The design of the drainage system shall be developed and evaluated by the applicant based on routing runoff hydrographs for the 2-year, 10-year, and 100-year, 24-hour events through its minor and major components. In addition, a general assessment of the impacts of flows exceeding the 100-year event shall be presented.
(A) Rainfall. Unless a contiguous simulation approach to the drainage system hydrology is used, all design rainfall events shall be based on the Illinois State Water Survey (ISWS) Bulletin 70. Third quartile median point rainfall distribution by Huff of the ISWS shall be assumed for discharge estimations. SCS Type II distributions shall also be acceptable for detention basin design.
(B) Stormwater runoff must be developed as part of the design of the drainage system's minor and major components. The SCS TR-55, TR-20, ACOE HEC-1, or other method approved by the Chief Inspector of the Board of Fire Prevention shall be used for the generation of hydrographs, subject to rainfall distribution modifications for all properties larger than five acres.
(C) Antecedent Moisture. All computations of runoff hydrographs which do not rely on a contiguous accounting of antecedent moisture conditions shall assume the following antecedent moisture conditions:
(1) SCS TR-55 - antecedent moisture condition two; and
(2) ILLUDAS - number three, "rather wet".
(Ord. 05-2528, passed 7-26-05)