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A broadly accepted paradigm is that vegetation reduces coastal dune erosion. However, we show that during an extreme storm event, vegetation surprisingly accelerates erosion. In 104-m-long beach-dune profile experiments conducted within a flume, we discovered that while vegetation initially creates a physical barrier to wave energy, it also (i) decreases wave run-up, which creates discontinuities in erosion and accretion patterns across the dune slope, (ii) increases water penetration into the sediment bed, which induces its fluidization and destabilization, and (iii) reflects wave energy, accelerating scarp formation. Once a discontinuous scarp forms, the erosion accelerates further. These findings fundamentally alter the current understanding of how natural and vegetated features may provide protection during extreme events. 

Abstract 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 theR_2%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.