Large parts of the Netherlands are vulnerable to flooding. Due to the great impact that a flood would have for the Dutch society, The flood safety standards are very strict, which means that the required failure probability of flood defences is very low. To keep the flood risk within acceptable limits of the hinterland, flood embankments are assessed on whether the probability of failure matches the stringent norms using the methods and rules which are stated in the legal assessment instrument (WBI). Within the WBI the probability of failure of the flood embankment equals the sum of the probability of occurrence of individual failure mechanism. This thesis specifically focuses on the failure mechanism: sliding of the landside slope cover due to wave overtopping of clay flood embankments. Due to wave overtopping the relative permeable top layer of the clay embankment becomes saturated. This leads to a shallow subsurface flow parallel to the landside slope cover which is unfavourable for the geotechnical stability of the landside slope cover. The stability assessment of this mechanism is currently assessed with the Edelman & Joustra formula. This formula method is not based on a probabilistic or semi-probabilistic analysis and therefore does not give an failure probability. Instead it uses safety factors. Levee managers therefore believe that the outcome is overly conservative (Waterschappen, et al., 2021). This thesis addresses the question: How can the stability assessment of the Edelman & Joustra formula can be optimized for clay flood embankments in the Netherlands within the legal assessment instrument? To answer this question, new semi-probabilistic safety factors were derived based on a full probabilistic assessment of the Edelman and Joustra formula. The outcomes show that the current approach is not always as conservative as is often assumed. Application of the newly derived safety factor allows levee managers to be more flexible in how to account for the contribution of this failure mechanism to the total levee failure probability. The newly developed method therefore prevents unnecessary rejection and over-dimensioning of flood embankments, so the available resources for flood embankment improvements can be used better.

A growing risk of flooding in coastal areas and the corresponding development of legislation drive continuous development in the field of hydraulic engineering. Failure due to
wave overtopping is not fully understood from a physical point of view. Existing knowledge gaps, processes that are not yet explained by physics or not even discovered, are
explained by empirical relations. The fact that knowledge on the relevant processes for overtopping load and soil resistance is fragmented and limited, causes limited application of it in practice.
To improve the understanding of failure caused by overtopping, a model that computes the soil stresses (soil stress is related to failure) in a dike cover, during overtopping load, is developed. It is possible that this model approach leads to additional knowledge on wave overtopping, e.g. new failure mechanisms or shift in normative mechanisms.

It is shown that overtopping has a large variety of appearances and so does failure due to overtopping. Literature study shows the large number of processes and characteristics, all with variable magnitude, that is relevant in the wave load and the soil strength during wave overtopping. Main wave load processes are shear stress (gradients), turbulence and impact. The considered soil strength process is the ratio between the stress (relative to the strength), for which the Young’s Modulus, Poisson’s ratio, soil weight, hydraulic conductivity and the subsoil stiffness are shown to be the main soil characteristics.

The developed model, a numerical 2D model with the model domain oriented parallel to the flow direction, computes the stress distribution and development in a loaded soil. For this case it is focused on dike land side slopes loaded by overtopping waves in particular, that is on dealing with clay, saturated conditions, fast varying load and a sloped surface. Stress equations in the model are derived, based on equilibrium of forces (horizontal and vertical) and motionless soil. The model is able to indicate initiation of failure for failure mechanisms that are associated with soil stresses, e.g. lifting of soil and head cut erosion. Soil in the model is loaded by normal and shear stress. The latter is, deviant from conventional methods, modelled as a shear stress gradient.

Test runs for verification and validation show that the model gives good results for cases with constant loads. Comparison of load cases with non-continuous overtopping wave loads shows both similarities and differences in the resulting soil stress. The most outstanding difference is the magnitude of the stress at depth. The differences indicate
starting points for further research.
Model employment demonstrates the future possibilities. A single wave load, computed by the model itself, is modelled. This run gives, at first view, plausible results.

The current model gives a reasonable representation of soil stress development. Further development is recommended to make the model useful for dike assessment and design. To do so, the model should be improved by probabilistic parameter definition, addition of soil stress damping and considering the impulsive nature and turbulent oscillations of wave load. Furthermore, it can be extended by the comparison between soil stress and strength, addition of spatial variability of the soil, enabling computations of non-planar parts of the slope and enabling the connection with other hydrodynamical models.

In the Netherlands flood protection is immensely important for the safety of the nation. The shocking outcome of the 1953 flooding proves this point. In modern days, the development of socioeconomic and climate change factors casts doubt on the effectiveness of conventional approaches to flood risk management. Consequently, this project explored new approaches to flood risk management.
An analysis of climate change effects led to estimation of future loading conditions. Subsequently, a detailed hydrodynamic analysis was conducted. It highlighted the significant levels of uncertainty that climate change introduces into loading conditions. Also, it confirmed the team’s perception, that the Westkapelle case region requires additional safety measures to guarantee an acceptable level of safety in the future. But how to guarantee the acceptable level of safety in the most efficient way? The team adopted the concept of robustness to find an answer. In a keynote publication Mens (2015) describes robustness in the following way: "Robust flood risk systems have some degree of resistance and some degree of resilience: the system can withstand some floods (no response), and for other (larger) floods impacts are limited and the system can recover quickly from the flood impact (response and recovery)." The team set out to include robustness as an integral part of the design process to handle uncertainties. The project shall be seen as an explorative study how this can be done, revolving around Westkapelle as a case study that proves the methodology’s feasibility. Robustness and uncertainty were included on multiple levels throughout the design process. Firstly, the range of uncertainties was quantified. Secondly, the effect, that single parameters have on the magnitude of uncertainties, was assessed. Thirdly, the system’s capacity was analysed to find the required overtopping reduction for guaranteeing sufficient safety. Fourthly, constructive measures were assessed on their robustness potential and satisfaction of stakeholder needs via a Multi Criteria Analysis (MCA). The MCA was then employed to select the type of constructive and non constructive measures to achieve the required levels of overtopping and safety. With the information on uncertainties, the measures were combined to form a robust design, consisting of living breakwater, dike heightening, surface protection and two policy measures. Probabilistic analysis was also done to see the sensitivity of the failure probability to sea level rise in different loading and design scenarios. A thorough comparison between the conventional design, that has been applied to the project location, and the robust design followed. The robust design came out on top. Robustness was found to be an effective tool in countering uncertainties. Where conventional design methodologies are lacking flexibility and precision, the robust design methodology makes use of the system and its resilience to find an optimal solution. Its applicability may not be limited to flood risk management only but stretch out to other civil engineering disciplines.