(a) Maximum Discharge Levels. All sections of this chapter are hereby qualified to prevent damages caused by accelerated storm water runoff, increased peak rates of runoff and volumes of runoff from developing areas. Storm water runoff shall be controlled, such that in no event shall the post development storm discharge exceed the pre-development discharge quantity at any point along the boundary of the development.
(b) Calculations.
(1) The volume, in cubic feet, of a retention/detention pond, shall be calculated as follows:
A. The peak rate of runoff from a Critical Storm (as determined below) and all more frequent storms occurring on the development area does not exceed the peak rate of runoff from a two year storm (in cfs) (Q2) over the same area under pre-development conditions.
B. Storms of less frequent occurrence than the Critical Storm, up to the one hundred (100) year storm, have peak runoff rates no greater than the peak runoff rate from an equivalent year storm under pre-development conditions.
(2) The Critical Storm for a specific development area is determined as follows:
A. Determine by appropriate hydrologic methods the total volume of runoff from a two year storm occurring over the development area before and after development.
B. From the volumes determined in subsection (b)(2)A. hereof, determine the percentage increase in volume of runoff due to development. Using this percentage, select the Critical Storm from the following table:
% Increase in Volume of Runoff | "Critical Storm" Discharge Limitation Year | |
(at least) | (but less than) | |
0 | 20 | 2 |
20 | 50 | 5 |
50 | 100 | 10 |
100 | 250 | 25 |
250 | 500 | 50 |
500 or more | 100 | |
(3) Retention/detention storage volumes shall be calculated by Soil Conservation Service (SCS) TR-55 or other approved methods.
(4) Retention/detention storage outlet pipes shall be designed for discharge based on the runoffs as calculated in subsection (b) hereof.
(5) Storage volume does not have to be provided for runoff from off-site upstream areas. Upstream runoff should be conveyed through the site in accordance with the current runoff conditions.
(6) No on-site drainage shall outlet downstream of the main retention/detention facility without providing supplemental retention/detention as per the above criteria, or an equivalent reduction in the retention/detention outlet.
(7) An overflow outlet shall be incorporated in the design of detention/retention basins to protect the surrounding structures in the event of an overflow.
(8) Maximum slope of the basin shall be a 3:1 grade.
CODIFIED ORDINANCES OF FOSTORIA