Field experiments of beach scarp erosion during oblique wave, stormy conditions (Normandy, France)

• Alongshore variability of artificial scarp changes on a macrotidal beach is described.
• Laser measurements show a strong connection between the beach change and scarp evolution.
• Scarp presence changes the swash dynamics through incident/reflected uprush interactions.
• Increasing tide level compensates the swash attenuation allowing berm destruction.

A field-based experimental study of beach scarp morphodynamic evolution was conducted on the shoreface of a macrotidal sandy beach subject to storms combined with spring tide events (Luc-sur-Mer, France). Both video and in-situ measurements on an artificial berm are used to understand beach scarp evolution over one tide during stormy conditions. Image time stacks are used to analyze the swash action on the beach scarp and topographical data of the scarp are recorded with a terrestrial scanner laser to quantify the morphodynamic response of the beach scarp to wave action. This work provides a new and unique dataset about beach scarp changes and berm morphology in particular under rising tide and oblique wind-wave conditions. During one stormy event, the berm was completely destroyed. However, contrasting alongshore changes were measured during the erosive phase with different crest and foot scarp retreats and eroded volumes between the west and the east side of the berm. The beach in front of the scarp also shows a contrasting residual evolution, indicating an evident longshore sediment transport on the study area as a consequence of incident oblique wave conditions. A strong connection between beach evolution and beach scarp changes is clearly identified. The scarp erosion increases on the west side of the berm when the beach level is lowered and reduces when the beach surface rises on the east side. The beach slope and foreshore elevation as a result of a longshore sediment transport between east and west profiles, influence swash activity. Overall, water depth and swash activity became progressively different along the scarp during the experiment.Swash measurements indicate that the presence of the beach scarp strongly influences the swash motion. At high tide, the reflection of the uprush on the scarp front induces a collision between the reflected backwash and the following uprush dynamic. These collisions reduce and sometimes stop the motion of the following uprush, reducing the incoming swash excursion. Consequently, the scarp presence modifies the swash interaction that normally appears on a planar beach surface. With a beach scarp, the swash energy level is substantially attenuated and its spectrum is characterized by a large band. The number of uprush impacts on the scarp front calculated from video images reaches about 25 per 5 min. In spite of the swash energy attenuation due to swash/swash interactions, these impacts provoke the berm destruction in about two hours. However, the onshore migration of the swash zone induced by the rising tide appears to be important to explain scarp destruction, compensating the attenuated swash activity due to backwash-uprush interactions.