinFlow printout INPUT: Basin: 7 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 0.00 0.00 0.0 1.00 186.00 2.3 2.00 241.00 10.2 3.00 270.00 19.6 3.75 276.00 27.2 5.80 235.00 46.6 7.60 0.00 51.8 3 Outlet Structures Outlet structure 0 Orifice name: 2yr I area (st)0.035 diameter or depth (in)1.000 width for rect. (in)5.000 coefficient 0.500 invert (ft)3.750 multiple 1 discharge into riser Outlet structure 1 Orifice name: emergency spillway area (sf) 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 5.800 multiple 1 discharge into riser Outlet structure 2 Culvert name: outlet multiple I discharge out of riser D (in)6.000 11 (In)0.000 Length (ft)47.000 Slope 0.023 Manning's n 0.013 Inlet coeff. Ke 0.500 Equation constant set 3 Invert (ft)3.750 I Inflow Hydrographs Hydrograph 0 Modified name: post dev captured C 0.900 Area (acres) 0.106 IDF file: a10 37.61 time of concentration (min)5.00 receding factor 1.670 time increment 1.00 time limit (min)200.00 fudge factor 1.00 storm: crit. duration (63 iterations) volume (Cy)37.61 peak flow (cfs)0.108 intensity (in/hr)1.131 time to peak (min)5.000 duration of peak (min)150.12 routed true OUTPUT: Routing Method: storage- indication Hydro graph 0 Routing Summa inflow (cfs) discharge (cfs) water level (ft) storage (cy) ry of Peaks: post dev captured 0.108 at 5.00 (min) 0.085 at 156.00 (min) 4.164 at 157.00 (min) 31.387 10 year peak storm BasinFlow printout INPUT: Basin: 7 Contour Areas Elevation(ft)Area(sf)Computed Vol.(cy) 0.00 0.00 0.0 1.00 186.00 2.3 2.00 241.00 10.2 3.00 270.00 19.6 3.75 276.00 27.2 5.80 235.00 46.6 7.60 0.00 51.8 3 Outlet Structures Outlet structure 0 Ori f icc name: 2yr area (sf) 0.035 diameter or depth (in) 1.000 width for rect. (in) 5.000 coefficient 0.500 invert (ft) 3.750 multiple 1 discharge into riser Outlet structure 1 Orifice name: emergency spillway area (sf)0.196 diameter or depth (in)6.000 width for rect. (in)0.000 coefficient 0.500 invert (ft)5.800 multiple 1 discharge into riser Outlet structure 2 Culvert name: outlet multiple 1 discharge out of riser D (in) 6.000 h (in) 0.000 Length (ft) 47.000 Slope 0.023 Manning's n 0.013 Inlet coeff. Ke 0.500 Equation constant set 3 Invert (ft) 3.750 1 Inflow Hydrographs Hydrograph 0 Modified name: post dev captured C 0.900 Area (acres) 0.106 IDF file: a10 time of concentration (min) 5.00 receding factor 1.670 time increment 1.00 time limit (min) 200.00 Fudge factor 1.00 storm: peak intensity (zero duration) volume (cy)9.43 peak flow (cfs)0.636 intensity (in /hr)6.666 time to peak (min)5.000 duration of peak (min)0.00 routed true OUTPUT: Routing Method: storage- indication Hydrograph 0 Routing Summa inflow (cfs) discharge (cfs) water level (ft) storage (cy) ry of Peaks: post dev captured 0.636 at 5.00 (min) 0.000 at 0.00 (min) 1.898 at 13.00 (min) 9.281 100 year critical duration storm BasinFlow printout INPUT: Basin: 7 Contour Areas Elevation(ft)Area(so Computed Vol.(cy) 0.00 0.00 0.0 1.00 186.00 2.3 2.00 241.00 10.2 3.00 270.00 19.6 3.75 276.00 27.2 5.80 235.00 46.6 7.60 0.00 51.8 3 Outlet Strictures Outlet structure 0 Orifice name: 2yr area (so 0.035 diameter or depth (in) 1.000 width for rect. (in) 5.000 coefficient 0.500 invert (ft) 3.750 multiple 1 discharge into riser Outlet structure 1 Orifice name: emergency spillway area (so 0.196 diameter or depth (in) 6.000 width for rect. (in) 0.000 coefficient 0.500 invert (ft) 5.800 multiple 1 discharge into riser Outlet structure 2 Culvert name: outlet multiple 1 discharge out of riser D (in)6.000 h (in)0.000 Length (ft)47.000 Slope 0.023 Manning's n 0.013 Inlet coeff. Ke 0.500 Equation constant set 3 Invert (ft)3.750 1 Inflow Hydrographs Hydrograph 0 Modified Rational name: post dev captured C 0.900 Area (acres) 0.106 IDF file: a100 time of concentration (min) 5.00 receding factor 1.670 time increment 1.00 time limit (min) 400.00 fudge factor 1.00 storm: crit. duration (1 1 1 iterations) volume (cy) 57.42 peak flow (cfs) 0.204 intensity (in /hr) 2.137 time to peak (min) 5.000 duration of peak (min) 120.04 routed true OUTPUT: Routing Method: storage- indication Hydrograph 0 Routing Summary of Peaks: post dev captured inflow (cfs) 0.204 at 5.00 (min) discharge (cfs) 0.156 at 126.00 (min) water level (ft) 5.049 at 127.00 (min) storage (cy) 39.858 Montessori Community School Post - Development Drainage Divide Without Cistern Drainage Area Map and calculations h v 3 ' a ` a 3 ' a ' s Q , 4 t I 2 AAA k 3 4 o 3Q3 9 R 3 3 f a S 3 n r I MONTESSORI SCHOOL m DRAINAGE AREAS Albemarle County, Virginia UNDETAINED DRAINAGE DIVIDE WITHOUT CISTERN D.A. a- 3 v _ Short Version BMP Computations For Worksheets 1 -1 Modified Simple Method Plan Montessori School Water Resources Area: Development Area Preparers . Alex Date:24- Sep -08 Project Drainage Area Designation D A. remaining excluding D A. captured by Cistern 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 =1.75 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 0 0 0 0 0 0 0 0 10777 Driveways Length Width no subtotal Length Width no. subtotal 0 10777 and walks - - 0 0 0 0 0 0 0 0 0 0 0 971 Parking Lots Area 1 Area 2 Area 3 Area 4 Area 1 Area 2 Area 3 Area 4 0 0 10777 0 0 0 10777 0 0 0 0 0GravelareasArea1Area2subtotalArea1Area2subtotal 14965 0 1 4965 x 0 70 = 10475.5 Structures 14965 0 14965 x 0 70 =10475.5AreanosubtotalAreano. subtotal 1350 1 1350 0 1 0 595 1 590 1 590 176 1 176 1 176 2116 1350 1 2121Actively- grazed pasture & Area Area yards and cultivated turf 0 x 0.08 = 0 0 x 0 08 =0ActivecroplandAreaArea 0x0.25= 0 Other Impervious Areas Area 1 Area 2 Area 3 0x0.25= Area 1 Area 2 Area 3 0 0 0 0 0 Impervious Cover 49% 0 0 0 0 31% Rv(post) V I(pre) e)I(post) 0.33 36.1 New Development (For Development Areas, existing impervious cover 20 %) C f I (pre) * Rv(pre) L(pre) L(post) RR RR Area Type0.70 1.00 20% 049 520 3.50 -1.70 49% Development Area0.35 1.00 0% 0.49 2.60 1.75 -0.85 49% Drinking Water Watersheds0.40 1.00 1 % 049 2 97 200 -097 49% 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 Type0700.90 20% 049 5.20 3 50 -1 18 34% Development Area0.35 085 0% 049 260 1 75 -046 26% Drinking Water Watersheds0400.85 1 % 049 2 92 2.00 -048 24% Other Rural Land rev 30 May 2002 Go parrjcKe:isiiz f esfgrisr :CeAz - -t f _:::::: >::s g624l >::: =: Source: Seelye Chart as modified in VDOT Drainage Manual (Appendix 6D -1) Notes: 1) Length of overland flow always <= 300 ft 2) Calculate a separate Tc for each ground cover condition along the flow path 280.00 Enter Length of Strip (ft) 11.00 Enter Slope (% or ft/100ft) 0.46 Enter Rational Method "C" value 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.0 Tc(TOTAL) = els, use 0.2*Tc Bottom Width (ft) Side Slope (Left) Side Slope (Right) Height (ft) Cross Sectional Area (ft ^2) Wetted Perimeter (ft) Hydraulic Radius (ft) Channel Slope Manning 'n' Velocity (ft/sec) Length of Flow (ft) Result Tc (minutes) 1 7.93 mins orrrpatiy :arsaii := fQ= 08.. :.. =»`: `. . TYPE AREA (A)PERCENTI C CA Buildings 2121.00 3%0.90 1908.90 Grass & Dirt 60075.00 80%0.35 21026.25 Concrete 2262.00 3%0.90 2035.80 Asphalt 10777.00 14%0.90 9699.30 Bio- filter 0.00 0.0%0.20 0.00 Total 75235.00 100%34670.25 sqft) f total land area (sqft) Average C = 0.46 Open Space 1 60075.00 1 80% Tc(TOTAL) _7.93 mins Return Period Enter Duration Calculated sity (b /(d+ (in /hr) 2 5 4.5 cfs 5 5.0 6.05 10 5.0 5.8 25 5.0 50 5.0 100 5.0 7.6 Acceptable values for duration are between 5 and 60 minutes. Use values outside this range at risk. Peak Discharge Q = CIA 000 C = 0.46 A = 1.73 1 acres 2 -Yr Peak Discharge Q =1 3.58 cfs 10 -Yr Peak Discharge Q =4.64 cfs 100 -Yr Peak Discharge Q100 6.05 cfs C = 0.46 A = 1.73 1 acres 2 -Yr Peak Discharge Q =1 3.58 cfs 10 -Yr Peak Discharge Q =4.64 cfs 100 -Yr Peak Discharge Q100 6.05 cfs Montessori Community School Proposed Swale Capacity Analysis JO Y' T F = ------- - - - - -- - - - - -- YO Z1 ------- - - - - -= Z2 B Grass Type IThick vegetation Limiting Manning 'n'0.035 