This paper deals with a probabilistic approach of the hydraulic loading conditions of the Oosterschelde storm surge barrier. The joint probability density function of the hydraulic boundary conditions (storm surge level , wave energy and water level at the Oosterschelde basin) is used as input. By introducing linear spectral transfer functions between the load and the hydraulic parameters , this density function can be transformed into the two-dimensional probability density function of wave- and static loads from which the probability distribution of the total hydraulic load can be derived by integration.

The needed transfer functions were determined with the aid of a mathematical model, which has been checkedb y a series of hydraulic model tests. By applying a probabilistic load determination as indicated above, the total horizontal load at the storm surge barrier was reduced by approximately 40%, as compared to the rather pessimistic outcome of a deterministic load determination, in which all unfavourable and unlikely events are assumed to coincide. The probabilistic load determination has also been used in a probabilistic approach of the behaviour of the storm surge barrier, in which the structural properties were treated as random variables, in addition to the loads. In this way, a risk analysis has been executed to find the failure probabilities of several parts of the barrier, which have to be in balance.

In the design of the Oosterschelde storm surge barrier semi-probabilistic methods have been used. the probabilistic load calculation requires knowledge of the three dimensional probability density function of storm surge level,basin leveland wave energy. However especially in the interesting regions of low probability of occurence the consequent,lack of measured data prevents a reliable estimate oft his function. In this paper a combination of purely statistical models and mathematical models, based on physical law sand checked with measured data,has been used. the probability density function of the storm surge level is based on a purely statistical model. A simplemathematical model,based on physical facts,is used to derive the conditional probability density function of the basin level on the storm surge level. the conditional probability density function of the wave energy on the storm surge level is found along the same lines. A mathematical model is developed based on the hypothesis, that the typical double peaked form of the wave spectrum is caused by the fact, that the wave energy originates from two sources. Waves,entering the estuary from deepwater via the shoals and waves,generated locally,from together the sea stateat the barriersite. the required three dimensional probability density function of storm surge level,basin level and wave energy is derived as the product of the probability density functions referred to above.