Risk Assessment, Not a Timeline

Contrary to popular belief, “100-Year Flood” does not refer to a flood that happens every century. Rather, the term describes the statistical chance of a flood of a certain magnitude (or greater) taking place once in 100 years. It is also accurate to say a so-called “100-Year Flood” has a 1 percent chance of occurring in a given year, and those living in a 100-year floodplain have, each year, a 1 percent chance of being flooded.

This basis gives the National Flood Insurance Program a standard for risk assessment, but does not mean that a 100-year flood will only take place once every century. Instead, it means that data from stream gauges in the area have been averaged over a long period of time for insurance companies and civil engineers to determine how likely it is a region will flood.

Individual, Community and Environmental Importance 

If a 100-year flood has already occurred in your lifetime, it can still happen again. Over a 30-year interval, a home in a 100-year floodplain has a 26 percent chance of being flooded at least once. For this reason, homeowners on 100-year floodplains are required to purchase flood insurance.

Infrastructure is designed with these risk assessments in mind, especially levees and dams. The city of Sacramento has had only about a 100-year level of flood protection, giving it a 1 percent chance of a major flood occurring each year, compared to St. Louis and Kansas City’s 500-year flood protection levels. To improve this, the Folsom Dam Auxiliary Spillway project in Sacramento is designed to upgrade the city’s 100-year flood protection to 200-year protection, as floods in past decades have proved dangerous to people and infrastructure alike. This way, the city’s risk of flooding is cut in half. Meanwhile, the Seven Oaks Dam in Southern California has 350-year flood protection, giving it a 0.3 percent chance of flooding in a given year. 

In the Delta, there are approximately 1,100 miles of levees protecting 700,000 acres of lowland from floods. To determine the magnitude of the Delta flood risks, the Delta Management Strategy (DMS) assessed major risks to Delta levees from floods, seepage, subsidence and earthquakes. In 2008, a report was released finding that if a major earthquake or flood occurs in the next 50 years, there is more than a 60 percent chance multiple levees will simultaneously fail. Water quality, water supply and animal habitats would be destroyed, costing the California economy up to an estimated $40 billion. 

Alternative Reporting

The timespan of 100 years is a given flood’s “recurrence interval,” or likelihood of happening once in this period. The United States Geological Survey (USGS) also calls this the 1-percent annual exceedance probability, or AEP. AEP can be used for other numbers as well, as “50-Year Floods,” would have a 2 percent chance of happening in a year; “25-Year Floods” would indicate a 4 percent probability. The Los Angeles Flood of 1938, for example, is considered a 50-Year Flood. One hundred years is a commonly reported recurrence interval that is used for ease of statistical communication.

Assessing Your Own Risk

Different locations along various rivers have their own 100-year flood magnitudes, according to the USGS. The Federal Emergency Management Agency (FEMA) provides flood maps for most communities. The California Department of Water Resources has developed a mapping project called Best Available Maps (BAM) to show land areas expected to experience 100-year, 200-year, and 500-year floods throughout California. Flood insurance policies have different coverages and expenses depending on the kind of floodplain on which a house is located.