PROCESS-FORM INTERACTIONS IN UNSTABLE SAND-BED RIVER CHANNELS - A NUMERICAL MODELING APPROACH

A deterministic numerical model of bed deformation and channel widenin g, which accounts for specific mechanisms of bank erosion and collapse , is used to analyze morphological and flow-energy parameters in adjus ting sand-bed channels, for a range of simulated fluvial environments. The model is based on a set of conservation and process equations sol ved in conjunction with a set of specific initial and boundary conditi ons. Simulated channels with a range of assumed boundary-material char acteristics were subjected to identical disturbances imposed by reduci ng the supply of sediment from upstream. Asymptotic reductions of the rate of energy dissipation (energy slope) and boundary shear stress we re found to be unifying characteristics of channel adjustment in all s imulations. Morphologic responses to an identical disturbance (reducti on of sediment supply from upstream), with identical bed material (d(5 0) = 1 mm) and channel gradient (S-b = 0.001), were diverse. The relat ive 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 cha nnels, where simulated adjustments of channel width were similar to si mulated vertical adjustments, because all components of total-mechanic al energy (flow depth or pressure head; bed elevation and channel grad ient or datum head; and flow velocity or velocity head) decrease simul taneously. Each simulated channel subjected to the baseline disturbanc e had an initial width-to-depth ratio (F) of 13.5. Asymptotic values o f energy dissipation rate, adjusted F-values, and time taken to reach equilibrium after the disturbance are shown to vary as a function of s imulated environmental boundary conditions. The importance of channel widening in controlling and reducing the flow depth of a given dischar ge and average boundary shear stress is highlighted for the silt-and s and-bank cases. Channel widening causes a shift from degradation to ag gradation during adjustment, because of delivery of sand-sized sedimen ts from failed bank material to the simulated channel. (C) 1997 Elsevi er Science B.V.