The evolution of sandbars along the Colorado River downstream of the Glen Canyon Dam

Abstract

The Colorado River in the Grand Canyon area has a lot of sandbars but they are eroding. Since the building of the Glen Canyon Dam and the Hoover Dam in the 1960s, sediment is trapped in the upstream lake. The inflow of water is clear, cold and nearly sediment free. The only noticeable remaining inflow of sediment in the Colorado River is by two tributaries which merge with the Colorado River. This sediment is mostly transported as bed load in downstream direction during normal flow conditions, and does not contribute to sandbar building. The erosion of sandbars has been researched by the United States Geological Survey for decades. Large field experiments are part of this research. The fourth experiment was the high-flow experiment of 2008. During the high-flow experiment of 2008 a dam release of 42000 cfs (1134 m3/s) was bypassed through the dam for 60 hours. The high velocities would bring sediment from the riverbed in suspension. The sediment is then deposited in low-velocity areas. The main question in this research is whether it is possible with a specified dam release, to create sandbars and how they can remain stable on the long term. To answer this question the software program Delft3D is used for the simulation of the sandbar creation. For the long term stability of the bars an image transformation was executed, and for the short term stability the geotechnical program PLAXIS was used. For the situation in the Willie Taylor and Eminence pools a dam release can be determined so sandbars will be created and remain stable for the long term. Sandbars can be created during a high discharge under sediment enriched conditions. The erosion of the sandbars can be set to a minimum when the drawdown of the water level takes 5 days, so the sandbars are almost fully drained. However, implications of erosion at other parts of the sandbar (other than the banks) should be considered using the Delft3D model. The daily fluctuations should be set to a minimum especially in the first period after the flood.