Reduced‐complexity modeling of free bar morphodynamics in alluvial channels

Abstract

Reduced‐complexity models of river behavior that neglect much of the physics governing fluvial processes have become increasingly popular in recent years. However, previous studies have demonstrated that channel morphology simulated by such models can be both unrealistic and highly sensitive to model grid resolution. A new reduced‐complexity model is presented here and applied to simulate the development and migration of free alternate bars in straight channels. This model incorporates a simple new treatment of lateral flow redistribution driven by topographic steering but neglects the role of momentum conservation, secondary circulation, and factors such as spatial variability in bed roughness. This approach is shown to replicate many of the characteristics of alternate bars observed in individual laboratory experiments conducted by Lanzoni (2000) and to produce results that are in agreement with more generic relationships evident in a larger experimental data set presented previously by Ikeda (1984). Moreover, model results are shown to be independent of grid resolution and are largely consistent with those obtained using more sophisticated models based on the depth‐averaged form of the Navier‐Stokes equations. There remains a need to evaluate and refine model process parameterizations for a wider range of conditions (including those in natural channels) and to examine model behavior in situations involving variable channel width and curvature. However, the results presented here provide the first evidence that such reduced‐complexity models may be able to simulate the vertical and streamwise scaling of free bars in alluvial channels and represent the morphodynamic feedbacks that control their temporal evolution.