Morphological diversity and complex sediment recirculation on the ebb delta of a macrotidal inlet (Normandy, France): A multiple LiDAR dataset approach
Résumé
The shoreline in the vicinity of inlets can exhibit considerable variability in morphology in both space and time.
Most studies on inlets and their adjacent shores have focused on the morphodynamics of sediment by-passing
mechanisms generated by longshore transport. For the first time, the morphology, sedimentary features, sediment
budgets and patterns of evolution of the shoreline and ebb delta in a macrotidal inlet system have been investigated
using seven LiDAR topographic surveys in Normandy, France, over a period of 3.7 years fromFebruary
2009 to October 2012. The ebb delta shows strong development on the northern flank of the inlet, expressed by a
large sand spit and two types of superimposed dynamic sandy features: eight long-crested and highly mobile
transverse bars and a large swash bar. Sand transport fromN–S on the updrift beach feeds the growth of the distal
part of the spit. This sand supply is further augmented by the onshore movement of a large swash barwelding to
the upper foreshore. However, the main topographic changes were induced by the northward migration of the
transverse bars on the ebb platform. This is driven by strong northward-directed tidal currents parallel to the
shore. The bars exhibit a more complex morphology and dynamics along the seaward margin of the ebb delta
where their mobility is controlled by wave action. Topographic measurements suggest a clear sand recirculation
pattern. In this morphodynamic model, sand coming from the updrift upper beach is transported southward
and deposited at the distal end of the spit, where it serves to construct transverse bars close to the tidal inlet.
Transverse bar migration ends in the wave-exposed northern margin of the ebb delta,where they are integrated
into the shallow dissipative shoreface sand sink. This sink nourishes the southward longshore transport to feed
growth of the large swash bar and southward spit elongation. This semi-circular recirculation cell model involves
an inversion of sand movement close to the inlet and emphasizes the combined role of tidal currents and waves
in the large-scale 3D ebb–delta sediment dynamics in this macrotidal setting, in contrast to the much more
commonly reported alongshore sediment by-passing mode of microtidal inlets.