The effect of sediment grain size on dune erosion

Abstract

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.