After the unprecedented flooding that took place in Houston as a result of Hurricane Harvey, questions are being asked about the effectiveness of the system used to predict flooding in the United States.
The storm’s relentless rainfall flooded areas around Houston that had rarely or never been underwater, an NPR article published Monday reported.
“When the numbers started coming in, it was a little scary,” Matt Zeve, director of operations for the Harris County Flood Control District, which includes Houston, told NPR. He referred to a hash mark on the White Oak Bayou bridge that indicated the water had reached 20 feet above the stream bed.
Floods such as those that resulted from Harvey are often referred to as 100- or even 500-year storms. Maps designed by the Federal Emergency Management Agency and states are intended to help predict where floods can occur. People who own property inside a “flood zone” usually have to purchase flood insurance from FEMA.
In Texas, however, areas that were well outside flood plains have flooded many times, including incidents in the Houston region in 2015 and 2016 that were considered major floods, NPR said.
County Judge Ed Emmett, who is Harris County’s chief executive and helps lead recovery efforts, said “we’ve had three 500-year events in two years … we’ve got to go back and look at what our flood plains are.”
That is already happening at Texas A&M University, where flood expert Samuel Brody studied more than 30 years of floods in the Lone Star State. He found that about half of the insurance claims made after the flooding in Houston were for properties considered outside the mapped flood plain. In some parts of Texas the rate was reportedly as high as 80 percent.
“We’re finding neighborhoods that are miles away from any FEMA-defined flood plain, and every house is flooding,” Brody told NPR. “It’s not just flooding once in these epic events,” he added. “These are chronic, repetitive events.”
Part of the problem is that Houston’s flood maps are considered out of date because they’re based on rainfall data compiled up to 1994. Scientists at the National Oceanic and Atmospheric Administration said Houston has seen many large storms since the 1990s.
Sanja Perica at NOAA’s Office of Water Prediction has been overseeing an effort to update the nation’s rainfall data. The updated rainfall estimates and new flood maps reportedly aren’t ready to be published, but Perica said “we looked at preliminary estimates in the Houston area [and] they will change significantly” when the new numbers are published.
Perica’s team estimated that big storms in Houston drop 30 to 40 percent more rainfall than they used to. This will more than likely result in big changes to the rainfall maps and increase the number of people who will have to purchase flood insurance.