Do all dike instabilities cause flooding?

One of the failure mechanisms of dikes is slope instability at the landward side. Often, one instability does not lead to flooding, and several successive instabilities are needed before the dike overtops, and erosion and breaching can occur, especially at lower water levels. In this paper we propose a method to estimate the probability of flooding, taking into account the residual resistance against flooding after the first instability. We base ourselves on basic probabilistic techniques and common slope stability analyses and estimate the probability of flooding by calculating the probability of several successive (conditional) instabilities. Because the geotechnical failure and dike failure is not the same for each water level, we evaluate the probability for different water levels. The case example shows that there is a considerable margin between the probability of geotechnical failure and the probability of flooding, especially at relatively low water levels. It also shows that the current practice of assuming that the probability of flooding is equal to the probability of instability is very conservative.