Rain flows down from a roof down

The Probability of Flood Risk

In some places, homes and businesses constructed in areas known as the 1-in-100-year flood plain have been hit by multiple floods in the same year. The assumption that if their area has experienced a 1-in-100-year flood, then for the next 99 years they do not have to worry about flooding is not correct.  While it’s unlikely that two large storms will happen in close succession, history has demonstrated that it is not impossible.

Confused by the term 1-in-100-year flood, many people begin to wonder what their flood risk really is. The definition of a 1-in-100-year flood is a flood that has a 1% chance of occurring in any given year. Understanding the probability of your flood risk can be a complex process, but the hydrologists at the U.S. Geological Survey (USGS) are striving to communicate risk more effectively, in part by transitioning away from the term 1-in-100-year flood and instead referencing multiple year flooding probabilities. For example, a home in the 1-in-100-year flood plain can be better understood as a home with a 26% chance it will flood over the course of a 30-year mortgage.

Providing residents and community leaders with a clearer understanding of the probability of flood risk for a location is important. It allows decisions to be made that will better protect themselves and the community. For more information on flood risk probability, visit the USGS website at https://www.usgs.gov/special-topic/water-science-school/science/floods-and-recurrence-intervals.