Author(s): N. J. Docherty; H. Chanson
Linked Author(s): Hubert Chanson
Keywords: Tidal bores; Breaking roller; Physical modelling; Bed roughness; Turbulence; Ensemble-average
Abstract: A tidal bore is an unsteady flow motion generated by the rapid water level rise at the river mouth during the early flood tide under appropriate macro-tidal and bathymetric conditions. The present study investigated physically the turbulent properties of breaking tidal bores. The results were based upon some experimental measurements of free-surface fluctuations and turbulent velocities conducted on smooth and rough beds. Using an ensemble-averaging technique, the free-surface fluctuations of breaking tidal bores were characterised. Immediately prior to the roller, the free-surface curved gradually upwards, and the passage of the bore roller was associated with some large water elevation fluctuations. The turbulent velocity measurements were performed at several vertical elevations during the breaking bore. Both the instantaneous and ensemble-averaged velocity data highlighted some strong flow deceleration at all elevations. Close to the bed, the longitudinal velocity component became negative immediately after the roller indicating the existence of a transient recirculation. The vertical velocity data presented some positive, upward motion beneath the roller with increasing maximum vertical velocity at increasing elevation above the bed. The transverse velocity data show some large fluctuations with a non-zero ensemble-average after the roller passage, suggesting some secondary turbulent motion advected behind the bore.
Year: 2011