There are three big problems happening with this metric, experts told me. “Doesn’t seem like it’s performing well,” he said. In the Houston area, he found that number to be closer to 30 to 40 percent. “Something like 15 to 20 percent of insured flood claims happen outside the floodplain,” Highfield said. These floodplain maps are best understood as estimates - and not necessarily very reliable ones. Plunk the flood depth estimates down on top of maps and you get a floodplain. Finally, a computer model helps turn those high flow rates (a measure that doesn’t tell you much about whether your couch will be underwater) into an estimated flood depth (which does). The USGS collects these flow estimates, plotting them over the years to find the normal amount of water that moves down a given stream - and what that flow looks like when it jumps to levels well above average. Throw that into a mathematical potpourri with other data points - what’s known about the shape of the stream, say - and you can come up with an estimate of flow, water measured in cubic feet per second. Geological Survey has deployed to collect real-time data on the depth and velocity of rivers and streams across the country. It begins with roughly 8,000 streamgages, sensors that the U.S. To get from point A to point B, scientists have to make a lot of assumptions and extrapolations, building in layers of uncertainty that mean the final determination of what is and isn’t in the floodplain should never be thought of as exact. Outside the floodplain, there would be no restrictions.īut there’s a gap between the data those maps are built on and the floodplain boundaries themselves. And you might not be able to build there at all. If a parcel of land fell inside the boundaries of where a 1-percent-annual-risk flood was likely to reach, any new buildings constructed there would have to be elevated and insured - and would therefore be more expensive. The term “100-year flood,” which Harvey has almost certainly caused, 2 entered the American vocabulary in 1973, when the federal government first defined which parts of the country would fall under new flood control regulations and which would not. It may be time for us to find a different way of evaluating that risk altogether.įloodwaters covered this neighborhood near Interstate 10 in Houston on Thursday. The concept of using a “100-year flood” as a benchmark for risk isn’t just misunderstood it obscures fundamental statistical problems in how we assess flood risks - problems that can lead to residents and homeowners believing themselves to live in a zone of safety that isn’t there. But there are still more holy offices to perform. Stories that emphasize this fact are “doing the Lord’s work,” said Wesley Highfield, professor of marine sciences at Texas A&M University at Galveston. Over the course of a 30-year mortgage, a house in a 100-year floodplain has a 26 percent chance of being inundated at least once. Instead, it refers to a flood that has a 1 percent chance of happening in any given year. In the wake of catastrophic flooding on the Texas coast, the media has been working hard to explain the term, turning out dozens of articles explaining that a “100-year flood” is not a flood that you should expect to happen only once every 100 years. That’s no surprise to experts, who say the concept of the “100-year flood” is one of the most misunderstood terms in disaster preparedness. Surveys have shown that even people who live across the street from the bureaucratically determined risk zones known as floodplains don’t understand how those boundaries were drawn or what the risk metric that defines them really means. But the risks that coastal Texans faced before the storm hit - and the probability that others will be dealt a similar fate - are still a confusing mess. Photos of water-covered neighborhoods and families riding floating refrigerators to safety have made clear the scale of Hurricane Harvey’s wrath.
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