The application of computational fluid dynamics to natural river channels:three-dimensional versus two-dimensional approaches

This paper addresses the recent trend in fluvial geomorphology towards usin g computational fluid dynamics to explore the adjustment between Row proces ses, sediment transport and river channel morphology. It aims to evaluate t he extent to which three-dimensional (3D) models improve predictive ability and prediction utility compared to two-dimensional (2D) applications. This is achieved through comparing the predictions of both 3D and 2D models wit h high-quality field data. Identical boundary conditions, obtained from a c onfluence within a gravel-bed river system with high relative roughness, ar e defined for each model. Evaluation of the 3D model suggests that there is a fundamental limitation upon model predictive ability due to problems of specifying topographic complexity. However, comparison with the 2D model sh ows that the 3D model has a higher predictive ability, particularly if the 2D model is not corrected for the effects on flow structure of secondary ci rculation. Further, the 3D model provides more reliable estimates of bed sh ear stress and other more useful information, such as the three-dimensional flow field important for mixing processes. This suggests that there is sig nificant merit in the move towards 3D models, but that research is required to incorporate methods developed in other fields for dealing with boundary condition uncertainties. (C) 1999 Elsevier Science B.V. All rights reserve d.