Stormwater runoff control addresses both peak rate and total volume of runoff.
(A) 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 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 just the predevelopment conditions. 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 downstream; thereby creating peak flows and the 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.
(1) Determine the total volume of runoff from a one-year frequency, 24-hour storm occurring over the area before and after development.
(2) Determine the percent of increase in volume due to development and using this percentage, pick the critical storm from the following table:
If the percentage of increase in volume of runoff is: | The critical storm for discharge limitation will be: | |
Equal to or greater than | and less than | |
-- | 10 | 1 year |
10 | 20 | 2 years |
20 | 50 | 5 years |
50 | 100 | 10 years |
100 | 250 | 25 years |
250 | 500 | 50 years |
500 | -- | 100 years |
(B) The peak rate of runoff from the critical storm occurring over the development shall not exceed the peak rate of runoff from a one-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 peak rate of runoff not greater than for the same storm under predevelopment conditions. As an example, if the total volume is shown to be increased by 35%, the critical storm is a five-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 one-year frequency storm under predevelopment conditions in the area. The runoff from a more intense storm need only be controlled so as not to exceed the predevelopment peak rate from the same frequency of storm.
(C) Storage volume does not have to be provided for off-site upstream areas. Flow from these areas will be routed through the drainage system in the development under consideration at a rate determined in the same manner as the on-site system. Off-site land use averaged over the last five preceding years before the development shall be considered as the predevelopment condition for the purpose of calculating changes in runoff.
(Ord. 16-96, passed 4-15-1996)