Louisiana flooding: What is a 500-year flood and why is it happening so much?

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Louisiana flooding: What is a 500-year flood and why is it happening so much?

As  of Wednesday morning, 11 people have died and more than 40,000 homes have been damaged in ongoing flooding  in southeastern Louisiana.

Up to two-and-a-half feet of rain that swelled rivers and swamped the area in and around Baton Rouge, La., has led the National Oceanic and Atmospheric Administration to classify the flooding as a once-in-every-500-years event.

Obviously, by definition, the events are rare – except this is the eighth time one of the 500-year events has happened in the United States in a little more than 12 months.

Six states – Louisiana, Texas, West Virginia, South Carolina, Maryland and Oklahoma --  have all had unprecedented rainfall events that, according to NOAA research, they should have only had a less than one percent chance of experiencing in any given year. 

So what is a 500-year flood and why are they happening more frequently? Here’s a quick look at what the historic rainfall means.

What is a 500-year flood?

The U.S. government, when creating the National Flood Insurance Program, used a measure called the 1-percent annual exceedance probability flood (AEP) to estimate the chance of repeat flooding of a certain level  in a certain area. The AEP defines a flood that, statistically, has a 1-in-100 chance of  being equaled or surpassed in any one year, thus the term “100-year flood” was born. The 500-year flood” is equal to an AEP of 0.2 percent, or a 1-in-500 chance an area will see a repeat of flooding at a certain level. 

In some areas of Louisiana, the flooding is being classified as a 1000-year-event – or an 0.1 percent chance of seeing flooding like that in any given year.

How are flood risks determined?

Scientists and engineers take annual measurements of the strength of the flow of a body of water and the peak height of the water as recorded by devices called streamgages. These devices are placed in spots along a river. They use those numbers, collected over time, to determine the probability (or chance) that a river will exceed those measurements during any given year.

Does a 500-year flood really mean that a flood of that type happens only once every 500 years?

No, not exactly. We are talking math. The term means  that, statistically, there is a 1-in-500 chance that an area will have a large flood in any given year. You could have a large flood two years in a row, but, chances are, you won’t. 

Why are we seeing eight such floods in the U.S. in a little over a year then? Does climate change have anything to do with it?

Climate scientists sure think it does. Many say they believe that global warming has everything to do with it and say we can look forward to more of these events. They have warned that warming temperatures on both land and sea, and the build-up of moisture in the atmosphere, will inevitably cause more large flooding events.

“We have been on an upward trend in terms of heavy rainfall events over the past two decades, which is likely related to the amount of water vapor going up in the atmosphere,” said Dr Kenneth Kunkel, of the Cooperative Institute for Climate and Satellites, told The Guardian.

“There’s a very tight loop – as surface temperatures of the oceans warm up, the immediate response is more water vapor in the atmosphere. We’re in a system inherently capable of producing more floods.”

David Easterling told The New York Times that the flooding “is consistent with what we expect to see in the future if you look at climate models. Not just in the U.S. but in many other parts of the world as well.” Easterling is a director at the National Centers for Environmental Information, which is operated by the NOAA.

Sources: NOAA; The New York Times; The Guardian; The Associated Press; The National Weather Service

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