Designing infrastructure to withstand the changing environmental climate presents a daunting and enigmatic task. A heavily researched and particularly sensitive element of our climate which directly affects the design, function, and failure of our civil infrastructure is our hydrosphere. However, with the combination of extensive historical data and stochastic climate model projections, we can confidently analyze tangible sustainability efforts in different facets of industry. The Illinois State Water Survey’s report, Projected Precipitation Frequency for Illinois (2025), colloquially known by its technical bulletin number, Bulletin 76, seeks to implement advanced climate modeling and statistical analysis to build upon the historical data represented in Bulletin 75 (2017) and project future precipitation frequency given trends of increased intensity and variability (Markus, M., Wu, S., 2025). Rainfall data is grouped into three projections: one for the year 2050, another for the year 2075, and the final for the year 2100.
Empirical trends measured across Illinois show increasing frequencies of heavy precipitation events and precipitation variability, which correspond to higher frequencies and magnitude of urban flooding events. Figure 1 shows that between 1990 and 2014, Illinois has experienced an above-average number of annual rain events above two inches in each five-year measurement period (Frankson et al., 2017). The most recent measurement period, 2010 to 2014, experienced a record number of events where stations averaged more than two 2-inch events annually (Bulletin 75 source). Per the Illinois Department of Natural Resources Report for the Urban Flooding Awareness Act (2015), flood damage claims involving public and private infrastructure within the six-county Chicago Metropolitan Area between the years of 2007 and 2014 totaled over $2.3 billion (Winters et al., 2015). National Flood Insurance Program and private insurance damage claims in themselves within this period were not enough to determine a trend. However, it was found that over 90% of damage claims came from areas outside the mapped floodplain, indicating widespread sewer and drainage infrastructure failure likely due to under-sizing.
One approach to improving the resiliency of stormwater management systems is increasing the site detention or retention capacity beyond the current ordinance requirements such that facilities can attenuate rainfall events projected by Bulletin 76 (2025). TERRA Engineering’s own John Helfrich, PE performed a study comparing the combined rate and volume control volumes calculated for an 11.36-acre site with 62.6% impervious area in the City of Chicago when using rainfall data from Bulletin 75, Bulletin 76 – 2050, Bulletin 76 – 2075, and Bulletin 76 – 2100. Using the Chicago Stormwater Tool R5102-01-2025 and factoring in a 15% contingency, it was found that a total combined rate and volume control capacity of 95,000 cubic feet would be required when using Bulletin 75 rainfall intensity data. When rainfall intensity data from Bulletin 76 – 2050, Bulletin 76 – 2075, and Bulletin 76 – 2100 was substituted, the required combined volumes jumped to 98,260 cubic feet, 102,400 cubic feet, and 107,000 cubic feet, respectively (Figure 2).
Contrary to our intuition, increasing the site’s retention capacity does not require sacrificing usable space or aesthetics. Practices which improve infiltration and add subsurface storage volume within soil matrix or aggregate, like bioinfiltration areas or rain gardens with native plantings, can add to site aesthetics and ecological value. Moreover, a stormwater management practice like increasing the aggregate depth underneath permeable pavers adds valuable subsurface storage volume while maintaining a usable surface area.
Whether we are designing a single catch basin for a parking lot, or a retention basin for a new subdivision, the quantity and frequency of precipitation directly affect the behavior of our surface water, which is of the utmost consideration to public safety and infrastructure longevity. Rather than relying on private or public flood insurance programs to aid in the rehabilitation of insufficiently sized infrastructure, designing with the future climate in mind can afford property owners the peace of mind and confidence that come with a resilient investment. While Bulletin 76 (2025) is unlikely to be adopted as a regulatory measure, using its data that projects future precipitation intensity marks a pivot away from reactionary design towards progressive stormwater management.