HR: 17:45h
AN: H44D-08    [Abstracts]
TI: Two-Dimensional Turbulence of Shallow, Plane, Jets and the Development of Subaqueous Levees
AU: * Rowland, J C
EM: rowland@eps.berkeley.edu
AF: Dept. Earth & Planetary Science University of California, Berkeley, 307 McCone Hall University of California - Berkeley, Berkeley, CA 94720 United States
AU: Dietrich, W E
EM: bill@eps.berkeley.edu
AF: Dept. Earth & Planetary Science University of California, Berkeley, 307 McCone Hall University of California - Berkeley, Berkeley, CA 94720 United States
AB: The application of jet theory to the formation of deltas dates back more than 50 years and conceptual models relating river mouth morphologies to jet mechanics were proposed in the 1970s. Despite advances in the understanding of hydrodynamics and stability of shallow plane jets since the 1970s, little progress has been made linking sedimentation patterns to observed jets characteristics. As a result, a morphodynamic explanation of channel formation by a sediment-laden jet entering still water is lacking. We have undertaken a series of experimental studies to investigate the development of leveed channels from sediment-laden jets entering still water. To study sedimentation from shallow jets, we introduce a flow laden with ground plastic particles (sg ~ 1.5) into a 3 by 8 by 0.4 m basin of still water. The inflow aspect ratio is scaled to match the width to depth ratio (5.5) observed in prototype floodplain "tie" channels occurring in lakes along lowland river systems. For all flows in which the inflow discharge is sufficient to entrain the particles into suspension, lateral deposition and levee formation occurs. The location and rate of levee formation, however, directly relates to the local characteristics of the jet. The most rapid, pronounced and continuous levee development occurs in the transition of jet turbulence dominated by 3-dimensional structures to quasi-2-dimensional structures. Three-dimensional turbulence scales with the flow depth and is generated by shear along the channel bed while the 2-D structures scale to the jet width and arise from instabilities generated by shear with and entrainment of zero-momentum ambient fluid along the margins of the jet. The development of 2-D vortices is well documented in laboratory studies of jets and has been observed in many field sites. A common characteristic of these flows, also observed in our experiments, is the "meandering" of the jet centerline associated with the onset of 2-D turbulence due to alternating sweeps of vortices across the body of the jet. The meandering sweeps greatly increases the cross-stream fluctuations in velocity and advect sediment from regions of high velocity, along the jet centerline, into slower moving fluid along the jet margins where particles rapidly settle out of suspension. The location of this transition zone relative to the jet outlet appears to scale with momentum of the inflowing jet. Increasing momentum flux pushes the transition zone basinward and a gap in lateral deposition occurs along regions of the jet dominated by the inflow inertia and 3-D turbulence. To develop a channel capable of propagating its form basinward, it is necessary to close this depositional gap. Decreases in jet momentum to reduce this gap, however, are limited by minimum shear velocity thresholds required to maintain entrainment of sediment in the jet. Therefore the creation of a self-formed channel by a sediment-laden jet depends on a balance between hydrodynamics of the jet and the entrainment and suspension properties of particles carried by the flow.
DE: 1815 Erosion
DE: 1824 Geomorphology: general (1625)
DE: 1861 Sedimentation (4863)
DE: 3020 Littoral processes
DE: 3022 Marine sediments: processes and transport
SC: Hydrology [H]
MN: Fall Meeting 2005