A theoretical analysis of river bars stability under changing channel width

Abstract

In this paper we propose a new theoretical model to investigate the influence of temporal changes in channel width on river bar stability. This is achieved by performing a nonlinear stability analysis, which includes temporal width variations as a small-amplitude perturbation of the basic flow. In order to quantify width variability, channel width is related with the instantaneous discharge using existing empirical formulae proposed for channels with cohesionless banks. Therefore, width can vary (increase and/or decrease) either because it adapts to the temporally varying discharge or, if discharge is constant, through a relaxation relation describing widening of an initially overnarrow channel towards the equilibrium width. Unsteadiness related with changes in channel width is found to directly affect the instantaneous bar growth rate, depending on the conditions under which the widening process occurs. The governing mathematical system is solved by means of a two-parameters (ε, δ) perturbation expansion, where ε is related to bar amplitude and δ to the temporal width variability. In general width unsteadiness is predicted to play a destabilizing role on free bar stability, namely during the peak stage of a flood event in a laterally unconfined channel and invariably for overnarrow channels fed with steady discharge. In this latter case, width unsteadiness tends to shorten the most unstable bar wavelength compared to the case with constant width, in qualitative agreement with existing experimental observations.