Process-form interactions in unstable sand-bed river channels: A numerical modeling approach

A deterministic numerical model of bed deformation and channel widening, which accounts for specific mechanisms of bank erosion and collapse, is used to analyze morphological and flow-energy parameters in adjusting sand-bed channels, for a range of simulated fluvial environments. The model is based on a set of conservation and process equations solved in conjunction with a set of specific initial and boundary conditions. Simulated channels with a range of assumed boundary-material characteristics were subjected to identical disturbances imposed by reducing the supply of sediment from upstream. Asymptotic reductions of the rate of energy dissipation (energy slope) and boundary shear stress were found to be unifying characteristics of channel adjustment in all simulations. Morphologic responses to an identical disturbance (reduction of sediment supply from upstream), with identical bed material (d50 = 1 mm) and channel gradient (Sb = 0.001), were diverse. The relative magnitude of simulated widening versus simulated bed-level change was greatest for sand-bank channels and least for clay-bank channels. Stable-channel dimensions were attained most rapidly for silt-bank channels, where simulated adjustments of channel width were similar to simulated vertical adjustments, because all components of total-mechanical energy (flow depth or pressure head; bed elevation and channel gradient or datum head; and flow velocity or velocity head) decrease simultaneously. Each simulated channel subjected to the baseline disturbance had an initial width-to-depth ratio (F) of 13.5. Asymptotic values of energy dissipation rate, adjusted F-values, and time taken to reach equilibrium after the disturbance are shown to vary as a function of simulated environmental boundary conditions. The importance of channel widening in controlling and reducing the flow depth of a given discharge and average boundary shear stress is highlighted for the silt- and sand-bank cases. Channel widening causes a shift from degradation to aggradation during adjustment, because of delivery of sand-sized sediments from failed bank material to the simulated channel.