Geomorphic Response of a Coastal Berm to Storm Surge and the Importance of Sheet Flow Dynamics
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AbstractDuring a storm, as the beach profile is impacted by increased wave forcing and rapidly changing water levels, sand berms may help mitigate erosion of the backshore. However, the mechanics of berm morphodynamics have not been fully described. In this study, 26 trials were conducted in a large wave flume to explore the response of a nearprototype berm to scaled storm conditions. Sensors were used to quantify hydrodynamics, sheet flow dynamics, and berm evolution. Results indicate that berm overtopping and offshore sediment transport were key processes causing berm erosion. During the morphological evolution of the beach profile, two sand bars were formed offshore that attenuated subsequent wave energy. The landward extent of that energy was confined to the seaward foreshore, inhibiting inundation of the backshore. Net offshoredirected transport was dominant when infragravity motions increased in the swash zone. Conversely, the influence of incidentband motions on sediment transport was relatively greater in the innersurf zone. Nearbed flow velocities and sheet flow layer thicknesses were larger in the swash zone than in the innersurf zone. This paper also provides a valuable analysis between morphologyestimated total sediment transport rates and rates derived from in situ measurements. Sheet flow dynamics dominated foreshore crossshore sediment processes, constituting the largest portion of the total sediment transport load throughout the berm erosion.
