Sediment Processes and Flow Reversal in the Undular Tidal Bore of the Garonne River

A tidal bore is a series of waves propagating upstream as the tidal flow turns to rising, and the bore front corresponds to the leading edge of the tidal wave in a funnel shaped estuarine zone with macro-tidal conditions. Some field observations were conducted in the tidal bore of the Garonne River on 7 June 2012 in the Arcins channel, a few weeks after a major flood. Despite the high initial water level and strong fluvial current, the bore front exhibited a sharp discontinuity in terms of free-surface elevation: the bore front was 0.45 m and 0.52 m high on the morning and afternoon respectively. The tidal bore was a flat undular bore with a Froude number close to unity: Fr1 = 1.02 and 1.19 in the morning and afternoon respectively. The field observations highlighted a number of unusual features on the morning of 7 June 2012 when detailed free-surface and velocity measurements were conducted simultaneously. These included (a) a slight rise in water elevation starting about 70 s prior to the front, (b) a flow reversal about 50 s after the bore front, (c) some large fluctuations in suspended sediment concentration (SSC) about 100 s after the bore front and (d) a transient water elevation lowering about 10 minutes after the bore front passage. The measurements of water temperature and salinity showed nearly identical results before and after the tidal bore: there was no evidence of saline or thermal front.