Petenwell Park Improvements | Adams County, WI

When is the next 100-year flood?

Eric Thompson, PE, CFM | with 0 Comments

When is the next 100-year flood?

Another 100-year flood! If we just had the 100-year flood, aren’t we safe for 99 more years?
In a recent post, How City Planners can Prepare for Large Storms, we noted the estimated rainfall depths corresponding to typical flood events (e.g. the 100-year rainfall) are significantly higher than reported in older documents.  A hydrologist or water resources engineer might respond by asking if you really had a 100-year rainfall or a number of smaller rainfalls that resulted in a 100-year flood.   
Wow, that’s confusing!  Let’s break it down a little by starting with the definition of ‘100-year’. This is a statistical categorization describing something that has a 1% probability of occurring in any given year. When you think in terms of probability, or odds, (as in games of chance) you can begin to see that it is possible to have 100-year occurrences in consecutive years, or even within the same year. There is nothing that defines what causes the situation, only the frequency of its occurrence.
Let’s move to 100-year flood. The generally accepted definition of the ‘100-year flood’ is the highest water surface elevation that a waterbody (stream, lake, pond, wetland) reaches in a 100-year period; or the water surface elevation that has an annual 1% chance of occurring.  Again, there is nothing stating why the water gets this high, only that it does get this high, on average, once every 100 years.
The generally accepted definition of the 100-year rainfall is described using two primary variables: total rainfall depth and the duration over which rainfall occurred.  Depending on the duration of the event, the depth occurring in any 100-year occurrence will be different than for any other duration event.
The average rainfall intensity is the depth of rain divided by the duration of the event.  In reality, rainfall intensity is rarely constant and there can be periods of very intense rainfall within a longer, lower density rain event.  Each of these periods of different intensity can be analyzed as individual events within the longer event.
With all these variables describing rainfall intensity; it is not hard to imagine a scenario where the occurrence of several rainfalls or different depth and intensity can saturate the soil and use up available floodplain storage such that subsequent rainfall event less than the 100-year rainfall might cause a 100-year flood.
Rainfall isn’t something we can control; we can, however, manage the stormwater runoff that results from it. The City of Dubuque is getting ahead of stormwater runoff by reconstructing their alleys with permeable pavers. The predicted reduction in stormwater runoff within the Bee Branch Watershed is as much as 80%. For more information regarding stormwater management, please contact Eric Thompson, PE, CFM. 

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