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Home My WebLink AboutSW8050242_COMPLIANCE_20050826STORMWATER DIVISION CODING SHEET POST -CONSTRUCTION PERMITS PERMIT NO. SW DOC TYPE ❑ CURRENT PERMIT ❑ APPROVED PLANS ❑ HISTORICAL FILE COMPLIANCE EVALUATION INSPECTION DOC DATE YYYYMMDD WET DETENTION POND ANALYSIS FILE NAME: PROJECT #: SW8 050242 PROJECT NAME: Westpark Plaza S:IWQSIPONDI050242.WK1 REVIEWER: L. Lewis DATE: 26-Aug-05 Receiving Stream: Shallotte River Class: SC HQW Drainage Basin: Lumber Index No. 15-25-2-(7.5) Site Area 23.00 acres Drainage Area 100,1880ia0 square feet Area in Acres 23.00 IMPERVIOUS AREAS Lots Road Pump Station Walk TOTAL SURFACE AREA CALCULATION % IMPERVIOUS 35,00% Des. Depth 6 TSS: 90 Rational C 763768.00 square feet 61-937.00 square feet 1845.00 square feet 18048,00 square feet square feet square feet square feet 51 506WI square feet Rational Cc= 0.03 SA/DA Ratio 4,90% Req. SA 49092 sf Prov. SA ' 7S40C sf VOLUME CALCULATION * place a "1" in the box if Rational is used Rational ?* Des. Storm Ii!i ;I'', 1;'00° inches Rv= 0.815 Bottom 7.5 msl Perm. Pool III!I;l,l '14'0.'; msl Design Pool 16.5 msl Storage met @elev 15.31 msl Req. Volume 00044':I cf Vol. Prov. at DPIll#`i i` i i 16:u2 cf ORIFICE CALCULATION Avg. Head = Flow Q2, cfs Flow Q5, cfs No. of Orifices Diameter, inches Drawdown = 0.92 ft 0.394 cfs 0.158 cfs C 3.331111111 weir HxW days FOREBAY Perm. Pool Volume= 375699 Req. Forebay Volume= 75139,8 Provided Volume-- 75140.00 Percent= 2.0°ro Elevation 14.50 17.00 Area 78400.00 89162.00 Inc. Vol. 209452.50 Acc. Vol. . 0.00 209452.50 Q2 Area = 12.30 sq. inches Q5 Area = 4.92 sq. inches Orifice Area 12.57 sq. inches Q= 0.402 cfs in x in Surface Area @DP = 87010 sq. ft. Falling Head Drawdown = 6.4 days L= -0.183 COMMENTS Surface Area , Volume and Orifice are within Design Guidelines Westpark Plaza 04016-0001 ,JAR Basin #1 Wet Detention Pond Calculations NC DENR Retention Requirements Objective: design a wet detention basin with the following characteristics: a permanent water pool depth between 3- and 6-feet, a surface area that meets TSS removal requirements (values set by NC DENR and included here), a forebay that is approximately 20% of the total pond volume, a temporary water pool sized to detain the initial 1 inch of rainfall runoff, an outlet device that drains the temporary water pool within 2-5 days, and a length -to -width ratio of approximately 3:1. Step 1: Determine the surface area required for 90% TSS removal Post -Development Conditions Total Drainage Area 23.0 ACRE Value from CAD Impervious Drainage Area 19.6 ACRE Value from CAD Impervious Cover 85.0% Impervious Cover=(Impervious Drainage Area)1(Total Drainage Area)*100% Elevation of Permanent Pool Surface 14.5 FT Value selected by designer Depth of Permanent Pool 6.0 FT Value selected by designer Elevation of Wet Detention Pond Bottom 8.5 FT (Bottom Elevation)=(Permanent Poo/ Surface Elevation) -(Depth of Permanent Pool) Approximate Pond Length 300 FT Value from CAD Approximate Pond Width 100 FT Value from CAD Len th:Width Ratio 3:1 Ratio=(Length)1(1Mdth):1 Required SA/DA Ratio for 90% TSS Removal 4.900 Value from chart. Reference: "90% TSSRemoval Required Permanent Pool Surface Area 49,092 SF Required Surface Area=(Required SAIDA Ratio)*(Total Drainage Area) Interpolated value from stage -storage Provided Permanent Pool Surface Area 78,400 SF calculations SteD 2: Determine the 1-inch runoff elevation within the wet detention pond. Runoff Coefficient, Rv 0.815 MIN Rv=0.05+0.009'(% Impervious) Required 1" Runoff Volume (Volume of Temporary 68,044 CF 1"Run off Volume =1 inch"Rv"I faot112 Pool) inch *(Total Drainage Area) Value from stage -storage calculations Volume Below Permanent Pool 375,699 CF (cumulative pond volume at permanent pool elevation) Tn4m1 Vnlnma 4n hn Cnntrnllad 68 044 'CF Total Volume to be Controlled=(Volume Below Permanent Pool.) +Runoff Volume) Storage Elevation at Required Volume 16.50 FT Value is interpolation based upon stage -storage values. See stage -storage calculations RECEIVRDV JUL 0 81005 PROD #osQvz4z `,gj41lIIIIP!