Numerical modelling of Colorado sandbar growth: An improved formulation of sediment transport and underwater slope slumping

In the Colorado River the Glen Canyon dam is located. The Glen Canyon dam is constructed in the Colorado river for the production of electricity. Due to the dam, nowadays only a fifth of the pre-dam sediment volume flows into the Colorado River through the Glen Canyon dam and the two major tributaries, the Paria River and the Little Colorado River. Due to the lack of sediment the sandbars present in the Colorado River started eroding. An attempt to solve this problem was by mimicking the pre-dam seasonal variation in discharge. High flood experiments (HFE) were conducted in 1996, 2004 and 2008. These HFE’s were partly successful; some sandbars grew, while other eroded during the HFE. A Delft3D model of two pools and sandbars was created for a better understanding of deposition of sediment on the sand bar. The resulting modelled topography showed 5 major differences with the measured bed levels. The banks in the model were higher than in reality. The main stream width became smaller as opposed to reality. The sandbar height was over-predicted and the sandbars were growing too much into the pool. The last difference is that the model did not show an erosion hole in the main stream as was visible in the measurements. To improve the shortcomings of the model two adjustments are implemented in the model. Turbulent velocities appear to be important because surface boils were seen during the HFE. In this thesis a sediment transport formula is created, which includes the turbulent velocity fluctuations on the sediment entrainment. The second adjustment is an avalanching and slumping formulation to overcome the shortcoming related to the narrowing of the main stream and to include the slope slumps as seen at the site. These near-bed velocities are described using a probability density function, which includes the turbulent velocity fluctuations. The near-bed concentrations are calculated using the probability of occurrence of an instantaneous velocity. This probability is multiplied with the concentration corresponding to that velocity. For every instantaneous velocity in the probability density function a concentration contribution is calculated. The sum of these contributions describes the concentration including the turbulent fluctuations. The avalanching is included by stating that no deposition can take place on slopes steeper than the angle of repose (32 degrees). Two methods are described: one where the sediment that cannot be deposited is re-suspended and one where it is transported as bed load. A third method simulates a large slope slump at a location where the bed slope was larger than the angle of repose. The slope is then changed to an equilibrium slope of 25 degrees. The sediment is transported as bed-load to the lower lying computational cells. With the inclusion of the two new formulations in the model that is applied to the Eminence pool, both formulations show to have an effect on the computed topography. When the near-bed turbulent velocities are included in the sediment transport the sandbar extends more into the pool, where the measurements show a shorter sandbar. The slope slumping method severely changed the computed topography; the Delft3D model became unstable. Both avalanching formulations showed a same influence on the final computed topography. The model results show reduced slope angles and an increase of the width of the stream. However, this is not to the extent that was measured.

Subject

Colorado
High Flood Experiment
Glen Canyon
Willy Taylor
Van Rijn
Sand bar

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master thesis

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