Effects of urbanization on the magnitude and frequency of floods in northeastern Illinois

Changes in land use associated with urbanization have increased flood-peak discharges in northeastern Illinois by factors up to 3.2.

Techniques are presented for estimating the magnitude and frequency of floods in the urban environment of northeastern Illinois, and for estimating probable changes in flood characteristics that may be expected to accompany progressive urbanization. Suggestions also are offered for estimating the effects of urbanization on flood characteristics in areas other than northeastern Illinois.

Three variables, drainage area, channel slope, and percent imperviousness (an urbanization factor), are used to estimate flood magnitudes for frequencies ranging from 2 to 500 years. Multiple regression analyses were used to relate flood-discharge data to the above watershed characteristics for 103 gaged watersheds. These watersheds ranged in drainage area from 0.07 to 630 square miles, in channel slope from 1.1 to 115 feet per mile, and in imperviousness from 1 to 39 percent.