NUMERICAL-SIMULATION OF WIDENING AND BED DEFORMATION OF STRAIGHT SAND-BED RIVERS .1. MODEL DEVELOPMENT

Many existing morphological models are limited by their inability to a ccount for changing channel width through time. In this and a companio n paper, the development and testing of a numerical model of river wid ening are reported. The new model is applicable to straight, sand-bed streams with cohesive bank materials, and nonuniform bathymetry and wi dth in the longitudinal direction. The flow field is obtained by solvi ng versions of the flow resistance, how momentum, and continuity equat ions, which account for the influence of gradually varied flow and lat eral shear in the near bank zones. Secondary and overbank flows are ex cluded. Predicted flows are used to account for streamwise and transve rse sediment transport fluxes. Numerical solution of the sediment cont inuity equation allows temporal variations in bed-material size, bed m orphology, and bank geometry to be simulated. Channel widening is simu lated by coupling bank stability with flow and sediment transport algo rithms. A probabilistic approach is used to estimate the longitudinal extent of mass failures within modeled reaches, and mixed layer theory is used to model the transport of the resulting bed and bank material mixture. Assessment of the accuracy of the new model, discussed in th e companion paper, indicates that the new model simulations of widths, depths, and deepening rates are within 15% of observed values, but wi dening rates are underpredicted by a factor of approximately 3.