Modeling sediment transport in eddy recirculation zones of the Colorado River in Grand Canyon.

摘要

In the Colorado River in Grand Canyon, sandbar beaches are formed beneath recirculation eddies during high floods. Numerous benefits are derived from these beaches including habit for endangered native fish and camping sites for visitors of the national park. The significant reduction in sediment supplied to the Colorado River in Grand Canyon, due to closure of Glen Canyon Dam in 1963, has resulted in significant erosion of these sandbar beaches over time. Currently, sand resources are only supplied from tributary floods. Thus the current strategy of rebuilding sandbars relies on beach formation by simulated floods following tributary inputs of sediment, and by reduction of beach erosion at other times. In November 2004, an experimental flood was released for rebuilding sandbars. This study aims to improve the understanding of sandbar rebuilding mechanisms using numerical models to provide precisely resolved information of flow and sediment transport in a recirculation area, based on the observed data of this flood. A Large Eddy Simulation (LES) model for recirculating flow, which directly integrates the spatially-filtered Navier Stokes equations, is developed and compared to laboratory experiments. Model results are visualized by the lambda2 method that identifies vortex structures. Visualizations of model output show that lateral vortex structures promote inward flow near the reattachment point. Laboratory experiments on the erosion properties of fine-grained Grand Canyon beach deposits show that sediment deposited during the experimental flood is relatively non-cohesive. The LES model is applied to calculate flow and sediment transport during the experimental flood. A particle-based suspended sediment model, using the velocity output from the LES flow model and random fluctuations to account for subgrid scale turbulence, is proposed. Results show that streamwise vorticity in the channel constriction episodically sheds intense vortex structures with vertical vorticity in the main channel. These separated vortex structures then produce intense flow and sediment transport into the reattachment zone. The dynamics of these large episodic vortex structures are critical to understand the process of beach formation in Grand Canyon.