Why scientists think FEMA floodplain maps may be flawed

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Research shows the 100-year floodplain maps by FEMA could be wrong for one reason.

FEMA's floodplain maps, which are the government's best estimates of who will flood in a massive storm, are said to be flawed.

A new study suggests FEMA failed to consider something significant when putting the maps together.

The question now is whether that could have resulted in a number of people getting caught off guard during Harvey.

The streets in Mary Carrillo's neighborhood in La Porte are lined with piles and piles of belongings.

"We're just throwing away everything," Carrillo said. "Everything was underwater."

Researchers from Rice University and Texas A&M Galveston looked at the Armand Bayou watershed in southeast Harris County, a 60-square-mile area which includes portions of Houston, Pasadena, Deer Park, La Porte and Taylor Lake Village.

Over a 10-year period, data showed many losses from storms, they say, even though the area is not in the 100-year floodplain as predicted by FEMA.

There were five major rain events in the study area from 1999 to 2009: Hurricane Ike in 2008, Tropical Storms Erin in 2007 and Allison in 2001, plus two major rain storms that caused flooding in 2006 and 2009. None of those events was considered a 100-year flood.

Carrillo's home off Somerton Drive flooded in Allison and now again during Harvey.

While she's not sure the reasons why, researchers say they have a good idea.

"Small changes in historical development can have large implications for the delineation of the floodplain," says Russell Blessing, a researcher from Texas A&M Galveston who is the lead author of the study.

He is a graduate student with joint appointments at the SSPEED Center and Texas A&M Galveston's Center for Texas Beaches and Shores.

Blessing says development is responsible for the flooding that has left so many piles of debris and so many homeowners as victims.

Studies from Rice University and Texas A&M University at Galveston looked at 10 years' worth of data. In 75 percent of flood claims, the damage happened outside FEMA's 100-year floodplain, Blessing said.

"I think that flaw is the change in land cover," Blessing said. "Those types of changes aren't adequately considered when they delineate the 100-year floodplain."

Homeowners like Mary Carrillo know that Harvey packed an even more powerful punch. No flood maps could have predicted what happened here, she says.

A FEMA spokesperson says a federal advisory committee created by Congress in 2016 found the National Flood Mapping Program produced technically credible flood hazard data and maps. The agency says they are currently engaged in response and recovery efforts for hurricanes Irma and Harvey and haven't had an opportunity to review this study.

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FEMAhouston floodfloodingresearchtexas newsu.s. & worldtexas a&m universityrice universityLa Porte
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