What does a '500-year flood' really mean?

HOUSTON – Living in southeast Texas means dealing with flooding. Most times the flooding is minor, but occasionally, the water can reach catastrophic levels.

Since 2015, the region has been through at least three significant floods – Memorial Day in 2015, Tax Day in 2016 and Hurricane Harvey in 2017.

Officials said the Memorial Day and Tax Day floods, when the region picked up more than 20 inches of rain, represented a 500-year flood event.

During Hurricane Harvey, an average of 35 to 45 inches of rain fell across Harris County during a four-day period. Nearly 50 inches of rain fell in some isolated locations. Officials said those rainfall amounts constitute a 10,000-year or 20,000-year flood event. In the extreme case, the amount marks a 40,000-year flood.

What is a recurrence interval?

According to the U.S. Geological Survey, the technical name for terms like “500-year flood” is “recurrence interval.” They are calculated through a statistical process called frequency analysis, and are used to estimate the probability of the occurrence of a given rainfall event.

The process takes into account not only the amount of precipitation a given area could receive in a given period of time, but also the amount of flow in nearby waterways that handle the runoff, according to the USGS.

What does it mean?

Without getting too technical, a 100-year flood means that type of flood has a one in 100 chance of occurring in any given year. In other words, there is a 1 percent chance that type of flood can happen in any year.

A 500-year flood means that type of flood as a one in 500 chance of occurring in any given year. In other words, there is a 0.2 percent chance of that type of flood occurring in any given year.

In Harvey’s case, that flood had a 1 in 10,000 chance of occurring in any given year, or a 0.01 percent chance.

What it doesn’t mean

Contrary to popular belief, the intervals do not mean that type of flood, for example, only happens once every 100 years. So, a region can experience, for example, multiple 100-year floods in any year.

If a homeowner settles in a 500-year flood plain, for example, that means the property has a one in 500 chance of flooding in any given year. However, it also means that 500-year flood, for example, can happen more than every 500 years.

Can intervals change?

According to the USGS, water levels that constitutes a flood interval can fluctuate based on changing climate.