(a) Quantitative Control. Editor’s Note: Stormwater quantity control shall be implemented pursuant to the criteria outlined as follows:
(1) Stormwater runoff control shall address both peak rate and total volume of runoff. The peak rate of runoff from an area after development shall not exceed the peak rate of runoff from the same area before development for all storms from one year up to a 100-year frequency, 24-hour storm. In addition, if it is found a proposed development will increase the volume of runoff from an area, the peak rate of runoff from certain more frequent storms must be controlled further. There are two reasons why increases in volume of runoff require a control standard more restrictive than controlling to the predevelopment condition. First, increases in volume mean runoff will be flowing for a longer period of time. When routed through a watershed, these longer flows may join at some point or points downstream thereby creating new peak flows and problems associated with peak flow (flooding). This is known as the “Routing Problem”. Second, longer flow periods of large runoff quantities place a highly erosive stress on natural channels. This stress can be minimized by reducing the rate of discharge. The permissible peak rate shall be determined as follows:
A. All development sites located outside the Hendron Ditch Watershed:
1. For the purpose of determining site pre-development condition a runoff curve number (RCN) of 77 shall be used.
2. Determine the total volume of runoff from a 1-year frequency 24-hour storm, occurring over the area before and after development.
3. Determine the percentage of increase in volume due to development and using this percentage, pick the critical storm from the following Table A:
TABLE A
If the percentage of increase in VOLUME [of] runoff is | The Critical Storm for discharge limitations 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 |
4. The peak rate of runoff from the critical storm occurring over the development shall not exceed the peak rate of runoff from a 1-year frequency storm occurring over the same area under predevelopment conditions. Storms of less frequent occurrence (longer return period) than the critical storm, shall have a peak rate of runoff not greater than for the same storm under predevelopment conditions. As an example, if the total volume is to be increased by 35%, the critical storm is a 5-year storm. The peak rate of runoff for all storms up to this intensity shall be controlled so as not to exceed the peak rate of runoff from a 1-year frequency storm under predevelopment conditions in the area. The runoff from a more intense storm up to a 100-year storm need only be controlled so as not to exceed the predevelopment peak rate from the same frequency of storm.
B. All development sites located within the Hendron Ditch Watershed:
1. For the purpose of determining the predevelopment runoff coefficient and RCN, undeveloped land within the Hendron Ditch Watershed shall be considered as undeveloped wooded land, in fair condition.
2. For single-family residential detached development:
a. The critical storm shall be as determined in Table A.
OR
3. For non-single family residential detached developments:
a. The critical storm shall be the 25-year frequency 24- hour storm event or the critical storm determined in Table A, whichever is greater.
(2) Storage volume, generally, does not have to be provided for off-site upstream drainage areas. However, flow from such areas will be routed through the drainage systems in the development under consideration, at its existing rate of flow.
(b) Qualitative Control. Stormwater quality control shall be implemented into sites within developing areas in accordance with general and specific requirements outlined in the latest edition of the Ohio EPA general (NPDES) permit for stormwater discharges associated with construction activity (See Part IIIG2e of the Ohio EPA’s NPDES Permit).
(Ord. 08-021. Passed 4-14-08.)