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?..r. Ordinance No. 99-130 Page 2 ten, twenty-five, fifty and one hundred year frequency storms, all storms from one to on hundred year frequency may be assumed to satisfy this requirement. (c) The run-off coefficients shall be as follows: Before development or redevelopment, Flat slope, less than 2 percent Average slope, 2 to 7 percent Steep slope, greater than 7 percent After development or redevelopment, Roof surface Asphalt surface Concrete surface Gravel roadway surface Grassed area Other surfaces 0.01 to 0.05 0.05 to 0.12 0.012 to 0.18 0.85 to 0.95 0.85 to 0.95 0.80 to 0.90 0.65 to 0.75 0.60 to 0.70 As approved by City Engineer Each type of area shall be calculated separately and the individual runoff quantities added together to give the total stormwater runoff. The runoff coefficient used for each site within each given range shall be subject to the approval of the City Engineer. (d) The time of concentration shall be determined a follows: (1) Residential areas: The concentration times to the critical inlet varies between 12 and 20 minutes with 15 minutes to be used as the general case based upon full development of the land. (2) IndustriaVcommercial, multi-family and school areas: The concentration time to the critical inlet varies between 10 and 15 minutes with 12.5 minutes to be used as the general case based upon full development of the land. (3) Major urban business areas and shopping centers: The concentration time to the critical inlet varies between 5 and 12 minutes with 10 minutes used as the general case based upon full development of the land. Runoff characteristics which vary from the expected for either residential, commercial, or industrial development will be supported by data submitted for approval by the City Engineer. (c) The rainfall intensity shall be based on Table 1 entitled "Standard Rainfall Intensity Duration Table". (d) If the volume of runoff is greater after development than before development, the peak rate of runoff shall be reduced. The critical storm shall be determined by the percent increase in runoff volume. The peak rate of runoff from the critical storm and all more frequent storms, shall be no greater than ninety percent (90%) of the peak rate of runoff from the one year storm occurring before any development. Runoff rates from less frequent storms (longer return periods) than the critical storm shall satisfy the requirements in Paragraph (b) of this Section.