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The term “500-year flood” makes the news as often as sandbag brigades work on levees. Offhand, the term seems to tell the last time and the next time for such a bad flood to hit.
In reality, the term is a beautiful tool for describing and managing risk.
The workaday term does its work mostly out of sight, busily dividing up flood risk among government, individuals, and the free market. The term measures and coordinates the portions of risk handled by each sector. Dividing up risk is a central reason societies organize.
With such a complex task, how the simple term functions is worth knowing. It works at the junction of science, engineering, economics, public policy and law. Let’s see.
Technically, a 500-year flood is the height of flood water expected to be equaled or exceeded every 500 years on average. The 500-year flood is more accurately referred to as the 0.2 percent flood, since it is a flood that has a 0.2 percent chance of occurring in any given year.
Bad floods also come in the smaller, “100-year flood” variety. They have a 1 percent chance of coming in any single year. That is a five-times higher chance.
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