The myth of the 100-year flood

By Ariel Wittenberg

With southeast Texas facing years of recovery after Tropical Storm Harvey's catastrophic floods, many ask whether the devastation could have been prevented or mitigated. 

Experts say yes. They blame how the government assesses and communicates flood risk.

Federal policies are built around the so-called 100-year floodplain, which is commonly and incorrectly understood as an area that would flood once every century. But it really means there's a 1 percent chance of flooding in any given year or that there's a 26 percent chance of being flooded at least once during a 30-year mortgage period, according to the Federal Emergency Management Agency.

Moreover, flood maps don't take into account pavement and development that takes place in wetlands or floodplains. Putting impervious pavement on top of natural sponges will only make flooding worse.

President Trump flubbed the numbers with a tweet describing Harvey as a "once in 500 year flood." But homes in a 500-year floodplain really have a 6 percent chance of flooding over a 30-year mortgage.

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