CHARACTERIZING HYDRAULIC PROPERTIES FOR SEDIMENT EROSION AND TRANSPORT; THE LOWER COLORADO RIVER

Diversion of flow along the Lower Colorado River as a result of damming and increased water demand has resulted in a change of its channel morphology and reduced the rivers ability to redistribute sediments deposited during floods. 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. Two million cubic feet of sediment were subsequently dredged from the channel to provide the capacity needed to prevent floods. This study details a survey conducted to determine the minimum discharge rate needed to maintain the active channel and transport excess sediment to prevent a reoccurrence.

The survey involved a detailed 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 insitu to determine flow parameters while particle sizes and size distribution where determined in the laboratory using the Malvern Mastersizer.

The cores contain coarse sediments, clay, and organic materials with the organic materials more prevalent at the surface of the stream sediment. The sediments analyzed from various locations within the study area were observed to have varying flow parameters and bulk properties. The plots obtained from the data show a general trend of a decrease in erosion rate and an increase in shear stress with increase in depth. However, sediments in areas with abundant organic materials draping the sediment tend to have a higher shear stress near the sediment surface.

Samples with interstratified gravels, sand, clay and organic materials which reduce sediment erodibility were found to exist primarily along the river bank and at mid-channel bars on the river channel. Also, the critical shear stress required to erode and transport sediments generally increased upstream from the dam to near where the river passes through a granite bedrock constriction at the city of Yuma then gradually diminished in value.