Permissible Velocity (fps)5.00 Design Information (Input Channel Invert Slope So =0.0120 ftlft Channel Manning's N N =0.035 Bottom Width B =0.0 ft Left Side Slope Z1 =3.00 ft/ft Right Side Slope Z2 =300 ft/ft Freeboard Height F =ft Available Channel Depth Y =0.35 ft Available Channel Depth H=1.00 Normal Flow Condition (Calculated) Channel Capacity (Discharge)Q =0.5 cfs Froude Number Fr =0.59 Flow Velocity V =1.4 fps Flow Area A =0.4 sq ft Top Width T =2.1 ft Wetted Perimeter P =22 ft Hydraulic Radius R =0.2 ft Hydraulic Depth D =0.2 ft Specific Energy Es =0.4 ft Centroid of Flow Area Yo =01 ft Specific Force Fs =0.0 kip Channel Normal Depth 9/26/2008, 10:49 AM Montessori Community School Storm Drain Calculations and Inlet Drainage Area Maps Ee ti it N 000 — Q ' / r . .. MONTESSORI SCHOOL DRAINAGE AREAS Albemarle County, Virginia 2 POST- DEVELOPMENT DRAINAGE DIVIDE zz n m a 0 z C) m m Ln G) z 0 m O m z 0 03 Rr " rn m n r 0 n u m m z 0 z n (n m m 0 z 0n 4J Q7 CD oM ;o8nn4 o ;o tt C C 00000000 0 a 0 M 0 L vm mo 1 o X C c00 c)n C>CD C)ZZ O M 00000 m z 0 m Mc 0 0 o o CD iP 0 Z C)CD (oC) 0 0 (1)0 jj 0CD 4 CD 4.O.mnT T COPI z m 00000c,c) co 0 CD CO 2N)z M I C: > C>r- 000C)C) 0 C,J -J 0M C,N)4 CD (D M 0 U00 0 0C) 0 0 000 0D Oi0 0 0) CY)a) 0)CY) a)a) D (D C)0 0 C)0 DI z ... zZ - nn > 0 0 c:> CD 0 C) M —BIDAlN)f 0 n CN0006coin00 M M Z m m 0 0 ID w q n 0Z7zDQto0wID mz tcnI z OD0 1-)1 "' 10, 1COFcccoP.wIn Z a G) ZE A C 1,Z C: C -0olIZ > M 0 p P CD P D 0 z Z U) A 0n0 C)0 CDcn 0R 0 4 N)N) 00 y)8cn uCm 0 . I 0 C >i'f A o) w N—rj r., >Io 9 C ' ) 4 0 0 O)D Mln -0 C/) 0 n 0 N)NI Z—ommw D 00I)l0j0M m z A m ok00000000 ni 03 Rr " rn m n r 0 n u m m z 0 Montessori Communitv School Erosion & Sediment Control Narrative Albemarle County, Virginia September 29, 2008 McKEECARSON LANDSCAPE ARCHITECTS - CONSULTING ENGINEERS - LAND PLANNERS 301 East High Street, Charlottesville, VA -22902 Tel: 434 - 979 -7522 Fax: 434-977-1194 www.mckeecarson.com Table Of Contents: l l'(1511)I1 lnl Scli(nIlt L(,ntr I i'lall Lhccl:l; ;t 1 -1'(1 1t,t1 dj1cj Sec11ll ent L((I1tr(d NarnitiVe Cdit1 ent trap de';i "11 Erosion and Sediment Control Plan Checklist ldt'``,11 A ... ,1a .` !' ;1'. . i t r , 7 RK, PIA NARRATIVE Project Description - Briefly describes the nature and pugmsc of the land disturhin activity, and the area (acres) to be disturbed. cam' Existing Site f im lihnns - A description of the misting topography, vcge'rak"i and drainage. Adjacent ,Areas - A description of neighboring areas such as streams, lakes, residential areas, roads, etc., which might be affected by the land disturbance. Off -Site Areas - Describe any off -site land disturbing activities that will occur including borrow sites, waste or surplus areas, etc.). Will any other areas he disturbed`? Soils - A brief description of the soils on the site giving such information as soil name, mapping unit, erodibilit_y, permeability, depth, texture aI]d soil structure. Critical Arcas - A description of areas on the site which have potentially serious erosion problems (e.g., steep slopes, channels, wet weather /underground springs, etc.). J Erosion Sediment Control u11eas - s h' methods )s . , description t the r c ] ds which .vill be used to c ontroi erasion and sedimentation on the site. (Controls should meet the specifications in Chapter 3 of the Handbook.) y Permanent Stabilization - A brief description, including specifications, 1)f how the site will be stabilized after construction is completed. Stormwater Ptnloff Considerati)_)ns - Will the developincilt site cause an increase in peak 12111off rates? Will the increase in runoff cause flooding or channel degradation do« nstrearu ? Describe the strategy to control stormy. ater runoff. Calculation` - Detailed for the dcsigll ()f tempOrar`" scdim ;lt permanent stcmli ',ater l;;'iCllmmi hanins, drG'.'rshms , ct1 mne1; , ctc. 111A& c1cultion` 14 Pic- nri d nnanco11ana - A schrh le T regukr in' , ti )n , ,. n ;)` rer ;lc- rc, !tr 1 c r):;ion cr)r.troI str)ICtIIrc:: ,ilc)],Ild he rth. Pernlancnt s "j,jizatkin shall R aPP oilisrawhai SC Cr an A.' 1 of the site- temporary soil ' s 1 that - 1 be is In, may not he at findenudedare "pplic(I "vitllin seen days to delgradcbut "Vill rennin dormant (undisturbed) for l than AIbethatare 'oPermanentstabilizationSllal be left dormant for more than Lyle year. L/ Sediment basins and tMPL perimeter dikes, sediment barr and other measures intended to tF, -11) shall be constructed as a Ill step in any land disturbing activity ancl shallbemadefunctionalbeforeupslopelanddisturbancetakesplace. 1.5(5): Stabilization measures shall'be applied to earthen structure J such as dams, dikes and diversions immediately after installation 1.5(17): wh construction vellicle access routes i pavedpublicroads, provisions shall he made to minimize the transport of sediment by tracking onto the paved surface. Where sediment is transported onto a public road surface, the road shall be cleaned thoroughly at the end of each day. S shall be by shove trremovedfromtheroadsors%vecping , and trn to a sediment con disposal area. Street washing shall be allowed only after sediment is removed in this i Tilis provision shall apply to individual subd ' v ' sloll lots a 'yell as t larger land disturbing activities 1.5118): All c a sediment control measures AM be removed within 30 days after Final site stabilization of after the teMPOral-Y measures are no 1011Qcr needed, unless fop FF PPedI othenvise authorized by the local program administrator - nistrato, . sediment and the disturbed soil areas resulting from the disposition of temporary measures Shall be PeFF"a" - C d to prevent further erosion and sedimentation. til 111es .,it lati )il h) ti'i .'ui Unlflil Ct ".;i. t( r InClUdt 111V llildnlal '.vhich nll`,_ assist in i( Caiiil till: Indicate Forth -The direction of north in relation to the site. Limits of Clearin Grading - :areas which arc to be cleared and gnWed. L Ex1St1nu COntOtlrS - Tile exlSting CllntonrS Of the Site. Final Contours - Changes to the existing contours, inclLiding, f11111 drainage patterllS. J Exisbng Ve - the existing tree lines, `,,rasscd areas, or unique ve " tltlon. Soils - The boundaries of different soil types. Existing Drainage Patterns - The dividing lines and the direction of flo%v for the different drainage areas. Include the size (acreage) of each drainage area. Critical Erosion Areas - Areas with P otentially serious erosion problems. (See fChapter 6 of the Handbook for criteria.) Site Development - Show all improvements such as buildings, parking lots, access roads, utility constriction, etc. L/ Location of Practices - The locations of erosion and sediment controls and stormwater management practices used on the site. Use the standard in(! abbreviations in Chapter 3 of the Handbook.) Off -Site Areas - Identify any oft -site land disturbing activities w.- borruw waste areas, etc.). Show location of erasion controls. A there sufficient information to assure adequate protection and stabilization'?) Detail Drawings - : \n,,' stmwural practices used that are not referenced to the ENA Handbook or local handbooks she ild be explained ZIA illustrated with detail drawings. Duet CortrOl Cann,trllcl C - n l- ntrance - `,'ash rack if wessarv. v Nate for the erosion control measures located on suhdivi:,ion lots: ""1_hc ucvcioper shall reserve the ri`aht to install and maintain all erosion control nicasures requirc by this flan rcgardlm of the We cif am lot. Ai IA `haste ;lreas - Must comply with all waste area re ulations (Sep Attachment #11 ), VOA Borrmv ;'areas - Must complti' with all borrow area regulations (See attachment #2). 