The gravel-sand transition: flume study of channel response to reduced slope

Abstract

This paper reports flume experiments with a bimodal gravel/sand bed and sediment feed, designed to investigate river channel response to declining slope. In successive runs an initial near-equilibrium channel was subjected to progressively bigger reductions in flume slope. This simulated a series of sites along a concave profile approaching a local base level and undergoing a transition from gravel to sand bed; it also models the effects of a rise in base level on a fixed site. Slope reduction led to immediate increase in depth and reduction in velocity and shear stress, hence progressive aggradation. Deposition was size-selective and led to the following sequence of bed texture change with increasing severity of slope reduction: (1) sand in lee of gravel clusters, (2) elongated sandy patches and thin streaks, (3) wider sand ribbons covering gravel completely and developing ripples. There was feedback from the gradual bed surface changes to hydraulic conditions: near-bed velocity increased, while depth, shear stress, and roughness height decreased. These feedbacks were more pronounced with a bigger reduction in slope. These results support the view that the gravel-sand transition along river channels with a local base level control is assisted by positive feedback between changes in bed surface texture and near-bed flow.