10% flood frequency means:

Concept:

The average number of years between floods of a certain size is the recurrence interval or return period. The actual number of years between floods of any given size varies a lot because of the naturally changing climate. In other words, the flood recurrence interval, also known as the return period, is a crucial concept used to assess the probability and magnitude of flood events for a specific location. It quantifies the average time period within which a flood of a particular magnitude or greater is expected to occur.

If flood frequency or flood probability is F, then the flood interval is \(1\over F\)

TF = 1, or \(T={1\over F}\)

Where T = recurrence interval or return period.

For example, a 100-year flood has a 1% chance of occurring in any given year, while a 10-year flood has a 10% chance of occurring annually, In simple words 10% flood frequency means flood occurs 10 times in 100 years.

Hence option (3) is the correct answer.