Channel dynamics and controls on planform changes along the Colorado and Pecos rivers.

摘要

Channel dynamics and controls on planform changes along the Colorado and Pecos Rivers Hydraulic forces exerted by flowing water on channel materials combined with the interaction of other channel properties work together to determine channel form dynamics. In the arid southwest United States this hydraulic effect creates a peculiar problem in terms of streams ability to transport sediment due to varying stream flow conditions. This dissertation integrates data obtained from two separate studies in order to estimate stable channel geometries and likely changes that may occur.; This research integrates a direct field measurement of the shear stress needed to erode and transport sediment within the channel and other observed and calculated parameters to estimate planform changes along a river. This method is unique to this study as previous studies do not incorporate direct field hydraulic measurements in sediment transport analysis.; The first study focuses on determining the effects of changes in sediment transport due to varying flow conditions in the lower Colorado River around Yuma, Arizona, while the second study focuses on the effect of removal of Tamarisk vegetation and its affect on channel geometry along the Pecos River.; The main hydraulic factor controlling mobility of sediment in the river channel is the shear stress. This was measured in the field using the Adjustable Shear Stress Erosion and Transport (ASSET) flume for the Colorado River while these were obtained from calculations based on grain size for the Pecos River. The calculated hydraulic parameters were combined with sediment mobility equations and interaction of bank properties to estimate channel dynamics for the two river systems.; Results from the lower Colorado River show a great spatial variability in hydraulic factors with channel materials, however, channel planform change is mainly in the form channel aggradation. The Pecos River channel morphology is mostly dependent on vegetation distribution. Channel planform changes will occur mainly through lateral migration in the form of expansion along reaches that lack vegetation while the reaches vegetated with abundant Tamarisk and other types of vegetation will undergo mostly channel deepening and contraction.; Results also show that the critical shear stress for sediment entrainment and transport in stream channels do not depend on grain size or clay content of the sediment but also depends on the presence of organic materials which enhance cohesion.