25` Slopes (Section 12.3 of Zoning Ordinance) 1 1/ 18/92 Erosion and Sediment Control Narrative Montessori Community School Erosion and Sediment Control Narrative June 10` f ', 2008 Project Description The project parcel TMP 78 -12A is zoned CO. It has a special use permit for the use of a school on -site. The site is 6.71 acres total, but only 1.41 acres would be disturbed during phase 1, as defined by the limits of construction. One sediment trap is proposed to capture the disturbed area, in addition to other silt fences. Existing Site Conditions The property currently is used by the school and is already developed. A gravel parking lot, temporary and permanent classroom buildings exist on the site. One building will be removed and one new building is proposed to be constructed. The existing gravel lot would remain. Adjacent Areas The site is located along Rt. 250 next to the Eckerd Pharmacy store along Rolkin Road. The vicinity map is attached on the cover sheet of the Final site plan. Off -site Areas No off -site areas will be disturbed as part of this project. Soils Soil information has been obtained from the Soil Survey GIS website of Albemarle County of Virginia. Soil boundaries are shown on the map and each soil type is identified by a symbol on the Erosion and Sediment Control Plan Detail sheet 11. Critical Areas There are critical slopes within the limits of construction. The preliminary site plan, Special Use Permit SUP) and critical slopes waiver have already been submitted and approved by the County. The slope along Rolkin Road is steep and the potential for erosion there is greatest, however, the majority of the slope will not be graded, a silt fence is proposed along the edge of grading near the top of the hill and the added vegetation, as specified in the Landscape Plan, sheet 06 should aid the current condition of the hill over time. Erosion and Sediment Control Measures Unless otherwise indicated, all erosion and sediment control practices will be constructed and maintained according to minimum standards and specifications of the Virginia Erosion and Sediment Control Handbook (1992). Safety Fence — E &S Std. & Spec. 3.0 A protective barrier is to be installed as specified on the plan. Montessori Community School Pa; e 2 Erosion and Sediment Control Narrative 2. Temporary Construction Entrance - E &S Std. & Spec. 3.02 i1 Temporary Construction Entrance shall be installed where indicated. The Construction Entrance shall be paved with a wash rack. Silt Fence with Wire Support- ES--S Std. & Spec. 3.05 Silt Fence with `vire support shall be installed in a sequenced manner as shown on the plan. 4. Storm Drain Inlet Protection — E &S Std. & Spec. 3.07 Storm Drain inlet protection shall be installed on the plan as specified. Culvert Inlet Protection — E &S Std. & Spec. 3.08 Culvert inlet protection shall be installed on the plan as specified. Temporary Fill Diversion — E &S Std. & Spec. 3.10 Temporary fill diversions shall be placed where indicated on the plan. Temporary Sediment Trap — E &S Std. & Spec. 3.13 The temporary sediment trap will be expanded as shown on the plans. Outlet Protection — E &S Std. & Spec. 3.18 Outlet Protection shall be installed where shown on the plans. Rip rap should be brownstone rip rap with the size shown on the plans. See attached calculations for dimensions. 9. Topsoiling - E &S Std. & Spec. 3.30 Topsoil will be stripped from areas to be graded and stockpiled for later use. Stockpile locations shall be located on -site and are to be stabilized with temporary vegetation. Selection of the seed mixture will depend on the time of year it is applied. Prior to land - disturbing activities, the contractor shall submit a supplementary E & S plan to the owner covering the off -site stockpile area which would have to be approved by the plan approving authority before any off -site activity commences. 10. Temporary Seeding: E &S Std. & Spec. 3.31 All areas temporary disturbed by construction will be stabilized with temporary seeding immediately following the conclusion of clearing operations. Unless otherwise specified, permanent seeding will be done with a seed mix approved by the Owner prior to application in accordance to E &S Std. and Spec. 3.31 of the 1992 Virginia Erosion and Sediment Control Handbook. 11. Permanent Seeding: E &S Std. & Spec. 3.32 All areas disturbed by construction will be stabilized with permanent seeding immediately following the conclusion of clearing operations, unless specified otherwise. Unless otherwise specified, permanent seeding will be done with a seed mix approved by the Owner prior to application in accordance to E &S Std. and Spec. 3.32 of the 1992 Virginia Erosion and Sediment Control Handbook. 12. Mulching: E &S Std. & Spec. 3.35 Areas which have been permanently seeded should be mulched immediately following seeding,. Straw is the preferred type of mulching. Montessori Con) Intl nity School Pale 3 Erosion and Sediment Control :narrative 13. Matting: E &S Std. & Spec. 3.36 The EC -2 matting shall be placed where indicated on Erosion & Sediment Control Plan. 14. Tree Protection: E &S Std. & Spec. 338 Tree Protection shall be installed prior to any earth disturbing activities as specified on the plan and in accordance with the Erosion and Sediment Control detail plan, sheet 11. 15. Dust Control - E &S Std. & Spec. 3.39 Dust Control practices shall be employed to prevent surface and air movement of dust from exposed soil surfaces and reduce the presence of airborne substances which may be harmful or injurious to human health, welfare, or safety, or to animal or plant life. Permanent Stabilization Permanent or temporary soil stabilization shall be applied to denuded areas within seven days after final grade is reached on any portion of the site. Temporary soil stabilization shall be applied within seven days to denuded areas that may not be at final grade but will rernain dormant for longer than 30 days. Permanent stabilization shall be applied to areas that are to be left dormant for more than one year. 2. During construction of the project, soil stockpiles and borrow areas shall be stabilized or protected with sediment trapping measures. The applicant is responsible for the temporary protection and permanent stabilization of all soil stockpiles on site as well as borrows areas and soil intentionally transported from the project site. A permanent vegetative cover shall be established on denuded areas not otherwise permanently stabilized. Permanent vegetation shall not be considered established until a ground cover is achieved that is uniform, mature enough to survive and will inhibit erosion. 4. Perimeter dikes, sediment barriers and other measures intended to trap sediment shall be constructed as a first step in any land - disturbing activity and shall be made functional before upslope land disturbance takes place. Stabilization measures shall be applied to earthen structures such as dikes and diversions immediately after installation. Minimum Standards Installation of all erosion and sediment control measures shall conform to the 1992 Virginia Erosion and Sediment Control Handbook. Critical measures required on this project are summarized below: Permanent or temporary soil stabilization shall be applied to denuded areas within seven days after final grade is reached on any portion of the site. Temporary soil stabilization shall be applied within seven days to denuded areas that may not be at final grade but will remain dormant for longer than 30 days. Permanent stabilization shall be applied to areas that are to be left dormant for more than one year. Montessori Con lilt] nity School Erosion and Sediment Control Narrative Pa; e 4 During construction of the pr01 ect, soil stockpiles and borrow areas shall be stabilized or protected with sediment trapping measures. The applicant is responsible for the temporary protection and permanent stabilization of all soil stockpiles on site as well as borrows areas and soil intentionally transported from the project site. A permanent vegetative cover shall be established on denuded areas not otherwise penmanently stabilized. Permanent vegetation shall not be considered established until a ground cover is achieved that, is uniform, mature enough to survive and will inhibit erosion. 