When Hurricane Harvey hit the Houston area, it brought heavy rain, severe flooding, and damaged thousands of homes. While Harvey was a severe storm, what made it particularly astounding was the amount of flooding it delivered to areas that don’t traditionally flood.
In fact, the flooding was so severe that it was called a 500-year event by weather experts. However, this phrasing is meant to convey that a storm of this magnitude is something that happens once every 500 years. Instead, it’s meant to communicate how likely a Harvey-sized storm is to happen each year.
What 500-Year Events Mean for the Likelihood of Severe Storms
Scientists use years to describe the probability of a storm occurring based on scientific data. So, the phrase “500-year event" actually means that’s a 1 in 500 chance that a massive storm like Harvey will occur each year. This definition applies to every type of storm. If one is described as a 10-year event, there’s a 1 in 10 chance that storm could have happened.
“People think: ‘Well, it’s a one-in-100-year flood. We had one last year, so it won’t happen again,’” Dr. Sandra Knight from the University of Maryland told the New York Times.
Dr. Knight’s insights are supported just by looking at data from Houston before Hurricane Harvey. Parts of the city saw 500-year flooding events in just before Hurricane Harvey. Places like Horsepen Creek had historic flooding in 2016, serving as a preview to what would happen to the rest of Houston just one year later. Now, weather experts are calling for a change in approach when it comes to describing storm severity.
“We’re looking at historical data when really we have something that is called non-stationarity,” Dr. Knight told The Times. “The world isn’t stationary anymore and the hydrology isn’t. The landscape isn’t. So why are we still presuming the future will look like the past?”
While a flood might be defined as a “500-year event", an outdated way of defining how likely strong storms are to happen might need to be updated with language that reflects our current reality.
Overdevelopment Is Linked to Historic Flooding
Some experts warn that overdevelopment is making 500-year flooding events more likely in places with booming populations such as Houston. Samuel Brody, director of the Center for Beaches and Shores at Texas A&M, was asked by TexasMonthly about what Houston can do to prevent flooding caused by excessive development.
"That might be the most important question. Houston is growing so fast.” Brody said. “The metro area has added well over 100,000 people per year [as of 2018] the past couple years. And we are at real peril if we don’t answer that question. Part of it will require retrofitting things that have already been built, and part of it will require recovering some of the land. And we have to think very carefully about where we’re going in the future, and how to go there in a more flood-resilient way"
Brody suggests that local governments need to plan more efficiently for floods, use materials that are flood-resistant while building infrastructure and incentivize developers to stop building metro areas of cities further and further out.
Storms Like Hurricane Harvey Are Likely to Happen Soon
With changing weather patterns and infrastructure that isn’t completely designed with storms in mind, it’s not a matter of if a 500-year storm will occur again—it’s a matter of how soon the next one will happen. As Houston residents, the best thing we can do is prepare for the worst. Check your home’s storm resistance, make sure you have the right insurance coverage, and remember to call Arnold & Itkin LLP if your insurance company isn’t being fair in the future. Our hurricane insurance lawyers are ready to help you recover.