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Sedimentologic Engineering in Grand Canyon

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suspended-sediment sampling in Grand Canyon
Sediment sampling: CMG and WRD crew deploys a USGS P-61 suspended-sediment-sampling device.
For 10 days in September, six CMG researchers joined colleagues from WRD to study sediment transport in Grand Canyon. Dave Rubin, Hank Chezar, and Robin Dornfest worked on suspended-sediment and bed-sediment measurements with a WRD crew led by David Topping and Jim Bennett, while Walter Barnhardt, Rob Kayen, and Diane Minasian surveyed sand bars using ground-penetrating radar (see following article). This field work followed several tributary floods that transported large quantities of sediment into the Colorado River. The purpose of the trip was to measure the rate at which the new sediment was transported downriver.

Some previous studies had suggested that most sand introduced by tributaries remains in storage in the channel. If this were the case, resource managers could wait for several years to allow tributary sediment to accumulate in the main channel before scheduling an artificial flood to rejuvenate the river habitat by redistributing the sediment. Results of this CMG-WRD project indicate that most tributary sediment does not remain in storage for years, but instead is transported out of the canyon relatively quickly. To have the maximum amount of sediment available, it is necessary to schedule habitat-enhancing floods shortly after tributary-influx floods.

underwater video image of sand grains
Imaging the bottom: Sample image of sand grains on the bed of the Colorado River. Image was taken in highly turbid water and is approximately 2 cm from top to bottom. The ring of light spots near the outside of the image is a reflection from LED lights built into the videocamera housing.
One purpose of this trip was to test a new device for measuring sediment grain size using an underwater video system that Hank Chezar helped design (modified from a waterproof videocamera used for examining the inside of plumbing pipes).

The lens of the video camera rests on the sediment bed, protected by a thin layer of clear plastic. Because the camera views through less than a millimeter of water, the system can function in water with extremely high turbidity.

The camera includes a LED (light-emitting diode) light source, is very small, and can be deployed rapidly and easily. Images from the camera will be processed digitally to determine grain size.

Additional work was performed with a suspended-sediment-sampling device. The sampler is lowered to the bed, and a solenoid opens a calibrated sampling nozzle. While the sampler is raised to the surface, it samples water and sediment at each depth in proportion to the speed of the water flowing at that depth.

The samples obtained by this procedure contain depth-integrated sediment and water (in the same proportion in which they are transported downstream). By knowing the water discharge, the sediment-transport rate can be calculated.

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