4. Sediment traps, perimeter dikes, sediment barriers and other measures intended to trap sediment shall be constricted as a first step in any land- disturbing activity and shall be made functional before upslope land disturbance takes place. 5. Stabilization measures shall be applied to earthen structures such as dams, dikes and diversions immediately after installation. 6. Sediment traps shall be designed and constructed based upon the total drainage area to be served by the trap. a. The minimum storage capacity of a sediment trap shall be 134 cubic yards per acre of drainage area and the trap shall only control drainage areas less than three acres. b. Surface nmoff from disturbed areas that is comprised of flow from drainage areas greater than or equal to three acres shall be controlled by a sediment basin. The minimum storage capacity of a sediment basin shall be 134 cubic yards per acre of drainage area. The outfall system shall, at a minimum, maintain the structural integrity of the basin during a twenty -five year storm of 24 -hour duration. Runoff coefficients used in runoff calculations shall correspond to a bare earth condition or those conditions expected to exist while the sediment basin is utilized. Cut and fill slopes shall be designed and constructed in a manner that will minimize erosion. Slopes that are found to be eroding excessively within one year of permanent stabilization shall be provided with additional slope stabilizing measures until the problem is corrected. Concentrated runoff shall not flow down cut or fill slopes unless contained within an adequate temporary or permanent channel, flume or slope drain structure. 9. Whenever water seeps from a slope face, adequate drainage or other protection shall be provided. 10. All stone sewer inlets that are made operable during construction shall be protected so that sediment -laden water cannot enter the conveyance system without First being filtered or otherwise treated to remove sediment. 11. Before newly constructed stormwater conveyance channels or pipes are made operational, adequate outlet protection and any required temporary or permanent channel lining shall be installed in both the conveyance channel and receiving channel. itlontessori Community School Erosion and Sediment Control Narrative Pale 5 l?. When work in a live watercourse is performed, precautions shall be taken to minimize encroachment, control sediment transport and stabilize the work area to the greatest extent possible during constriction. Nonerodible material shall be used for the construction of causeways and cofferdams. Earthen fill may be used for the strictures if armored by nonerodible cover materials. 13. When a live watercourse must be crossed by construction vehicles more than twice in any six - month period, a temporary vehicular stream crossing constructed of nonerodible material shall be provided. 14. All applicable federal, state and local regulations pertaining to working in or crossing live watercourses shall be met. 15. The bed and banks of a watercourse shall be stabilized immediately after work in the watercourse is completed. 16. Underground utility lines shall be installed in accordance with the following standards in addition to other applicable criteria: a. No more than 500 linear feet of trench may be opened at one time. b. Excavated material shall be placed on the uphill side of trenches. Effluent from dewatering operations shall be filtered or passed through approved sediment trapping device, or both, and discharged in a manner that does not adversely affect flowing streams or off -site property. d. Material used for backfilling trenches shall be properly compacted in order to minimize erosion and promote stabilization. e. Restabilization shall be accomplished in accordance with these regulations. f. Applicable safety regulations shall be complied with. 17. Where construction vehicle access routes intersect paved or public roads, provisions shall be made to minimize the transport of sediment by vehicular tracking onto the paved surface. Where sediment is transported onto a paved or public road surface, the road surface shall be cleaned thoroughly at the end of each day. Sediment shall be removed from the roads by shoveling or sweeping and transported to a sediment control disposal area. Street washing shall be allowed only after sediment is removed in this manner. This provision shall apply to individual development lots as well as to larger land- disturbing activities. 18. All temporary erosion and sediment control measures shall be removed within 30 days after final site stabilization or after the temporary measures are no longer needed, unless otherwise authorized by the local program authority. Trapped sediment and the disturbed soil areas resulting from the disposition of temporary measures shall be pernanently stabilized to prevent further erosion and sedimentation. Montessori ('onununit School Page 6 Erosion and Sediment Control larratiNe 19. Stormwater Runoff Consideration (NIS -19) Properties and waterways downstream from development sites shall be protected froth sediment deposition, erosion and damage due to increases in volume, velocity and peak flow rate of' stormwater runoff for the stated frequency storm of 24 -hour duration in accordance with the following standards and criteria: Concentrated stormwater runoff leaving a development site shall be discharged directly into an adequate natural or man -made receiving channel, pipe or storm sewer system. For those sites where runoff is discharged into a pipe or pipe system, downstream stability analyses at the outfall of the pipe or pipe system shall be performed. b. Adequacy of all channels and pipes shall be verified in the following manner: 1) The applicant shall demonstrate that the total drainage area to the point of analysis within the channel is one hundred times greater than the contributing drainage area of the project in question; or 2) (a) Natural channels shall be analyzed by the use of a two -year storm to verify that stormwater will not overtop channel banks nor cause erosion of channel bed or banks; and b) All previously constructed man -made channels shall be analyzed by the use of a ten - year storm to verify that stormmvater will not overtop its banks and by the use of a two - year storm to demonstrate that stonnwater will not cause erosion of channel bed or batiks; and c) Pipes and storm sewer systems shall be analyzed by the use of a ten -year storm to verify that stormwater will be contained within the pipe or system. c. If existing natural receiving channels or previously constructed man -made channels or pipes arc not adequate, the applicant shall: 1) Improve the channel to a condition where a ten -year stone will not overtop the banks and a two -year storm will not cause erosion to the channel bed or banks; or 2) Improve the pipe or pipe system to a condition where the ten -year storm is contained within the appurtenances; or Develop a site design that will not cause the pre - development peak runoff rate from a two -year storm to increase when runoff outfalls into a natural channel or will not cause the pre - development peak runoff rate from a ten -year storm to increase when runoff outfalls into a man -made channel; or d) Provide a combination of channel improvement, storm water detention or other measures which is satisfactory to the plan - approving authority to prevent downstream erosion. d. The applicant shall provide evidence of permission to make the improvements. Montessori Community School Erosion and Sediment Control Narrative Page 7 e. All hydrologic analyses shall be based on the existing watershed characteristics and the ultimate development of the subject project. f. If the applicant chooses an option that includes storm water detention he shall obtain approval from