/P�� ''` �► CAR�yI RSSS y SEAL • RD C04,.�` ��lllll111� � / Westpark Plaza 04016-0001 JAR Step 3: Calculate the recuired forebav volume (18-22% of total pond volume) and compare to the forebav volume provided Total Pond Volume 375,699 CF Value from stage -storage calculations Required Forebay Volume 75,140 CF Forebay Volume=(Total Pond Volume)*20 Provided Forebay Volume 75,725 CF Value from stage -storage calculations Provided Forebay Volume:Totai Pond Volume 20% (Provided Forebay Volume)/(Total Pond Volume) *100 Step 4: Verifv that time recuired to drawdown the 1-inch runoff volume is within 2 to 5 days Diameter of Proposed Low -flow Orifice 4.00 IN Value chosen by designer 1" Treatment Volume Elevation 15.35 FT Value chosen by designer (Total Elevation Head Above Orifice)=(Weir Total Elevation Head Above Orifice 0.85 FT Elevation) -(Elevation of Permanent Pool Surface) (Average Elevation Head Above Orifice)=[(Storage Elevation at Required Average Elevation Head Above Orifice 0.43 FT Volume)+(Elevation of Permanent Pool Surface)f/2-(Storage Elevation at Required Volume) Cd, Coefficient of Discharge 0.60 Value chosen by designer Q, Flowrate Through Low -flow Orifice 0.27 CFS Q=Cd*(Pi)*[(Diameter of Orifice) *(I ft/92 in)I"2/4* 2*32.2*(Aver e Head) ^112 Drawdown Time for 1-inch Runoff 2.88 DAYS (Drawdown Time)=(1"Runoff Volume)/Q*(1 day/86400 seconds) RECEIVED JUL 0 g 2005 DWQ 1?RQJ # i Westpark Plaza 04016-0001 JAR Wet Detention Pond #1 Calculations Stage -Storage Calculations for Proposed Wet Detention Pond determine the storage capacity and characteristics of the proposed wet detention pond. Pond side slope above Permanent Pool 3 Planted shelf 2' below Permanent Pool 6 Side Slope below planted shelf 3 incremental and cumulative storage of the wet detention pond per contour interval. Stage/Storage Above Permanent Pool (Including Forebay) Cumulative Contour Incremental Volume, S Contour Area (SF) Volume (CF) (CF) 14.5 78,400 0 0 Permanent Pool 15.0 80,089 39,622 15.1 80,770 81043 47,665 15.2 81,111 8,094 55,759 15.3 81,453 8,128 63,887 15.35098 81,628 4,157 68,044 -T' Treatement 15.5 82,140 12,202 80,247 16.0 85,615 41,939 122,185 17.0 89,162 87,388 209,574 17.25 90,060 22,403 231,976 RECEIVED J U L 0 4 2005 DWQ PROJ # Westpark Plaza 04016-0001 JAR Wet Detention Pond #1 Calculations Stag e-Storage Calculations for Proposed Wet Detention Pond )/Storage Total Pond (Including Forebay) Cumulative Contour Incremental Volume, S Contour Area (SF) Volume (CF) (CF) 8.5 48,400 0 0 Pond Bottom 9.0 53,824 25,556 25,556 10.0 56,644 55,234 80,790 11.0 59,536 58,090 138,880 12.0 62,500 61,018 199,898 12.5 65,536 32,009 231,907 13.5 71,824 68,680 300,587 14.5 78,400 75,112 375,699 —-Permanent Pool 15.0 80,089 37,978 338.565 16.0 89,162 83,781 459,480 17.25 90,060 112,014 571,494 Forebay Only (20% of Total Pond) Cumulative Contour Incremental Volume, S Contour Area (SF) Volume (CF) (CF) 8.5 9,680 0 0 --Bottom Elev. 9.0 10,765 5,111 5,111 10.0 11,329 11,047 16,158 11.0 11,907 11,618 27,776 12.0 12,500 12,204 39,980 12.5 13,107 6,402 46,381 13.5 14,365 13,736 60,117 14.5 15,680 15,022 75,140 -Forebay Volume RECEIVED JUL 0 8 2005 nwQ PROD #