In dike safety assessment, failure probabilities are scaled up from the cross-sectional level to the trajectory level. The current Dutch guidelines simplify the upscaling process by relying on rules of thumb. This research introduces a new probabilistic assessment method, named the fragility curve method, which more accurately accounts for the correlation between dike sections and failure mechanisms. The fragility curve method combines fragility curves to transition from the cross-sectional level to the trajectory level. The application of fragility curves allows the water level to be separated from the other variables. Consequently, the correlation of the water level can be regarded independently of the correlation of the remaining variables. In the fragility curve method, the water level is fully positively correlated in space and between failure mechanisms, while the remaining variables are assumed to be independent.
The main objective of this research is to compare the fragility curve method with the current Dutch guidelines for dike safety assessment. To achieve this, both methods are applied to two hypothetical sea dike trajectories. The first trajectory is relatively uniform, comprising dike sections that differ only slightly from one another. The second trajectory contains one dominant dike section. Three failure mechanisms are considered in the assessment, namely: inner slope stability, piping and overtopping. The conclusion of this research is that the fragility curve method is recommended over the current Dutch guidelines to compute the trajectory failure probability of the geotechnical failure mechanisms inner slope stability and piping. The current Dutch guidelines are recommended for computing the trajectory failure probability of the overtopping failure mechanism. Finally, when the failure mechanisms have comparable trajectory failure probabilities, the fragility curve method is recommended for calculating the total trajectory failure probability.

The sea level is rising, possibly by one meter, by the end of this century. The Netherlands is preparing for this scenario through Kennisprogramma Zeespiegelstijging (KP-ZSS). One of the goals is to determine the hydraulic effects of sea level rise (SLR) on the current system. For the flood risk analysis within
KP-ZSS, the current strength of the flood defences needs to be described. A set of typology-defined fragility curves based on river dikes in river areas describes the current strength of all the primary flood defences in the Netherlands. However, it is unknown if these fragility curves are applicable to sea dikes
due to shorter high water and differences in soil parameters in sea/tidal areas. The research described in this thesis aims to evaluate if site-specific fragility curves for sea dikes result in significantly different predictions of future reinforcement cost for sea dikes compared to the typology-defined fragility curves. The analysis is centred around three dike sections in trajectories 29-3, 30-3 and 32-4 in the Western Scheldt. Initially, an evaluation is conducted on the fragility curves related to the geotechnical failure mechanisms of macro-instability and backward erosion piping. Subsequently, the height requirement is taken into account during the cost calculation. The typology-defined fragility curves underestimate the strength of macro-stability in two out of three
cases with a factor 104 and factor 102, while underestimating it for one section with a factor 105. For the piping failure mechanism the typology-definde fragility curves overestimate the strength in two out of the three cases with a factor 103 and factor 102, but underestimates it for one section with a factor 103.
The pre-overburden pressure appeared to be the most important factor influencing the failure probability of the macro-stability failure mechanism, with higher occurring values for the sections in the Western
Scheldt. The failure mechanism of piping was influenced most by the hydraulic conductivity, where the
encountered soil in the Western Scheldt consisted of finer soils with lower hydraulic conductivity.
From the results, it can be concluded that the use of site-specific fragility curves resulted in a decrease of 13% in net present value, averaging over all SLR scenarios for an analysis until 2200. Considering the most probable SLR scenario resulted in a 12% decrease. If the height requirement is included in the cost calculation, the use of site-specific fragility curves results in a 7% decrease in net present value on average and a 12% decrease for SLR scenario low. For the low SLR scenario, the cost of relocating the road infrastructure in and around the expansion zone of the dike is dominant. With increasing SLR scenarios, the increase in crest height becomes the most important factor, with the revetment the dominating factor cost-wise, leading to minimal differences in reinforcement cost between the fragility
curve approaches.