Probabilistic design of breakwaters in shallow, hurricane-prone areas

Abstract

One of the failure mechanisms of a rubble mound breakwater is the failure of its armour layer. In order to determine the stability of an armour layer, the design load has to be defined, which is in fact the wave that attacks the structure. Being a highly stochastic phenomenon, the wave action is not easily defined, while there is always some uncertainty inherent to its definition. In a deterministic calculation this uncertainty is totally overlooked, as the possible variations of the design wave height are not taken into account. In order to incorporate uncertainties into the design process, and therefore increase its reliability, probabilistic design methods should be applied. A commonly used approach is a semi-probabilistic computation on level 1, which introduces the application of partial safety coefficients, yet the indicated methods to derive and apply them do not clarify the uncertainties incorporated, adding an undefined degree of safety in the process, or end up with incorrect results under certain conditions. Another approach is a fully probabilistic computation on level 2 or 3. This type of design tackles explicitly a great deal of uncertainties, hence its results can be considered much more accurate. However it is not commonly used, due to the fact that there are not straight forward guidelines to support it, and therefore a number of critical decisions by the designers are required. The main objective of this study is to indicate the weaknesses of the existing design methods, and to suggest a design approach that is both attractive to designers and sufficiently reliable. This is achieved through application of the existing methods in an example case, whose features facilitates a critical assessment, and enables formulation of an improved approach. The chosen case is the jetties at the entrance of Galveston port, in the Gulf of Mexico, and the features of interest are the hurricane-dominated hydraulic climate and the fact that the structure is located in shallow water, meaning that the design load is determined by depth-limited waves. The design methods that are demonstrated are a classical deterministic design, a semi-probabilistic calculation on level 1 as proposed by PIANC in 1992, and a fully probabilistic calculation on level 3 with a Monte Carlo simulation. Based on the evaluation of the three design processes and the results, the new approach can be developed, which suggests a rational framework for deriving safety factors. According to it, a set of safety factors is generated which incorporate the same uncertainties as a fully probabilistic design; hence an equally reliable result is extracted. The final product is a guideline for code makers indicating the procedure to derive the safety factors and a guideline for future designers indicating the analytic steps for a proper use of the safety factors. In addition a large number of concluding remarks are summarized, which can contribute in optimizing the performed analysis. The concluding remarks refer in particular to the determination of hydraulic boundary conditions, the application of the design methods, the probabilistic model used for Monte Carlo simulation, the proposed design approach, and the safety factors derived with this approach.