Thanks to the devastation brought by Hurricane Harvey, we have been hearing a lot about so-called 500-year-floods. A 500-year-flood isn’t the worst flood an area has seen in 500 years; rather, a 500-year-flood is a term used to describe the likelihood that a flood of that severity will occur. So, a 500-year-flood is a flood that, based on historical floods, is likely to happen every 500 years. This means that a 100-year-flood has a one percent chance of happening each year. There is a .2 percent chance of a 500-year-flood happening each year.
When the government classifies the seriousness of a flood, its classification is based on local flooding. Flooding that would equal a 500-year-flood in New York City, a place that doesn’t have many floods, may not be a serious flood in Houston.