Designing for a changing climate is not an abstract idea anymore. It is a design condition. And for architects and developers working in Illinois, a new scientific benchmark is making it harder to ignore: the Illinois State Water Survey Bulletin 76.
What Bulletin 76 Actually Says
Published in 2025, Bulletin 76 is the first rainfall frequency study in Illinois history to use forward-looking climate projections rather than historical data alone. Drawing on CMIP6 global climate modeling data, the study projects design storm depths for the years 2050, 2075, and 2100. It builds directly on Bulletin 75 (2020), the current ordinance standard across most of northeastern Illinois.
Results are presented across 10 climatic sections statewide, covering the same storm durations and return intervals used in Bulletin 75, making comparisons direct and actionable. Findings were clear and design storm depths are increasing. Rainfall intensities that engineers are currently designing to will be undersized for the infrastructure going in the ground today, especially on projects with a 30-to-50-year service life.
The Numbers Behind the Trend
The trend is already visible in the historical record. The number of extreme precipitation days in Illinois, defined as days with 2 or more inches of rain, has increased by roughly 40% since the early 1900s. From 1990 to 2014, every five-year period recorded above-average frequencies of these events, with the 2010-2014 window setting a record (See Figure 1). Design storm estimates for northeast Illinois have climbed steadily over decades: from 6.46 inches for a 24-hour, 100-year event in 1961 to 8.25 inches in Bulletin 75, and Bulletin 76 projects continued increases through mid-century.
The cost picture is stark. Urban flooding caused at least $2.4 billion in documented damages in Illinois between 2007 and 2014. The July 2023 Chicago flooding event alone carried an estimated $500 million price tag. Critically, more than 90% of flood damage claims in the Chicagoland area fall outside mapped floodplains, pointing not to fringe risk, but to infrastructure that was simply not designed for the rainfall Illinois now regularly sees.
What This Means for Projects in Design Today
Bulletin 76 is not yet a regulatory requirement, but it is a clear signal of where ordinances are heading, and it is already a useful lens for evaluating risk. At TERRA Engineering, we applied the Bulletin 76 projections to an 11-acre site study in Chicago. Comparing required storage volumes under Bulletin 75 (current code) against the 2100 projection, we found approximately a 13% increase. That may sound modest, but it has real design implications: underground structure sizing, grading, coordination with utilities, and long-term adaptability.
For a developer or architect, that 13% gap is the difference between a site that performs as designed over its lifespan and one that will need to be retrofitted in 20 years. When resilient stormwater management is incorporated at the start of the design process, the numbers are manageable. When it becomes a late-stage coordination problem, the costs compound quickly.
Stormwater Control Structure - Dineen Park, Milwaukee, WI
Resilience Does Not Have to Fight Good Design
The strategies that add stormwater capacity are often the same ones that add site value. Rain gardens, bioswales, native planting zones, and infiltration areas can increase storage volume meaningfully while improving the character and ecology of a project. Even incremental moves, like increasing the aggregate depth beneath permeable pavers, can expand capacity without changing how the space reads or functions.
Done well, these elements become interpretive features that support a project's broader story, whether that is a sustainable campus, a health-forward campus, or a mixed-use development marketing itself around outdoor livability. Resilient stormwater design is not a compliance burden layered onto a finished concept. It is a design input that, introduced early, gives the site more options and fewer constraints.
The Time to Think About It Is Now
The infrastructure being designed and permitted today will be managing stormwater in 2050, 2075, and beyond. Bulletin 76 gives us a credible, research-backed picture of what those conditions will look like. It is not about over-engineering every project. It is about understanding the risk gap between what code requires today and what the climate will demand, and making a deliberate decision about how to close it.
At TERRA Engineering, resilient site design is part of how we approach every project, from initial feasibility through construction documentation. If you want to understand how Bulletin 76 might affect a project you are planning, we are glad to run the numbers with you.
Reference: Markus, M. and Wu, S. (2025). Projected Precipitation Frequency for Illinois (ISWS Bulletin 76). Illinois State Water Survey. https://hdl.handle.net/2142/125352