URBANA - Flood peaks in the Chicago metropolitan area are higher than they used to be, and they are also higher than estimates currently used by water managers, according to an Illinois-Indiana Sea Grant study.
"Estimating future flood peaks accurately is critical in terms of allocating resources to minimize damage from these events," said Momcilo Markus, a researcher at the Illinois State Water Survey who studied Chicago area flood trends using data from the U.S. Geological Survey and NOAA. "Underestimating or overestimating 100-year flood levels can result in large economic losses on one hand or increased environmental degradation on the other."
He found that the steady increase in flood discharges in small streams over the past 100 years is due to increases in urbanization and precipitation, with urbanization playing the major role.
It's no surprise that urbanization has increased dramatically in the region. "Between 1954 and 1999, urbanization, on average, increased from about 11 percent to 62 percent in the 12 Chicago area watersheds in our study," said Markus.
Urban areas, unlike agricultural or forested areas, have hard surfaces such as roofs, parking lots and sidewalks, which cause water from large storms to rush into nearby storm sewers and waterways instead of being absorbed into the ground. Add to this an increase in frequency and intensity of heavy precipitation and the result is higher flood levels.
Precipitation records in the Chicago area generally date back about 100 years. "At the Aurora College rain gauging station, the 10 largest historical storms recorded have been since 1950, and these storms were much larger than any in the previous 50 years," explained Markus.
Flood flow estimates are reviewed by the Illinois Department of Natural Resources, Office of Water Resources and are published by the Federal Emergency Management Agency (FEMA). The extent of flooding shown on Flood Insurance Rate Maps guides development and insurance purchases. Flooding estimates published by FEMA are used to design bridges and culverts as well as plan development.
This study shows that these estimates need to be updated. "Many regulatory discharges have not been revisited since the 1980s and 1990s when the studies were conducted. Evidence shows that since then, heavy rainfall has increased, as has urbanization in northeastern Illinois," said Markus. "Present day flood discharges are, on average about 15 percent larger than currently certified estimates. If you account for ongoing urbanization, the flood peaks will become even higher."
In addition to incurring economic costs in terms of property damage and insurance rates, high flood peaks can be ecologically harmful, which is ultimately costly as well. Rainwater flowing into waterways from parking lots and other urban surfaces can carry a variety of contaminants and litter. Plus, stream banks suffer increased erosion, which further degrades water quality and washes away valuable land.
Storm water managers can design structures, such as detention ponds that lessen the impact of flooding. "To address the problem effectively, accurate predictions of future flood peaks are critical," said Markus.
The Illinois-Indiana Sea Grant College Program is one of 30 National Sea Grant College Programs. Created by Congress in 1966, Sea Grant combines university, government, business and industry expertise to address coastal and Great Lakes needs. Funding is provided by the National Oceanic Atmospheric Administration (NOAA), U.S. Department of Commerce, the University of Illinois at Urbana-Champaign and Purdue University at West Lafayette, Indiana.