Turbulent mixing of particles under tidal bores: an experimental analysis

Abstract

A tidal bore develops in an estuary when the tidal range exceeds 4.5–6 m and the estuarine bathymetry amplifies the tidal wave. The bore is an abrupt rise in water depth associated with a discontinuity in velocity and pressure fields at the front. Herein the free-surface properties and the turbulent mixing of light-weight particles were investigated during the passage of tidal bores. The free-surface properties were recorded using a non-intrusive technique, while particle tracking was performed under undular and breaking bores. A basic result was the identification of a broad spectrum of particle trajectories, linked with the existence of large-scale vortical structures. These turbulent structures were responsible for the vertical water mixing as a tidal bore propagates upstream in an estuary. The large-scale eddies were also responsible for the rapid longitudinal dispersion of particulates, such as fish eggs, with some form of preferential motion, depending upon the particle's vertical elevation.

Keywords:

• Particle dispersion
• physical modelling
• tidal bore
• turbulent mixing
• unsteady flow

Acknowledgements

The authors acknowledge the technical assistance of Graham Illidge and Clive Booth, University of Queensland.