Importance of infragravity waves for the generation of washover deposits

Washover deposits are among the most remarkable sedimentary signatures of dune breaching and coastal inundation. Overwash of the barrier leading to washover development can be related either to storms or tsunamis but the distinction between these two processes based on the depositional record is still a matter of debate. Hence, studying, recent well-documented washover deposits, combining sedimentological and hydrodynamic investigations is a key approach to identify the processes governing the washover deposition and to analyze their sediment record within the washover. During the winter of 2013-2014, the coasts of the Bay of Biscay experienced an exceptional wave climate, leading to dune breaching and washover deposition at many locations, including the Gatseau Spit (South of Oleron Island, France). Field observations, aerial photographs, topographic measurements, GPR investigations, trenches and cores, grain size analysis and magnetic fabric measurements were carried out and complemented with hydrodynamic modeling of tides, storm surges, short waves and infra gravity waves in order to compute maximum runup during the winter of 2013-2014. Such an interdisciplinary approach led to propose that: (1) large overwash events occurring during the winter of 2013-2014 were mainly driven by infra gravity (IG) waves combined with high tides; (2) the washover elongated outer shape was strongly controlled by the antecedent morphology of the back-barrier area; (3) one overwash flow driven by one IG wave is recorded by one normally graded lamina composed of quartz sand at base and heavy minerals enriched sand at top; (4) dominant settling of the particles from a suspension during sedimentation occurred in the end of each overwash flow in response to flow velocity decrease; (5) laminae can be grouped in laminasets generally composed of thinning upward laminae, related to overwash flow depth decrease during falling tide and to erosion of basal laminae by highest overwash flows occurring more likely around the high tide; (6) the number of laminasets in a proximal position is in the same order of magnitude as the number of modeled highest runup events, offering an almost complete record of overwash events that occurred during the winter 2013-2014.