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100 Year Rain Storm, Again?

Thursday, September 14, 2017

We have classifications for earthquakes, hurricanes, and tornadoes that speak to energy released (magnitude) and/or damage, but the costliest natural disaster in most years (floods) has a classification that does not directly classify floods by similar means. Instead, we use unintentionally misleading phrases. By combining hydrology statistics and our risk tolerance we end up with, for instance, the “100-year flood.”  This flood event terminology is more of a statistical analysis (probability) rather than describing magnitude and damage, which in turn tends to mislead property owners.

All too often property owners confuse this terminology by thinking that flooding is cyclical and that we are able to predict the future.  I say this because an all too common statement heard is “We just had a 100-year event last year, we won’t have one for another 100 years”, or “we haven’t had a flood in quite a while, we’re due.”  This reference to the probability of it occurring misinforms most.

We don’t call “box cars” the “36 roll dice roll” or a “full house” the 700 deal poker hand; so perhaps we should reconsider using terminology related to probabilities when talking about floods.

In short, we are dealing with a natural event which is hard to predict.

We can only wish that we could predict a flood, but the reality is that we are dealing with a complex event that includes some amount of rain over a period of time falling onto some land size that has grass, pavement, buildings, ponds, lakes, storm drains etc.., which then drain to our creeks and streams.  In turn, these creeks and rivers all react differently to the different rainfalls and land uses. 

I believe by developing a system that speaks to severity and damages, similar to how our other natural disaster ratings are organized, will both minimize the misunderstanding that a particular rain event won’t happen again for some time and attempt to better describe the destruction the storm can or has had on the local and regional area.

To do this we need to have continued dialog with all stakeholders in order to help identify what makes sense for everyone.  With that I ask, “what makes sense to you?”  Feel free to post a response or email me at  I’ll assemble all of your responses and include them in my future blogs.


Derek O' Sullivan is a Senior Project Specialist at REHCE and in his prior capacity as the  Assistant Director to the Will County Stormwater Management Planning Committee, he was charged with implementing the County-wide Stormwater program.