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100-Year Floods

What are 100-year and 500-year floods?


Floodwaters of river Leine, near Hanover
Karl Johaentges/LOOK/Getty Images
The use of the term 100-year flood is really a statistical way of saying a flood is rare. Many people will talk of 50-year floods, 500-year floods, and 10-year floods. Each flood type is actually a way to discuss the chances of a flood happening in the same region. In other words, a 100 year flood does not happen once in 100 years. Instead, a 100 year flood has a 1 in 100 or 1% chance of occurring in any particular year. A 500-year flood only has a .2% chance of occurring in any given year.

Flooding determinations and probabilities are based on recurrence intervals. The United States Geological Survey (USGS) defines a recurrence interval as the average number of years between floods of a certain size. In other words, how long is the average return period of a flood to the same region?

The National Flood Insurance Program (NFIP) also uses recurrence levels to determine whether a home owner should have flood insurance. The 100-year flood (1% chance of returning) is the standard used for floodplain management and to determine the need for flood insurance. For example, a structure located within a special flood hazard area shown on an NFIP map has a 26 percent chance of suffering flood damage during the term of a 30 year mortgage.

Weather Statistics

In meteorology, a chance is basically a common way to explain a statistic to people in a forecast. For example, a chance of rain has a statistical analysis associated with it. Because meteorology data is constantly collected and recorded, the chances of a flood happening in a region can also change over time.

The actual number of years between floods of any given size varies a lot because of naturally changing climate conditions. Increased urbanization in a region will also factor into the chances of a major flood striking an area. In the case of flooding, the probability of a flood happening again are listed in the table below.

Factor Affecting Flood Recurrence Intervals

Other factors can increase or decrease the probability of a flood occurring in a given region twice in a row. For instance, if the ground is saturated, the chances of a river cresting and causing extensive flooding are greater.

Prolonged storms and torrential rains can also cause extensive flooding. If the meteorological conditions are right, a storm can hang over an area for an extended period. Statistically speaking this can happen any time. Meaning the chances of having two 100-year or 500-year floods in a row are highly unpredictable.

So, can a 500-year flood happen two times in a row? The answer is yes. The recurrence of a major flood in a region simply means that although the odds were against it, the floods can happen. Think of it this way, while your chances of winning the lottery may be 1 in several hundred million, there have been people who have won the lottery 2 or more times!

Flood Probabilities

Chances of a Major Flood in a Region
Return Period Chances
500-year 1 in 500 (0.2%)
100-year 1 in 100 (1%)
50-year 1 in 50 (2%)
25-year 1 in 25 (4%)
10-year 1 in 10 (10%)
5-year 1 in 5 (20%)
2-year 1 in 2 (50%)