Turbulent advances of a breaking bore: Preliminary physical experiments

• The propagation of breaking bore roller toe was investigated in laboratory.
• The toe perimeter shape fluctuated rapidly with transverse distance and time.
• Large fluctuations in bore celerity were observed in terms of both time and space.
• Instantaneous longitudinal roller profiles showed temporal and spatial fluctuations.
• Distinctive air bubble entrainment was measured at the toe of the roller.

In an estuary, a tidal bore may be generated at the leading edge of the flood tidal wave during the early flood tide under spring tide conditions into a narrow funnelled channel. For Froude numbers greater than 1.4–1.6, the leading edge of the bore is characterised by a breaking roller. The roller is characterised by a sudden increase in water depth, a highly turbulent flow with large-scale vortical structures, some kinetic energy dissipation, a two-phase air–water flow region and strong turbulence interactions. New experiments were conducted in a large canal with a focus on breaking bore roller propagation. The upstream propagation of the roller toe was highly turbulent. The toe perimeter shape fluctuated rapidly with transverse distance and time. The celerity of the roller toe changed rapidly with time and space, although in a quasi-two-dimensional manner on average. The instantaneous longitudinal free-surface profile of the roller showed significant temporal and spatial fluctuations. New air–water flow measurements highlighted some distinctive air bubble entrainment at the toe of the roller. Bubbles with larger chord times were detected at higher vertical elevations in a more intermittent manner. Overall the study demonstrated that the propagation of breaking bore is a very turbulent, three-dimensional process.