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Home My WebLink AboutSW8021126_COMPLIANCE_20030121STORMWATER DIVISION CODING SHEET POST -CONSTRUCTION PERMITS PERMIT NO. SW gm ZZO? DOC TYPE ❑ CURRENT PERMIT ❑ APPROVED PLANS ❑ HISTORICAL FILE COMPLIANCE EVALUATION INSPECTION DOC DATE YYYYMMDD I DETENTION POND ANALYSIS FILE NAME: G:IDATAIWPDATAIWQSIPOND1021126.WK1 PROJECT #: SW8 021126 REVIEWER: L. Lewis PROJECT NAME: Dollar General -Hampstead DATE: 21-Jan-03 Receiving Stream: Harrisons Creek Class: C Sw Drainage Basin: Cape Fear Index No. CPF23 18-74-49 Site Area 2.69 acres Drainage Area 11177. square feet Area = 2.69 acres IMPERVIOUS AREAS Rational C Building 12985.00 square feet Parking 51832.00 square feet SW 1691.00 square feet Future 15000,00 square feet square feet square feet TOTAL 8154$ 00' square feet Rational Cc= 0.06 SURFACE AREA CALCULATION IMPERVIOUS 69.56% Des. Depth 5 TSS: 90 VOLUME CALCULATION Rational ?* Des. Storm .,.�.��:.-":inches Rv= 0.68 Bottom 43 msl Perm. Pool$ Design Pool 49.5 msl Design SA 12473 sf Req. Volume 6601! cf Prov. Volume .....:1 > cf ORIFICE CALCULATION Avg. Head = Flow Q2, cfs Flow 05, cfs No. of Orifices Diameter, inches Drawdown = 0.72 ft 0.038 cfs 0.015 cfs weir HxW days SA/DA Ratio 4.97% Req. SA 5823 sf Prov. SA681< sf * place a "1" in the box if Rational is used Z*P : s 1 Perm. Pool Volume= 32153 Req. Forebay Volume= 6430.6 Provided Volume= 6335.00 Percent=.1 °a SA @ _ Elevation= Q2 Area = Q5 Area = Orifice Area Q= in COMMENTS Surface Area , Volume and Orifice are within Design GuidelineE 1.34 sq. in. 0.54 sq. in. 1.23 sq. in. 0.035 cfs x FTTU in TRIPP ENGINEERING, P.C. 41 419 ChCstnut StIVUt Viilmington, North Carolina 2W] Phone: (910) 763-7'100 • 1"AX: (910) 763-5631 November 12, 2002 I NCDENR Division Water Quality 127 Cardinal Drive Wilmington, NC 28405-3845 Attn: Ms. Linda Lewis Re: Dollar General - Hampstead Pender County, NC TE 02067 Dear Linda: NOV 13 Z00Z Enclosed please find two (2) sets of plans, one (1) signed original and one (1) copy of the application form with narrative, one (1) wet detention basin supplement, two (2) sets of calculations and one check for the application fee. Please review for approval and contact us with any questions or comments. Thank you. Sincerely, Tripp Engineering, P.C. 2)qy,4/ Bapaw David Brown DBB:dcb Enc. November 12, 2002 Stormwater Narrative Dollar General - Hampstead Pender County, NC TE 02067 Dollar General Hampstead is an existing vacant site of 2.69 acres. A proposed development of 12,985 sf of commercial building with 51,832 sf of asphalt parking and 1,691 sf of concrete sidewalk will be treated by wet detention. The pond is sized to treat the State's i" requirement for the proposed development and a future 15,000 sf. Outflow from the proposed pond will discharge to Harrison Creek, Classification C-SW. %1W 1111212002 CDC Total Drainage Area s . ft. acres 117,177 2.69 Im envious Area Land Use s . ft. acres buildings 12,985 0.30 arkin las halt 51,832 1.19 sidewalks 1,691 0.04 Future 15.000 0.34 Total 81,508 1 1.87 Stormwater Calculations: Time of Concentration Hydraulic Length (L) = 600 ft. change in Height (H) = 2 