20 to 7-year storms (7/30/2021)

There was a report in one of the professional publications that I read that predicts that what we call in our profession 20-year storms are shifting and becoming 7-year storms. Now that is an interesting concept that I teach in my classes on stormwater, in particular in the class entitled “Where the Water Goes.” The class has a subtitle Hydrology for ESC and SW Inspectors. ESC stands for Erosion and Sediment Control and SW for Stormwater.

So, what does a x-year (24-hour) storm means and what do these values mean? I tell my students that those famous 100-year storms do not occur only once every 100 year. I remember well that in 2003 we had two 100-year storms in one week, followed the next week by hurricane Isabel, which dumped another 100-year storm in our area. As you can imagine that September the ephemeral ponds behind our home were full to the brim, a thing that usually only occurs in February. They are usually dry in the month of September. Those two 100-year storms in a row actually created a major issue during Isabel; the soil was saturated, and the trees were extremely unstable because of this. Trees were falling all over the place during the hurricane. I lost 13 trees in my back yard that day.

In a simple explanation, I tell my students that the concept of so many year storms is based on statistics. For example, a 100-year storm tells us that the storm has a likelihood of 1% (or 100%/100) of occurring any day of the year. A 20-year storm has a chance of occurring 5% (100%/20) any day of the year, and so on. Somehow, NOAA or the National Oceanic and Atmospheric Administration, could not find a better name for this phenomenon than calling it a so-many-year storm. In particular, since if it occurred today, it has the same probability of occurring again tomorrow. So, although rare what happened in 2003 having three 100 year storms in two weeks, it was statistically explainable.

So, I looked at the data for my home. A 20-year 24-hour storm at my home dumps around 6.5 inches of rain or 165 mm in those 24 hours, according to NOAA. In the good old days, a 7-year storm drops 5 inches or 127 mm in the same 24 hours. In other words, as a result of climate change the 7-year storm at my home would increase 1.5 inches or almost 40 mm of rain. Surprise, this is something we are all experiencing, thunderstorms are increasingly getting stronger and more severe.

I realize this is only an example for one place, but in the past weeks we have seen similar examples in Germany, Belgium, Arizona, and China just to name a few where we are seeing that storms are getting increasingly severe and dumping more water. Naturally this is not helped by all the development around us and the imperviousness that we are creating in our watersheds. It all means more runoff and flooding. The problem on top of this is the imperviousness that we appear to have created and are increasingly creating in neighborhoods of disadvantaged racial minority groups.

Some of these things going on with our climate and our earth are most likely very difficult to reverse. We will have to learn to live with them and adapt to them. That is part of the job I do professionally. I am part of a group that teaches stormwater management. We teach designers, developers, and regulators that it is best to try to infiltrate all that stormwater, by minimizing the impervious areas such as parking lots and roads. We also tell them that they need to preserve the soil, not compact it, so that water can infiltrate. Plant trees and shrubs, minimize lawns. Trees and shrubs intercept rain in their canopies and often only 40% of the rain makes it down to the ground. Are my students listening? Some, few are. But we keep working on them. But we don’t get discouraged, it is the right thing to do.

This is a picture I took a few years ago of a rain garden/bioretention area in Charlottesville, Virginia.  We teach students that the placement of ponds like these help with the infiltration and cleanup of stormwater and hopefully reduce the flooding danger.