Sand bars in tidal channels part 2. Tidal meanders
Careful analysis of new field observations on the geometry of meandering tidal channels reveals that the values of meander wavelengths as well as curvatures, conveniently scaled by local channel width, fall within a fairly restricted range, suggesting the existence of some mechanistic process controlling meander formation. A three-dimensional model is then proposed to predict flow and bed topography in weakly meandering tidal channels. The analysis is developed for meander wavelengths much smaller than the tidal wavelength and assuming that sediment is transported both as bedload and as suspended load. Both conditions are typically met in nature. Due to the symmetry of the meander pattern investigated, the theory indicates that oscillations associated with the basic flow give rise to symmetric oscillations of the point bar-pool pattern around the locations of maximum curvature. However, no net migration in a tidal cycle is present, at least for periodic tides with zero mean. Suspended load leads both to an enhanced bottom deformation and to a downstream shifting of the position of the point bar. The model then provides the basis of a planimetric instability theory of the type developed for river meanders (Blondeaux and Seminara 1985). Though the available data do not yet allow a detailed quantitative comparison, it is shown that the wavelengths selected by the 'bend mechanism' are somewhat larger than those typically encountered in nature. The geomorphology of the process of meander formation in tidal environments is then discussed and, upon comparison with observational evidence, points out the need for various developments of the present model.