How can we estimate the risk of flooding over a period of years if we know the flood return period?

If the return period is denoted by T, then the probability of a flood in any one year is 1/T. The cumulative probability P of at least one flood over a period of N years is given by the formula

P = 1 - exp(-N/T)

assuming that the risk of flooding is reasonably small. The relationship is shown in this graph:

Example

Probability of flooding in one year: 0.005
Return period: T = 200 years
Period of interest: N = 50 years
Calculate: N/T = 50/200 = 0.25
Probability of flooding in 50 years (from the graph): 22%