If the probability of a 20-year flood is 1 in 20 per year, what does this imply about its frequency?

If the probability of a 20-year flood is 1 in 20 per year, what does this imply about its frequency?

• A. It is expected to occur once every 20 years on average
• B. It will occur every year
• C. It will only occur during wet years
• D. It will never occur

Answer: (A)

In flood-frequency terms, a 20-year flood has an annual exceedance probability of 1/20, or 5%. This means there’s a 5% chance it happens in any given year. The frequency is understood as the average time between such events, so with a 0.05 probability each year, the expected interval is 1 / 0.05 = 20 years. So, on average, these floods occur about once every two decades. It’s not a guaranteed schedule and isn’t tied to wet years only, nor does it imply it never occurs; it’s an average rate over many years.

In flood-frequency terms, a 20-year flood has an annual exceedance probability of 1/20, or 5%. This means there’s a 5% chance it happens in any given year. The frequency is understood as the average time between such events, so with a 0.05 probability each year, the expected interval is 1 / 0.05 = 20 years. So, on average, these floods occur about once every two decades. It’s not a guaranteed schedule and isn’t tied to wet years only, nor does it imply it never occurs; it’s an average rate over many years.