ELISE HU, HOST:
A lot of people and scientists in South Texas are wondering, what good are flood maps anymore? It's a big question after the shocking amounts of rain and flooding that came with Hurricane Harvey in August. NPR's Christopher Joyce recently went to Houston to find out what scientists have learned from Harvey and what it could mean for the city's future.
CHRISTOPHER JOYCE, BYLINE: Blues man Stevie Ray Vaughan spoke for a lot of Texans when he sang the song "Texas Flood."
(SOUNDBITE OF SONG, "TEXAS FLOOD")
STEVIE RAY VAUGHAN: (Singing) Yeah, flood water keep a rolling - man, it's about to drive poor me insane.
JOYCE: When Hurricane Harvey approached last August, it was so big even seasoned flood watchers were worried.
MATTHEW ZEVE: When the numbers started coming in, it was a little scary.
JOYCE: Matthew Zeve is operations director for the Harris County Flood Control District. I met him at White Oak Bayou, one of the many streams that crisscross Houston. He shows me a pink hash mark on a bridge.
ZEVE: This is how high the water got during Harvey in this location.
JOYCE: And looking down at where the stream is, that's a good 10 feet.
ZEVE: And we're probably - that's probably 15, 20 feet. This is the first time in our recorded history that the entire county - all of Harris County - experienced record-breaking rainfall and flooding.
JOYCE: And it came on the heels of two previous huge floods. These are supposed to be rare events, the kind that happen only once a century or even 500 years. Maps show where these floods are likely to occur. The Federal Emergency Management Agency works with states to make those maps. They determine whether you have to buy flood insurance. But in Texas, it keeps flooding way outside of those mapped flood plains. County Judge Ed Emmett, who helps lead recovery efforts in Harris County, wonders if the maps are any good.
ED EMMETT: You know, we've had three 500-year events in two years. Does that mean our definition of a 500-year event is wrong? Clearly we've got to go back and look at what our flood plains are.
JOYCE: Some flood scientists already have. Sam Brody at Texas A&M University studied over 30 years of floods in South Texas. He found that about half of the insurance claims made after floods in Houston were for properties outside the mapped floodplain. And Harvey looks to be just as bad.
SAM BRODY: We're finding neighborhoods that are miles away from any FEMA-defined flood plain. And every house is flooding, and why is that the case? And it's not just flooding once in these epic events. These are chronic, repetitive events.
JOYCE: Much the same is happening in places like Baton Rouge and Chicago.
BRODY: A hundred-year boundary is not going to capture the amount of risk and impacts from flooding now and certainly not moving forward.
JOYCE: One problem with Houston's flood maps is they're out of date. They're based on rainfall data before 1994. So are engineering standards for construction. But scientists at the National Oceanic and Atmospheric Administration point out that Houston has seen numerous huge storms since the 1990s. Sanja Perica at NOAA's Office of Water Prediction has factored in rainfall numbers from those more recent storms.
SANJA PERICA: We have looked at preliminary estimates. In Houston area, for example, they will change significantly.
JOYCE: How significantly - her team calculates that big storms in Houston are now dropping 30 to 40 percent more rainfall than they used to.
PERICA: Thirty or 40 percent increase in precipitation estimates will probably affect flood estimates as well a lot.
JOYCE: And the other problem with flood maps is they don't consider climate change. Gerry Galloway is a flood scientist at the University of Maryland.
GERRY GALLOWAY: We've always figured out the probabilities of flood events by looking back at the past. That doesn't work anymore.
JOYCE: A warmer planet means rainfall patterns are changing. And that makes a flood plain a moving target.
GALLOWAY: When somebody says this is a hundred-year flood, it's a guess. It's a concept. We know now that we're dealing with a new world.
JOYCE: Galloway and Brody are part of a team of scientists advising Texas on how to prepare Houston for more big storms. In the meantime, NOAA will deliver its new rainfall data to Texas authorities later this year. The new information could mean that the city will have to build more reservoirs and flood water diversions. New highways and houses may have to be built higher. More people will have to buy flood insurance. And it will all cost a lot of money. Christopher Joyce, NPR News.
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