(A) For the determination of detention and depressional storage requirements, an approved hydrograph-producing runoff calculation method shall be used. Approved methods include HEC-1, Soil Conservation Service TR-20, or Soil Conservation Service TR-55 tabular method. The use of methods other than those listed above must be approved by the village.
(B) Runoff hydrographs shall be developed incorporating the following assumptions of rainfall amounts and antecedent moisture:
(1) Unless a continuous simulation approach to stormwater management system hydrology is used, all design rainfall events shall be based on the Illinois State Water Survey Bulletin 70.
(2) The first quartile point rainfall distribution shall be used for the design and analysis of conveyance systems with critical durations of less than or equal to six hours.
(3) The second quartile point rainfall distribution shall be used for the design and analysis of conveyance systems with critical durations of greater than six hours and less than or equal to 12 hours.
(4) The third quartile point rainfall distribution shall be used for the design and analysis of conveyance systems with critical durations of greater than 12 hours and less than or equal to 24 hours.
(5) The fourth quartile point rainfall distribution shall be used for the design and analysis of conveyance systems with critical durations of greater than 24 hours.
(6) The first, second, third and fourth quartile distributions described by Huff are presented in Table 37 of Bulletin 70. The SCS Type II distribution may be used in lieu of the Huff distributions.
(7) Computations of runoff hydrographs which do not rely on a continuous accounting of antecedent moisture conditions shall assume an antecedent moisture condition of two as a minimum.
(Ord. 07-02, passed 4-16-2007)