The 5.59 inches of rain reported at the Springfield Airport on Friday night fell in 6 hours. This represents the 100-year storm for that duration in central Illinois, according to Bulletin 70. The results were dramatic, as reported by the Springfield Journal Register.
The concept of the 100 year storm is commonly used by engineers for assessing the risk of heavy rainfall. The 100-year storm is more completely described as the storm expected to have a return period of once every 100 years on average. The phrase “on average” being key. It does not mean the storms are exactly 100 years apart. Instead it means that if you look at rainfall statistics long enough the average frequency of such a storm would be 100 years. Unfortunately, we don’t have hundreds of years of rainfall data. Instead we estimate the values based on fitting a statistical model to the observed data.
While the phrase “100-year storm” is eye catching, it does not do a good job conveying the risk of such an event. A better way of describing it would be the “1% chance storm”, meaning it has a 1% chance of occurring in any given year. It correctly conveys the message that these storms can happen at any time and that we aren’t safe for another 99 years.
Beside the total rainfall, the storm duration is an important factor as well. Three inches of rain in one hour may actually cause more flooding than 3 inches in 6 hours or 24 hours as the rapid runoff may overwhelm the stormwater system.
Here are the hourly rainfall reports taken just before the top of the hour at the Springfield airport:
- 7 pm: 3.44″
- 8 pm: 1.73″
- 9 pm: 0.23″
- 10 pm: 0.16″
- 11 pm: 0.01″
- 12 pm: 0.02″
While the storm lasted 6 hours, much of the rainfall fell in the first two hours, which is pretty typical for storms of this duration. As mentioned earlier, Bulletin 70 (1989) is the design standard for Illinois, published in 1989. That report indicated that this storm had a 100-year return period. In fact, the 6-hour, 100-year storm is 5.59 inches – exactly matching the total on Friday night. I’ve never seen that happen before.
The more recent NOAA Atlas 14 (2004) yielded a much longer return period for this storm, something greater than 200 years (5.36″) but less than 500 years (6.13″). NOAA Atlas 14 has struggled with underestimating the rainfall at the longer return period, as discussed here. For example, their estimate of the 6-hour, 100-year storm is only 4.83 inches. That means that any stormwater structure designed using NOAA Atlas 14 would be smaller than one designed using Bulletin 70, making it more likely that the stormwater structure will fail to prevent flooding.
Heavy rainfall in Illinois has been increasing over time as shown in the Urban Flood Awareness Report (2015). In addition, Bulletin 70 was published in 1989. Illinois needs to update Bulletin 70 to reflect the current heavy rainfall climatology and prepare for possible future changes.