Probability of Flooding Due to Instability of the Outer Slope of a Levee

The Netherlands is protected against major floods by a system of primary flood defenses (levees, dunes and hydraulic structures). These must comply with standards defined in terms of maximum allowable probabilities of flooding. Therefore, a new assessment framework for the main failure mechanisms is based on a probabilistic approach. One of the failure mechanisms which is not yet following such a probabilistic approach is instability of the outer slope of a levee. This failure mechanism is of importance after a rapid water level drop after a high-water event. In such an event, the pore water pressures in the levee are still high, and if an instability happens flooding can occur when (1) there is no time to take emergency measures before a second high-water event follows and (2) there is insufficient residual strength to prevent flooding during this consecutive high-water event. Levee reinforcement projects in the Netherlands allocate significant resources to resolve the presumed lack of safety of the levee due to the outer slope instability mechanism. Hence, this paper discusses how outer slope stability safety can be assessed probabilistically. A failure due to outer slope instability depends, besides the characteristics of the levee, on the peak water level, the water level drop velocity, the inter-arrival time between two consecutive high-water events and the time needed to take emergency measures. In this paper, a framework based on event trees is presented, using Intensity-Duration-Frequency-curves to include the time dependent statistics of the water level drop. This is a novel approach for outer slope instability in the Netherlands and results in less conservatism in assessments and designs, and therefore less required resources to mitigate the mechanism.