the locality of a plan for maintenance of the detention facilities. The plan shall set forth the maintenance requirements of the facility and the person responsible for performing the maintenance. g. Outfall from a detention facility shall be discharged to a receiving channel, and energy dissipaters shall be placed at the outfall of all detention facilities as necessary to provide a stabilized transition from the facility to the receiving channel. h. All on -site channels must be verified to be adequate. i. Increased volumes of sheet flows that may cause erosion or sedimentation on adjacent property shall be diverted to a stable outlet, adequate channel, pipe or pipe system, or to a detention facility. j. In applying these storm water runoff criteria, individual lots or parcels in a residential, commercial or industrial development shall not be considered to be separate development projects. Instead, the development, as a whole, shall be considered to be a single development project. Hydrologic parameters that reflect the ultimate development condition shall be used in all engineering calculations. k. All measures used to protect properties and waterways shall be employed in a manner which minimizes impacts on the physical, chemical and biological integrity of rivers, streams and other waters of the state. General construction notes for Erosion and Sediment Control plans The plan approving authority must be notified one week prior to the pre- construction conference, one week prior to the commencement of land disturbing activity, and one week prior to the final inspection. All erosion and sediment control measures will be constructed and maintained according to minimum standards and specifications of the Virginia Erosion and Sediment Control Handbook and Virginia Regulations VR 625 -02 -00 Erosion and Sediment Control Regulations. All erosion and sediment control measures are to be placed prior to or as the first step in clearing. 4. A copy of the approved erosion and sediment control plan shall be maintained on the site at all times. Prior to commencing land disturbing activities in areas other than indicated on these plans including, but not limited to. off -site borrow or waste areas), the contractor shall submit a supplementary erosion control plan to the owner for review and approval by the plan approving authority. 6. The contractor is responsible for installation of an_v additional erosion control measures necessary to prevent erosion and sedimentation as determined by the plan approving authority. 1lontessori Community School Erosion and Sediment Control Narrative Page 8 All disturbed areas are to drain to approved sediment control measures at all times during land disturbing activities and during site development Until Final stabilization is achieved. During dewatering operations, water will be pumped into an approved Filtering device. 9. The contractor shall inspect all erosion control measures periodically and after each runoff producing rainfall event. Any necessary repairs or cleanup to maintain the effectiveness of the erosion control devices shall be made immediately. 10. All fill material to be taken from an approved, designated borrow area. 11, All waste materials shall be taken to an approved waste area. Earth fill shall be inert materials only, free of roots, stumps, wood, rubbish, and other debris. 12. Borrow or waste areas are to be reclaimed within 7 days of completion per Zoning Ordinance section 5.1.25. 13. All inert materials shall be transported in compliance with section 13 -301 of the Code of Albemarle. 14. Borrow, fill or waste activity involving industrial -type power equipment shall be limited to the hours of 7:OOam to 9:OOpm. 15. Borrow, fill or waste activity shall be conducted in a safe manner than maintains lateral support, or order to minimize any hazard to persons, physical damage to adjacent land and improvements, and damage to any public street because of slides, sinking, or collapse. 16. The developer shall reserve the right to install, maintain, remove or convert to permanent stormwater management facilities where applicable all erosion control measures required by this plan regardless of the sale of any lot, unit, building or other portion of the property. 17. Temporary stabilization shall be temporary seeding and mulching. Seeding is to be at 75 lbs /acre, and in the months of September to February to consist a 50/50 mix of Annual Ryegrass and Cereal Winter Rye, or in March and April to consist of Annual Rye, or May through August to consist of German Millet. Straw mulch is to be applied at 801bs /100sf. Alternatives are subject to approved by the County erosion control inspector. 18. Permanent stabilization shall be lime and fertilizer, permanent seeding, and mulch. Agricultural grade limestone shall be applied at 901bs /1000sf, incorporated into the top 4 -6 inches of soil. Fertilizer shall be applied at 1000lbs /acre and consist of a 10 -20 -10 nutrient mix. Permanent seeding shall be applied at 180lbs /acre and consist of 95 Kentucky 31 or Tall Fescue and 0 -5 Perennial Ryegrass or Kentucky Bluegrass. Straw mulch is to be applied at 80lbs /l00sf. Alternatives are subject to approved by the County erosion control inspector. Montessori Community School Erosion and Sediment Control Narrative Page y 19. Maintenance: All measures are to be inspected weekly and after each rainfall. Any damage or clogging to structural measures is to be repair immediately. Silt traps are to be cleaned when 50` /i, of the wet storage volume is filled Nvith sediment. All seeded areas are to be reseeded when necessary to achieve a good stand of grass. Silt fence and diversion dykes which are collecting sediment to half their height must be cleaned and repaired immediately. 20. All temporary erosion and sediment control measures are to be removed within 30 days of final site stabilization, when measures are no longer needed, subject to approval by the County erosion control inspector. Management Strategies Construction should be sequenced so that grading operations can begin and end as quickly as possible. The job contractor shall be responsible for the installation and maintenance of all erosion and sediment control practices. Sediment trapping measures shall be installed as a first step in grading and shall be seeded and mulched immediately following installation. 4. Areas which are not to be disturbed shall be clearly marked by flags, signs, etc. Soil stockpiles and borrow areas Topsoil will be stripped from areas to be graded and stockpiled for later use. Stockpile locations shall be located on -site and are to be stabilized with temporary vegetation. Selection of the seed mixture will depend on the time of year it is applied. No off -site areas will be disturbed as part of this project. Construction Schedule It is our understanding that site work construction will be beginning once final approvals have been received. Construction Sequence The project shall be constructed in the order as mentioned below: Install the construction entrances. Clear for and install all erosion and sediment control measures. as shown on the plan. Stockpile topsoil and stabilize as required. Perform grading (cut /fill) on the site for road and stabilize them with Construction Road Stabilization (CRS), Erosion Control Blanket (EC?), etc. Install all culverts before /simultaneously with the grading as shown on the plan. Place culvert inlet protection, as shown on the plan, immediately after installation of proposed culverts and bridges. 