A 100-year flood designation in drainage design indicates a flood event with what annual probability?

A 100-year flood designation in drainage design indicates a flood event with what annual probability?

• A. 10% annual probability
• B. 5% annual probability
• C. 0.5% annual probability
• D. 1% annual probability

Answer: (D)

In drainage design, a 100-year flood designation means the flood event has a 1 percent chance of being exceeded in any given year. This comes from the idea of a 100-year return period, which corresponds to an annual exceedance probability of 1/100 = 0.01, or 1%. It doesn’t guarantee one such flood every 100 years; it’s a probabilistic measure for any single year. The other options map to different return periods: 10% per year is a 10-year event, 5% per year is a 20-year event, and 0.5% per year is a 200-year event.

In drainage design, a 100-year flood designation means the flood event has a 1 percent chance of being exceeded in any given year. This comes from the idea of a 100-year return period, which corresponds to an annual exceedance probability of 1/100 = 0.01, or 1%. It doesn’t guarantee one such flood every 100 years; it’s a probabilistic measure for any single year. The other options map to different return periods: 10% per year is a 10-year event, 5% per year is a 20-year event, and 0.5% per year is a 200-year event.