HR: 0800h
AN: H11D-0784    [Abstracts]
TI: The Influence of Large-Scale, Meandering, Shear-Induced Instabilities on Jet Hydrodynamics and Sediment Transport
AU: * Rowland, J C
EM: jcrowland@stanford.edu
AF: Dept. Geological and Environmental Sciences, Braun Hall, Building 320 Stanford University, Stanford, CA 94305,
AU: Stacey, M
EM: mstacey@berkeley.edu
AF: Dept. Civil & Environmental Engineering, 665 Davis Hall UC Berkeley, Berkeley, CA 94720,
AU: Dietrich, W E
EM: bill@seismo.berkeley.edu
AF: Dept. Earth & Planetary Science, 307 McCone Hall UC Berkeley, Berkeley, CA 94720,
AB: Sedimentation leading to the development of leveed, prograding floodplain tie channels arises from the interaction of a sediment-laden jet with quiescent lake waters. In the field, a meandering, coherent turbulent instability generated by shear along the margins of the jet is a dominant feature of tie channel jets. Spatially, this instability scales with the width of the jet which is five to ten times the flow depth and appears to be quasi-two dimensional. In the laboratory, we have conducted physical experiments on tie channel morphodynamics that reproduce these quasi-two dimensional, large-scale, coherent turbulent structures. Using acoustic doppler velocimetry (ADV) we collected detailed measurements on these experimental jets and quantified the influence of the meandering instability on the jet hydrodynamics. Based on spectral and autocorrelation analysis of the velocity time series data, we analyzed the velocity data at two timescales; one corresponding to the large-scale, quasi-two dimensional turbulent structures generated by shear at the jet margins, and the second at the shorter timescales of the three-dimensional turbulence generated by bed-induced shear. Our results indicate that the presence of the meandering structure doubles the magnitude of both the streamwise and cross-stream turbulent intensities and boundary shear stresses across the jet. Estimates of lateral momentum diffusivity that include the meandering structure are an order of magnitude greater than diffusivities generated by bed shear alone. Suspended sediment and deposition rate measurements show that the coherent meandering structure dominates the lateral transport of sediment to the jet margins and the developing channel levees. The apparent lateral diffusivity of sediment increases down-jet and reaches a value 20 times the diffusivity of momentum. The settling of sediment particles through the water column leads to an additional timescale (the settling time) that is relevant to the dynamics of lateral transport. By comparing the settling timescale to the local timescale of the jet meander, we can estimate a ratio of sediment diffusivity to momentum diffusivity (beta) and treat lateral sediment transport by the large-scale turbulence as a diffusive transport mechanism.
DE: 1825 Geomorphology: fluvial (1625)
DE: 3270 Time series analysis (1872, 4277, 4475)
DE: 4528 Fronts and jets
DE: 4568 Turbulence, diffusion, and mixing processes (4490)
DE: 4863 Sedimentation (1861)
SC: Hydrology [H]
MN: 2007 Fall Meeting