Dune erosion during storm surges

Dunes often act as the primary line of defence for low-lying hinterland against storm surges with dune erosion in the swash-dune collision regime. In the swash-dune collision regime, an elevated total water level, consisting of tide, surge, and wave setup, temporarily submerges the beach. As a consequence, the incident swash may run up to the dunes and collide with the dune face. The damage to dunes following from these collisions can be severe. In the most extreme of cases, storm surges with dune erosion in the swash-dune collision regime can lead to dune breaching or failure with flooding of the hinterland as a consequence. These floods can have devastating economic effects and potentially lead to loss of life.

The risk assessment of areas protected by dunes is often performed using predictive dune erosion models. Such models first use possible storm conditions as input, then use a set of physics based and empirical equations to model the physical processes based on that input, and finally use a measure of impact to the dunes from the model results to estimate the amount of damage to the dunes. However, not all physical processes are currently fully understood, complicating accurate reproductions in models and the risk assessment of areas protected by dunes.

This dissertation aims to study two such physical processes that are relevant to dune erosion during storm surges in the swash-dune collision regime. The first process is the suspension of sediments in the inner surf zone. The total amount of sediment in the water column affects the sediment transport rates. As a consequence, sediment concentrations can have a substantial impact on the magnitude and speed with which sediments eroded from the dune face are transported offshore, and thus on the total amount of dune erosion.

The second process studied is sediment transport due to soil instabilities, i.e. the slumping (or avalanching) of sediments from the dune face. Sediment transport due to soil instabilities leads to dune scarping and a gradual retreat of the dune face. If this type of transport persists for a prolonged period of time, the retreat of the dune face can continue until there is no more dune to erode. As a consequence, the dune breaches and enters the overwash regime, and complete failure of the dune may follow.

To study both processes, a prototype scale field experiment was conducted in the winter of 2021-2022. Two artificial dunes were constructed in close proximity to the high water line on a sandy beach. This increased the probability that the total water level driven by a storm event would result in dune erosion within the swash-dune collision regime. The dunes were monitored for a period of three months and within this period three such events occurred.

The suspension of sediments in the inner surf zone was studied by comparing variability in measured, wave-averaged (i.e. 20 min mean) suspended sediment concentrations. These variations were compared to variability of hydrodynamic drivers that are known from literature to govern sediment suspension during storm conditions. Overall, sediment suspension due to bore turbulence appeared the dominant suspension driver during energetic events representative of storm conditions. During such events, wave energy was saturated in the inner surf zone, and almost all waves were breaking and contributed to the generation of bore turbulence at the free surface. The outcome of the first study suggests that, based on the events analysed, dune erosion models may achieve more accurate results if computations of the magnitude of suspended sediment concentrations were to include a bore-induced turbulence term. If such a term is already included, models should properly address the relative importance of bore-induced turbulence when compared to other drivers.

Sediment transport due to soil instabilities was studied by analysing profile crosssections of the dune face during two storm events. Overall, the morphodynamic behaviour of the upper dune face and dune crest was primarily steered by the morphodynamic behaviour at the dune base. The morphodynamic behaviour (i.e. erosion rate) of the dune base correlated well with the elevation difference between the dune base and the incident total water levels, specifically the square of the total water level that was exceeded for 2% of the time. The slumping events that occurred during both storms likely occurred when sediments from previous slumps at the dune base were nearly depleted by the persisting erosion rate. As a consequence, under similar erosion rates, a new slumping event occurred sooner when the volume of the preceding slump was smaller. A clear relationship could not be established between hydrodynamics seaward of the dune and the volume of individual slumps.

Different model approaches are currently being used to implement sediment transport due to soil instabilities. These different approaches all use the persisting erosion rate of the submerged part of the dune base to steer the erosion of the upper dune face. Therefore, according to the field experiment results, these different approaches may all be able to achieve accurate dune erosion volume magnitudes. Still, depending on the application (e.g. one-dimensional versus two-dimensional modelling), one approach might be more suitable than others.