Flume experiments on flow and sediment supply controls on gravel bedform dynamics

Abstract

We present results from a flume experiment in which we investigate how the formation and dynamics of gravel bedforms are affected by changes in discharge and sediment supply. We conducted experiments in a straight rectangular sediment feed flume using a unimodal mixture of fine gravel and coarse sand. The experiment consisted of three phases: (1) equilibrium sediment supply and steady flow, (2) equilibrium sediment supply and repeated hydrographs, and (3) doubled sediment supply and repeated hydrographs. During the experiments, low-amplitude, migrating bedforms resembling alternate bars developed and their dynamics were characterized and tracked via collection of repeated structure-from-motion topographic data sets. Channel-scale morphology was essentially the same for steady and unsteady flow at the same sediment supply, but the bedform celerity was much lower with unsteady flow than it was under constant discharge. Bedform amplitudes increased on the rising limb of the hydrograph and declined on the falling limb, but their wavelength was largely insensitive to flow variation. When the sediment supply was increased, the dominant channel response was an increase in slope. The bedform celerity increased, but not to the same rate as under steady flow. Although our experiment developed alternate bars, the width-to-depth ratio of 8 indicates that present theory would not predict the occurrence of bars for our conditions. Our observations show that alternate bars may develop under low width-to-depth conditions and suggest that further theoretical development is needed to identify the mechanism responsible for their formation.