HR: 11:40h
AN: H52A-06    [Abstracts]
TI: Erosion characteristics of fine-grained, beach-building sediment along the Colorado River in Grand Canyon
AU: * Akahori, R
EM: ryosuke.akahori@asu.edu
AF: Department of Geography, Arizona State University, P.O. Box 870104, Tempe, AZ 85287-0104 United States
AU: Schmeeckle, M W
EM: schmeeckle@asu.edu
AF: Department of Geography, Arizona State University, P.O. Box 870104, Tempe, AZ 85287-0104 United States
AU: Topping, D J
EM: dtopping@usgs.gov
AF: U.S. Geological Survey, Grand Canyon Monitoring and Research Center, 2255 N. Gemini Dr., Flagstaff, AZ 86001 United States
AB: In the Grand Canyon segment of the Colorado River, eddy sandbars, which form in lateral recirculation eddies, are important for endangered fish habitat, riparian habitat, protection of archeological sites, and recreation. By virtue of the 1963 closure of Glen Canyon Dam, sediment (i.e., sand, silt, and clay) supply to the Colorado River at the upstream boundary of Grand Canyon National Park has been reduced to about 5% of the pre-dam supply. This has caused substantial reduction in the size of eddy sandbars. The major supplier of sediment in the first 123 km downstream from Glen Canyon Dam is the Paria River, and its sediment consists mainly of clay, silt, and finer sand. During large floods on the Paria River, about 50% of the load is silt and clay, and the median size of the sand is about 0.11-0.12 mm. In order to restore the eroded eddy sandbars in the upper portion of Grand Canyon, an experimental controlled flood, i.e., Beach Habitat Building Flow (BHBF), has been proposed following enrichment of the sediment supply by flooding on the Paria River. Deposits produced by this BHBF should be fine-grained and cohesive. Understanding the sediment-transport behavior of this cohesive sediment is essential for the prediction and evaluation of the influence of the BHBF on rebuilding bars and increasing turbidity in the main channel. In this study, cohesive sediment samples of beach bars were collected from bars in the Colorado River in the Lake Mead delta. Laboratory experiments have tested the bulk density, erosion rate, and critical shear stress of these collected samples. The erosion rate of each sample was tested several times at different boundary shear stresses in a laboratory flume, allowing for estimation of the critical shear stress. Samples were placed in a 10-cm diameter cylinder below the flume. The sample was pushed out of the cylinder as it was eroded, such that the sample surface remained at the same height as the flume floor. Boundary shear stresses were estimated from near-bed, acoustic Doppler velocimetry measurements. Erosion rates were measured by comparing digital pictures of a laser line on the sediment surface at fixed time intervals. Results of experiments show that erosion rates of silt and sand rich samples rapidly increase when bottom shearing stress exceeds about 0.5(N/m$^{2}$), and those rates range from 0.0001(cm/sec) to 0.0008(cm/sec). On the other hand, erosion rates of clay rich samples and organic rich silt are much smaller at these stresses, and very small amounts of erosion is observable at even very low boundary shear stresses. Also, results show that the erosion rate decreases over time for a range of shear stresses.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting