Inception of bed load motion beneath a bore

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.