Characterisation of unsteady turbulence in breaking tidal bores including the effects of bed roughness

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. With time, the leading edge of the tidal wave becomes steeper and steeper until it forms a wall of water that is the tidal bore. Herein the turbulence in breaking tidal bores was investigated experimentally under controlled conditions with two types of bed roughness: smooth PVC and fixed gravel bed (ks = 3.4 mm). Some qualitative observations were conducted with both undular and breaking bores. The tidal bore flow patterns were independent of the bed roughness, as well as of the initial steady flow conditions, while the free-surface properties were close to earlier findings. Using an ensemble-averaging technique, the free-surface fluctuations of breaking bores were characterised. Immediately prior to the roller, the free-surface curved gradually upwards and the gentle surface elevation rise was about 0.1×do where do is the initial water depth. The passage of the bore roller was associated with some large water depth fluctuations. Some detailed turbulent velocity measurements were performed at several vertical elevations during and shortly after the breaking bore passage. Both the instantaneous and ensemble-averaged velocity data highlighted some seminal features of breaking bores. Namely a strong flow deceleration was observed at all elevations during the tidal bore passage. Close to the bed, the longitudinal velocity component became negative immediately after the roller passage implying the existence of a transient recirculation "bubble". The vertical velocity data presented some positive, upward motion during the bore front passage with increasing maximum vertical velocity with increasing distance from the bed. The vertical motion was believed to be linked with some streamline curvature. The transverse velocity data presented some large fluctuations with a non-zero ensemble-average after the roller passage that highlighted some intense secondary motion advected behind the bore front. A comparison between ensemble-average (EA) and variable interval time average (VITA) velocity data was performed. The EA and VITA results showed some comparable velocity pattern with some relatively-long-term data trend superposed to some high-frequency turbulent fluctuations. The data showed however that the VITA calculations for a single experiment presented some non-negligible difference with the EA median value for all velocity components. Overall the study demonstrated the intensive turbulence and turbulent mixing under a breaking tidal bore