Flow Sediment Transport and Morphological Evolution of Lateral Separation Eddies in the Grand Canyon

Project: Research project

Description

Glen Canyon Dam operations under the current Record of Decision (ROD) restrict the rate of discharge increases and decreases, and one of the reasons for these restrictions is to limit sandbar erosion. While failure of steep beaches during a rapid downramp is likely, the rate of sediment mass loss for moderately-sloped beaches under differing dam operation scenarios remains largely unknown. This question is critical to minimizing the mass loss during daily fluctuating flows and following high flows. Previous research has resulted in the creation of a computer model to predict the stability of beach faces under differing dam operation scenarios. Validation of the stability model and incorporation of the stability model into a morphological eddy sandbar model is critical for predictive application to Grand Canyon sand bars.

Description

Important sandbar resources in the Colorado River in Marble and Grand Canyons occur primarily in lateral separation eddies downstream of tributary debris fans. 30 of these lateral separation sandbar sites have been monitored for over two decades. The sandbar volume change of individual sandbars to a given water and sediment discharge event shows some consistency. However, the sandbar response across different sandbar sites to the same water and sediment discharge events is substantially different. Why are there significantly different erosion/depostion responses among different sandbars to the same flow and sediment discharges? Geometric differences between the sandbar sites must drive this variable response between sites, but previous modeling efforts at different sites have not shown significant across site response variance. In general, previous modeling efforts have not accurately predicted erosion and often over-predict deposition. The typical bathymetric and flow features of a lateral separation eddy are depicted in Figure 1a. Constricted flow in the rapid produces a jet of high velocity fluid entering the zone of channel expansion. Interaction between the jet and return current produces low-frequency, episodic fluctuations of velocity in the separation eddy. The length parameters depicted in Figure 1b) , as well as the pool depth are hypothesized to control the strength and fluctuation of the return current and the flow near the point of reattachment. These areas are critical for the rate of sediment input and output of the separation eddy. Large eddy simulation (LES) is a computationally intensive modeling technique in which turbulence larger than the scale of the grid is directly calculated. Current parallel algorithms employed on supercomputers are now able to perform simulations of turbulence and suspended sediment transport that directly calculate fluxes of momentum and mass into and out of the lateral separation eddies, rather than relying on eddy diffusivity parameterizations th

Description

There have been three high-flow experiments (HFEs) in Grand and Marble Canyons. A chief aim of these experiments was to explore the feasibility of using such flows to restore sandbars within recirculation eddies downstream of channel constrictions in the Colorado River. The general timeaveraged, recirculation flow structure of these recirculation eddies has been modeled using timeaveraged two- and three-dimensional numerical models. However, these models have failed to predict many of the detailed flow features found in field data and descriptions. Further, previous eddy sediment transport models have generally over-predicted the rate of deposition when there are substantial suspended sediment concentrations supplied to the eddy and under-prediction of erosion when mainstem sediment concentrations are low. As a result, prediction of the eddy sandbar building phase during an HFE cannot be predicted very well and the erosion of sandbars during low periods of low sediment concentration is very poor.
StatusFinished
Effective start/end date8/2/112/25/16

Funding

  • DOI: US Geological Survey (USGS): $97,291.00

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sandbar
canyon
sediment transport
eddy
erosion
sediment
beach
dam
marble
suspended sediment
turbulence
modeling
experiment
large eddy simulation
flow structure
volume change
prediction
river
diffusivity
tributary