4. Remove all the erosion and sediment control measures when site is completely stabilized. Montessori Community School Erosion and Sediment Control Narrative Maintenance 1'ae 10 All erosion and sediment control measures will be checked weekly, after each runoff - producing rainfall event and repaired as necessary. Seeded areas will be checked regularly to ensure that a good stand is maintained. Reseeding and straw mulch will be applied as necessary. Sediment Trap Design 1) Enter Project No.: 2) Enter trap identification number: 3) Enter additional trap identification (e.g. centerline stationing): 4) Enter area draining into trap (AC) ( <3.0): storage required (CY): storage required (CF): wet storage required (CF): dry storage required (CF): 0706 ST 0.35 47 1259 630 630 5) Enter depth of excavation (ft) ( < =4.0):2.00 6) Enter side slopes of excavated wet storage area (_:1):1.00 area of top of wet storage (1st try) (SF):456 1st try width of wet storage (ft):15 1st try length of wet storage (ft):30 7) Enter a new value for width:10.00 8) Enter a new value for length:45.60 top area based on new width & length (SF):456 bottom area (SF):250 average area (bottom, top) (SF):353 volume based on average area (CF):706 volume required (CF):630 okay?YES 9) Repeat steps (8) and (9) until volume requirement is exceeded and dry storage 11w => 2 10) Enter H(0), which is also D(2) (ft) (0.5— H(0) -4.0): 1.00 11) Enter side slopes of dry storage volume (_:1): 2.00 width of basin (ft): 15 length of basin (ft): 70 dry storage 11w: 4.67 volume based on average area (CF): 753 dry storage volume required (CF): 630 okay? YES ST AREA =0.35 Ac. ELEVATION L Avg. W D Z BOTT 50 5 2.00 1.00 WET 46 10 1.00 2.00 DRY 70 15 WEIR 2.1 2.0 Montessori Community School Stormwater Management Narrative Albemarle County, Virginia June 10, 2008 McKEECARSON LANDSCAPE ARCHITECTS - CONSULTING ENGINEERS - LAND PLANNERS 301 East High Street, Charlottesville, VA -22902 Tel: 434 - 979 -7522 Fax: 434-977-1194 www.mckeecarson.com TABLE OF CONTENTS Introduction Project Description Methodology Summary Pre - Development Drainage Divides Map and runoff calculations Post - Development Drainage Divides Map and runoff calculations Cistern Drainage Area Drainage Area Map Removal Rate spreadsheet Runoff contribution calculation Cistern Routing results Post - development Drainage Divide Without Cistern Drainage Area Drainage Area Map Removal Rate spreadsheet Runoff contribution calculation Storm Drain Calculations INTRODUCTION The Montessori Community school is proposing the construction of one school building during Phase 1 of a four phase construction process. Currently the school is served by temporary modular type buildings and the new building would produce a more permanent teaching environment. The stormwater management plan utilizes a cistern to capture all of the runoff from the new building and surrounding collonades. The Modified Rational Method recommended by both the state of Virginia and Albemarle County) is used for hydrologic analysis. PROJECT DESCRIPTION Montessori is located in Albemarle County at TMP 78 -12A. The proposed development is located in a CO Commercial office zoning region and has obtained a special use permit (SP- 2006- 00038, Approval date: 03/14/07) for the current use. The site includes a total of 6.71 acres. It is located along Rt. 250 on Pantops. Other adjacent properties include TMP 78 -11, 78 -11A, 78- 11 B, 78 -12. METHODOLOGY Pre - Development Hydrology The school site is located on the top of a hill and the drains roughly in 4 different directions providing 4 separate drainage divides. Phase 1 corresponds to one of these drainage divides, which accounts for 1.83 ares. The site is currently developed. One building is being demolished, while one new building is proposed to be built. Therefore, the increase in overall runoff is not significant. However, the school has decided to install a cistern to harvest rainwater from the rooftop of the new building and surrounding collonades. This water will be re -used inside the building to flush toilets and to irrigate the surrounding landscape. The drainage divides are analyzed using the Modified Rational Method. The 2 -yr and 10 -yr runoff in the pre - development condition are 3.85 cfs and 4.98 cfs respectively. Post - Development Hydrology Cistern Drainage Area The proposed cistern receives a total drainage area of approximately 0.106 acres. This is all rooftop rainwater and will be routed directly to the cistern. No surface water will be directed to the cistern. It is proposed that 8000 gallons be a permanent storage supply within the cistern that will hold rainwater and will be replenished with tap water, when the supply is low, via a float. An air gap system will prevent any contamination or backflow of the cistern water. Above the 8000 gallon mark, a 1" orifice will provide an outlet for the remaining 3700 gallons, provided for the purpose of detention. An overflow would exist at the top of the cistern for emergency release. For the purposes of this analysis, the cistern was assumed to be already 40% full at the beginning of both storms. The cistern begins to release from the orifice and act as a detention no longer retention) facility once 67% full. This condition seemed realistic and was therefore modeled as under this scenario. The cistern critical duration 2 -yr and 10 -yr routed discharge are 0.082 cfs and 0.128 cfs respectively. See attached summary and calculations Remaining Drainage Divide, excluding Cistern D.A. The total post - development drainage divide is approximately 1.94 acres. However, with the subtraction of the drainage area that the cistern captures, the remaining drainage area is 1.83 acres. Due to the fact that the cistern is capturing a large part of the added impervious area, the impervious area in the remaining drainage divide (27 %) is less than under the pre - development condition (48 %) and therefore requires a -39% removal rate or 0 %. Likewise, no detention is proposed for this drainage divide. The 2 -yr and 10 -yr discharges from this drainage area are 3.76 cfs and 4.86 cfs respectively. SUMMARY In summary, adding the routed discharge from the cistern to the remaining drainage divide yields a 2 and 10 year peak discharge of 3.84 cfs and 4.99 cfs, respectively. These runoffs are maintained to the pre - development 2 and 10 year discharges of 3.85 cfs and 4.98 cfs. See below for a summary of the discharges. Drainage Area /Divide Pre - development Area Acre) 2 -Yr Runoff 10 Yr Runoff Pre -devlo ment DA 1.831 3.851 4.98 Post - development Cistern 1 0.106 0.082 0.128 D.A. excluding cistern 1.83 3.76 4.86 Total 1 1.94 3.84 4.99 Table: Summary of Peak Discharges for site Notes: 1) Length of overland flow always <= 300 ft 2) Calculate a separate Tc for each ground cover condition along the flow path 260.00 Enter Length of Strip (ft) 12.80 Enter Slope (% or ft/100ft) 0.47 Enter Rational Method " value 7.31 Result Tc (minutes) 0.94 Velocity (ft/s) IF UNPAVED 1.18 Velocity (ft/s) IF PAVED 0.00 Result Tc (minutes) IF UNPAVED 0.00 Result Tc (minutes) IF PAVED Source: Kirpich Chart as modified in VDOT Drainage Manual (Fig. 1.5.1.2 on Page 1 -14) Notes: 1) For small drainage basins. z) t--or concrete cnanner. 0.00 B 5.00 S 5.00 S 0.80 H 0.00 C 0.00 VI 0.00 0.00 0.00 0.00 0.00 0.00 Tc(TOTAL) - use 0.2 *Tc Dttom Width (ft) de Slope (Left) de Slope (Right) eight (ft) ross Sectional Area (ft ^2) Petted Perimeter (ft) ydraulic Radius (ft) hannel Slope anning 'n' elocity (ft/sec) ength of Flow (ft) esult Tc (minutes) 7.31 1 mins Source: Seelye Chart as modified in VDOT Drainage Manual (Appendix 6D -1) Girl Meee isulgnr- e crsrat- a1re, :fl6f it ... ::'`': TYPE Enter Duration TOTAL PERCENT C CA 4.6 Buildings 5.0 3418.00 cfs 0.90 3076.20 6.0 Grass & Dirt 25 64634.00 0.35 22621.90 Concrete 100 971.00 7.8 0.90 873.90 Pervious Parking 0 0.40 0.00 Asphalt 10777.00 0.90 9699.30 Total 79800.00 100%36271.30 sqft)of total land area sqft) Average C =0.45 Tc(TOTAL) =7.30 mins Return Period Enter Duration Intensity Intensity (b /(d +Tc) in /hr)2 5.0 4.6 5 5.0 6.51 cfs 10 5.0 6.0 25 5.0 50 5.0 100 5.0 7.8 00 C = 0.45 A = 1.83 acres 2 -Yr Peak Discharge 0 =1 3.85 cfs 10 -Yr Peak Discharge Q10 =1 4.98 cfs 100 -Yr Peak Discharge Q100 =6.51 cfs C = 0.45 A = 1.83 acres 2 -Yr Peak Discharge 0 =1 3.85 cfs 10 -Yr Peak Discharge Q10 =1 4.98 cfs 100 -Yr Peak Discharge Q100 =6.51 cfs A t ol DRAINA ARE A - 1. 