Further perspectives on the evolution of bed material waves in alluvial rivers

In one-dimensional mathematical models of fluvial flow, sediment transport and morphological evolution, the governing equations based on mass and momentum conservation laws constitute a hyperbolic system. Succinctly, the hyperbolic nature excludes dispersion or diffusion operators, which is well known in the context of differential equations. There is no doubt that the so-called 'dispersion' argument for bed material wave evolution is questionable, as we have explicitly asserted. Surprisingly, in a recent communication, the authors of the 'dispersion' argument suggest that dispersion is not precluded in hyperbolic systems. We provide herein further perspectives to help explain that the dispersion argument is neither appropriate nor necessary for interpreting bed material wave evolution. Also the continuity equations involved are addressed to prompt wider understanding of their significance. In particular, the continuity equation of the water-sediment mixture proposed by the authors of the 'dispersion' argument is proved to be incorrect, and inevitably their reasoning based on it is problematic. Copyright (C) 2005 John Wiley Sons, Ltd.