Particle Dispersion under Tidal Bores: Application to Sediments and Fish Eggs

A tidal bore is a surge of water propagating upstream as the tidal flow turns to rising into an estuary with a tidal range larger than 5 to 6 m and a bathymetry that amplifies the tidal wave. The bore front is a shock characterised by a singularity of the free-surface and pressure and velocity fields. This study aims to characterise the tidal bore propagation and the induced turbulent mixing under controlled flow conditions. Some physical modelling was performed based upon a Froude similitude and the tracking of light particles was conducted with both undular and breaking bores. Some large fluctuations of horizontal and vertical particle velocity components were observed during the undular bore propagation beneath the undulations. A major result was the identification of large-scale vortical structures generated below the front of the tidal bore. These large coherent turbulent structures must be responsible for some bed erosion and vertical mixing of the water column when a tidal bore propagates upstream in the estuarine zone of a natural system. The large-scale eddies are also responsible to the rapid longitudinal dispersion of fish eggs reducing the impact of predators, with some form of preferential motion depending upon the eggs' vertical elevation.