8 3 ``r A E C OA- 5 Q2 = 3: 8 IFS } Q 10 - 4 9 8 CG F S f' s ! I a - - 1, McKee Co— and N,fd )port Coot, Lt0 f. 1111 theu common law copVnoht and other prop, l nghN in These drowings Sections, details, and plans. 1e the properly of McKee Carson and Lio i —Non; —1. to be rep changed, o coped in any form o, manner I feu r,K C— —.2 I SsoGoa9 nor are they to b, assigned to any 4 sd onrty w (hoof first obtaining Me express perm , and consent of either McKee cc... or Field spot Concepts, Litl McKEECARSON LU D D LU O Q O H o QOQ W m D H Q Z 11J W we O o g O W, Q O N Z W — o W Q E > Z jX a W O a C C W Z N X W Montessori Community School Post - Development Drainage Divide Map and runoff calculations Short Version BMP Computations For Worksheets 1 -1 Modified Simple Method Plan: Montessori School Water Resources Area: Development Area Preparer: Alex Date: 10- .Jun -07 Project Drainage Area Designation RR for whole drainage 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 = 1.94 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) 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 0 0 0 0 0 0 0 0 10777 0 10777 Driveways Length Width no. subtotal Length Width no. subtotal and walks - - 0 -0 0 0 0 0 0 0 0 0 0 971 0 2262 Parking Lots Area 1 Area 2 Area 3 Area 4 Area 1 Area 2 Area 3 Area 4 0 0 0 0 0 0 0 0 0 0 Gravel areas Area 1 Area 2 subtotal Area 1 Area 2 subtotal 14965 0 14965 x 0.70 = 10475.5 14965 0 14965 x 0.70 =10475.5 Structures Area no. subtotal Area no. subtotal 1596 1 1596 1 1046 1 1046 1 590 1 590 4539 1 186 1 186 3418 595 1 595 5134 Actively - grazed pasture & Area Area yards and cultivated turf 0 x 0.08 = 0 0 x 0.08 =0 Active crop land Area Area 0x0.25= 0 0x0.25=0 Other Impervious Areas Area 1 Area 2 Area 3 Area 1 Area 2 Area 3 0 0 0 0 0 0 0 0 Impervious Cover 34%36% I(pre) e)I(post) Rv(post) V 0.37 46.9 19354 New Development (For Development Areas, existing impervious cover <= 20 %) C f I (pre) * Rv(pre) L(pre) L(post) RR % RR Area Type 0.70 1.00 20% 0.36 3.78 4.45 0.66 15%Development Area 035 1.00 0% 0.36 1.89 2.22 0.33 15%Drinking Water Watersheds 0.40 1.00 1% 0.36 2.16 2.54 0.38 15%Other Rural Land min. values Redevelopment (For Development Areas, existing impervious cover > 20 %) C f [(pre)* Rv(pre) L(pre) L(post) RR % RR Area Type 0.70 0.90 20% 0.36 3.78 4.45 1.04 23%Development Area 0.35 0.85 0% 0.36 1.89 2.22 0.62 28%Drinking Water Watersheds 0.40 0.85 1% 0.36 2.12 2.54 0.74 29%Other Rural Land rev. 30 May 2002 TYPE Enter Duration AREA (A)PERCENT C CA 4.6 Buildings 5 5134.00 6% 0.90 4620.60 10 Grass & Dirt 6.0 64692.00 78% 0.35 22642.20 50 Concrete 2262.00 3% 0.90 2035.80 7.8 Asphalt 10777.00 13% 0.90 9699.30 Bio- filter 0.00 0.0% 0.20 0.00 Total 82865.00 100%38997.90 sqft)f total land area sqft) Average C =0.47 78%Open Space 64692.00 Tc(TOTAL)=7.30 mins Return Period Enter Duration Calculated sity (b /(d+ (in /hr) 2 5.0 4.6 cfs 5 5.0 7.00 cfs 10 5.0 6.0 25 5.0 50 5.0 100 5.0 7.8 Acceptable values for duration are between 5 and 60 minutes. Use values outside this range at risk. arge Q = CIA C = 0.47 A = 1.90 acres 2 -Yr Peak Discharge Q =1 4.14 cfs 10 -Yr Peak Discharge Q, =5.36 cfs 100 -Yr Peak Discharge Q,00=7.00 cfs I hN n these 0—nl All ideo desgns 5a,-, d, aL, and Puns are ma Pro Percy C—, -d Fald ,., Ca-011. 01 x ,, P,-, ax rot to A of manner hotsoe— n are they to , to any tn Part S "W C~ 7 16 bt—Ing and consent of comer -K- Cason v I.Id Sport Cance McKEECARSON D j LU 0 Cn 0 Z V > OC LU 00 u z LU vJ Z 7 LLJ 0 E 0 0 Z 0 C Q < LU Lu 4A 0 CL G— CH—ED 11— Pnir oe h Ltd Montessori Community School Cistern Drainage Area Map and calculations w McKEECARSON Ir y + ++ ++ 1 V ^_ ` • + rry " ---- E f P r I r i$ I p t n j r 0 dw,• € HC VAN ` AISLE u o au a /i Q ad CISTERN DRAINAGE AREA .; __ , o : WZ E Z A = 0, 1055 ACRES Q Lu Z W n W C = 0. 90 °° a ' , d N wwM 02 = 0. 49 CFS Q10 = 0. 63 CFS @$ ' r pp + ;; FbwF. v cer ww vow r ax' .iro. _+.,. rNxo. n.o tw oe... o.oxw+• o t ` CHECSEO PBe E sCnE r.7 bAiE o6 -let Sox r,as cwr w..,r ROGER am HEEf rruMBEu hts m Yraie Aovnngs. All rdeos desgns, sachons, details, and p ons ore the gope tY of McKee Carson and Fuld Sport Concap•s. Lrc These ?loos ora no• to be ep' odocatl, changed, a coped in onv lone or monner vha! soa.er, nor are they to be assianen to anY thrd party .,.h -f first obtaining the express pertnbstan gtl cor:sent of e:iha• McKee Corson a Freltl Sport Concepts. ltd McKee \.Oem and f.d Sport Co co[ , Ltd. I-- ccmmon law copynght and other popenl ' a Short Version BMP Computations For Worksheets 1 -1 Modified Simple Method Plan: Montessori School Water Resources Area: Development Area Preparer: Alex Date: 10- Jun -08 Project Drainage Area Designation D.A. consisting of rooftop, colonnades and porch to Cistern 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 = 0.11 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)/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 0 0 0 0 0 0 0 0 0 0 0 Driveways Length Width no. subtotal Length Width no. subtotal and walks - - 0 -0 0 0 0 0 0 0 0 0 0 0 0 0 Parking Lots Area 1 Area 2 Area 3 Area 4 Area 1 Area 2 Area 3 Area 4 0 0 0 0 0 0 0 0 0 0 Gravel areas Area 1 Area 2 subtotal Area 1 Area 2 subtotal 0 0 0x0.70= 0 0 0 0x0.70=0 Structures Area no. subtotal Area no. subtotal 0 1 0 4487 1 0 1 0 2 0 1 0 0 1 0 0 1 0 4595 Actively - grazed pasture & Area Area yards and cultivated turf 4487 x 0.08 = 358.96 0 x 0.08 =0 Active crop land Area Area 0 x0.25= 0 0 x0.25=0 Other Impervious Areas Area 1 Area 2 Area 3 Area 1 Area 2 Area 3 0 0 0 0 0 0 0 0 Impervious Cover 8%100% I(pre)post) Rv(post) V 0.95 7.1 New Development (For Development Areas, 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 0.15 0.62 0.47 76%Development Area 0.35 1.00 0% 0.12 0.04 0.31 0.27 87%Drinking Water Watersheds 0.40 1.00 1% 0.12 0.05 0.35 0.31 87%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 0.15 0.62 0.48 78%Development Area 0.35 0.85 0% 0.12 0.04 0.31 0.27 89%Drinking Water Watersheds 0.40 0.85 1 % 0.12 0.05 0.35 0.31 89%Other Rural Land rev. 30 May 2002 09 Pot TYPE Enter Duration AREA(A)PERCENTI C CA 5 Buildings 10 4595.00 100% 0.90 4135.50 50 Grass & Dirt 100 0.00 0% 0.35 0.00 Concrete 0.00 0% 0.90 0.00 Asphalt 0.00 0% 0.90 0.00 Bio- filter 0.00 0.0% 0.20 0.00 Total 4595.00 1 100%4135.50 sqft)f total land area sqft) Average C =0.90 0%Open Space 0.00 Tc(TOTAL)=5.00 mins Return Period Enter Duration Calculated sity (b /(d+ (in /hr) 5.2 6.7 8 .8 2 5.0 5 5.0 10 5.0 25 5.0 50 5.0 100 5.0 000 Acceptable values for duration are between 5 and 60 minutes. Use values outside this range at risk. Peak Discharge Q = CIA C = 0.90 A = 0.11 acres 2 -Yr Peak Discharge Q =0.49 cfs 10 -Yr Peak Discharge Q =0.63 cfs 100 -Yr Peak Discharge Q,00 0.83 cfs Acceptable values for duration are between 5 and 60 minutes. Use values outside this range at risk. Peak Discharge Q = CIA C = 0.90 A = 0.11 acres 2 -Yr Peak Discharge Q =0.49 cfs 10 -Yr Peak Discharge Q =0.63 cfs 100 -Yr Peak Discharge Q,00 0.83 cfs Montessori Community School Post - Development Drainage Divide Without Cistern Drainage Area Map and calculations Short Version BMP Computations For Worksheets 1 -1 Modified Simple Method Plan: Montessori school Water Resources Area: Development Area Preparer: Alex Date: 10- Jun -08 Project Drainage Area Designation D.A. remaining excluding D.A. captured by Cistern 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) 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 =1.83 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) 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 0 0 0 0 0 0 0 0 10777 0 10777 Driveways Length Width no. subtotal Length Width no. subtotal and walks - - 0 0 0 0 0 0 0 0 0 0 0 971 0 0 Parking Lots Area 1 Area 2 Area 3 Area 4 Area 1 Area 2 Area 3 Area 4 10777 0 0 0 10777 0 0 0 0 0 Gravel