2022 18 ORDINANCE - Laserfiche WebLink

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11 ORDINANCE NO. 2022-18 Page 18 requirements for stormwater 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. 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. The Critical Storm for each specific development drainage area shall be determined as follows: a. Determine, using a curve number -based hydrologic method or other hydrologic method approved by the 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) Drainage area maps for both pre -developed and post - developed conditions shall include area, curve number, and time of concentrations. Time of concentration shall also show the flow path and the separation in flow type. (4) Use the Standard Rainfall Intensity — Duration Tables from the Uniform Standards for Sewerage Improvements, Latest Edition for stormwater design. (5) Use the SCS Type 11 rainfall distribution for all design events with a recurrence interval greater than 1 year. Include lot coverage assumptions used for full build out of the proposed condition.