Numerical and Physical Experiments on the Effect of Variations in Channel Width on Gravel-Bed River Morphodynamics

River channels are often characterized by downstream variations in channel width. The effect of width variations on bed topography and sorting patterns, however, remains poorly understood, especially under conditions of changing sediment supply. In this study we use two-dimensional numerical and physical modeling to explore how channel width variations and changes in sediment supply influence channel morphology. Using Delft3D we systematically explore how the amplitude and wavelength of sinusoidal width variations affect the shape and location of bars, and sorting patterns between wide and narrow sections in a coarse-grained channel. We perform simulations with constant sediment supply, no sediment supply, and a sediment pulse with no background sediment feed. The numerical experiments are used to guide ongoing laboratory flume experiments. Preliminary results indicate that width variations force riffle-pool topography with riffles co-located with greater channel widths and pools located at narrow sections. The amplitude of the width variations is the dominant factor controlling riffle-pool relief and also influence whether central or side bars develop in riffle sections. We hypothesize that greater amplitude of width variations increases pulse dispersion. These ongoing numerical and physical simulations can potentially be used to guide stream restoration and river management objectives under conditions of varying sediment regimes.