Flow, sediment transport, and bed topography in straight and curved gravel-bed channels

Abstract

In recent years, many river restoration projects have aimed to restore natural channel stability and dynamism by re-establishing channel meanders lost to historical channelization. An understanding of meandering channel behavior is crucial to successful restoration of these rivers. Meandering and straight channels differ greatly in terms of sediment transport, velocity, and flow patterns under equilibrium conditions. The primary objective of this study was to investigate the mechanisms responsible for sorting patterns in mixed-grain straight and curved channels using flume experiments. After an absence of sorting was observed in the flume experiments, the study objective was modified to: 1) investigate the formation, behavior, and dynamics of free and forced bars within a straight channel with and without an upstream barrier and 2) explore the mechanism that accommodates for spatial boundary shear stress variations in curved gravel bed channels. The flume experiments involved detailed measurements of bed topography, velocity, and sediment transport in both a curved channel and straight channel with and without an upstream obstruction. It was expected that the gravel bed meandering river would compensate for spatial variability in boundary shear stress through surface grain size adjustment (sorting), as opposed to sediment transport convergence. Instead, the data reveal sediment transport divergence as the primary mechanism for balancing shear stress variability. The lack of sorting may likely be attributed to low excess shear stress and steady, rather than unsteady flow conditions. Regarding free and forced bar behavior, no stability was achieved in the straight channel without an obstruction. This can be attributed to a range of factors related to upstream boundary conditions, shear stress, and lack of forcing topography. It is suggested that future studies utilize both higher excess shear stress and unsteady flow conditions in investigating shear stress variability in curved gravel-bed channels.