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.