Theory and experimental observations on channel bifurcations.

The present work is motivated by the observation (Mosley, 1976) that streamline divergence is one of the main causes of channel bifurcation in braided rivers. We then built a laboratory model of a braided river, and observed that the most common mechanism leading to the formation of channel bifurcations consists of the creation of a central bar in a divergent channel. Hence we performed experiments in a channel whose width increased linearly downstream and observed that erodible diverging streams are subject to two distinct modes of instability that seemed to depend on the Shields stress value. A theoretical investigation of the stability of flow and bed topography in an indefinitely divergent cohesionless channel connected at some initial cross section with a straight channel located upstream was then undertaken. Results show that various lateral ‘modes’ are simultaneously unstable but in the linear regime, i.e. when perturbations have small amplitudes, instability selects the first lateral mode as the fastest growing in time. A non linear numerical analysis of flow and bed topography is in progress in order to determine the final, finite amplitude configuration of the bed pattern and the actual development of channel bifurcation.