Morphological description of river bifurcations in gravel-bed braided networks

摘要

In this work we provide a detailed description of the morphology of Y-shaped bifurcations in gravel-bed braided networks, focussing the attention on their planimetric configuration and on sediment transport processes within different branches. We discuss the results of a series of experimental runs performed in a wide laboratory flume, with a movable bed. The experiments have been designed in order to trace the evolution of a braided network subject to constant liquid and solid discharges. Sequences of time lapsed images have been acquired, through which the width and the orientation of single branches have been measured. The bifurcations have been classified depending on the occurrence of bed load transport in the branches. On the average only 20% of the bifurcations displayed three morphologically active channels (type AAA), while one of the distributaries was not active in 50% of the monitored bifurcations (type AAN). In the latter case, the asymmetrical behaviour of the node is also reflected by the unequal width of downstream branches and by their orientation with respect to the incoming flux. Moreover, the relative distribution of AAA and AAN bifurcations within the entire network was found to evolve toward an asymptotic state, depending upon the stream power of the total flow. A relationship of the active braiding intensity (defined as the number of active branches for each cross section) and the total braiding intensity can be inferred by the abov analysis.