First, what is a 100 year flood?
In the 1960’s, the United States government decided to use the 1-percent annual exceedance probability (AEP) flood as the basis for the National Flood Insurance Program. Because the 1-percent AEP flood has a 1 in 100 chance of being equaled or exceeded in any 1 year, it often is referred to as the “100-year flood”.
What are the chances of my home being flooded by a 100 year flood?
The 1-percent AEP flood has a 1-percent chance of occurring in any given year; however, during the span of a 30-year mortgage, a home in the 1-percent AEP (100-year) floodplain has a 26-percent chance of being flooded at least once during those 30 years! As many have learned after Hurricane Harvey, flood predictions and floodplain maps are based on the available data, which are sometimes inadequate.
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Was the flooding from Hurricanes Katrina and Harvey 500 Year Floods?
Hurricane Harvey has brought “500-year” rainfall and flood conditions to the Houston area according to officials at the Harris County Flood Control District. As of August 31 2017, widespread areas around Houston have experienced flooding reaching 1,000-year thresholds or more.
But 500-year floods, as it turns out, happen more frequently than you might expect. The Houston area alone has seen no fewer than three such events in the past three years, according to local officials: Memorial Day floods in 2015 and 2016, followed by Hurricane Harvey’s torrential rains this year.
So how often do “500-year” floods actually happen?
A 500-year flood isn’t necessarily something that happens once every five hundred years. Rather, a 500-year flood is an event that has a 1 in 500 chance of occurring in any given year. “For a 500-year flood, there is a 0.2 percent chance of having a flood of that magnitude occurring” in any given year, according to the National Weather Service. The risk of having the flood in any given year is the same,” regardless of whether it occurred recently. Ditto for 500-year floods.
Is my property in a flood zone?
The National Weather Service web site provides flood zone maps. We recommend you check the map for your area, especially if you are near a body of water, river or stream.
Prepare yourself for a weather emergency by using the information on the Weather.gov web site. https://www.weather.gov/safety/