Extreme Rainfall: When It Rains, It Pours

It is the time of year when we see flooding pop up in the news almost every day. And it has been a crazy year for flooding. In the last two weeks, we’ve seen terrifying floods move through Kentucky, and less devastating but still unbelievable flooding in Denver, Boston, New York City…the list goes on. The odd thing is, however, that this type of flooding isn’t so odd anymore. 

Now, the Midwest is at about the highest risk in the U.S. for flooding related to extreme rainfall (defined: when the amount of rain experienced in a location substantially exceeds what is normal).  

Source: https://www.epa.gov/climate-indicators/climate-change-indicators-heavy-precipitation

The metropolitan statistical areas that are most impacted by heavy rainfall include five MSAs in West Virginia, Virginia, and Kentucky, as well as seven more in Ohio and Indiana. That’s 4.7 million people who are experiencing—and will continue to experience—rainfall that exceeds anything they’ve known before. 

In the U.S., precipitation in the contiguous 48 states has increased at a rate of 0.2 inches per decade (the global average rate is 0.04 inches per decade); some areas in the U.S. have seen an increase of 30% since 1901, while others have seen a 30% drop. The shifts in weather patterns are only expected to become stronger and more divergent. And that’s a problem in both rural and urban communities. 

Here’s why. 

First, consider the fact that most of our stormwater infrastructure is at least 50 years old. Even a 10% increase in precipitation puts an incredible amount of stress on these systems; a doubling of major floods and an increase of severe storms by 40% is untenable. (and yet it’s happening). And that’s assuming our infrastructure is in great shape.  

It’s not. According to the newly released 2021 Infrastructure Report Card, America’s stormwater infrastructure received a grade of D. 

While stormwater utilities are on the rise, with more than 40 states having at least one, the impervious surfaces in cities and suburbs are also expanding, exacerbating urban flooding, which results in $9 billion in damages annually. Stormwater also affects water quality as polluted runoff from pavement enters water bodies. Nearly 600,000 miles of rivers and streams and more than 13 million acres of lakes, reservoirs, and ponds are considered impaired. Federal funding, though up in recent years, averages about $250 million annually, which leaves a growing annual funding gap of $8 billion just to comply with current regulations.  

—Report Card for America’s Infrastructure, ASCE 2021 

We’re dealing with a few key issues: 

  • Aging stormwater infrastructure that—due to budget issues and limited availability of funding—has been managed according to a run-to-failure model. This is not the fault of the communities who are managing this infrastructure; it’s the fault of an insufficiently funded system, and that is a result of a common mindset that says if you can’t see it, then it’s probably fine. That mindset isn’t working anymore. 

Source: https://www3.epa.gov/npdes/pubs/assetmanagement.pdf

  • In addition to it just getting, well, old, our stormwater infrastructure was designed a long time ago, when rainfall patterns were very different. And in the case of areas at high risk for extreme rainfall, those patterns can be dramatically different. So, if your stormwater system was designed to handle an inch every 5 hours, and now it’s getting an inch an hour, you’re going to have a problem. 
  • In urban areas, urbanization increases, putting more stress on our infrastructure simply by adding volume. And not just rainfall volume, but runoff from more pavement and more people, not to mention the addition of garbage. In rural areas, systems tend to be open; they get overgrown or lost or forgotten until it’s too late. 

As a result, the capacity of our stormwater infrastructure is often less than its design standard (and that’s assuming the design standard of 50 years ago…or even 10…still applies).  

What to do. It’s a big problem; but with information, we can take steps to solve it.  

We’ve seen several communities address it successfully by: 

  • Tracking performance of stormwater systems during all types of rain events, not just statistically significant storms. This includes time to peak flow, minimum available capacity, and return to baseline. 
  • Tracking frequency of statistically significant storm events. 
  • Evaluating how a system performs based on the type of storm (significant or otherwise): is it the intensity? Duration? The frequency that causes flooding? Do upstream conditions affect available capacity?  
  • Understanding the social impact. Are poorer communities affected more than wealthier areas in the same town? What other parameters correlate to the measured differences? 
  • Comparing system performance relative to the characteristics of the community. Does greenspace make a difference? Or is the flooding all combined with tidal impacts, or upstream flows?  
  • Understanding what type of mitigation strategies work given relative risk, land use, cost/benefit, and zoning. 

Again, it’s a big problem. But it’s absolutely manageable one community, and one neighborhood, at a time. All we need is to be able to measure it. And we can do that together. 

About the author

Erin Rothman

Talk stormwater with erin@stormsensor.io With more than 15 years of environmental consulting experience, Erin observed so many opportunities for innovation in the stormwater industry. With those in mind, she founded StormSensor to enthusiastically embrace new technology to help solve the problems of an age-old industry.