Geomorphic Response of a Coastal Berm to Storm Surge and the Importance of Sheet Flow Dynamics.

During 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 near‐prototype 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 offshore‐directed transport was dominant when infragravity motions increased in the swash zone. Conversely, the influence of incident‐band motions on sediment transport was relatively greater in the inner‐surf zone. Near‐bed flow velocities and sheet flow layer thicknesses were larger in the swash zone than in the inner‐surf zone. This paper also provides a valuable analysis between morphology‐estimated total sediment transport rates and rates derived from in situ measurements. Sheet flow dynamics dominated foreshore cross‐shore sediment processes, constituting the largest portion of the total sediment transport load throughout the berm erosion. Plain Language Summary: Coastal berms are natural sand barriers between the ocean and the back beach. During storms, berms may help prevent or slow down the erosion of the beach. A large‐scale laboratory experiment was conducted to study sediment transport and how berms erode. Sensors were used to measure water levels, flow velocities, sediment concentrations, and beach profile changes. The findings indicate that berm erosion was primarily caused by swash processes occurring as waves overtopped the berm. Offshore sediment transport resulted in the formation of two sand bars that helped dissipate subsequent wave energy. Overall, by quantifying processes leading to berm erosion, we can enhance our understanding of coastal dynamics, implement effective measures to protect beaches during storms, and safeguard against flooding. Key Points: Wave overtopping, runup, and low‐frequency motions are the main processes of berm erosionDuring early storm stages, near‐bed flow velocities and sheet layer thicknesses are larger in the swash zone than in the inner‐surf zoneQuantification of sediment transport rates indicates the dominance of sheet flow layer load at the foreshore, throughout the berm erosion [ABSTRACT FROM AUTHOR]