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51 S. Forrestal Cir RES20-0122 6 of 16 drainage calcs_17 of 21DEVELOPED BY: Comp. By:Reynold Peterson, P.E. Date:5/8/2020 Company:ALPHA SOUTHEAST, INC Public Works Department City of Atlantic Beach Permit No:_____________________________ Address:51 Forrestal Circle S Required Storage Volume Criteria: Section 24-66 of the City of Atlantic Beach's Zoning, Subdivsion, and Land Development Regulations requires that stormwater runoff from impervious areas be stored onsite. Volume to be retained is as follows: V = CAR/12 which is the Modified Rational Method for estimating stormwater runoff Where:V = Volume of Runoff to be stored (cubic feet) C = Runoff Coefficient, 0.92, the difference between impervious area (C=1.0) and undeveloped conditions (C=0.08). A = Impervious Area (square feet) R= 25-yr / 24-hr rainfall depth (9.3 inches for Atlantic Beach) Onsite Storage Volume Required for Impervious Area: Lot Area = 20,048 ft2 Impervious Area (A) = 3,991 ft2 =19.9% V = 0.92 x 3,991.0 x 9.3 /12 V = 2,846 ft3 Provided Storage: Area 1 - Relative Elev.Area Storage Sideslope:8 :1 (ft)(ft2)(ft3) 5.50 1,363 BOTTOM size:34 X 17 6.00 2,397 TOB size:124 X 20 Area 2 - Relative Elev.Area Storage (ft)(ft2)(ft3) BOTTOM size:0 X 11.5 TOB size:X Area 3 - Relative Elev.Area Storage (ft)(ft2)(ft3) 0.0 0 BOTTOM size:0 X 0 0.0 0 TOB size:0 X 0 Inground Storage: = A*d/pf Total Storage Area at TOB (A) =2397.0 ft2 Depth to ESHWT from BOTTOM (d) =2.5 ft, default is 2.0 ft, verify onsite ESHWT Pore Factor (pf) = 0.4 default is 0.4 Inground Storage Provided =2397.0 ft3 Required Treatment Volume =2,846 ft3 Supplied Treatment Volume =3,337 ft3 940 0 0 Retention COAB_STORAGE CALCS_51 Forrestal Cir S 5/8/2020