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.

Along the Dutch coastline, coastal dunes act as the primary sea defence to protect the low-lying areas behind it from flooding. To ensure that these dunes are strong enough to meet national safety standards, models are used to assess their strength. Even though our modelling capabilities have significantly improved over the years, model uncertainties remain because specific processes are not yet fully understood.

One of those uncertainties is the effect of sediment grain size on dune erosion. The relative importance and effect of this parameter differs significantly among dune erosion models. It is however expected that this parameter influences the morphological response of dunes during storm conditions.

Manipulative field experiments to study the effect of sediment grain size on dune erosion were conducted from September 24 until September 27 2022. In total, six experiments were conducted in which four different grain sizes were tested. Two open shipping containers were placed in the intertidal zone and acted as coastal wave flumes. Inside both containers, artificial dunes were built, with median grain sizes ranging from 0.21 to 0.31 mm, and were subjected to wave attack during high water. Cameras were used to calculate erosion volumes at a high sampling frequency. Environmental conditions were measured to capture water levels and wave conditions.

Dune erosion was measured with cameras, providing a low-cost and user-friendly method. This method can accurately and continuously capture dune profiles, particularly when there is a distinct color contrast between the dune profile, incoming swash, and backdrop. Analysis of the profile development reveals that the dune toe linearly translates over time in both the cross-shore and vertical directions. The translation in the vertical direction is also correlated with the rising mean water level.

The analytical model derived by Larson et al. (2004) based on wave impact theory was used to relate measured erosion volumes to incoming hydrodynamic conditions and to isolate the relative importance of sediment grain size on dune erosion. The considered time interval used in this study differs from the original approach, as this study focuses on short-term timescales of subsequent events during the experiment rather than the total erosion of an experiment. The application of this newly introduced timescale requires less data and gives more insight into the involved processes for subsequent erosion events. A linear trend is obtained between erosion volume and hydrodynamic forcing but the model cannot be used as an accurate prediction model, it tends to overestimate erosion volumes. The effect of grain size is incorporated in the transport coefficient. The transport gradient decreases when the sediment grain size increases. A negative correlation that converges as grain size increases was found between the two
parameters, demonstrating a non-linear, monotonic dependence.

This study reveals that erosion rates are smaller for dunes composed of coarser grains than finer grains. This is indicated by both the cumulative erosion volumes for identical hydrodynamic conditions and the values of the transport coefficient in the analytical model. Additionally, the formation of steeper
slopes is observed for dunes composed of coarser grains. These findings could be attributed to the fact that coarser grains have larger fall velocities and therefore are less easy transported by the waves and currents. This study enhances our comprehension of the effect of sediment grain size, ranging from 0.21 to 0.31 mm, on dune erosion and can contribute in resolving discrepancies between model predictions and field measurements to improve future assessments of dune erosion with models.

The coastal safety of the dunes are of high importance for low laying countries (Chen et al., 2017). This is especially true for the Netherlands as it is used officially as a safety measure against flooding from extreme weather since 1984 (Rijkswaterstaat, 1989). As of current 1D modelling of the waves is an acceptable method for the Dutch coast to assess the safety of the dunes (Den Heijer, 2013). For assessing the dune coastal safety the 98t h percentile of the runup R2% (Stockdon et al., 2006) is used for defining the wave response during a storm on the dunes. The current dune coastal safety studies consider bound waves (infragravity and sea- swell waves) propagating from offshore to nearshore. However, due to the Dutch coast being located at a very large semi-enclosed basin additional free infragravity waves can be present during a storm next to the bound waves that are reflected from other coasts (Daouk, 2020) as leaky waves (Herbers et al., 1995), and potentially increasing the R2% magnitude. Here we show that the R2% with the additional free infragravity waves in the wave domain can have a significant response on the dune coastal safety. The additional free infragravity waves can have tens of centimetres of R2% magnitude increase that is dependent on the slope of the domain. For the additional free infragravity waves a higher R2% is observed for steeper slopes, while a larger R2% growth is observed at the milder slopes when the free infragravity waves are initially introduced to the slope. The dune coastal safety assessments conducted thus far do not consider the effects from leaky waves from other coasts, therefore underestimating the safety provided from the dunes against flooding. With the research conducted by considering additional free infragravity waves with the traditionally bound waves originating in a storm, a better insight can be provided for dune coastal safety. As a result, companies and governmental bodies such as Rijkswaterstaat in the Netherlands that assess the coastal safety of the dunes can benefit by having a more complete representation of the coastal hydrodynamics that can as a result prevent flooding or provide dune nourishment after the episodic event (Sallenger, 2000).

The Netherlands has 254 km of coastline that is covered with sandy dunes. As these dunes function as primary flood defences, intensive research has been conducted into dune erosion in recent years. This research mainly took place in laboratories, because all boundary conditions could be controlled and measurements could take place very accurately. However, it remains difficult to simulate a natural wave climate in a flume. In addition, there is also the risk of forgetting certain (as yet unknown) natural processes. Partly because of this, the demand for accurate field data and field measurement techniques has increased over the years. This thesis proposes a new experimental method to investigate dune erosion in the field, in which a 40 feet open-top shipping container is used as an in-field wave flume. Two containers are placed in between the low- and high waterline of the Sand Engine, near Kijkduin. The experimental method is validated by looking at the hydrodynamics within the container and by comparing dune erosion within this container with dune erosion from existing field and laboratory studies. This validation consists of four experiments in which several research hypotheses are tested to see if the wave propagation and erosion processes inside the containers work as expected. In general, it can be concluded that the experiments carried out inside the containers were in good agreement with the tested theories. These theories included that: the avalanching process is present, higher dunes have a bigger eroded volume, the dune toe follows the rising water level and the slope of the foredune remains similar during the erosion process. As this was the first time this method has been used, some recommendations are made. One of those findings is that the use of the measurement systems should be the same in both containers. A good reason to use this contained erosion method again is for the investigation of the grain size influence in erosion processes.