Storm surges cause elevated water levels, potentially leading to dune erosion, which is crucial to understand due to the protective function of dunes and sandy coastal defences against flooding. Dunes are dynamic systems, extensively studied through flumes and field observations. Collecting hydrodynamic and morphological data from erosion-prone land reclamations can inform dune research and design optimisations. Loss of volume in land reclamation projects can result in setbacks, increased costs, and emissions. While dune erosion models estimate volume changes, they may deviate significantly under annual storm conditions due to calibration with normative storm conditions. Dune erosion typically occurs when the initial dune foot elevation is exceeded, resulting in a post-storm dune foot elevation at the maximum water level within the storm. Therefore, the (initial) dune foot might play an important role in the resulting erosion volume. The current knowledge on the dune foot behaviour of sandy coastal defences due to annual storm conditions is too limited and therefore the main research question of this thesis is: ‘How does the dune foot of sandy coastal defences behave due to annual storm conditions and what is the influence on dune erosion?’

This thesis performs an analysis of two field sites with hydrodynamic and morphological measurements, RealDune-REFLEX on the Holland coast and Land Reclamation Philippines in Manila Bay in the Philippines. For Land Reclamation Philippines, the hydrodynamics are transformed using two hydrodynamic instruments and validated using an ERA5 wind-driven SWAN model. The definition of the dune foot position is based on the maxima of curvature of the dune profile above a certain threshold following the second derivative method. The maximum total water level elevation is approximated with the measured water level elevation and an empirical parametrisation of the wave runup based on offshore wave conditions and the foreshore slope.

For RealDune-REFLEX, negligible erosion occurred for maximum total water level elevations far below the dune foot. For elevations just below and exceeding the dune foot, it shows that the post-storm dune foot correlates with the maximum total water level elevations. The dune foot could translate upwards by dune erosion and downward by bed level lowering and occasional avalanching. At Land Reclamation Philippines, it was found that pre-storm, more alongshore variability was observed in the vertical and horizontal position of the dune foot compared to post-storm. Minor coastal features eroded and resulted in larger dune foot retreats on those transects. Major coastal features remained present and influenced the post-storm dune foot position.

Concluding, an increase in the maximum total water level elevations above the initial dune foot height led to the highest erosion quantities of the dune front. However, no significant linear relation was found between the initial dune foot position relative to the maximum water level elevation and the resulting dune erosion. The post-storm dune foot elevation was found to correlate significantly to the maximum total water level elevation, approximated with an empirical runup formula, for upward and downward dune foot translation due to an out-of-equilibrium upper foreshore slope. Change of the horizontal dune foot position was found to relate to some extent to the storm duration and intensity of the total water level elevation exceeding the dune foot. Dune foot retreat magnitudes can be of the same order for small and large values of this quantification method and therefore exceedance of the dune foot does not provide a proper estimation of the dune erosion volume. It is found that the mismatched volume of the initial profile with the equilibrium condition, defined as an equilibrium slope reaching from the intersection of the initial profile with the maximum total water level elevation, is a more accurate estimation of the dune erosion volume. This methodology provides a practical way of predicting erosion volumes in sandy coastal defences using simple initial conditions, making it applicable to engineering practice.

For further research, it is recommended to improve the mismatch method based on the shape and size of the equilibrium profile. Second, study the dune foot dynamics within a storm using continuous LiDAR laser measurement and therewith validate dune erosion models. Third, civil contractors should enhance the quality of surveys in periods where dune erosion is expected to establish valuable datasets. Lastly, numerical modelling of complex typhoon-induced nearshore hydrodynamics could lead to more accurate insights into the alongshore behaviour of typhoon-induced dune erosion and the validity of the methods used.

