(A) For areas of 200 acres or less, the rational method may be used to determine the required volume of storm water storage. The following 11-step procedure may be used to determine the required volume of storage. Other design methods may also be used, subject to approval of the Board, and as described in § 155.076.
(B) The steps of the procedure follow, in order:
(1) Determine total drainage area in acres A.
(2) Determine composite runoff coefficient Cu based on existing land use (undeveloped).
(3) Determine time of concentration tc in minutes based on existing condition.
(4) Determine rainfall intensity Iu in inches per hour, based on time of concentration and using data given in Appendix A, Table 6 for the 10-year return period.
(5) Compute runoff based on existing land use (undeveloped) and 10-year return period:
Qu=Cu IuA
(6) Determine composite runoff coefficient Cd based on developed conditions and a 100-year return period.
(7) Determine the 100-year return period rainfall intensity Id for various storm durations td up through the time of concentration for the developed area using Appendix A, Table 6.
(8) Determine the developed inflow rates Qd for various storm durations td, measured in hours:
Qd=CdIdA
(9) Compute a storage rate Std for various storm duration td up through the time of concentration of the developed area:
Std=Od-Qu
(10) Compute required storage volume SR in acre-feet for each storm duration td. This assumes a triangular hydrograph of duration (2*td) hours with the peak flow of Std at td hours:
SR=Std (td/12)
(11) Select the largest storage volume computed in step 10 for detention basin design.
(Ord. 1995-10 § XIV E, passed 10-13-1995)