The 100-year flood plain is defined as an area with what annual probability of flooding?

• A. 1 percent
• B. 5 percent
• C. 10 percent
• D. 50 percent

Answer: (A)

The idea being tested is how flood risk is expressed as a probability per year. The 100-year flood plain is defined by an annual exceedance probability of 1%—that is, there is a 1 in 100 chance of flooding in any given year. This doesn’t mean floods occur exactly once every hundred years; it means the flood level used to map the area has a 1% chance of being exceeded in any single year. Other probabilities correspond to more frequent events: a 5% annual probability (20-year flood), a 10% annual probability (10-year flood), or a 50% annual probability (two-year flood). So the correct interpretation is a 1% annual probability.