Extreme value processes and the evaluation of risk in flood analysis

A analysis of hydrological risk is presented associated with decisions based on stochastic flood models. The maxima of a stream-flow are described by a marked Poisson process with a cyclic trend and exponentially distributed marks. Typical design criteria like the expected largest exceedance of a fixed level in a given period are derived from the extreme value process. The approach adopted is based on the whole record of flood data, which consists of the number, the occurrence times and the exceedances of the maxima in the observation period. Thus, compared to the series of largest annual exceedances more information is extracted. This yields an improvement in the evaluation of risk.