Appendix 304. Design Methodologies.

For single-family developments under five acres in size, and for all other developments less than one acre in size, detention requirements may be calculated using the modified rational method as described in the Illinois Department of Transportation Drainage Manual. For larger sites, a hydrograph method incorporating reservoir routing or equal must be used. When considering the hydrologic loading for the design, the data used must be based on the following sources:

Rainfall Depths: All rainfall depths shall be based on the Illinois State Water Survey's Bulletin 71 published depths for Illinois, Zone 1.

Rainfall Distributions: All rainfall distributions shall be based on the Illinois State Water Survey's Bulletin 71, except that the SCS Type II rainfall distribution may be used as an alternate to the Huff distributions. Where the Huff distribution is used, the first quartile, second quartile, third quartile, and fourth quartile distributions shall be used as follows:

A.

The first quartile distribution shall be used for storm events with durations of less than 6.0 hours.

B.

The second quartile distribution shall be used for storm events with durations of between 6.1 and 12.0 hours.

C.

The third quartile distribution shall be used for storm events with durations of between 12.1 and 24.0 hours.

D.

The fourth quartile distribution shall be used for storm events with durations greater than or equal to 24.1 hours.

Methods for Generating Runoff Hydrographs: Runoff hydrographs shall be developed incorporating hydrograph calculation methods that include, but are not necessarily limited to HEC-1, HEC-HMS, SCS TR-20, and SCS TR-55. Where the method incorporates an Antecedent Moisture Condition (AMC), a conservative wet AMC shall be used.