Impacts of urbanization and climate variability on floods in northeastern Illinois

Trend analysis of annual flood peaks on 12 small urbanizing watersheds in northeastern Illinois indicated that annual peaks, and thus frequency and impact of flooding, increased over the past several decades. An increase in flood peaks could be attributed to intensive urbanization and increasing incidences of heavy rainfall. Average urbanization of the 12 watersheds increased significantly from 10.6% in 1954 to 61.8% in 1996. In addition, numerous studies have reported increasing frequency and intensity of heavy rainfall in the region. This outcome is consistent with lower design rainfall estimates produced by older studies, such as U.S. Weather Bureau Technical Paper No. 40 (TP-40), compared to more recent sources, such as National Oceanic and Atmospheric Administration Atlas-14. This study used a design storm approach and the Hydrologic Engineering Center for Hydrologic Modeling System (HEC-HMS) model to calculate design flood peaks. Hydrologic model parameters were calibrated using hourly rainfall-runoff data of two large regional floods, observed in 1954 and 1996 at 12 small urbanizing watersheds in the metropolitan Chicago area. A sensitivity analysis was performed to evaluate the effects of urbanization and climate variability on increasing flood peaks. Results indicated that, on average, urbanization caused a 34% greater increase in peak flows than climate variability. In addition, this study indicated that present discharges are, on average, at least 19% larger than regulatory discharges. Ongoing urbanization may cause flood peaks to become even higher. The proposed framework can be used to provide input for flood study prioritization by comparing published regulatory discharges and flood discharges computed for current conditions, and investigating potential impacts of future land use changes and precipitation on flood peaks.