The Truth about 100-Year Floods

 Whenever a strong hurricane or storm causes significant flooding, there is often talk of a 100-year flood. When people hear this term they think a flood of that magnitude occurs once every hundred years, so they should be safe from harm for the next hundred years. Unfortunately, some base their decision to buy flood insurance on this false belief, putting them at risk of a catastrophic loss.

 The reality of this is different. It involves frequency analysis and statistical probability. In order to determine the frequency of floods of a given level, scientists create a history of the area. Generally, scientists start with a minimum of ten years of information. The longer the record, the better and more accurate predictions. They look at the frequency of different size floods and the average number of years between them in order to develop the probability of a given size flood in any given year.

 In order to consider a flood extreme, experts start by determining the normal amount of rainfall and stream stage for a given body of water. Stream stage is the level of water, measured in feet, above a certain arbitrary point. The measurement is the distance from the riverbed to the height of the water. A stream or river in normal conditions could have a stage of two feet, meaning the stream is two feet deep. If a big storm hits suddenly the stream stage could quickly rise to fifteen or twenty feet. A flood occurs when the stream or river overtops the natural or artificial banks of the stream. Rain volume alone does not always create a flood; the condition of the ground before the rain starts is critically important. Has the area received a lot of rain recently and is the ground already saturated? Less rain is needed to create a flood under those conditions. Likewise, if the area has been under a drought and the rain is falling fast, it is possible for the water to run off instead of soaking into the ground. This can lead to mud and landslides, depending on the ground in the surrounding area.

 If a river basin is altered such that it affects the flow of water, scientists will re-evaluate the frequency. The addition of dams and levees will change the odds. Once a flood has receded, high-water marks are surveyed to estimate maximum flows that cannot be measured during an active storm. Streamgauges record the height and quantity of flow. As rivers and streams flow and flood, the beds and flow of water change causing sediment to flow downstream and the bed itself to change, which can change the flow of water.

 The accumulation of data allows for a determination of the annual probability that a given stream or river will exceed its banks. This is known as annual exceedance probability or AEP. A one-percent AEP flood has a 1 in 100 chance of occurring every year, with an average recurrence every 100 years. This is the 100-year flood. A 500-year flood has an AEP of 0.2 percent chance, or a one in 500 chance of occurring in any given year. It is entirely possible for there to be 100-year or 500-year floods in consecutive years. Because of the confusion and misunderstanding the term 100-year flood generates, the United States Geological Survey is encouraging the use of the AEP terminology, so the phrase 1-percent AEP flood is preferred over a 100-year flood. During the span of a thirty-year mortgage, there is a 26 percent chance of a home in a 1-percent AEP, or 100-year flood area, being flooded.

The National Weather Service provides a calculator so it can be determined just how often a storm of a given strength will occur over a given time period. In the following chart we have calculated the risk of a 100-year and other floods over 1, 10, 20, 30, and 50 years.

 

Time 10 year flood 25 year flood 50 year flood 100 year flood Total Percent Risk
1 year 10% 4% 2% 1% 17%
10 years 65% 34% 18% 10% 127%
20 years 88% 56% 33% 18% 195%
30 years 96% 71% 45% 26% 238%
50 years 99% 87% 64% 39% 289%

As you can see, areas susceptible to a 1% chance of a 100-year flood are at significant risk of some type of water incident much sooner than 100 years; at the ten-year mark, the chances of some sort of water damage is 127%. If I had a 127% chance of winning the lottery I'd definitely buy a ticket. At 20 years the chance of some water damage is 195%. It may not be catastrophic, but water damage is expensive, especially in warmer climates where mold can grow quickly and cause even more damage.

This premium content is locked for FC&S Coverage Interpretation Subscribers

Enjoy unlimited access to the trusted solution for successful interpretation and analyses of complex insurance policies.

  • Quality content from industry experts with over 60 years insurance experience, combined
  • Customizable alerts of changes in relevant policies and trends
  • Search and navigate Q&As to find answers to your specific questions
  • Filter by article, discussion, analysis and more to find the exact information you’re looking for
  • Continually updated to bring you the latest reports, trending topics, and coverage analysis