In the Netherlands dikes are assessed on piping using the model developed by Sellmeijer. This model is calibrated on experiments with permeable, homogeneous, fluvial sand with no fines and covered by an impermeable cohesive layer. In tidal areas, however, the sand deposits are strongly heterogenous, contain fines and are influenced by biochemical effects. It is therefore expected that the Sellmeijer model overpredicts the piping vulnerability of tidal sands. This thesis is aimed to improve the piping assessment of levees situated on tidal sediments. A large-scale experiment on piping in the Hedwigepolder on tidal sediments is planned by Deltares, Fugro and Waterschap Hollandse Delta for 2021. A design of this experiment is made, substantiated by findings from literature and the modelling and analysis of small-scale experiments on tidal sands.

In the Rhine-Meuse Delta over 100 scour holes are located. Many of these scour holes have dynamic behaviour and are growing in size and depth. A scour hole can lead to an increased probability of flooding. One of the potential hazards of the presence of scour holes is the occurrence of flow slides on the foreshore next to scour holes. Depending on the distance between the dike and the river, a flow slide can affect the water-retaining function of a dike. Flow slide is an indirect failure mechanism and can be used as a scenario in the safety assessment of direct failure mechanisms.

In the research, a data-driven method has been developed for the risk assessment of scour holes. The method is elaborated for flooding due to the failure mechanism overtopping. The development of a scour hole is coupled with the safety assessment regarding overtopping for the dike next to a scour hole. In the method, the development of a scour hole and the future dimensions are predicted in a probabilistic way by extrapolation of historically measured bed level data. With the predicted future scour hole dimensions, the probability of occurrence of a flow slide and the probability of several post flow slide profiles is calculated with the methods given in the Wettelijk Beoordelingsinstrumentarium (WBI). Based on the post flow slide profiles, the probability of exceeding the critical overtopping discharge is determined. Combining the probability of exceedance of the critical overtopping discharge with the consequences of a flood gives the flood risk due to the presence of a scour hole.

The developed data-driven method is applied to one case study, a scour hole in the river Spui. For this particular scour hole, the flood risk is assessed. A prediction is made for the future dimensions of the scour hole. The flood risk is determined with the probability of occurrence of flow slide, the probability of exceedance of the critical overtopping discharge and the expected consequences of a flood. The assessed flood risk is evaluated and risk mitigation measures are proposed. The economic feasibility of risk mitigation measures is analysed with a cost-benefit analysis.

With the shift of the safety standards for levees from exceedance probability (standard 1996) to flooding probability (new standard), the part of the levee failure process after the initial occurrence of a failure mechanism becomes more important for the calculation of the strength and thus, reliability of the levee cross-section. Interactions between failure mechanisms can take place in this part of the levee failure process. An interaction refers to one failure mechanism influencing the probability of another failure mechanism, where positive interaction is defined as a reduction and a negative interaction as an increase of the failure probability. The main problem is a lack of knowledge about the interactions of failure mechanisms and their influence on the failure probabilities of levees. Several studies (Calle, 2002; ’t Hart et al., 2016; Kok et al., 2017) suggest that there is an interaction between different failure mechanisms. However, only the effects of overtopping on slope instability (de Visser et al., 2018) and a reduction of the shear strength in slope instability due to uplift (Kanning and van der Krogt, 2016) is currently included in the statutory assessments of levees (Rijkswaterstaat WVL, 2017a,d). This research aims to quantify the influence of the interaction between the failure mechanisms backward erosion piping (BEP) and slope instability (SI) on the safety of a levee. The parametric study of this research assesses whether the occurrence of a failure mechanism (BEP or SI) affects the parameters' present states in the limit state functions of the other failure mechanism. A model was created to calculate this interaction between SI with BEP. After a slope instability, the remaining profile is assumed to contain a berm. This change in levee profile, does not result in the immediate breaching of the levee, it can however influence the probability of BEP. The model sequentially executes a stability analysis, a displacement analysis of the sliding plane and a BEP analysis. The degree of interaction is defined as the difference between the safety factor of BEP of the original profile (prior to sliding) and the safety factor of BEP of the remaining profile (after sliding). The modelling study shows that the degree of interaction from SI to BEP is significantly affected by the presence of cracks in the berm. A consequence of these cracks is that the exit point moves and is now located in the berm. The degree of interaction also depends on the shape and size of the sliding plane, because it determines the size of the berm of the remaining profile. A vast majority (50\% to over 75\%) of the considered scenarios of the seven considered levee profile cases result in a positive interaction. However, in all the considered cases, negative interactions were found that give a reduction of 50\% to 2\% on the original safety factor, which could lead to an overestimation of the safety in the assessment of the levee. All of these negative interactions result from the assumption that large cracks can form in the soil after sliding.