We present new observations on the dynamics of spring tides propagating in the Gironde estuary and the Garonne River up to 160 km from the estuary mouth. The amplification and distortion of the tide lead to the formation, in the Garonne River, of well-developed undular tidal bores, which propagate over more than 30 km. New results about the structure and the dynamics of the secondary wave field associated with tidal bores are presented.
A tidal bore is a positive surge propagating upstream that may form when a significant amplitude rising tide enters shallow, gently sloping and narrowing rivers. Tidal bores have been widely observed worldwide in estuaries and rivers in regions with large tidal amplitudes (e.g. Tricker, 1965, Lynch, 1982, Simpson et al., 2004, Wolanski et al., 2004). The study of this surge wave phenomenon is motivated by its significant impact on the river ecosystem behavior, and also because tidal bores present analogies with tsunami-induced river bores (Higman et al., 2004). Such tsunami bores can cause tremendous damage when propagating up rivers (e.g. Tsuji et al. (1991) and Tanaka et al. (2008)). Even if the physical mechanisms controlling the formation of tidal bores and tsunami bores are different, their propagation up rivers shows many similarities. The complex competition between energy dissipation, nonlinear and dispersive effects, which govern bore dynamics, makes the prediction of their evolution a challenging task for numerical models. Contrary to tsunamis, tidal bores are mainly related to a predictable phenomenon that is the astronomical tide. Thus, to analyze large amplitude bore propagation in the field, we conducted in 2010 two intensive field experiments devoted to tidal bore propagation in the Garonne River (see Bonneton et al., 2011b) that revealed that, in this river, tidal bores were ubiquitous as observed for a large majority of tides.