areas Area 1 Area 2 subtotal Area 1 Area 2 subtotal 14965 0 14965 x 0.70 =10475.5 14965 0 14965 x 0.70 =10475.5 Structures Area no. subtotal Area no. subtotal 1596 1 1596 0 1046 1 1046 0 590 1 590 186 1 186 3418 1 0 595 Actively - grazed pasture & Area Area yards and cultivated turf 0 x 0.08 =0 0 x 0.08 =0 Active crop land Area Area 0x0.25=0 0x0.25=0 Other Impervious Areas Area 1 Area 2 Area 3 Area 1 Area 2 Area 3 0 0 0 0 0 0 0 0 Impervious Cover 48%27% I(pre)I(post) Rv(post) V 0.30 33.7 New Development (For Development Areas, existing impervious cover 20 %) C f I (pre) * Rv(pre) L(pre) L(post)RR RR Area Type 0.70 1.00 20% 0.48 5.15 3.33 1.82 55%Development Area 0.35 1.00 0% 0.48 2.58 1.67 0.91 55%Drinking Water Watersheds 0.40 1.00 1% 0.48 2.94 1.90 1.04 55%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% 0.48 5.15 3.33 1.30 39%Development Area 0.35 0.85 0% 0.48 2.58 1.67 0.52 31 %Drinking Water Watersheds 0.40 0.85 1% 0.48 2.89 1.90 0.55 29%Other Rural Land rev. 30 May 2002 TYPE AREA (A) PERCENTI C CA Buildings 539.00 1%0.90 485.10 Grass & Dirt 66165.00 83%0.35 23157.75 Concrete 2262.00 3%0.90 2035.80 Asphalt 10777.00 14%0.90 9699.30 Bio- filter 0.00 0.0%0.20 0.00 Total 79743.00 100%j 35377.95 sqft) f total land area (sqft) Average C = 0.44 Open Space 1 66165.00 1 83% Tc(TOTAL)= 7.30 mins Return Period Enter Duration Calculated sity (b /(d+ (in /hr) 4.6 6.0 7.8 10 -Yr Peak Discharge Q, =4.86 e b d 2 5.0 0.82 49.02 10.5 5 5.0 10 5.0 0.73 46.95 9.5 25 5.0 50 5.0 100 5.0 0.55 31.09 5 Acceptable values for duration are between 5 and 60 minutes. Use values outside this range at risk. Peak Discharge Q = CIA C = 0.44 A = 1.83 acres 2 -Yr Peak Discharge Q =1 3.76 cfs 10 -Yr Peak Discharge Q, =4.86 cfs 100 -Yr Peak Discharge Q,00 6.35 cfs N E F F E) A i N i a G F" t u IUL I AIIN ou CISTERN)AREA (WITH A - 1. 83 ACRES 02, - 3, 76 CFS 4 4,,86 CFS Can sqd c—t, Ltd low copy" oh -0 'h P, oP.M -9 AV 10005 d- 0 p ccmmon b.,. I•- d-- d --l— c ---" '- - I McKEECARSON an-, or Feld SPQl Concepts, 7777 M,, p .... P : - GNKKEo 3HEF NUM— z o LLJ V1 U 0 0 0 LU u vs Q LU LU 0 0 u V) n Z ul Lu : I.- E 0 z ir.Z 0 lx LU z Lu a z an-, or Feld SPQl Concepts, 7777 M,, p .... P : - GNKKEo 3HEF NUM— Cistern routing results 2 year critical duration storm BasinFlow printout INPUT: Basin: 6 Contour Areas length (ft)2.000 Elevation(ft)Area(sf)Computed Vol.(cy) 3.00 200.00 0.0 4.00 200.00 7.4 5.00 200.00 14.8 6.00 200.00 22.2 7.00 200.00 29.6 7.50 200.00 33.3 3 Outlet Structures Outlet structure 0 Weir name: emergency spillway length (ft)2.000 coefficient 3.300 invert (ft)6.500 multiple 1 discharge through dam Outlet structure 1 Orifice name: 2yr area (sf)0.035 diameter or depth (in)1.000 width for rect. (in)5.000 coefficient 0.500 invert (ft)5.000 multiple 1 discharge into riser Outlet structure 2 Culvert name: name multiple 1 discharge out of riser D (in) 6.000 h (in) 0.000 Length (ft)50.000 Slope 0.010 Manning's n 0.013 Inlet coeff. Ke 0.500 Equation constant set 3 Invert (ft)0.000 1 Inflow Hydrographs Hydrograph 0 Modified Rational name: post dev captured C 0.900 Area (acres) 0.106 IDF file: a2 time of concentration (min) 5.00 receding factor 1.670 time increment 1.00 time limit (min) 200.00 fudge factor 1.00 storm: crit. duration (95 iterations) volume (cy) 21.43 peak flow (cfs) 0.111 intensity (in /hr) 1.166 time to peak (min) 5.000 duration of peak (min) 80.04 routed true OUTPUT: Routing Method: storage- indication Hydrograph 0 Routing Summa inflow (cfs) discharge (cfs) water level (ft) storage (cy) ry of Peaks: post dev captured 0.111 at 5.00 (min) 0.082 at 86.00 (min) 5.388 at 87.00 (min) 17.687 10 year critical duration storm BasinFlow printout INPUT: Basin: 6 Contour Areas Elevation(ft) Area(sf) Computed Vol.(cy) 3.00 200.00 0.0 4.00 200.00 7.4 5.00 200.00 14.8 6.00 200.00 22.2 7.00 200.00 29.6 7.50 200.00 33.3 3 Outlet Structures Outlet structure 0 Weir name: emergency spillway length (ft) 2.000 coefficient 3.300 invert (ft) 6.500 multiple 1 discharge through dam Outlet structure 1 Orifice name: 2yr multiple area (so 0.035 diameter or depth (in)1.000 width for rect. (in)5.000 coefficient 0.500 invert (ft)5.000 multiple 1 discharge into riser Outlet structure 2 Culvert name: name multiple 1 discharge out of riser D (in)6.000 h (in)0.000 Length (ft)50.000 Slope 0.010 Manning's n 0.013 Inlet coeff. Ke 0.500 Equation constant set 3 Invert (ft)0.000 1 Inflow Hydrographs Hydrograph 0 Modified Rational name: post dev captured C 0.900 Area (acres) 0.106 IDF file: a10 time of concentration (min) 5.00 receding factor 1.670 time increment 1.00 time limit (min) 200.00 fudge factor 1.00 storm: crit. duration (89 iterations) volume (cy) 29.25 peak flow (cfs) 0.184 intensity (in/hr) 1.924 time to peak (min) 5.000 duration of peak (min) 65.04 routed true OUTPUT: Routing Method: storage- indication Hydrograph 0 Routing Summa inflow (cfs) discharge (cfs) water level (ft) storage (cy) ry of Peaks: post dev captured 0.184 at 5.00 (min) 0.128 at 71.00 (min) 5.891 at 73.00 (min) 21.418 Critical Storm Duration T = 2Aac(b -t, /4) -b qo Drainage Area (A) =1.90 acres a =161.60 60* [(QioTd) +(Qty / b =18.73 1 475.04 C =0.47 Time of Concentration (Tc) =7.30 mins 10 -Year Peak Outflow (q0) =4.98 cfs Critical Storm Duration (Td) = 12.60 mins 10 -Year Storm Intensity(i0) I a/ b +Td 5.16 in. /hr Quantitv Storaqe Required Peak Discharge Q (CIA) =4.62 cfs V =60* [(QioTd) +(Qty / Quantity Storage Required =1 475.04 Ift3 Montessori Community School Storm Drain Calculations 6/ 10/2008 BASED ON 10 -YEAR DESIGN FLOW FREQUENCY MANNING' S "n" = 0. 010 FULL FLOW I IFULL FLOW I FULL OR PARTIAL PIPE ID UPPER STRUCT.LOWER STRUCT. TYPE STRUCT. MANNING'S n" DRAINAGE AREA AC.) RUN -OFF COEFF C CA LOCAL Tc MIN.) RAINFALL INTENSITY I (IN /HR) RUNOFF Q) CFS) INVERT ELEVATION LENGTH FT) SLOPE ACTUAL) UNIT /UNIT) SLOPE MIN. REQ' D) UNIT /UNIT) MIN. DIA. REQ' D AT ACTUAL SLP. IN) DIA. ACTUAL IN) CAPACITY CFS) VELOCITY FPS) FLOW TIME MIN.REMARKS Q <Capacity NO.UNIT TYPE STRUCT.INCRE- MENT IACCUM- JULATED UPPER END FT) LOWER END FT) INC. S rS A IN -11 TO OZ- 1 0. 01 aC „'.'_' Or' -`: 0. 01 0. 01 5 00 670 0. 09 1078. C-107. 701 9046 0. 88% 0. 0001% 2. 7 1 5.00 79 0. 1 21. 63 OK C' s' -A CV -A1 TO O°= -2 0. 01 TO 3! 1; 0. 10 0110 70 670 0. 65 108. 0:- 107. 70 58. 58 0. 51 00059%6. 5 15. 00 60 05 1. 85 OK Eh -G w EX -1 Tol C P -3 001 C7 0 45 0. 32 032 5 'JC 670 2. 13 107, 5 85, 15 163, 61 1166%0 0640%5. 5 15, 00 31. 1 17 1. 57 OK STS -L IN -2 TO 1 >B -1 0. 01 5 019 1 5 0. 03 003 00 670 0. 23 118. 50 1 111, 03 60. 47 12. 40%0 1004%2. 4 6. 00 26 1 1. 2 0. 85 OK STS -13 EB -1 TO 3 1 0. 01 1 0 06' 3 C .',0. 03 003 00 670 0.23 110, 8,1 10' 4 59 40. 34 2. 96%00008%32 15. 00 14. 5 1 0. 2 3. 60 OK BASED ON 100 -YEAR DESIGN FLOW FREQUENCY MANNING' S "n" = 0. 010 UPGRADED PIPE SIZE FULL FLOW I FULL FLOW I FULL OR PARTIAL SEQUENTIAL UPPER STRUCT.LOWER STRUCT.DRAINAGE MANNING' S AREA n" (AC.) RUN -OFF COEFF C CA LOCAL Tc MIN.) RAINFALL INTENSITY I (IN /HR) RUNOFF Q) CFS) INVERT ELEVATION LENGTH FT) SLOPE ACTUAL) UNIT /UNIT) SLOPE MIN. REQ' D) UNIT /UNIT) MIN. DIA. REQ' D AT ACTUAL SLP. IN) DIA. ACTUAL IN) CAPACITY CFS) VELOCITY FPS) FLOW TIME (MIN.) REMARKS Q <CapacityNO.UNIT TYPE STRUCT.TYPE STRUCT.INCRE- MENT ACCUM- ULATED UPPER END FT) LOWER END FT) INC. ST -A 1'4 -11 TO Or' -`: 0. 01 ? "?1. 4 001 0. 01 0 0 8. 80 0. 11 108. 50 107, 70 90. 46 0. 88% 0. 0002%3 0 1 5,00 79 01 1647 OK CV -A C" -A!TO OF -2 0. 01 GO 2 C 010 0. 10 c 00 8 80 0. 85 108. 00 10710 58. 6;, 0. 51% 1 0. 0102%72 15, 0C 6. 0 0. 7 141 OK EX- CVI3 LX -1 TO C p O? C0I n' L0;' 4 0 32 0. 32 8 80 2. 80 107. 50 3. 13 1# 3 1 13. 66%0 1105%6 1 5. 00 31. 1 23 1. 19 OK Si5 -03 IN- 2 i0 ED -:L% ".135 0. 03 0. 03 8. 80 0. 30 11:;. 50 1 "t 0-60. 47 12. 40%01732%2. 7 6. 00 2. 6 1. 6 0. 65 OK S 1 S. E EB -1 TC}O -v'. v =3 0. 03 0. 03 8. 80 0. 30 1C 3G 109. 59 40. 84 2. 96%0 0013%3. 5 15. 00 14 5 0. 2 274 OK Montessori School Storm Drain Analysis