Trajectories of river channel adjustments from nonequilibrium to equilibrium: Rate law evaluation using sand‐bed flume data

Abstract River systems are often in nonequilibrium because of changes in water and sediment inputs and channel‐boundary conditions, but it remains unclear how the changes affect the trajectories of river channel‐form adjustment from nonequilibrium to equilibrium. Using the data collected from a series of experiments conducted in sand‐bed flumes and the generalized model of the rate law, this study presents a detailed investigation of the trajectories of channel‐bed elevation adjustments from aggradation to equilibrium. For 15 runs of the experiments conducted respectively in the 2‐ and 4‐m‐wide sand‐bed flumes at two flow discharges (30 and 60 L·s −1 ) and four sediment concentrations (5, 10, 20 and 30 kg·m −3 ), the temporal variations of average channel‐bed elevations at both inlet and outlet cross‐sections in the 2‐m‐wide flume all take exponential decay trajectories and can reach equilibrium within a short period. In contrast, the temporal variations in the 4‐m‐wide flume take exponentially or linearly increasing trajectories in many runs, without a tendency of reaching equilibrium. Crucially, we found that under the combined influence of flow discharge, sediment concentration and channel width, the adjustment trajectories tend to shift from gradual increase to decay as the unit sediment transport rate increases. These trajectories arise from complex spatiotemporal interactions among flow conditions, sediment transport and channel‐bed morphology. Further investigation is needed to identify the processes and conditions under which river channel‐form adjustments evolve along nondecaying trajectories.