An experimental study of flow, bedload transport and bed topography under conditions of erosion and deposition and comparison with theoretical models

Abstract

ABSTRACTThe nature of flow, sediment transport and bed texture and topography was studied in a laboratory flume using a mixed size‐density sediment under equilibrium and non‐equilibrium (aggradational, degradational) conditions and compared with theoretical models. During each experiment, water depth, bed and water surface elevation, flow velocity, bed shear stress, bedload transport and bed state were continuously monitored. Equilibrium, uniform flow was established with a discharge of about 0.05 m3 s−1, a flow depth of about 0.01 m, a flow velocity of about 0.81–0.88 m s−1, a spatially averaged bed shear stress of about 1.7–2.2 Pa and a sediment transport rate of about 0.005–0.013 kg m−1 s−1 (i.e. close to the threshold of sediment transport). Such equilibrium flow conditions were established prior to and at the end of each aggradation or degradation experiment. Pebble clusters, bedload sheets and low‐lying bars were ubiquitous in the experiments. Heavy minerals were relatively immobile and occurred locally in high concentrations on the bed surface as lag deposits.Aggradation was induced by (1) increasing the downstream flow depth (flume tilting) and (2) sediment overloading. Tilt‐induced aggradation resulted in rapid deposition in the downstream half of the flume of a cross‐stratified deposit with downstream dipping pebbles (pseudo‐imbricated). and caused a slight decrease in the equilibrium mean water surface slope and total bedload transport rate. These differences between pre‐ and post‐aggradation equilibrium flow conditions are due to a decrease in the local grain roughness of the bed. Sediment overloading produced a downstream fining and thinning wedge of sediment with upstream dipping pebbles (imbricated), whereas the equilibrium flow and sediment transport conditions remained relatively unchanged. Degradation was induced by (1) decreasing the downstream flow depth (flume tilting) and (2) cutting off the sediment feed. Tilt‐induced degradation produced rapid downstream erosion and upstream deposition due to flow convergence with little change to the equilibrium flow and sediment transport conditions. The cessation of sediment feed produced degradation and armour development, a reduction in the mean water surface slope and flow velocity, an increase in flow depth, and an exponential decrease in bedload transport rate as erosion proceeded. A bedload transport model predicted total and fractional transport rates extremely well when the coarse‐grained (or bedform trough) areas of the bed are used to define the sediment available to be transported. A sediment routing model, MIDAS, also reproduced the equilibrium and non‐equilibrium flow conditions, total and fractional bedload transport rates and changes in bed topography and texture very well.