Field observations of turbulence, sediment suspension, and transport under breaking tidal bores

Abstract

Turbulence dynamics and sediment suspension, under intense tidal bore conditions are investigated using field observations from the Qiantang Estuary, China. During the passage of the bore the tidal current bottom generated turbulence is augmented by turbulence injection from the surface as part of the breaking processes of both bore front and the bore associated secondary waves. These processes lead to enhanced vertical mixing and vertical distribution of the advected sediments. The latter contributes to increased sediment transport in the direction of the bore propagation that potentially modifies the normal tidally-driven estuarine sediment dynamics and morphology. The results show that turbulence intensity increases with increased tidal currents and oscillating motions associated with the breaking processes. At the early stage of the tidal bore, the vertical mixing of suspended sediment is more convective than diffusive due to the elevated length scales associated with bore and breaking secondary wave generated turbulence. Turbulent momentum and vertical sediment flux are determined by intermittent, large magnitude turbulent events (i.e., turbulent bursting). At early period of the tidal bore (~ 40s), inward and outward interactions contribute most to the Reynolds stress and vertical sediment flux, in contrast to later periods when sweep and ejection become more important. Negative vertical sediment flux (i.e., toward the bed) is observed, indicating that the sediment buoyancy acts as a source of turbulent kinetic energy, possibly due to the modification of vertical distribution of suspended sediment by the bore. These observations, in flow conditions under intense tidal bore breaking, reveal different sediment dynamics from traditional boundary process by injecting breaking induced turbulence which modifies the distribution and transport of the suspended sediment.