Multi time-scale evolution of a wide estuary linear sandbank, the Longe de Boyard, on the French Atlantic coast

Abstract

This trans-disciplinary work combines modeling and observational approaches and offers a unique dataset to study the behaviour of a wide estuary linear sandbank over different time-scales. The sandbank studied, the Longe de Boyard, lies in a macrotidal estuary environment off the French Atlantic coast. Side scan sonar data combined with shipek grab samples and numerical modeling of waves and tides revealed its short-term dynamics. Historical (1824) and present-day (2000 and 2003) bathymetric data combined with numerical simulations of waves and tides and tide-related sand transport in 1824, and seismic profiling, were then used to demonstrate the long-term evolution of the sandbank and how this correlates with the short-term dynamics. The geological evolution (centuries to millennia) was finally deduced from seismic stratigraphy combined with an analysis of vibrocore samples. Most of the long-term morphological changes in the ‘Longe de Boyard’ can be explained by the short-term dynamics involving sand transport convergence driven by both tides and waves. Seaward, the changes in the axial part of the bank correspond mainly to erosion and can be explained by wave and tide ravinment. Shoreward, sediment accretion is related to the convergence of tide-related sand transport during ebb and flood due to the dam-effect of the crest of the bank. The changes in the sandbank since 1824 can also be related to a decrease in the tidal channel cross section. The latter was the result of a 10% decrease in tidal currents and tidal prisms subsequent to the rapid sediment infill, and a related 20% reduction in the water volume of the estuary system where the sandbank lies. Seismic stratigraphy and cores showed that the modern sandbank consists of upper clinoforms made of fine sand built up over a core made of coarse sand and gravel related to high-energy environments. Hence, the Longe de Boyard is not only the result of sand convergence driven by both tides and waves but also integrates decreases in the tidal prism subsequent to sediment infilling of the surrounding estuaries on a century and millenia time-scale.