(a) 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 2-year frequency, 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 storm have a peak runoff discharge shall have 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 in designing and developing to meet this standard.
(c) The critical storm for a specific development drainage area is determined as follows:
(1) Use the SCS TR-20 Hydrologic Analysis Model or other appropriate and approved hydrologic simulation model along with 24-hour rainfall data obtained The Rainfall Frequency Atlas of the Midwest as authored by Huff & Angel, to determine the total volume (acre-feet) of runoff from a 2-year development. Include clearly in the calculations the coverage assumptions used for full build out of the proposed condition. Curve numbers for the pre-development condition must reflect the average type of land use over the past ten years and not only the current land use.
A. For sites which are currently developed and are scheduled to be redeveloped, the pre-developed condition shall be defined to be 100% of the site as grassland for critical storm and volume storage calculations.
B. From the volumes determined in division (c)(1) of this section, determine the percent increase in volume of runoff due to development. Using this percentage, select the 24 hour critical storm from the following table:
TABLE 1: CRITICAL STORM DETERMINATION TABLE
| ||
If the Percentage of Increase in Volume of Runoff is:
| The Critical Storm Will Be: | |
Equal to or Greater Than: | Less Than | |
TABLE 1: CRITICAL STORM DETERMINATION TABLE
| ||
If the Percentage of Increase in Volume of Runoff is:
| The Critical Storm Will Be: | |
Equal to or Greater Than: | 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 |
(Ord. 15-O-2820, passed 5-19-2015)