Determination of recurrence intervals at the Cologne gauge (Rhine) by means of plotting positions, four different fit functions and yet another resulting fit function (Merz 2005 - modified).

The return period T is the return value of the exceedance probability PE: T = 1/PE. The return period is the average intervals between events of a certain strength. A ten-year flood event therefore occurs about ten times in one hundred years. The interval between two ten-year events may vary greatly, however. This purely statistical value therefore says nothing about the actual interval between two events. It also describes a stationary condition, i.e. the underlying conditions are assumed to be constant. This assumption does not hold over any length of time, however because the contributing circumstances (e.g. climate) change in the course of time.