Field study of post-overtopping flow impacts on a vertical wall at the crest of an upper-beach dike

In many urbanized shorelines, upper-beach sea-defenses and coastal buildings can be exposed to the impact of wave induced post-overtopping flow during energetic storm conditions. In order to properly anticipate the risk exposure of these seafront infrastructures, it is crucial to characterize and quantify the resulting assailing forces. Current knowledge on this type of configurations is mainly based on downscale laboratory experiments. Moreover, the case of sea-defense structures with an emergent toe is still scarcely documented in the literature. This study aims to provide an original insight into this topic using a new dataset collected during an experiment carried out in real conditions at a field site in the southwest of France. The instrumented site corresponds to a semi-reflective beach with a very shallow foreshore. The deployment was performed during an energetic storm (offshore significant wave height $H_S=4.4$ m and peak period $T_P>15s$) concomitant with spring tide that resulted in recurrent wave-induced overtopping events, while the overtopped upper-beach seawall toe always remained emergent. The experimental setup included an innovative wave impact measuring station composed of a self supporting structure equipped with a series of high frequency pressure sensors. The station measured the loads induced by overtopping flow impacts at the crest of an upper-beach dike. Additionally, the swash flow characteristics prior to each impact and the runup against the station were captured using synchronized video recordings, complemented by an array of pressure sensors deployed on the beach foreshore. The magnitude of the measured impact force ranges between 0.38 and 9.70 kN/m. A majority of the recorded events have a twin-peak signal shape similar to that observed in laboratory studies. The joint analysis of the vertical pressure distribution and of the synchronized video images also allows to highlight different impact phases, which align with results from previous downscale experiments. However, in contrast with existing laboratory observations, our measurements show that the force peak measured during the reflection phase, which follows the maximum runup, deviates significantly from the hydrostatic prediction. The greater the intensity of the force, the more pronounced the deviation. The analysis of the pre-impact flow properties suggests a relation between the maximum impact force, the overtopping discharge and the momentum flux, while the deviation from the hydrostatic prediction seems more related to the swash flow height. Finally, the order of magnitude of the measured maximum force of the single impact events was shown to be reasonably estimated by applying an hydrostatic-like formulation to the maximum runup height, provided the application of an empirically fitted reduction coefficient. These results provide a first description of overtopping wave impacts on an upper-beach structure with an emergent toe in real field conditions. It paves the way for further measurement efforts to extend the obtained characterization to different water level and wave energy conditions.