3.2.1 Design Storms

Rainfall frequency and duration information from the metropolitan area is commonly taken from the U.S. Department of Commerce's Weather Bureau Technical Paper No. 40 (TP-40); however, another more recent source of data is the Illinois State Water Survey's Rainfall Frequency Atlas of the Midwest (1992).  The statistically derived data are used for determining critical storms that represent varying design conditions or levels of service.  Additional references include the National Oceanic and Atmospheric Administration's Technical Memorandum NWS Hydro-35: 5- to 60-Minute Precipitation Frequency for the Eastern and Central United States (June 1977) and the Metropolitan Council's Precipitation Frequency Analysis for the Twin Cities Metropolitan Area (January 1989).

For regulatory purposes, the District has derived from these sources critical event rainfall depths of 2.4? in 24 hours for a one-year event, 4.1? in 24 hours for a 10-year event, and 5.9? in 24 hours for a 100-year event.  These critical events are used to calculate stormwater quantity and quality impacts of new development and redevelopment.