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. ... Read more

In the coastal defence of the Netherlands, sand nourishment is a common practice. Part of the Dutch coastal system, the Wadden Sea coast, is a very complicated coastal system in terms of hydrodynamics and morphodynamics. Many different processes are playing a role in the coastal dynamics in the Wadden Sea area. The execution of a mega nourishment in this area is intended. To make this possible, the dynamics of the system must be understood better than they are today.

Part of the coastal dynamic system is the intrawave sediment transport. To improve understanding of this phenomenon, research is done on the wave conditions in a tidal inlet of the Wadden Sea. In the accompanying field campaign use is made of unanchored WaveDroids, used as wave resolving drifter. This is the first time unanchored use of WaveDroids is carried out. The use of a moving measurement device gives rise to differences in measurement results compared to the known approach with fixed measurement devices.

This research shows the difficulties that must be overcome to process raw measurement data in such a way that the data becomes suitable for analysis and interpretation. The data is filtered on frequency and on wave height to give a useful representation of the wave field. The processed data is assessed on the energy density spectrum, the wave height distributions and the time series of wave heights and surface elevation. Subsequently, in the interpretation of the selected data, insight is given in the measured wave field.

From this research is concluded that the use of WRD’s is suitable to measure the vertical component of wave displacement. The effect of a Doppler shift, originating from the use of different frames of reference, is not significant is common sea states. The measurement of the horizontal component of wave displacement requires more research on correcting processing steps. ... Read more