Morphology and internal structure of mixed bio-siliciclactic coastal barriers: flume experiments
Résumé
Wave-dominated coastal barriers are dynamic systems responding to sea-level fluctuations at different time scales induced by storm surges, tides (from diurnal to pluri-annual cycles), and climate (century to millenia). They are composed of siliciclastic sand, bioclastic debris or a mixture of both. Sediment-starved epicontinental platforms of temperate areas are frequently the location of intense biologically-controlled carbonate production. Consequently, coastal sediments of these systems are mixed, composed of siliciclastic sand (mostly quartz grains) and shell debris, essentially derived from mollusc shells. In some environments of very high biological productivity, such as the English Channel and more specifically the Normandy-Brittany Gulf, shell debris can largely dominate the overall composition of coastal sediments. With time, due to natural factors (climate changes, sea-level fluctuations) and human activities (shellfish farming, fishing, species introduction, marine aggregates extraction) the proportion between siliciclastic and bioclastic grains can vary. It is critical to understand how these modifications in sediment composition can affect sedimentary processes involved in the construction of coastal barriers and potentially their stability. This is the general objective of the present study mainly based on physical modelling.
After two experimental studies performed in a settling tube and in a flume under unidirectional current, which show the complex hydrodynamic behaviour of different mollusc shell species (Rieux et al., 2019), a set of three wave-tank experiments were performed under varying mean water level and regular wave forcing, in order to reproduce barrier morphologies using bioclastic (B) / siliciclastic (S) sediment mixture in different proportions. Bioclastic debris were sampled in the Mont-Saint-Michel bay (NW France) whereas siliciclastic particles consist in a mix of different marine sands. Three sediment compositions have been created: 90% B – 10% S; 50% B – 50% S; 25% B –75% S. Barrier morphology was surveyed using a laser telemeter and Structure-from-Motion photogrammetry. The internal architecture was studied using side-photographs and lacquer-peels. The results show a great variability of migration rate, morphology and internal structure of barriers depending on the sediment composition. These variations can be explained by the modification of the permeability and by sorting processes occurring in the breaker and swash zones where the settling velocity of sediment particles plays a leading role. Barriers rich in bioclastic debris absorb the energy of the waves, which involve an aggradation of bioclastic particles on the upper beach, a steeper beach face, infrequent ridge overwash and subsequently a shorter internal face. On the contrary, a sediment composition dominated by siliciclastic grains, involve a less permeable sediment, a less steep beach face, many ridge owerwash and a longer internal face.