Series: After the Flood The Challenge of Rebuilding as the Climate Changes

The Federal Emergency Management Agency’s flood insurance maps are critically important for millions of Americans who live in flood-prone areas. The maps determine the annual premiums for flood insurance, which is required by law for homeowners with federally backed mortgages who live in high-risk areas. But many of the nation’s flood maps are woefully out of date.

ProPublica talked with David R. Maidment, a professor at the University of Texas at Austin who has advised FEMA on flood mapping, about why the data behind modern maps is 10 times as accurate as the older data and why half of Texas still doesn’t have up-to-date maps.

How did you become involved with flood mapping?

I was the drainage engineer for a small community out in Rollingwood, Texas. It’s a small suburb of Austin, and it was hit by a big flood in 1981, which is when I joined the University of Texas at Austin. They didn’t have much of any regulation at the time, and so people did more or less whatever they wanted, and they put buildings wherever they wanted. It was clear that we needed a better system.

Why is it important to have up-to-date flood maps?

Flooding is the natural disaster that impacts people more than any other.

Since federal disasters started being declared in 1952, two-thirds of them have involved flooding. The next one after that is fire. There have been about 300 federal disasters declared from fire and 1,600 from flooding. Fire is a very horrible thing, but flooding occurs much more frequently, and it’s also much more widespread. That’s the fundamental reason. Floods devastate human lives to a greater extent than any other national hazard.

What’s changed since you started studying flooding?

The big change, which happened about 10 years ago, was the conversion of maps from a paper system into a digital system. That was a very large investment of federal funds. It’s considered to be the largest civilian mapping program in the world.

How up to date are FEMA’s flood maps now?

It varies quite a bit. The data in cities is generally pretty good. It’s once you get outside of the cities that things start falling away. The original question that we were asked to investigate was whether it was possible to produce updated flood maps with out-of-date terrain and panametrics information. Flood maps have two-dimensional information that says, ‘Where are things on the ground?’ And they have three-dimensional information, which says, ‘How high is the land?’ We found that the two-dimensional data — where are things horizontally? — is actually pretty accurate, because it’s being done with image mapping now, and the emergence of Google Earth and other things like it have made image mapping really accurate. What we found was out of date was the nation’s three-dimensional elevation data, which in many cases was drawn from U.S. Geological Survey topographic maps that were developed 30 or 40 years ago, using mapping processes that aren’t as accurate as what is now possible.

What makes the new maps more accurate than the old ones?

The big improvement that’s happened is in the vertical elevation data. There’s a new technology that’s emerged called lidar, and what it involves is having an airplane flying over the land and firing out a bunch of laser pulses in kind of a beam. The beam comes out of the airplane and hits a mirror that goes left and right as the airplane goes forward. So there are billions of pulses that hit the ground and bounce back to the airplane again. About a third of the country has been covered with lidar now. The three-dimensional elevation data that it generates is about 10 times more accurate than the old data.

If you live somewhere prone to flooding, what do these improvements mean?

There’s a much more precise definition of what the real flood risk is. Also, they make it possible to define how deep the water will be. In the old flood plain maps, the only question they answered was, ‘Is your building anywhere within the floodplain boundary?’ And with the more accurate three-dimensional data, different questions can be asked: What is the likelihood that your building will flood? And if it does flood, how deep will the water be? It wasn’t possible to answer any of those questions before.

And FEMA couldn’t do that with the old maps?

No. The old maps were 2D — they were paper maps. There was no 3D aspect to them at all.

How much of the country has these newer, more accurate flood maps?

The original intention was to cover the whole United States, coast to coast, with digital flood maps. But it turned out they had a billion dollars — $200 million a year, over five years — to do that, and it wasn’t a billion-dollar job. It was probably a $10 billion job, or a $20 billion job. It turned out to be much more expensive to do that than what had been anticipated at the beginning. And so FEMA cut back on the original goals, and instead focused on the areas of highest flood risk, which generally are associated with high-population zones and zones near the coast. They quantified the flood risk in about 65 percent of the country that contained 92 percent of the population. That means that in the populated areas, the flood maps are actually in pretty good shape now. But in the unpopulated areas, in many cases there are no updated maps at all. Half of Texas, for example, has no flood maps. And I’m distressed about that, because we’re still not where we need to be.

This interview has been edited for clarity and length.