The event has been called a “500-year flood”— a flood event that has a one-in-500 chance of occurring in any given year in that location. Residents in 500-year flood zones aren’t required to purchase flood insurance, even though there is more than a 5 percent chance, more than one in 20, of a 500-year flood over a 30-year mortgage.

The Federal Emergency Management Agency (FEMA) mandates that people living in the high-risk 100-year flood zone purchase flood insurance. While a 1 percent chance every year sounds unlikely, consider that it equates to roughly a 25 percent chance over a 30-year mortgage. A 500-year flood has roughly a 5 percent chance.

Hundreds of thousands of people live in flood-prone areas like Houston, Miami, New Orleans, Tampa Bay and New York.