To provide adequate stormwater management and to prevent off-site damage from accelerated stormwater runoff from development areas, the increased peak rates and volumes of runoff shall be controlled such that:
(a) The peak discharge rate of runoff from the critical storm and all more frequent storms occurring under post-development conditions does not exceed the peak discharge rate of runoff from a one-year frequency, 24-hour storm occurring on the same development drainage area under pre-development conditions. Pre-development shall be defined for runoff control as site conditions at the time of adoption of the City's original Stormwater Ordinance 1989-34, passed December 29, 1989.
(b) Storms of less frequent occurrence (longer return periods) than the critical storm up to the 100-year storm have peak runoff discharge rates no greater than the peak runoff rates from equivalent size storms under pre-development conditions. Consideration of the 1, 2, 5, 10, 25, 50 and 100-year storms will be considered adequate for designing and developing methods to meet this standard.
(c) The critical storm for a specific development drainage area is determined as follows:
(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 ten (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.
(5) Use the Natural Resources Conservation Service (NRCS) TR-55, Urban Hydrology for Small Watersheds, to determine the total volume (acre-feet) of runoff from a one-year, 24-hour storm occurring on the development drainage area before and after development.
(6) From the volumes determined in (5), determine the percent increase in volume of runoff due to development and using this percentage, select the 24-hour critical storm from the following table:
Table 920.19.1: 24-Hour Critical Storm
If the Percentage of Increase in Volume of Runoff is: | ||
Equal to or Greater Than: | and Less Than: | The Critical Storm will be: |
--- | 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 |
(For example, if the percent increase between the pre- and post-development runoff volume for a 1-year storm is thirty-five percent (35%), the Critical Storm is a 5-year storm. The peak discharge rate of runoff for all storms up to this frequency shall be controlled so as not to exceed the peak discharge rate from the 1-year frequency storm under pre-development conditions in the development drainage area. The post-development runoff from all less frequent storms need only be controlled to meet pre-development peak discharge rates for each of those same storms.)
(d) Methods for controlling increases in the rate and volume of stormwater runoff include many options. Some of those options are described in the Ohio CGP. Acceptable design criteria for these options are included in the Rainwater and Land Development Manual and the City's Stormwater Utility's Rules and Regulations as promulgated by the Utility Commission.
(Ord. 2019-35. Passed 11-12-19.)