Mapping For Future Floods

Jun 2, 2019
Originally published on June 2, 2019 1:55 pm
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LULU GARCIA-NAVARRO, HOST:

Homes across the central U.S. are underwater. But look at the federal government's flood insurance maps, and many of them are in what's called areas of minimal flood risk. That's because the maps are old - sometimes 50 years old or more. That means hundreds of thousands of Americans are at risk for flooding, and they don't even know it. Here's NPR's Rebecca Hersher on a piece of technology used by scientists that could help.

REBECCA HERSHER, BYLINE: The technology is a way to map the surface of the Earth and see even tiny little bumps. It's called LiDAR, short for light detection and ranging. And for people who study rocks and ruins and rivers, it's like having a superpower.

ROBERT YOUNG: The story of LiDAR, as a research tool, is really a happy one.

HERSHER: Robert Young is a geologist at Western Carolina University. He uses LiDAR data all the time for his research. It's so detailed it can show where ancient rivers used to flow by mapping subtle changes in elevation. And the government also uses LiDAR to map areas along the coasts.

YOUNG: The United States Geological Survey began a lot of the early LiDAR studies that allowed us to quantify the volume change in a barrier island during a hurricane, you know, like, really neat things that we would never have been able to do before.

HERSHER: But until recently, that information was still mostly for research. It was only in the last few years that local governments have realized, hey. We also want a super-detailed map of the land we live on. Maybe if we know exactly how high things are, we'll be able to predict where water will go during a flood. In recent years, state and city governments have started collecting LiDAR data. North Carolina, Delaware, big chunks of Florida and major cities like Houston and Atlanta are all doing it. Lou Gritzo is the lead engineer for flood mapping at the private insurance giant FM Global.

LOU GRITZO: I think the most important thing is that, you know, cities are understanding the value of having data like LiDAR. And there's lots of different reasons why they would do that - for urban planning, for development. Flood is one of the biggest reasons.

HERSHER: His company is the biggest provider of private, commercial flood insurance in the country, which means they're really focused on making accurate flood maps. He says having more accurate data can be useful. But collecting LiDAR data is not enough on its own. You also need good computer models and old-fashioned humans to look for things that even LiDAR can't see.

GRITZO: There can be lots of little features that are even difficult to pick up with high-resolution LiDAR data. And those can be drainage pipes. They can be a covered area where there's a passageway into a basement, where there's high-value property.

HERSHER: Basically, LiDAR is helpful. But it's not enough to just use LiDAR to figure out how high or low every building is and then assume that the low ones will flood and the high ones will be safe. As states and cities update flood maps, they've also had to make sure that they backup their LiDAR data with enough manpower. Geologist Robert Young says that lesson has been showing up in some parts of North Carolina.

YOUNG: LiDAR is being misused, in my opinion.

HERSHER: Many coastal flood maps in the state have been updated in recent years using LiDAR data about elevation. If the LiDAR says the house is higher, it's assumed to be at lower risk, which sounds logical until you look at what some homes are sitting on top of.

YOUNG: Those are not granite peaks sitting out there on the barrier islands in North Carolina. They're sand dunes.

HERSHER: And, of course, sand dunes can erode.

YOUNG: You know, even my 11-year-old can tell you that you can make a 10-foot-high sandcastle on the oceanfront if you want to, but the waves are still going to chew it apart. So, you know, we have this tool that allows us to map elevation in ways we've never been able to before, but you still have to have a basic understanding of what that elevation means.

HERSHER: Many local officials say the new flood maps are still more accurate and easier to use than the old ones. But understanding that LiDAR data needs to be accompanied by strong models is increasingly important. That's because Congress is currently deciding on big changes to the National Flood Insurance Program. And so far, every plan has set aside big money for mapping technology like LiDAR.

Rebecca Hersher, NPR News.

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