Probabilistic Design of Berm Breakwaters

In mechanical engineering the approach of optimal maintenance is mostly based on the empirical failure rate. Due to the unique character of hydraulic engineering structures, empirical failure data are lacking and a fundamental approach is needed. For the modelling of the aging process the serviceability limit state models are applied to the initial resistance of the structure taking account of the uncertainties involved. The probability of failure as a function of time is calculated by confronting the diminishing resistance with the loading. Within this model the usefulness of optimal repair intervals will be shown for a berm breakwater. In the last decade a renewed interest is shown for the berm breakwater concept. In the design of a berm breakwater, many limit states have to be checked by the designer: the stability of the armour cross and longshore, the compliance of the subsequent layers with filter rules, the stability of the toe under wave action, the geotechnical stability, overtopping, etc. In this paper attention will be focussed on the stability of the outer armour. Special attention is given to the longshore transport case. It will be shown that only a probabilistic approach gives insight into the sensitivity of this failure mechanism.