Meeting:Abstract 2013 CSDMS meeting-093
Numerical modeling of turbulence and sediment transport in lateral recirculation zones along the Colorado River in Grand Canyon
[[Image:|300px|right|link=File:]]A number of two- and three-dimensional models are currently available to calculate sediment transport and channel change in rivers. These three-dimensional models rely on time-averaging and parameterization of the turbulence. Available depth-averaged, two-dimensional models also rely on simple boundary stress closures. In relatively simple channels these models have predictive capability, but they often perform poorly when there is large-scale flow separation or when secondary circulation is strong. Sharp meanders, channel constrictions, many engineering structures, vegetation, and certain types of bedforms all cause flow separation, secondary circulation, and free shear layers. Turbulence-resolving flow and sediment transport models may do better at predicting channel change in complex channels, but at a substantially larger computational cost. With parallelization, turbulence-resolving models can now be developed and applied to refractory fluvial morphodynamic problems.
Detached-Eddy Simulation (DES) is a hybrid large eddy simulation (LES) and Reynolds-averaged Navier Stokes (RANS) method. RANS is applied to the near-bed grid cells, where grid resolution is not sufficient to fully resolve wall turbulence. LES is applied further from the bed and banks. A one equation turbulence closure model with a wall-distance dependence, such as that of Spalart and Allmaras (SA), is ideally suited for the DES approach. The rough wall extension of the SA model is utilized herein. Our river DES numerical modeling system was developed in OpenFOAM. The model resolves large-scale turbulence using DES and simultaneously integrates the suspended sediment advection-diffusion equation, wherein advection is provided by the DES velocity field minus particle settling, and diffusion is provided by the sub-grid or RANS eddy viscosity. As such, turbulent suspension throughout most of the flow depth results from resolved turbulent motions. A two-dimensional, depth-averaged flow model, also written in OpenFOAM, determines the local water surface elevation. A separate program was written to automatically construct the block-hexagonal, computational grid between the calculated water surface and a triangulated surface of a digital elevation model of the given river reach. Domain decomposition of the grid is employed to break up the integration between multiple processors, and Open MPI provides communication between the processors. The model has shown very good scalability up to at least 128 processors.
Results of the modeling system will be shown of flow and suspended sediment model in lateral separation eddies in the Colorado River in Grand Canyon. The eddy recirculation zones exist downstream of channel constrictions from tributary debris fans. The modeling system is currently being developed and validated to be used in designing discharges from Glen Canyon Dam for the preservation of sandbar beaches, which are critical habitat for endangered fish.
Keywords: fluvial geomorphology, sediment transport, lateral separation zones. Movie at: https://csdms.colorado.edu/mediawiki/images/GrandCanyonDES.avi