Effects of Vegetation-Sediment Interactions on the Morphological Evolution of Coarse-Bedded Rivers: Results from Flume Experiments

Abstract

Laboratory experiments over the past two decades have demonstrated the crucial role of vegetation in determining the morphological characteristics of river channels. Through a series of flume experiments, this research builds on previous studies by examining the respective role of both vegetation and sediment load composition on resulting channel planform style and dynamics as vegetation density increases. I used a 1.5 m by 6 m flume filled with well-sorted quartz sand (D50 = 0.5 mm) to simulate a gravel-bedded river. Each experiment simulated a series of 4-hour floods, after each of which the flume was seeded with alfalfa (Medicago sativa) so that vegetation density increased with each flood. The only variable between the two experiments was the composition of the sediment feed. We fed only bed load in Experiment 1, and fed both bed load and suspended load in Experiment 2. Results confirm the findings of earlier experiments in that vegetation progressively stabilized the surface of the flume, limiting the number of channel threads until only one remained. The resulting channel was deeper, faster, and narrower than the unvegetated channel. Results suggest that vegetation-sediment interactions can produce widely differing channel morphology depending on the composition of sediment load and the frequency of overbank flows. They also demonstrate that while bed load transport within a channel with strengthened banks is critical for the generation of a meandering pattern by building bars, fine sediment may play an equally important role in adjusting floodplain topography. The amount, distribution, and storage of sediment trapped in the riparian corridor were directly related to vegetation density and the presence of overbank flow conditions.