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.