Experimental observations on channel bifurcations evolving to an equilibrium state

摘要

Bifurcation is the process that generates and maintains a braided pattern and determines the distribution of flow and sediment transport along the downstream branches. According to the results of recent theoretical investigations, under bedload dominated conditions bifurcations are likely to display an unbalanced configuration, which results in an uneven partition of flow discharge in downstream channels (Bolla Pittaluga et al. 2003). In this work we present the results of two series of experimental runs which have been planned with the aim of investigating how the hydraulic parameters and the morphological response of the upstream channel affect the geometry, the water partition and the stability of the node. We have devoted specific attention to examine the role of free bars developing in the incoming channel on the stability of the node. Moreover, we have quantified the characteristics time scales of the evolutionary process of a bifurcation. We have found that two different effects control water and sediment distribution in the case of a self-formed Y-shaped bifurcation. Initially the migration of free bars induces relatively rapid fluctuations of the discharge partition in the downstream branches. Once a strongly unbalanced configuration establishes at the node, the upstream morphodynamic influence induced by the bifurcation slows down the speed of bars and modifies their structure. As a result, further evolution of the bifurcation occurs on a longer timescale.