Beach and Dune Subsurface Hydrodynamics and Their Influence on the Formation of Dune Scarps

Erosive beach scarps influence beach vulnerability, yet their formation remains challenging to predict. In this study, a 1:2.5 scale laboratory experiment was used to study the subsurface hydrodynamics of a beach dune during an erosive event. Pressure and moisture sensors buried within the dune were used both to monitor the water table and to examine vertical pressure gradients in the upper 0.3 m of sand as the slope of the upper beach developed into a scarp. Concurrently, a line-scan lidar tracked swash bores and monitored erosion and accretion patterns along a single cross-shore transect throughout the experiment. As wave conditions intensified, a discontinuity in the slope of the dune formed; the discontinuity grew steeper and progressed landward at the same rate as the R2% runup extent until it was a fully formed scarp with a vertical face. Within the upper 0.15 m of the partially saturated sand, upward pore pressure gradients were detected during backwash, influencing the effective weight of sand and potentially contributing to beachface erosion. The magnitude and frequency of the upward pressure gradients increased with deeper swash depths and with frequency of wave interaction, and decreased with depth into the sand. A simple conceptual model for scarp formation is proposed that incorporates observations of upward-directed pressure gradients from this study while providing a reference for future studies seeking to integrate additional swash zone sediment transport processes that may impact scarp development. Dunes are important protective features of many sandy coasts around the world. Often, when dune erosion occurs, steep faces known as scarps form. Scarps can erode landward quickly, and it is therefore useful to understand when and where they might form. Here, a large-scale laboratory experiment was used to study the processes involved in the erosion of a dune. The water pressure and moisture content within the dune sediments, and the runup extent and changing elevation of the sand were observed simultaneously. During the experiment, a scarp developed as the wave heights, periods, and water levels increased. As waves passed over the sand surface, the water pressure within the sand changed rapidly, sometimes causing differences (gradients) in pressure between sand at different depths. If the pressure gradients became high enough and were directed upwards, they put an upward force on the sand grains, potentially making them prone to erosion. Upward-directed pressure gradients were observed as wave conditions grew more energetic. To show how the processes observed in this study may have contributed to the formation of the scarp in the experiment, a conceptual model of scarp formation is proposed. A 1:2.5 scale laboratory experiment was used to study the subsurface hydrodynamics of a beach dune during an erosive eventDestabilizing pressure gradients were identified within the sand surface, and were impacted by swash conditions and depth within the sandA conceptual model for scarp formation is proposed that incorporates observations from this study and could guide future work