Short term morphodynamics of an intertidal bar on megatidal ebb delta
Résumé
The morphodynamics of an intertidal bar located on an ebb delta of a megatidal inlet system in Normandy, France, was examined during four short experiments under low to high energy wave conditions and spring or neap tide contexts. Although there have been numerous studies on ebb-tidal bar morphology and on the processes affecting such bars, these concern only microtidal or mesotidal settings, and are mainly based on observations. No detailed work involving hydrodynamic conditions embracing the whole tidal cycle has been carried out so far on the mechanisms of onshore migration of these coastal accumulation features, especially along macro and megatidal coasts. In this study, the morphological response of the bar was coupled with the residence times and intensities of each hydrodynamic process (swash, surf, shoaling waves) over a representative bar cross-shore profile, and sediment transport patterns deduced from fluorescent tracer. The results show that the bar migrated exclusively shoreward during moderate to storm conditions (Hs > 0.7 m at the bottom of the bar seaward slope). During low energy wave conditions, no bar movement was observed. Swash action was not the dominant process, mainly due to its duration, which did not exceed 8 min in the course of a semi-diurnal tidal cycle. The movement of sediments and the bar morphology is induced mainly by surf processes and their associated currents. The increase in the significant wave height disturbed the general mean current behaviour, which was parallel to the bar crest during low energy conditions. Under surf conditions, mean flows are directed onshore, with an absence of bed return flow; this allows sediment to be transported towards the bar crest causing a landward bar migration. An offshore-directed cross-shore current (not favourable for bar migration) is recorded with larger water depths and no breaking waves (shoaling conditions). This study also highlights the influence of the tidal water level fluctuations in such a large tidal range setting, which induce a rapid shift in processes (such as bed shear and sediment re-working) across the shore-normal profile. These conditions explain the relative low rate of migration of the bar over the delta platform (about 33 m/year). This rate is much less than those reported from micro-mesotidal environments. Calculations of bed shear stress also show that the mean currents alone are not responsible for onshore bar movement, notwithstanding the megatidal character of the field site.