What 500 and 100-Year Flood Really Means
A 500-year flood does not mean that it only happens once every 500 years. In theory, a 500-year flood is something that has a 1-in-500 chance of happening in any given year. Thus, it’s an event with such a low probability of happening that it doesn’t even make sense to plan around its occurrence. However, the issue with this term is that the so-called 500-year floods are happening at a greater rate than these given odds — particularly in flood-prone cities. Flood prevention and planning in cities like Houston haven’t advanced to acknowledge that a 100-year or 500-year flood isn’t really that inconceivable to expect annually. In fact, parts of Houston saw “500-year flooding” just last year.