Non-hydrostatic modelling of extreme water levels on Banneg Island, France

Extreme water level events were observed during recent winters on Banneg Island [Ardhuin et al., 2011; Suanez et al., 2009], a small island off western Brittany well exposed to the large North Atlantic swells and characterized by steep rocky cliffs on its western part. Based on geomorphic evidences (bedrock scars, overturned boulders, debris lines) and hydrodynamic data, Fichaut and Suanez [2011] investigated the quarrying and transport of cliff-top storm deposits induced by giant wave events and partial flooding of the island (Fig.1), while Suanez et al. [2009] provided a retrospective analysis of extreme water levels based on a 30-year wave model hindcast and empirical run-up formula. Later, Sheremet et al. [2014] deployed pressure sensor measurements to this site and applied a 1D nonlinear mild-slope model to reveal that the highest water levels could exceed 6.5 m above the astronomical tide during major storms, mainly induced by large infragravity waves. In order to improve our understanding of the storm-induced hydrodynamics in the Iroise Sea, a wave buoy was deployed to the west of Banneg and additional pressure sensors were installed on the western part of the island (Section 2). In addition, a phase-resolving wave model based on the nonlinear shallow water equations, including non-hydrostatic pressure was applied to the study site and the model results were validated against in-situ observations (Section 3). The hydrodynamic data collected during the major storms of February 2014 were analyzed and the model was forced with the hydrodynamic conditions of the morning high tide of February 5, when some of the highest water levels were observed on the island (Section 4). Based on this combined model-data analysis, the incident and infragravity wave dynamics in the nearshore, and the associated sporadic flooding of the island were investigated in the light of previous studies on Banneg Island (Section 5)