While these phrases can make it sound like the flooding is cyclical, only happening once every 100 or 500 years, it is not. Storms are not on a schedule. Rather, these terms refer to statistical probabilities. For example, the “100-year flood” means there is a 1% chance of a flood of that magnitude happening in any given year in a given location. For the 500-year flood, the probability is 0.2%.
Given the right conditions, it is possible to see multiple 100-year or 500-year flood events in a relatively short period of time. Ellicott City, MD, for example, recently saw two 1000-year flood events only two years apart. For more information, watch the video below from the NWS.