Formation and migration of transverse bars along a tidal sandy coast deduced from multi-temporal Lidar datasets

Abstract

A field of long-crested transverse bars was monitored from a 2.5-year series of topographic Lidar surveys in the vicinity of a tidal inlet on the macrotidal (mean spring tidal range = 7 to 12 m) west coast of Cotentin (Normandy, France). The bar field, the alongshore extent of which is about 1.8 km, is composed of a total of 8 bars with lengths varying from 320 m to 1300 m and mean heights comprised between 0.5 m and 2.5 m. Bar cross-sections are variable between bars and for a single bar, and also over time. The surveys show a consistent northward migration of the bars at a mean rate of about 2 m/month, but the rate is larger in winter than in summer. The Lidar observations show that the tidal inlet, located at the southern limit of the bar field where the bars start forming, comprises a large sediment platform that acts as a source of sand for the bars. The ebb jet debouching from the inlet is deflected northward by the ambient strong shore-parallel tidal currents in this large tide-range setting, and this may be the primary mechanism leading to the emplacement of the bars. Smaller wave-formed swash bars that further feed the development of these large transverse bars have also been observed. Monitoring of bar migration in the course of six consecutive spring tides with fair-weather conditions showed that strong spring tidal currents are sufficient to drive bar mobility in the absence of waves. Storm wave resuspension of sand is thus expected to enhance bar mobility rates, as shown by the higher rates of winter bar migration compared to the summer rates. The ebb jet explains the slower bar migration rates at the vicinity of the inlet, these rates increasing with distance northward of the inlet as the tidal currents become unimpeded. The main difference between these macrotidal transverse bars and their counterparts in microtidal settings resides in these strong tidal currents that are the essential driver of bar migration, unlike the wave-driven migration of microtidal bars. The large tidal range, in conjunction with storm wave activity, also induces longshore and seasonal variability in bar morphology. The transverse bars of Normandy appear to be inscribed in a sand circulation system involving the west Cotentin coast, the large ebb tidal delta from which they are formed, and the central Cotentin embayment where they are ultimately incorporated into the nearshore sand pool. Longer-term field hydrodynamic monitoring and modelling will be required in order to further elucidate the mode of formation of these transverse bars.