It often seems that a month cannot go by without reading about catastrophic flooding somewhere. These floods can present high risk of property damage and even loss of life. When you are considering purchasing property that is in a flood plain or otherwise vulnerable to flooding, it is important to understand the level of risk you are taking on. The odds of floods are typically expressed in terms of years. You will often see terms such as “100-year flood” or “500-year flood.” Decoding these can help you better understand the odds and the risk to your property.

What do they mean by 100-year flood?

Hydrologists typically use the term “100-year recurrence interval” instead of “100-year flood” because the term is more descriptive. This is because a 100-year flood does not describe an event that can happen only every hundred years.

Instead, the term describes the odds of a specific degree of flooding happening in any given year. An area in a 100-year flood zone has a one percent chance of flooding in any given year. Because of this, you can see multiple “100-year floods” in a given area.

Flood frequency is determined based on historical data over the longest period of time for which we have records. In general, a minimum of 10 years of information is needed to perform a frequency analysis to tell us the odds of flooding in any given year. Of course, more data is better. A more accurate recurrence interval can be found when there is 30 or 40 years of flood data instead of only 10.

Does a 100-year storm mean a 100-year flood?

The intensity of a storm is not the only factor that contributes to flooding risk. The length of the storm plays a part. If soil is already saturated because of prior rainfall, there is also a higher chance of flooding.

By taking all of these factors into account and looking at what has happened in the past, researchers can provide better information about risks. Understanding what goes into calculations improves your understanding of your exposure at any given time and how likely it is that flooding can damage your property.