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.