2012-076 Ordinance - Laserfiche WebLink

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DRAFT Storm Water Management and Urban 66M Sediment Pollution Abatement S stems 927.06 Each type of area shall be calculated separately and the individual runoff quantities added together to give the total storm water 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 as 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) Industrial/commercial multi-familv and school areas: The concentration time to the critical mlet 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) Maior urban business areas and shopping centers: The concentration time to the critical mlet 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. (e) The rainfall intensity shall be based on Table 1 entitled "Standard Rainfall Intensity Duration Table". (f) 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 mcrease in runoff volume. The peak rate of runoff from the critical storm and all more frequent storms, shall be no greater than ninety (90%) percent 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. (g) The critical storm for a specific development area is determined as follows: (1) Determine the total volume of runoff from a twenty-four-hour one year frequency storm, occurring on the development area based on the difference between after development and ninety (90%) percent of before any development. (2) From the volumes in Paragraph (c) of this Section, determine the percent of increase in volume of runoff due to development and, using this percentage, select the critical storm from the following table: If the percentage increase in volume of runoff is: The "critical storm" Equal to or and for discharge Greater than Less than limitation will be: 0 10 1 Year 10 20 2 Years 20 50 5 Years 50 100 10 Years 100 250 25 Years 250 500 50 Years 500 --- 100 Years