The formation of beach scarps is a challenging morphodynamic phenomenon that the coastal community has yet to capture in coastal change models. Understanding scarp formation is crucial to accurately predicting coastal erosion and vulnerability during extreme events, as models without parameters for scarp formation and development severely underpredict total erosion volumes. In models, the transition from planar beach slope to scarp is triggered by the exceedance of an empirical user-defined beach slope. However, little is known about the subsurface physical processes that can precipitate the formation of these highly erosive features.
Our work presents results from a near-prototype experiment undertaken to examine both the subsurface and subaerial hydrodynamics involved in the erosion of a beach dune under hurricane conditions. Data was collected during a scaled simulation of Hurricane Sandy on a 1:2.5 scale beach dune in the NSF NHERI O.H. Hinsdale Wave Research Laboratory Large Wave Flume. Pressure and moisture sensors (f = 100 Hz) buried within the dune tracked the location of the water table over the course of the experiment and captured the influence of wave runup events on pore water pressure and moisture content within the dune. A line-scan lidar (f = 2 Hz) determined the runup elevation of each bore and tracked erosion along a single cross-shore transect throughout the experiment.
During the experiment, a vertical scarp formed on the beach face as the water level and wave height increased. Throughout the period of scarp formation, a local increase in the total hydraulic head developed underneath the swash zone. Moisture sensors confirm that the sand in the swash zone was saturated, which indicates a reduction in matric suction from the partially saturated state. Partial momentary liquefaction events, which destabilize surficial sediments on the beach face, were also observed as the scarp forms. Partial momentary liquefaction events are positively correlated with both the total hydraulic head and the swash bore depth. While a reduction in matric suction and an increase in partial momentary liquefaction events can explain some of the slope steepening observed during the experiment, the initial slope discontinuity that developed into the scarp was observed prior to the occurrence of our observations of these instabilities.