The application of computational fluid dynamics to natural river channels: Eddy resolving versus mean flow approaches

In the last decade, as computing power has increased, there has been an explosion in the use of eddy-resolving numerical methods in the engineering, earth and environmental sciences. For complex geomorphic flows, where accurate field investigations are difficult to perform and where experiments may be difficult to scale, these numerical approaches are beginning to give key insights into the nature of these flows. Eddy-resolving methods such as Large and Detached Eddy Simulation (LES/DES) may be contrasted with the time-averaged, three-dimensional simulations that only really began to be applied seriously in geomorphology fifteen years ago. While the potential of LES for geomorphology has been examined previously, DES is a relatively recent method that deserves further consideration. In this paper, we explain the method and then utilise examples from meander and confluence flows, as well as flow near the bed of a gravel bed river, to highlight the improvements to both the representation of the mean flow, and to the representation of time-varying processes, that result from the use of LES/DES. Some suggestions are provided for the future use of such techniques in geomorphology.