(A) Event hydrograph routing methods shall be used to calculate design runoff volumes for site runoff facilities. The methods must be HEC-1, (SCS methodology), HEC-HMS, TR-20, or TR-55 tabular method. For sites of 40 acres or more the methods used must be acceptable to FEMA. Event methods shall incorporate the following assumptions:
(1) Antecedent moisture condition = 2;
(2) Appropriate Huff rainfall distribution; and
(3) 24-hour duration storm with a 1% probability (100-year frequency) of occurrence in any one year as specified by Illinois State Water Survey Bulletin 70 Northeast Sectional rainfall statistics.
(B) Figure 1, based upon generic hydrograph curves developed by the Will County Stormwater Management Committee, may be used for sites of 20 acres or less when acceptable to the Administrator. The Rational Method is not acceptable. (See Appendix E following this chapter.)
(Ord. 06-0383, passed 3-15-06)