Morphodynamic response of a variable‐width channel to changes in sediment supply

Abstract

AbstractRiver channels commonly exhibit downstream variations in channel width, which can lead to the development of alternating shallow and deep areas known as riffle‐pool sequences. The response of these channels to variations in sediment supply remains largely unexplored. Here we investigate the morphodynamic response of a variable‐width channel to changes in sediment supply through laboratory experiments conducted in a straight flume in which we imposed sinusoidal variations in width. We first developed equilibrium conditions under a constant sediment supply and then eliminated the sediment feed to create a degraded, armored bed. This sediment‐starved bed was subjected to two types of sediment supply increases: a return to the initial constant supply, and the introduction of a well‐sorted sediment pulse (analogous to gravel augmentation). Riffles and pools formed in wide and narrow areas, respectively, and the location of and relief between riffles and pools remained the same throughout all experimental runs, regardless of the sediment supply. The primary channel response to changes in supply was adjustment of the overall slope. The sediment pulse evolved primarily through dispersion rather than translation, which contrasts with prior gravel augmentation experiments conducted in constant‐width channels and suggests that width variation and resulting riffle‐pool topography enhances pulse dispersion. Our results indicate that width variation is a primary control on the location and relief of riffles and pools in straight channels, and sediment supply changes are unlikely to affect riffle‐pool morphology when bank geometry is fixed and water discharge is steady.