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HomeMy WebLinkAboutZMA200600001 Stormwater Management Calculations Study Zoning Map Amendment 2006-03-29 PRELIMINARY STORMWATER MANAGEMENT CALCULATIONS FOR FACILITIES B & C WESTHALL V ZMA 2006-00001 Prepared by: W"—: ENGINEERS SURVEYORS PLANNERS ASSOCIATES 147 Mill Ridge Road 1402 Greenbrier Place Lynchburg.VA 24502 Charlottestiille,VA 22901 Phone:434-582-6175 Phone:434-984-2700 a-way.zrwassocia tes.net March 29, 2006 Project Number: 205161.02 Stormwater Management Calculations WWA No.205161.02 Table of Contents Stormwater Management Stormwater Narrative Figure 1: Pre-development Drainage Area Map Figure 2: Post-development Drainage Area Map Stormwater Computations 1 Stormwater Management Calculations WWA No.205161.02 Stormwater Narrative Two stormwater management facilities will provide quantity (2-year storm) and quality (BMP) controls for Westhall V. Existing Facility B constructed with Westhall I & II will be relocated and expanded to provide the prescribed quantity and quality (extended detention with forebay) controls for the Westhall I-IV drainage areas currently directed to the existing facility as well as the southern portion of Westhall V. Facility B will be within Open Space Parcel A. Facility C will be located in Open Space Parcel D and will provide the prescribed quantity and quality(bio-filter) controls for the northern portion of Westhall V. 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POST-DEVELOPED FIGURE 2 A__OcITE_ REVIEWED BY- „FW/alw HYDROLOGY CONDITIONS SHEET NUMBER: "nOid"v^� WWA NUMBER: FILE NAME: 'DISCIPLINE. SCALE: DATE: REVISION BY DATE N0. REVISION 4.r.4v.Hfm t d.�,ew vema NO. ^-..,. �•„�_w.,�� 2 OF 2 BY DATE �5/81D0 51Y-/gSTN CIVIL f Y.50' 03/29/06 Stormwater Management Calculations WWA No.205161.02 General Total Site Area= 8.96 ac(Facility B = 6.34 ac, Facility C=2.62 ac) Lot Assumptions: Building Footprint 1200 SF Driveway 20' x 15' 300 SF Sidewalk 20' x 5' 100 SF Patio 10' x 10' 100 SF 1700 SF 2000 SF Facility B Some calculations extracted from Westhall Phase I & II Facility B Stormwater Calculations dated October 21, 2005 Drainage Area Pre-Development= 6.1 ac (Westhall I&II) Post-Development = 6.4 ac + 4.4 ac (Westhall V) = 10.8 ac Ground Cover and C-Factor Westhall I&II Pre-development: Forest= 1.1 ac C = 0.30 Pasture=5.0 ac C = 0.45 Cpre= (1.lac)(0.30)+(5.0ac)(0.45)_ 0.39 6.6ac Post-development: Pavement= 1.82 ac C = 0.90 Townhouses=0.52 ac C = 0.65 6,000 sf lots= 3.71 ac C = 0.50 Lawns=0.35ac C= 0.30 Cpost = (1.82ac)(0.90)+(0.52ac)(0.65)+(3.71ac)(0.50)+(0.35ac)(0.30) =0.62 6.4ac Westhall V Pre-development: Wooded= 6.34 ac C = 0.25 Cpre=0.25 Post-development: Road=45,160 s.f. = 1.04 ac C =0.90 30 Lots= 60,000 s.f. = 1.38 ac C =0.90 3 Stormwater Management Calculations WWA No.205161.02 Pervious Area= 3.92 ac C = 0.30 Cpost = (1.38ac)(0.90)+(1.04ac)(0.90)+(3.92ac)(0.30) = 