First order control of alluvial cover morphology by channel sinuosity and sediment supply rate

Abstract:

The formation and characteristics of the alluvial cover are an important aspect of the morphology and dynamics of rivers with non-alluvial boundaries, such as natural bedrock or engineered channels. Experimental studies of alluvial cover in fixed-bed channels have largely focused on straight channels. Here, we present results from a series of flume experiments in a sinuous fixed-bed channel designed to determine the role of sediment supply rate and planform geometry on the alluvial cover formation as a major element of channel morphology and morphological processes in this channel type. Experiments were conducted within a regularly-sinuous channel with dimensions, bend geometry, discharge, and supply material grain size distribution scaled from a small urban channel with a gravel cover and cohesive glacial till bed. A range of sediment supply rates were fed to the channel while the extent and topography of the alluvial cover were mapped using photogrammetry. In all experimental runs, the areal extent, mass, and volume of the alluvial cover initially increased rapidly before reaching a stable value that increased with sediment supply rate following a logarithmic relation. Sediment output rates revealed that the stable alluvial cover corresponded to sediment transport equilibrium. A complete areal cover was never achieved, even when the supply rate exceeded the estimated transport capacity of the channel. While supply rate controlled the total extent, mass, and volume of the alluvial cover, the channel sinuosity ‘fixed’ the location of the cover and influenced cover morphology. Cover developed as a periodic point bar-riffle morphology, similar to alluvial meander bed morphology, but with persistent areas of bare bed along the outside of bends downstream of the bend apex. The alluvial cover texture had distinct spatial sorting patterns controlled by planform with finer bar tops and coarser connecting riffle features. The experiments revealed the interacting but distinct roles of sediment supply rate and channel planform geometry on the alluvial cover dynamics of sinuous fixed-bed channels. Results can help inform and guide sediment supply management strategies to mitigate the consequences of sediment starvation seen below dams and in urban rivers and improve geomorphic and ecological function.