Influence of sediment composition on morphology and internal structure of mixed siliciclastic-bioclastic coastal barriers: Contribution of flume experiments

Coastal barriers are dynamic systems, the morphology and architecture of which are controlled by local hydrodynamics, sea-level fluctuations at different timescales, geological heritage and sediment composition. Coastal barriers may be composed of siliciclastic sediments, bioclastic sediments, or a mixture of both. Mixed siliciclastic-bioclastic sediments (as common as 'pure' sediments) are still little represented in the literature. Changes in sediment composition could affect sedimentary processes which are involved in the construction and stability of coastal barriers due to a different hydrodynamic behaviour between bioclastic and siliciclastic particles. In this study, wave-flume experiments were used to investigate the role of sediment compositional mixing on the morphology and architecture of coastal barriers. Three different siliciclastic/bioclastic mixtures were exposed to regular wave forcing, together with mean water level fluctuations to create regressive and transgressive depositional units. Compositional mixtures responded similarly in first-order to mean water-level fluctuations, with the formation of a bar at low water level. Its subsequent on-shore migration and reworking as berm deposits during rising water level stages, and with the formation of washover deposits during high water level stages. In detail, the increasing content of bioclastic sediment increased the beachface slope and reduced the length of washover deposits. The faster aggradation of washover sheets with bioclastic-rich mixtures accelerated the barrier recovery after a submersion and breaching event. A strong segregation between the siliciclastic and the bioclastic grains was observed in the different depositional units, which is attributed both to the coarse size (grain-size control) and to the flat-shape (compositional control) of the bioclastic particles.