A tidal bore is a series of waves propagating upstream in the river mouth as the tidal flow turns to rising in macro-tidal conditions. Some related geophysical processes include the tsunami-induced bores and uprushing bores on beaches. In the present study, the inception of sediment motion beneath tidal bores was investigated physically. No sediment motion was observed in the initially steady flow and beneath undular bores. A transient sheet flow motion was observed beneath breaking bores and the onset of sediment motion was closely linked with the passage of the roller toe. The forces acting on the movable gravel bed particles were estimated from the physical measurements. The results showed that the longitudinal pressure gradient force was the dominant contribution de-stabilising the particles and inducing the onset of sediment motion. The drag force added a sizeable contribution to maintain the upstream particle motion, although the entire sheet flow motion was brief.

A tidal bore is a positive surge taking place during the flood tide with a large tidal range and the bore corresponds to the leading edge of the tidal wave propagating upstream. In this study, some physical modelling was performed to investigate the upstream bore propagation over fixed and movable gravel beds. Both undular and breaking bores were tested. The free-surface and velocity measurements were complemented by some observations of particle motion beneath the bore front. In the initially steady flow and beneath undular bores, no sediment motion was observed for the experimental setup. Beneath breaking bores, on the other hand, some upstream gravel bed load motion was observed behind the bore. The gravel bed particles were de-stabilised by the roller toe passage and advected upstream. The unsteady velocity data showed some damping of the transient recirculation on the movable gravel bed.

A tidal bore may develop in an estuary during the spring tide conditions when the tidal range exceeds 5-6 m and the flood tide is confined to a narrow funnelled estuary with low freshwater levels. The tidal bore is of great importance for the geomorphology of the estuarine zone. In this study, some physical modelling was performed to investigate the sediment motion inception beneath a tidal bore on a movable gravel bed. The results show the significant impact of breaking bore propagation on the gravel bed motion. The dominant contribution to sediment transport inception is the longitudinal pressure gradient force, while the transient recirculation motion next to the bed yields to a drag force acting in the upstream direction and contributing to sediment motion.

A tidal bore is a natural estuarine phenomenon forming a positive surge in a funnel shaped river mouth during the early flood tide under spring tide conditions and low freshwater levels. The bore propagates upstream into the lower estuarine zone and its passage may induce some enhanced turbulent mixing, with upstream advection of suspended material. Herein the flow field and turbulence characteristics of tidal bores were measured using both numerical (CFD) and physical modelling. This joint modelling approach, combined with some theoretical knowledge, led to some new understanding of turbulent velocity field, turbulent mixing process, Reynolds stress tensor, and tidal bore hydrodynamics. The numerical CFD tool Thétis was used herein. Thétis is a CFD model using the volume of fluid technique (VOF) to model the free-surface and Large Eddy Simulation technique (LES) for the turbulence modelling. Physical data sets were used to map the velocity and pressure field and resolve some unusual feature of the unsteady flow motion. A discussion will be provided to explain why a detailed validation process is crucial, involving a physical knowledge of the flow. Comparison of the numerical model results and experimental data over broad ranges of conditions for the same flow is mandatory. The validation process from 2D to 3D will be commented and difficulties will be highlighted.

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