Turbulent advances of a breaking bore: Preliminary physical experiments

Highlights

• 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.

Abstract

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.

Introduction

A sudden increase in flow depth in an open channel induces a positive surge [13], [3], [19]. In an estuary, a positive surge of tidal origin is called a tidal bore which may be generated by the early flood tide propagating upstream into a narrow funnelled channel under a large tidal range [34], [8]. After formation, the bore may be analysed as a hydraulic jump in translation [30], [20]. The shape of the surge front is a function of its Froude number Fr₁ [25], [9]. For a rectangular channel, Fr₁ equals:

$${\mathit{Fr}}_1=\frac{V_1+\bar{U}}{\sqrt{g\times d_1}}$$

where V₁ is the initial flow velocity positive downstream, $\bar{U}$ is the mean bore celerity positive upstream, g is the gravity acceleration, d₁ is the initial flow depth (Fig. 1A). For Fr₁ > 1.4–1.6, the leading edge of the bore is characterised by a breaking roller. Fig. 1B presents a photograph of a large breaking tidal bore in China. The bore 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 with the free surface associated with splashes and droplet ejections.

In this contribution, a physical investigation was conducted in laboratory with a focus on the bore roller properties. Detailed measurements were performed in a relatively large facility. The observations included a series of video observations of propagating breaking bores to characterise the roller toe perimeter, the bore front celerity and their fluctuations, as well as some preliminary unsteady air entrainment measurements in the bore roller using a dual-tip phase-detection probe. It is the purpose of this contribution to study thoroughly the upstream roller propagation and its turbulent fluctuations.