Changing Climate = Changing Standards

At Wilmette stormwater meetings, residents often complain that stormwater projects are based on outdated standards. For example, the Option 3 Neighborhood Storage project is supposed to serve most of the westside for a “10-year storm,” but a “10-year” storm now seems to occur every couple years.

I share this concern, but for Option 3, engineers didn’t just rely on standard precipitation data. They also used flood reports from local residents, topographic and soil maps, and flow metering data from existing sewers to pinpoint when surcharging and flooding occur.

Nonetheless, it’s true that precipitation data is important for evaluating the performance and cost/benefits of stormwater projects.

Illinois State Water Survey (ISWS)

In our state, the Illinois State Water Survey (ISWS), Bulletin 70, has been the main resource, required by law, for calibrating stormwater plans.

The water survey breaks the state down into 10 geographic sections, with Wilmette located in the Northeast (Section 2).

For each section of the state, precipitation is calculated for a range of storms: from a short storm with little rain that is likely to occur frequently, to a long storm with lots of rain that rarely recurs. Tables of data analyzing storm intensity and frequency help engineers plan capacity for stormwater projects.

Considering the importance of the water survey, some good news is that it was recently updated.

Bulletin 70 Update

Previously, the Illinois State Water Survey (ISWS), Bulletin 70 (Huff and Angel, 1989) was based on data from an 83-year time period, ending thirty years ago (1901-1983).

In March, the state released Bulletin 70-Revised (Angel and Markus, 2019), updating the precipitation tables using more recent sources, including daily rainfall from the National Oceanic and Atmospheric Administration (NOAA), tracked over the past seven decades (1948 to 2017).

Current Rainfall Trends

No surprise: the updated version of Bulletin 70 shows significant increases in the intensity and frequency of rain events, as compared with the old survey.

For example, I believe that the design storm for our westside Option 3 stormwater project is a storm lasting 3 hours, and likely to recur every 10 years (aka 3 Hour, 10-Year Storm). That storm’s rainfall was previously expected to be 2.86 inches, but is now 3.30 inches. As shown in the diagram, the precipitation for a 3 Hour, 10-Year Storm in Wilmette has increased by .44 inches (15 percent):

An increase of .44 inches of rainfall might not seem like much, but consider what that means in terms of stormwater volume. Here’s an extremely rough estimate for the additional stormwater volume for the entire village:

Wilmette land area (5.4 sq miles or 3456 acres) x Rainfall (.44 inches or 0.0367 feet) = Additional Stormwater Volume (126.8 acre-feet)

Let’s say the westside is half that, and runoff mostly a problem on the 32 percent of impervious land (developed or paved). That could still mean about 20 acre-ft more stormwater for a 3 Hour, 10-Year Storm, which could impact the westside Option 3 project.

Precipitation Stats in a Time of Climate Change

Unfortunately, one day (or even one year) of rainfall data does not reveal rainfall trends. That’s what makes this so tricky. Stormwater projects require up-to-date precipitation data, but also data that reliably represents long-term trends.

As noted in the updated Bulletin 70:

The changing climate of heavy precipitation observed in Illinois and the Midwest presents a significant challenge for storm water management. The observed increases noted in this report, along with the expectation of continued increases over the 21st Century […], will necessitate more frequent assessments of precipitation frequency…

A good question locally is: How will the updated Bulletin 70 be used by Wilmette’s Department of Public Works and consulting engineers for planning stormwater projects?

Ideally, the new data will be used to inform, not derail, progress on stormwater management.


If you want to read the updated Bulletin 70 yourself, there’s plenty of updated data to dig into.

For example, detailed charts show the precipitation increases for various storm frequencies, as compared with the previous water survey.

I’ve selected three examples (below) for storms that last 2 hours, 12 hours, and 24 hours. Notice that for each storm duration, the top left diagram shows the storm with the least rainfall and most frequent recurrence. The bottom right diagram shows the storm with the most rainfall and least frequent recurrence. In between are diagrams showing intermediate rainfall/recurrence.

Update on 2-Hour Storms (inches of additional rainfall)

Update on 12-Hour Storms (inches of additional rainfall)

24-Hour Storms (inches of additional rainfall)

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