THE MECHANICS OF TURBULENCE AND SEDIMENT TRANSPORT: PHYSICALLY-BASED NUMERICAL MODELING OF FLOW, SEDIMENT AND BED EVOLUTION IN THE THE COLORADO RIVER ALONG THE MARBLE CANYON

Eddy resolving three-dimensional simulations combined with field and laboratory observations allow elucidating key elements of fluid dynamics and sediment transport in fluvial environments. We will be presenting a parallelized, three-dimensional, turbulence-resolving model using the Detached- Eddy Simulation (DES) technique, tested at the scale of the river-reach in the Colorado River. DES is a hybrid large eddy simulation (LES) and Reynolds-Averaged Navier Stokes (RANS). RANS is applied to the near-bed grid cells, where grid resolution is not sufficient to fully resolve wall turbulence. LES is applied in the flow interior. We utilize the Spalart-Allmaras one equation turbulence closure with a rough wall extension. The flow solver based on DES is coupled with a sediment solver to predict the sediment concentration in the computational domain. This model is also coupled with a mobile bed module that allows studying morphodynamic changes. The Smith and Mclean suspended sediment boundary condition is used to calculate the upward and downward settling of sediment fluxes in the grid cells attached to the bed. Model results compare favorably with ADCP measurements of flow taken on the Colorado River in Grand Canyon during the High Flow Experiment (HFE) of 2008. The model accurately reproduces the size and position of the major recirculation currents, and the error in velocity magnitude was found to be less than 17% or 0.22 m/s absolute error. Large-scale turbulence structures with vorticity predominantly in the vertical direction are produced at the shear layer between the main channel and the separation zone. However, these structures rapidly become three-dimensional with no preferred orientation of vorticity. Lateral separation eddies are more efficient at storing and exporting sediment than previously modeled. The input of sediment to the eddy recirculation zone occurs in the interface of the eddy and main channel. Pulsation of the strength of the return current becomes a key factor to determine the rates of erosion and deposition in the main recirculation zone. This numerical model has the potential to be used for a range of applications in fluvial geomorphology over a wide range of scales.