PLEA Network

28 August 2017, VOX, The “500-year” flood, explained: why Houston was so under prepared for Hurricane Harvey. It’s the city’s third “500-year” flood in the past three years. It’s difficult to comprehend the scale of the flooding and devastation that Hurricane Harvey and its aftermath are wreaking on the Houston area. Weather experts call the storm unprecedented, and note that it’s gone beyond even the most pessimistic forecasts. In the final reckoning, it’s certain that Harvey will be classified a 500-year flood — and maybe even a 1,000-year flood. But those terms can be a bit misleading — especially when high-profile people, like the president of the United States, confuse the issue by calling Harvey “a once in 500 year flood.” In theory, a 500-year flood is something that has a 1-in-500 shot of happening in any given year — in other words, the sort of event that’s so rare that it might not make sense to plan around the possibility of it happening. The problem is that 500-year floods are happening more often than probability predicts — especially in Houston. And, especially in Houston, prevention planning hasn’t evolved to acknowledge that a “500-year” flood isn’t really a 1-in-500 chance anymore. “500-year” floods are based not on history, but on probability. The severity of floods tends to get put in terms of years: a 100-year flood, a 500-year flood, a 1,000-year flood. But this isn’t an assessment of “the worst flood in” that time — places like Houston don’t actually have detailed weather records going back to 1017 AD, after all. Read More here