0.53 6.34ac Cposttotal —_ (6.4ac)(0.62)+(4.4ac)(0.53)= 0.58 10.8ac Time of Concentration Westhall I&II Pre-development(two parts): 1) Seelye Method for Overland Flow (G ---* I) TC1=0.225Lo.42S°'9C1.0 L=200 ft S = 0.01 C=0.30 To= 16.7 min 2) Kirpich Method for Shallow Concentrated Flow (I -* J L) TC1= 0.000948H-o.381, 1.13 H= 660 ft—525 ft= 135 ft L= 605 ft TC2=2.1 min tc Tc1 + TC2= 18.8 min (Pre-Developed) Post-Developed (three parts): 1) Seelye Method for Overland Flow (G -*H) TC1= 0.225Lo.42S -19C1.0 L= 100ft S =0.01 C=0.50 TC1 =7.5 min 2) Kirpich Method for Shallow Concentrated Flow (H -* J -+K) TC2=0.009481 "o.38 L1.13 H= 661 ft-637ft=24ft L= 535ft TC2= 3.5 min 3) Conduit Flow (K —>L) L TC3 = V V=4 fps pr 240 ft/min L=275 ft TC3= 1.2min 4 Stormwater Management Calculations WWA No.205161.02 tc= Tcl + Tc2+ TC3= 12.1 min (Post-Developed) Westhall V Pre-development(two parts): 1) Seelye Method for Overland Flow (D —>E)(Plate 5-1, VESCH) L= 150ft S = 0.047 C= 0.25 To= 15.0 min 2) Shallow Concentrated Flow (E —4 F) (Plate 5-2, VESCH) L= 370ft S = .0676 Avg. Velocity, V= 5.40 fps (Unpaved) TC2 60V = 1.14min tc To + TC2= 16.14 min (Pre-Developed) Post-Developed Length of Pipe=730' Assume TC1= 5 min.. TC2= 730' /4 fps/60 sec/min= 3 min t, Tc, + TC2= 8.0 min (Post-Developed) Use Westhall I & II time of concentrations Intensity(VSECH Appendix 4D—Albemarle County IDF) 2yr(te= 18.8 min) = 3.0 in/hr Rational Method Q= CIA (Allowable Discharge) 2 yr: Q2 =(0.39)(3.0 in/hr)(10.8 ac.) = 12.6 cfs Modified Rational Direct Solution; (VSMH p. 5-23) 1) Define 2-year critical storm duration. T iii2CAa(b—tc l 4) b d2 _— qo2 C = 0.58 A= 10.8 ac to= 12.1 min qo2= 12.6 cfs a2= 106.02 5 Stormwater Management Calculations WWA No.205161.02 b2= 15.51 2(0.58)(10.8)(106.02)(15.51- 12.1)4 Td2 = 1 —15.51 = 21.6 min 11 12.6 2. Solve for the 2-yr critical storm intensity. 12 = a2 = 106.02 =2 86in/hr b+Td2 15.51+21.6 3. Determine 2-yr Peak Inflow Qi2 =Cl2A =(0.58)(2.86)(10.8) =17.9cfs 4. Determine the 2-yr required storage volume V2 = Q�2Td2 + Qtc _ R'o27'd2 — 3R'o2tc 60 4 2 4 Qi2 = 17.9 cfs C =0.58 A= 10.8ac Td2 = 21.6 min to = 12.1 min qo2= 7.1 cfs VZ = [(17.9)(21.6)+ (17.9)(12.1) (12.6)(21.6) — (3)(12.6)(12.1) 60 4 2 4 1\72 = 11,422 cf (Water Quality Volume Required) For water quality, the target removal rate is 57% (Albemarle County Modified Simple Method spreadsheet for BMP computations). An extended detention pond was chosen for this facility with at least 50%target removal rate(Albemarle County Design Standards Manual,p. 5-41). Facility C 6 Stormwater Management Calculations WWA No.205161.02 Drainage Area Pre-Development= 2.62 ac Post-Development = 2.62 ac Ground Cover and C-Factor Pre-development: Wooded=2.62 ac C = 0.25 Cpre= 0.25 Post-development: Road=24,450 s.f. = 0.56 ac C= 0.90 7 Lots= 14,000 s.f. =0.32 ac C= 0.90 Pervious Area= 0.99 ac C = 0.30 Cpost = (0.56ac)(0.90)+(0.32ac)(0.90)+(0.99ac)(0.30) _ 