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 &
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.