Optimal maintenance decisions for hydraulic structures under isotropic deterioration

The Dutch lowlands are protected against flooding by dyke rings consisting of dunes, dykes, and special water-retaining structures (e.g. the Eastern-Scheldt stormsurge barrier). As soon as a structure deteriorates to such a degree that it fails to meet its main requirements, it should be maintained preferably against minimal costs. The Ph.D. thesis is devoted to determining cost-optimal maintenance decisions for hydraulic structures subject to deterioration. In hydraulic engineering, a distinction can often be made between a structure's resistance (e.g. the crest-level of a dyke) and its design stress (e.g. the maximal water level to be withstood). A failure may then be defined as the event in which--due to stochastic deterioration--the resistance drops below the stress. Even though it is common to model a deterioration process mathematically as a so-called 'Brownian motion with drift', this model is inadequate in describing the deterioration of hydraulic structures. To illustrate, a dyke whose height is subject to a Brownian deterioration can, according to the model, spontaneously rise up, which cannot occur in practice. Furthermore, in most appreciations there is only information available in terms of the uncertain average rate of deterioration. In order that stochastic deterioration processes have the desired properties, they have been considered as so-called 'generalised gamma processes'. Examples are the deterioration processes of ongoing coastal erosion (dunes), crest-level decline (dykes), longshore rock transport; (berm breakwaters), scour hole development (the block mats of the Eastern-Scheldt barrier), and current-induced rock displacement (the rock dumping of the Eastern-Scheldt barrier). On the basis of generalised gamma processes, tailor-made models have been built and implemented to determine the following cost-optimal maintenance decisions: optimal sand nourishment sizes and optimal dyke heightenings which balance the initial cost of investment against the future cost of maintenance, optimal inspection intervals for berm breakwaters and for the block mats and the rock dumping of the Eastern-Scheldt barrier whose expected costs of maintenance and failure are minimal; and optimal decisions that reduce flood damage along the Meuse river. These decision models can be applied to many engineering systems for the purpose of maintenance optimisation and life cycle costing.