0.49 2.62ac Time of Concentration Pre-development(two parts): 1) Seelye Method for Overland Flow (A B)(Plate 5-1, VESCH) L= 150ft S = 0.05 C= 0.25 To= 14.7 min 2) Shallow Concentrated Flow (B C) (Plate 5-2, VESCH) L=225 ft S= .122 Avg. Velocity, V= 5.70 fps (Unpaved) TC2 60V = 1.14min tc Tc1 + TC2= 15.4 min (Pre-Developed) Post-Developed Assume TC. = 5 min. (small area) tc TC, = 5.0 min (Post-Developed) Intensity (VSECH Appendix 4D—Albemarle County IDF) 2yr(te= 15.4min) = 3.4in/hr 7 Stormwater Management Calculations WWA No.205161.02 Rational Method Q= CIA (Allowable Discharge) 2 yr: Q2 =(0.25)(3.4 in/hr)(2.62 ac.) = 2.2 cfs Modified Rational Direct Solution; (VSMH p. 5-23) 1) Define 2-year critical storm duration. T _ 2CAa(b—t,l4) b d2 — qa2 C= 0.49 A=2.62 ac tc= 5.0 min qo2 =2.2 cfs a2= 106.02 b2= 15.51 2(0.49)(2.62)(106.02)(15.51—5) Td2 = i 22 —15.51 = 26.5min 2. Solve for the 2-yr critical storm intensity. I2 — a2 = 106.02 =2 52in/hr b+Td2 15.51+26.5 3. Determine 2-yr Peak Inflow Qi2 = Cl2A =(0.49)(2.52)(2.62) =3.2cfs 4. Determine the 2-yr required storage volume V2 = QJ2Td2 + Q2tc — g02Td2 —3go2tc 160 4 2 4 Qi2 = 3.2cfs C = 0.49 A=2.62 ac Td2 =26.5 min tc= 5 min q02=2.2 cfs V2 =[(3.2)(26.5)+ (3.2)(5) — (2.2)(26.5) — (3)(2.2)(5)J160 4 2 4 2= 3084 cf Per VSMH, p. 3.11-14—The surface area of the bioretention bed is a minimum of 2.5% of impervious area or 1090 square feet per impervious acre: A=0.025 (38,450 s.f.)= 961 sf or A= 1,090 (38,450 s.f./43,560)= 962 sf 8 Stormwater Management Calculations WWA No.205161.02 For water quality, the target removal rate is 32% (Albemarle County Modified Simple Method spreadsheet for BMP computations). A bioretention area was chosen for this facility with at least 50%target removal rate(Albemarle County Design Standards Manual,p. 5-42). 9 C4 0 LENGTH " L" OF STRIP IN FEET ,c�D, I 1 1 1 ► I ► ► ► ► I 1 I ► I l 1 1 ( 1 1 1 1 ► I i 1 , ► ► n ► ► i ► I ► ---- d - / 0 N (A' — N W .p 01 01 11 WWO 0 p' 0 0 0 0 0 00000 / 0 0 C 0 D c X 9,/O Agi/T. 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' ' ",'' - — * ',.%!',,,-.-.1:,,!;,:=-.1.,;,,,...`.:4.,:,•P"1.4z;Ze-4.,:7-',,,,$'''' '''•:`..„-';'',:',..--i'--•"-”i"11-*-ili•.5,,,h,W,A,,,--1,-,'-'";,,,.-'xi' "."' - — - -'- ^ - ' Short Version BMP Computations For Worksheets 2 -6 Albemarle County Water Protection Ordinance: Modified Simple Method Plan: Westhall V Water Resources Area: Development Area Preparer: ACW Date: 27-Mar-06 Project Drainage Area Designation DA#2 rL2.C I i t 5 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 I percent imperviousness P annual precipitation,43"in Albemarle A project area in acres in subject drainage area, A= 10 80 $ C pollutant concentration,mg/I or ppm target phosphorus f factor applied to RR ✓ required treatment volume in cy,0.5"over imperv.area= A(I)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 01 235224 0 0 0 0 0 0 O 0 0 0 O 0 0 235224 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 0 0 O 0 0 0 Parking Lots 1 2 3 4 1 2 3 4 0 0 0 0 0 Gravel areas Area Area ' __ x 0.70= 0 x 0.70= 0 Structures Area no. subtotal Area no. subtotal 0 0 0 0 1 0 0 0 0 0 0 0 .._.._- 0..._...