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365 W 3rd St RES20-0082 3rd submittal 4 of 7 drainage narrative_1 2650 ROSSELLE STREET, Suite 2A Jacksonville FL 32204 Ph: 904-551-4945 Civil Design and Construction Management Professionals July 1, 2020 Atlantic Beach Site Drainage Improvements In-Fill Lots Lot Address: 365 3RD STREET WEST Atlantic Beach, FL NARRATIVE: The attached calculations are provided to support the lot drainage design at 365 3rd Street West in the City of Atlantic Beach, FL. The calculations were performed utilizing the equations in the city ordinance sec 24-66(b) and bulletin 1-20. The proposed project will construct a home, driveway, and stormwater storage facility. The runoff coefficient was provided based on the impervious values and associated coefficients of sec 24-66(b) of the code. ______________________________ Reynold D. Peterson, Sr Project Manager FL P.E. # 84559 This item has been digitally signed and sealed by: Reynold D. Peterson, P.E. FL PE NO 84559 Printed copies of this document are not considered signed and sealed and the signature must be verified on any electronic copies Comp. By:Reynold Peterson, P.E. Date:7/1/2020 Company:ALPHA SOUTHEAST, INC Public Works Department City of Atlantic Beach Permit No:_____________________________ Address:365 3rd Street West 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 = 12,099 ft2 Impervious Area (A) = 3,067 ft2 =25.4% V = 0.92 x 3,067.1 x9.3/12 V = 2,187 ft3 Provided Storage: Area 1 - Side **Area Storage Sideslope:6.7 :1 (ft) (ft 2) (ft 3) 4.0 771 BOTTOM size:97.94 X 7.94 4.9 2,115 TOB size:110 X 20 Area 2 - Area Storage (ft) (ft 2) (ft 3) 0.0 0 BOTTOM size:0 X 0 0.0 0 TOB size:0 X 0 Area 3 - Area Storage (ft) (ft 2) (ft 3) 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) =2115.0 ft2 Depth to ESHWT from BOTTOM (d) =2.0 ft, default is 2.0 ft, verify onsite ESHWT Pore Factor (pf) = 0.4 default is 0.4 Inground Storage Provided =1692.0 ft3 Required Treatment Volume =2,187 ft3 Supplied Treatment Volume =2,991 ft3 1,299 0 0 **AREAS PROVIDED ABOVE ARE MEASURED IN AUTOCADD DRAWINGS Retention COAB_STORAGE CALCS_365 3RD ST W 7/1/2020