2011 018 Ordinance - Laserfiche WebLink

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(TMDL) has identified water quality problems in the receiving surface water of the State. D. Storm Water Quantity Control: The Comprehensive Storm Water Management Plan shall describe how the proposed storm water management practices are designed to meet the following requirements for storm water quantity control for each watershed in the development: The peak discharge rate of runoff from the Critical Storm and all more frequent storms occurring under post-development conditions shall not exceed the peak discharge rate of runoff from a 1-year, 24-hour storm occurring on the same development drainage area under pre-development conditions. 2. Storms of less frequent occurrence (longer return periods) than the Critical Storm, up to the 100-year, 24-hour storm shall have peak runoff discharge rates no greater than the peak runoff rates from equivalent size storms under pre-development conditions. The 1, 2, 5, 10, 25, 50, and 100-year storms shall be considered in designing a facility to meet this requirement. 3. The Critical Storm for each specific development drainage area shall be determined as follows: a. Determine, using a curve number-based hydrologic method that generates hydrographs, or other hydrologic method approved by the Mayfield Village Engineer, the total volume (acre-feet) of runoff from a 1-year, 24-hour storm occurring on the development drainage area before and after development. These calculations shall meet the following standards: (1) Calculations shall include the lot coverage assumptions used for full build out as proposed. (2) Calculations shall be based on the entire contributing watershed to the development area. (3) Curve numbers for the pre-development condition must reflect the average type of land use over the past 10 years and not only the current land use. (4) To account for future post-construction improvements to the site, calculations shall assume an impervious surface such as asphalt or concrete for all parking areas and driveways, regardless of the surface proposed in the site description. b. From the volume determined in Section 1128.09(D)(3)(a), determine the percent increase in volume of runoff due to development. Using the percentage, select the 24-hour Critical Storm from Table 3.