Dune foot behaviour and erosion of sandy coastal defences in annual storm conditions

Abstract

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.