100-year flood. 500-year flood. These are the curious and somewhat misleading terms that meteorologists, emergency response organizations, and resiliency planners use to discuss flood events. A 100-year flood is one that reaches a level that, statistically speaking, stands a good chance of occurring once in a hundred years. Likewise, a 500-year flood might occur once every 500 years. However, this is not a schedule; It’s merely a statistic based on historical data and future projections.