__.. 0 0 0 0 Actively-grazed pasture& Area Area rds and cultivated turf 0 x 0.08= 0 235224 lax 0.08= 18817.92 Active crop land Area Area x0.25- 0 -ix 0.25= 0 Other Impervious Areas 0 0 Impervious Cover 0% , IA I(pre) ((post) Rv(post) V ..__._ 0.54 392.0'I New Development(For Development Areas,existing impervious cover<=20%) - C-- f I(pre)* Rv(pre) L(pre)_1..._L(post) RR %RR rArea Type 0.23 15.25 1 35.54 20.29 57% Development Area 0.05 1.66 17.77 16.11 91% Drinking Water Watersheds 0.06 2.24 20.31 18.07 89% Other Rural Land -*min.values Redevelopment(For Development Areas,existing impervious cover>20%) C f I(pre)* Rv(pre) L(pre) L(post) RR I %RR Area Type 0.70 ' 0.90 20% 0.23 15.25 35.54 21.81 61% Development Area 0.35 0.85 0% 0.05 1.66 17.77 16.36 92% Drinking Water Watersheds 0.40 0,85 1%0 0.06 2.24 20.31 18.41 91% 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: Westhall v Water Resources Area: Development Area Preparer: ACW Date: 24-Mar-06 Project Drainage Area Designation DA# I Fact 1 t 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 I percent imperviousness P annual precipitation,43"in Albemarle A project area in acres in subject drainage area, A= _ 2.62 C pollutant concentration,mg/I or ppm target phosphorus f factor applied to RR ✓ required treatment volume in cy,0.5"over imperv.area= A(I)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 LengthWidth subtotal 0 0 0 0 0 24450 0 0 0 0 0 0 O ^^ 0 0 0 O 0 0 24450 Driveways Length Width no. subtotal Length Width no. subtotal and walks 0 0 0 0 0 0 0 2800 0 0 0 0 0 0 0 0 0 0 0 2800 Parking Lots 1 2 3 4 1 2 3 4 __-i_-__•.. 0 _I - 0 0 _ 0 0 Gravel areas Area Area x 0.70= 0 x 0.70= 0 Structures Area no. subtotal Area no. subtotal O 0 0 0 1 9100 0- 0 0 0 0 0 O 0 0 0 0 9100 Actively-grazed pasture& Area Area yards and cultivated turf 0 x 0.08= 0 0 x 0.08= 0 Active crop land Area Area x0.25= 0 x0.25= 0 Other Impervious Areas 0 0 Impervious Cover ' v 325q I(pre) Roost) Rv(post), _V 0.34 56.1- New Development(For Development Areas,existing impervious cover<=20%) C f I(pre) 1 Rv(pre) L(pre) L(post) ! RR 1 %RR 'Area Type 1.00 20% 0.23 1 3.70 5.41 ' 1.72 32% Development Area ., 1.00 0% 0.05 1 0.40 2.71 I 2.31 I, 85% Drinking Water Watersheds e, 1.00 1% 0.06 ( 0.54 3.09 ! 2.55 A 82%j Other Rural Land *min.values Redevelopment(For Development Areas,existin• impervious cover>20%) C f I (pre)* Rv(pre) OM= L(post) RR - %RR 1 Area Type 0 70 0.90 20% 0.23 3.70 5.41 2.09 39% Development Area 0.35 0.85 0% 0.05 0.40 2.71 2.37 87% Drinking Water Watersheds ------- - ---- - 0.40 0.85 1% 0.06 0.54 3.09 2.63 Other Rural Land Interim Manual,Page 70 rev. 16 Feb 1998 GEB