The recurrence interval (or return period) T of a flood of a given magnitude is the reciprocal of its annual exceedance probability P, i.e. T = 1/P. This gives the average number of years between floods that equal or exceed that level, though it does not mean such floods occur at exactly regular intervals.
Given that the annual probability of the water level equaling or exceeding the marked high-flood level is P = 0.01: Recurrence interval T = 1...
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