Researchers update Illinois standards for storm frequencies

CHAMPAIGN - As heavy rainstorms become more frequent and stronger than in the past, municipal drainage systems designed from outdated standards often fail, resulting in flooding and financial losses.

Researchers at the Illinois State Water Survey (ISWS) have recently applied newer data to update ISWS Bulletin 70, the publication that provides the state standards for expected extreme storms.

Published in 1989, Bulletin 70 was based on Illinois rainstorm data from 1901 to 1983. Current research shows that the number of storms in Illinois producing over 2 inches of rain has nearly doubled over the past century.

“One of the most striking findings from our study is that the upper third of Illinois had the largest increase in the amount of rain and number of heavy storms,” said recently retired Illinois State Climatologist Jim Angel, co-author of the updated Bulletin 70. “Incidentally, that’s the most heavily populated part of the state.”

After the ground is saturated, any additional rain runs into storm sewers. In cities, the hard surfaces of roads and parking lots cause more runoff.

Engineers who design these sewers and culverts are typically required by county or community ordinances to use data from Bulletin 70 to build adequate structures based on a predefined magnitude and duration of storms.

Momcilo Markus, co-author and ISWS hydrologist, said one of the most commonly used is the 24-hour 100-year storm, or a 1 percent chance that a storm of this duration will occur in Illinois in any year. Extreme storms are becoming more common, so water structures are failing more often and at greater costs.

“Flood protection structures are designed to fail from time to time,” Markus said. “The expense of building sewers or other structures to accommodate large volumes of water may outweigh the risk of economic losses from what have been considered rare storms. In allocating funding, city and county planners consider a cost-benefit analysis, weighing the costs of building storm structures that can handle large storms against other demands, such as resources for hospitals and schools.”

For the new publication, Frequency Distributions of Heavy Precipitation in Illinois: Updated Bulletin 70, Angel and Markus used an additional 34 years of data from the original Bulletin 70, analyzing more recent records up to 2017 to better reflect the current risk of heavier precipitation events.

Statewide average annual precipitation has increased 11 percent over the past century, and temperatures have risen by 1.2 degrees.

This trend in heavier rainfall will continue in the future, the researchers say.

“The heavy precipitation in Illinois and the Midwest presents a significant challenge in terms of storm water management,” Markus said. “To address the problem effectively, more accurate predictions of future rainfall intensity and frequency are critical and require up-to-date assessments of current climate conditions and model-generated data for future horizons.”

Frequency Distributions of Heavy Precipitation in Illinois: Updated Bulletin 70 is available at


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