The statistics are staggering. From a recent article in Engineering News Record (as of Sept 11): at least 136,000 homes were damaged (Harris County). 27% of Houston’s commercial buildings may have flooded (CoStar). 35 water systems are still shut down, 136 have “boil water” notices, and 35 wastewater treatment plants are inoperable (TCEQ). 26 roads remain closed due to floodwaters (TxDOT). The Harris County Flood Control District, FEMA and the city of Houston are preparing a home buyout program to move people out of floodplains (more info on the program to be released Sept. 18). Read the full ENR article here.
And from a recent article in the Washington Post: A new analysis from the University of Wisconsin’s Space Science and Engineering Center has determined that Harvey is a 1-in-1,000-year flood event that has overwhelmed an enormous section of Southeast Texas equivalent in size to New Jersey. A 1,000-year flood event, as its name implies, is exceptionally rare. It signifies just a 0.1 percent chance of such an event happening in any given year. “Or, a better way to think about it is that 99.9 percent of the time, such an event will never happen,” researcher Shane Hubbard said.
Apart from Harvey, there’s simply no record of a 1,000-year event occupying so much real estate.
While no one questions the exceptional nature of Harvey’s rainfall, the concept of a 1,000-year flood event has been criticized by some academics and flood planners. For one, rainfall and flood data generally go back only 100 years or so, so statistical tricks must be applied to determine what 500-year and 1,000-year events actually represent. But Hubbard, who analyzes geographic information to help decision-makers plan for floods, stands by the use of these return interval metrics despite their shortcomings. “For a community, they help put these events into perspective and understand the impact of a flood,” he said.
He added that they have “tremendous” value for flood planning and designing infrastructure to be able to withstand events up to a certain intensity. “Decision-makers have to be able to pick a number and say this is the number we need to be prepared for,” he said. “If we debate and belabor the accuracy of these estimates, the community will not have a value to plan for.”