Abstract:A detailed field study was carried out on a tidal bore to document the turbulent processes and sediment entrainment which occurred. The measured bore, within the Arcins Channel of the Garonne River (France), was undular in nature and was followed by well-defined secondary wave motion. Due to the local river geometry a collision between the Arcins channel tidal bore and the bore which formed within the main Garonne River channel was observed about 800 m upstream of the sampling site. This bore collision generated a transient standing wave with a black water mixing zone. Following this collision the bore from the main Garonne River channel propagated 'backward' to the downstream end of the Arcins channel. Velocity measurements with a fine temporal resolution were complemented by measurements of the sediment concentration and river level. The instantaneous 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, including during the 'backward' bore propagation. Large suspended sediment concentration estimates were recorded and the suspended sediment flux data showed some substantial sediment motion, consistent with the murky appearance of the flood tide waters.

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A tidal bore is an unsteady rapidly-varied open channel flow generated by the swift advance of the early flood tide in a funnelled shape 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 bore's 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 characterised 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.

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