(A) For areas greater than 200 acres, methods other than the rational method for determining runoff and routing of storm water may be used to determine the storage volume required to control storm water runoff. The procedures or methods used must receive the prior approval of the Board. The ILLUDAS and TR-20 models are approved by the Board for appropriate use in analysis of the runoff and routing of storm water. The use of these models or other approved procedures can be defined in a 7-step procedure to determine the required storage volume of the detention basin.
(B) The steps of the procedure follow, in order:
(1) Calibrate the hydrologic/hydraulic model that is to be used for prediction of runoff and routing of storm water.
(2) For each storm duration listed in Table 6, perform steps (3) through (6).
(3) Determine the 10-year, undeveloped peak flow. Denote this flow by Qu10.
(4) Determine the 100-year runoff hydrograph (Hd100) for developed conditions.
(5) Determine the hydrograph that must be stored (Hs100) by subtracting a flow up to Qu10 from the hydrograph (Hd100) found in step 4.
(6) Determine the volume of water (Vs) to be stored by calculating the area under the hydrograph Hs100.
(7) The detention basin must be designed to store the largest volume (Vs) found for any storm duration analyzed in step 6.
(Ord. 1995-10 § XIV F, passed 10-13-1995)