ft, Tc = (C lH)'S°' 1 128 9.68 min. Flow for 2 year, 6 hour Storm (02): QPNE _ (CPRE) x (110) x (Area) 2,86 cfs QPoST _ (CPoST) x 010) x (Area) 8,46 cfs ,]oil story o Ga acl Tor [ year Jlorm, CN = (98)(% imp.) + (60)(1-% imp.) 87 P = 3.30 in. for2yr, 6 hr storm S= 10001CN-10 1.49 Depth (D)= (P-(0.2)(S)121 (P+(0.8)(S)] 2.00 in. Time to Peak Ftow: TP = ((43.5)(D)(Area)] 1 02,Pov 27.71 min, Detention Pond Design: Pond Size: Elevation ft. Surface Area s , ft. Normal Pool 48.00 10,608 Flood Pool 49.50 12,473 Top of Bank 50.00 13.123 Bottom of Pool 43.00 2.253 Dollar General Hampstead Stormwater Calculations % impervious = 0.696 = 69.6% say 70.0% CPRr: = 0,25 CPosT = (% imp.)(.95)+(1-% imp.)(.25) 0,74 12 = 4,25 inlhr I,o = 4.92 in/hr Flow for 10 year, 6 hour Storm (Q,o): QPRE _ (CPRF) x (125) x (Area) 3,31 cfs QPosT = (CPosT) x (125) x (Area) 9.79 cfs Soil Storage Capacity for 10 year Storm: CN = (98)(% imp.) + (60)(1-% imp.) = 87: P = 4.82 in. for 10 yr, 6 hr storm S = 10001CN - 10 1.49 Depth (D)= [P-(0.2)(S)]2 1 [P+(0.8)(S)] 3.40 in. Time to Peak Flow: TP = [(43.5)(D)(Area)1 1 Q,e,POST 40.60 min. state SurtaCe Area tie ulrement at normalpool: Pond side slopes = 3 :1 Depth below N.P. = 6 ft. (Chart for 90% TSS Removal for Wet Detention Pond without Vegetative Filter Required SA = (SA1DA) x (Total Drainage Area) 4,570 s . ft. State Volume Required for Storage of first 1" of runoff: Provided Story a Volume: Rv = ,05+ ,009(% imp) Vol. _ ((Normal pool SA + Flood Pool SA) 121 = 0.676038 x (F.P, elev. - N.P. elev.) V= (Design rainfall)(Rv)(Drainage Area) 17,311 cu. ft. 11Lt(VI!!f!!1 — 6,601 cu. ft. 'FFS$/p• 9 RECEIVED 17374 NOV 13 2002 %OREG DWQ PROJ # SLvB �l• �Z•oz 02067pond 1 of 3 02067 11/12/2002 Dollar General 02067 CDC Hampstead Stormwater Calculations Pond Volume Bottom SA = 2,253 sq. ft. Normal Pool SA= 10,608 sq. ft. Vol. = ](N.P. SA + Bottom SA)1 21 x (N.P. el. - Bottom al.) = 32,153 cu. ft. Foreba : Required Volume Vol. = 20% of storage volume — 6,431 cu. ft, Outlet Structure Desi n• Flow for 2-day drawdown.- for 1st V of stormwater, Q = Required Vol. 1 172,800 sec. (2-days) = 0.038 cfs Required Area of Pipe for 2-day drawdown; A= Q/ [Cd x SQRT(2 x g x h)] Cd = 0.6 g = 32.2 ft.ls' h = (F.P. eiev.- N.P. elev.)12 0.75 ft. A = 0.0092 s5j. ft, Diameter of Pipe for 2-day drawdown: DIA. = SQRT[(A x 4) / pi] = 0.1080 ft. 1.30 in. 'Use: 1,25 in diameter pipe Outlet Structure Check; Using a 4' x 4' Basin, Perimeter L) = 16 ft. Q = CiA C = (% imp.)(,95) + (1-% imp.)(.25) 0.74 1 = 4.92 in. for a 25 yr. Storm A = 2.69 Acres 9.79 cfs Q= CwxLxHi5 H = [Q ! (Cw x L)]2r3 Cw = 3 = 0.35 ft. Peak elevation = flood pool elevation + H 49.85 ft. Provided Volume Bottom SA = 1,156 sq. ft. Normal Pool SA = 3,067 sq. ft. Vol. _ [(N.P. SA + Bottom SA) / 21 x (N.P. at. - Bottc 6,335 cu, ft, Flow for 5-day drawdown: for 1st 1" of Stormwater, Q = Required Vol. / 432,000 see. (5-days) = 0.015 cfs Required Area of Pipe for 5-day drawdown A= 01[CdxSQRT(2xgxh)] Cd = 0.6 g = 32.2 ft.lsz h = (F.P: elev. - N.P. elev.) 12 0.75 ft. A= 0,0037 sq. ft. Diameter of Pipe for 5-day drawdown: DIA. = SORT[(A x 4)1 pi] 0.0683 ft. = 0.82 in. 02067pond 2 of 3