In addition to the information contained in the SWP3, the applicant shall submit supplemental information regarding post development runoff as part of the Storm Water Control Plan. The Storm Water Control Plan shall describe how the proposed storm water management practices are designed to meet the following requirements for storm water quantity for each watershed in the development:
(a) The peak discharge rate of runoff from the Critical Storm and all more frequent storms occurring under post-developmentl 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.
(b) 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.
(c) The Critical Storm for each specific development drainage area shall be determined as follows:
(1) Determine, using a curve number-based hydrologic method that generates hydrographs, or other hydrologic method approved by the City Engineer, the total volume (acre-feet) of runoff from a l-year, 24-hour storm occurring on the development drainage area before and after development. These calculations shall meet the following standards:
A. Calculations shall include the lot coverage assumptions used for full build out as proposed.
B. Calculations shall be based on the entire contributing watershed to the development area.
C. 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.
D. 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.
(2) From the volume determined in Section 1367.l2(c) (1) determine the percent increase in volume of runoff due to development. Using the percentage select the 24-hour Critical Storm from Table 3.
Table 3: 24-Hour Critical Storm
If the Percentage of Increase in Volume of Runoff is: | The Critical Storm will be: | |
Equal to or Greater Than: | and Less Than: | |
If the Percentage of Increase in Volume of Runoff is: | The Critical Storm will be: | |
Equal to or Greater Than: | and Less Than: | |
--- | 10 | 1 year |
10 | 20 | 2 year |
20 | 50 | 5 year |
50 | 100 | 10 year |
100 | 250 | 25 year |
250 | 500 | 50 year |
500 | --- | 100 year |
(d) A computer program acceptable to the City Engineer shall be used for all detention or retention basin designs.
(e) A narrative signed and sealed by a Professional Engineer registered in the State of Ohio describing procedures used in design of storm water quantity control devices and summarizing data, including any necessary supplemental information shall be submitted with Preliminary and Final Plans. The Narrative shall be in a format acceptable to the City Engineer.
(f) The design and details of the storm water retention and detention basins shall be in accordance with standards approved by the City Engineer and Service Department.
(Ord. 2008-18. Passed 2-26-08.)