David Reungoat

Field Measurements of Unsteady Turbulence in a Tidal Bore: the Garonne River in October 2013

ABSTRACT A tidal bore is an unsteady rapidly-varied open channel flow generated by the swift advance of the early flood tide in a funnel-shaped river estuary when the tidal range exceeds 4.5 to 6 m. This contribution presents a detailed field investigation conducted on the tidal bore of the Garonne River (France). The bore was undular and the bores leading edge was followed by well-defined secondary waves, or whelps. The instantaneous ADV velocity data indicated large and rapid fluctuations of all velocity components during the tidal bore. Large Reynolds shear stresses were observed during and after the tidal bore passage. The investigation characterized some unusual transient turbulence caused by the bore propagation in a large river system, and the results suggested the advection of large-scale eddies in the wake of the bore front. The present study highlighted the need for detailed field measurements with fine temporal resolution, to characterize the highly unsteady rapidly-varied nature of tidal bore flows.

Macroscopic analysis of a turbulent round liquid jet impinging on an air/water interface in a confined medium

In this paper, a submerged vertical water jet impinging on an air/water free surface is experimentally and numerically investigated. On the one hand, a two dimensions-two components particle image velocimetry technique is carried out to perform velocity measurements from the jet injection to the unsteady free surface. The motion of this interface is recorded by a biaxial shadowgraph imaging system. On the other hand, volume of fluid and large eddy simulation methods are coupled to compute the interaction between the jet dynamics and the interface behavior. Numerical and experimental results are confronted in both the single-phase jet area and the impinging region beneath the free surface. A statistical analysis of the interface motion is led through the maximum elevation point to highlight its organized time and spatial evolution in terms of a characteristic lifetime.

Tidal bore hydrodynamics and sediment processes: 2010–2016 field observations in France

ABSTRACT A tidal bore is a compressive wave, advancing upstream in an estuary when the flood tidal flow starts. It is observed when a macro-tidal flood flow enters the funnel shaped river mouth with shallow waters. Its upriver propagation impacts the natural system, with sediment scouring and suspension. The tidal bores of the Garonne and Sélune Rivers in France were extensively investigated between 2010 and 2016. Instantaneous velocity measurements were conducted continuously at high-frequency (50 to 200 Hz) during each bore event. In the Garonne River, instantaneous sediment concentration data were obtained and the sediment properties were systematically tested. The nature of the observations was comprehensive, regrouping hydrodynamics and turbulence, sedimentology and suspended sediment transport. The key outcomes show that the tidal bore occurrence has a marked effect on the velocity field and suspended sediment processes, including a sudden flow deceleration and flow reversal during the bore passage. The turbulent Reynolds stresses present large instantaneous amplitudes, with rapid fluctuations, during the tidal bore. The sediment flux data imply considerable mass transport rates during the first hour of flood tide. This unique review of field data further shows a number of common features, as well as the uniqueness of each individual event.