2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 41
Presentation Time: 1:30 PM-5:30 PM


NZEWUNWAH, Chima1, LANGFORD, Richard1 and ROBERTS, Jesse2, (1)Department of Geological Sciences, Univ of Texas at El Paso, 500 University avenue, El Paso, TX 79902, (2)Sandia National Labs, Carlsbad Programs Group, 4100 National Parks Hwy, Carlsbad, NM 88220, chima@geo.utep.edu

In 1996, a major flood on the Gila River aggraded the Colorado River Channel by 1.3 m along a distance approximately 10 km downstream from the confluence. Over a million cubic meters of sediment were dredged to provide the capacity needed to prevent floods. This investigation is designed to determine the minimum discharge required to maintain the active channel and transport excess sediment. A study of the sediments along a 30 kilometer long stretch of the river around Yuma Arizona using the Adjustable Shear Stress Sediment Erosion and Transport (ASSET) Flume, used to measure insitu sediment erosion properties at various shear stresses at different flow conditions and depth. Twenty-six core samples along a stretch of the river were analyzed to determine flow parameters while particle sizes and size distribution where determined. The cores contain coarse sediments, clay, and organic materials with the organic materials more prevalent at the surface. Sediments from 4 different segments of the river; were observed to have different bulk and hydraulic properties. Grain size ranges from 65 to 542 microns and show an increase in erosion rate and an increase in critical shear stress with depth below the surface and also with increasing grain size. However, sediments in areas with abundant organic materials draping the sediment have a higher shear stress near the sediment surface. A well-developed floodplain coincides with the region with lowest critical shear stress and highest erosion. Zones requiring critical shear stresses of 0.5 to 1.1 Pascals exist primarily along the river bank and at mid-channel bars comprising interstratified gravels, sand, clay and organic materials. The erosion rate generally increased upstream from 0.005 to 0.01 cm/second below the dam to 0.105 cm/seconds near the confluence with the Gila River.