Wave runup is known to depend on offshore wave conditions and coastal morphology. While most field studies on wave runup have focused on low‐to‐mild‐sloping sandy beaches, runup measurements on steep and irregular rocky cliff profiles are still very scarce. Here we investigate the physical processes controlling wave runup in such environments and the range of applicability of empirical runup formula. This study focuses on the steep rocky cliffs (0.1 < tanβ < 0.4) of Banneg Island, a small island located in the Molène archipelago, Brittany, France, occasionally flooded during extreme water level events. A statistical parameter for extreme runup is derived from the measurements of pressure sensors deployed in the intertidal zone. Deep water wave parameters are used to force a high‐resolution wave model, and nearshore wave parameters and high‐resolution topographic data are analyzed concurrently with runup time series in order to assess the dependence of the runup on hydrodynamic conditions and morphological parameters. The wave runup is shown to be strongly related to the square root of the offshore significant wave height times the offshore wavelength. The measurements also reveal the depth dependence of the runup, which is mainly attributed to the curvature of the foreshore profile. In comparison to empirical relation obtained for a mild‐sloping beach, the present data show a significant reduction in normalized wave runup, that is attributed to enhanced bottom friction over the rocky bottom.

Plain Language Summary

When waves reach the shores, they travel up and down the beach before being reflected seaward. The maximum vertical excursion of the waterline relative to the still water level, called the wave runup, is a key parameter for the design of coastal structures and the prediction of overtopping volumes during storm events. Most runup studies in natural environments have focused on smooth and mild‐sloping sandy beaches, and empirical formula to predict the wave runup has been derived for these environments. Here we study the process of wave runup over steep rocky cliffs. Using pressure sensors deployed in the intertidal zone of Banneg Island—a small island located west of Brittany, France, exposed to very high waves—we measured runup events. Then we investigated the link between these runup data, offshore wave parameters, and cliff slopes. Our results reveal that the wave runup is linearly dependent to the square root of the offshore wave height times the offshore wavelength. In addition, a significant reduction of the runup is found compared to sandy environments, which is attributed to the frictional effect exerted by the rocky bottom on the flow.