Fieldwork Hydraulic Engineering’ is a course given at Delft University of Technology for the MSc Hydraulic Engineering. In collaboration with local experts Boyan Savov and Traian Marin, a team of 8 students guided by Mark Voorendt was sent to investigate the local conditions at Asparuhovo beach in Varna, Bulgaria, for purely educational purposes. Before 2019, Asparuhovo beach used to have a stable coastline with some seasonal variations. However in 2019, the Karantinata port was constructed and disturbed the equilibrium state of the beach. Rapid sedimentation occurred near the port and the port entrance. Due to this excessive sedimentation near the port entrance, the fishing port has lost almost all of its intended functionality, as minimum water depths in the port entrance approach 0.3m. The port was originally designed for larger fishing boats, which are currently not able to enter and making the port lose functionality. It is yet unknown how this sedimentation trend is formed with the construction of the fishing port. The main objective of the research was to examine the current sedimentation near Karantinata port by executing a measuring campaign during the Hydraulic Fieldwork and by setting up a 5 year monitoring program for the marine environment. By doing so, the processes which lead to sedimentation can be understood and a model can be made. With this model, adjustments to the port layout can be examined which are potentially needed for the port to operate at full functionality. To tackle these problems, the students performed the fieldwork. With 2 days of beach and foreshore measurements the research question was assessed. With the acquired data of the system, supported by additional lab sieving analysis, data processing and modelling in Delft, the students formed theories on the origin of the sedimentation problem. With these insights, recommendations for the area can be suggested. The measurements are performed on multiple locations and at each location multiple variables were researched. For Asparuhovo beach and foreshore these are the bathymetry, waterline position, wave climate, beach profile, sediment characteristics and ecology. At the fishing port Karantinata these are the port characteristics such as functions, planning and infrastructure, port entrance, bathymetry and breakwater design. At the Asparuhovo breakwater it is the top protection layer, damage assessment and measures of improvement. At Veteran beach this is the soil samples for grain size distribution. At Martsiana quarry the length to thickness ratio, blockiness were researched, as well as the diameter to check if potentially suitable for breakwater material. With this information, a preliminary model in Delft3D has been set up with the land boundaries, grid and bathymetry file. With knowledge of the coastal processes and the processed data, potential causes of sedimentation are speculated on. It is unlikely that such large amounts of sediment are coming from outside of Asparuhovo beach system, as there are no sediment rich rivers nearby, the sedimentation occurred in a very short period of two years and the sediment would mostly not be able to cross the deep navigation channel as it would settle due to lower flow velocities. It is expected that large parts of the settled sediment near the port entrance is from the beach itself. This is also more likely due to the two closed boundaries of the beach, the Asparuhovo breakwater and the Karantinata port. This was checked by analyzing the grain size diameter compared to other locations at the beach and looking at the waterline developments. It was found that the grain size at the middle of the beach was 1.8 mm and at the port entrance between 0.2 and 0.3 mm. It was suspected that the fine sediments of the middle of the beach are eroded and deposited at the port entrance as the sediment can settle at the Southeastern part behind the port breakwater due to sheltered conditions. The mechanisms that could have induced this are: rip currents, longshore currents and the different wave patterns. To examine and validate these findings, a monitoring plan for the coming 5 years is proposed. This is very important to create an understanding of the systems parameters and behaviour. Without monitoring, adjustments to the port cannot be tested in a correctly calibrated model. The parameters which need continuous measurement are the wave parameters, sea level measurements and visual beach observations. Biannual measurements are needed for currents, hydrographic works, visual observations with a drone, bathymetry and sediment parameters. Before the port is fully operational again, the port entrance needs dredging. This can be done in this time span of 5 years to ensure the passage of fishing boats. After dredging a short survey of the area needs to be performed to incorporate the changes into the model. These are the bathymetric survey, visual observations and sediment samples all around the dredging area. There are three potential solutions incorporated in the report, which can be modelled with the findings of the monitoring plan. The first potential solution to make the port fully operational again is relocating the port entrance with a curved breakwater stretching into the sea. The second solution is a combination of the entrance relocation and water flow through the port. The third is a blocking groyne stretching from Asparuhovo beach into the sea, blocking the sedimentation going into the port entrance. For all these potential solutions, dredging works are needed to reensure the required water depth for the vessel draught. The least costly and most promising is the relocation of the port entrance design. It can be noted that continuous dredging is not a sustainable solution as the sedimentation keeps occurring near the port entrance as the hydrodynamic conditions will not change. It is recommended to first find the source of the settled sediment by comparing control volumes of sediment on the beach over the years. Then a model should be set up of the Asparuhovo beach and foreshore and Karantinata port to give insights in the processes. This model needs to be validated and calibrated with input from a monitoring campaign. With a working model, the causes of sedimentation can be found and further research can be done whether the potential solutions are appropriate.