HR: 14:25h
AN: NG12A-04    [PDF]
TI: Instantaneous Initiation of Bedforms from a Flat Sand Bed
AU: * Venditti, J G
EM: venditti@geog.ubc.ca
AF: University of British Columbia, Dept. of Geography, Vancouver, BC V6T 1Z2 Canada
AU: Church, M A
EM: mchurch@geog.ubc.ca
AF: University of British Columbia, Dept. of Geography, Vancouver, BC V6T 1Z2 Canada
AU: Bennett, S J
EM: seanb@buffalo.edu
AF: State University of New York at Buffalo, Dept. of Geography, Buffalo, NY 14261 United States
AB: Sediment transport in sand bedded alluvial channels is strongly conditioned by bedforms, typically ripples or dunes. Consequently, the origin of bedforms has attracted interest for over a century. Recent research has suggested that there may be several bedform initiation modes. Bedform fields may be produced by the propagation of defects via flow separation processes and local sediment transport. Alternatively, bedform fields have been observed to develop instantaneously, covering the entire bed surface under general sediment transport conditions. This latter initiation mode has not been investigated extensively. A series of experiments were designed to examine bedform initiation on a flat sand bed composed of homogeneous 0.5 mm sand. The narrowly graded sand bed was subjected to a 0.155 m deep, non-varying mean flow ranging from 0.30 to 0.55 m/s in a 1 m wide flume. Bedform initiation was monitored using overhead video. At lower flow strengths, with sporadic sediment transport, the defect propagation process was observed to dominate. At larger flow strengths, where general sediment transport occurs, bedform initiation began with the imprinting of a cross-hatch pattern on the flat sediment bed which leads to chevron shaped forms that migrate independently of the initial structure. The chevron shapes are organized and form incipient crestlines that quickly develop into two-dimensional (2D) bedforms. Once the 2D bedforms are organized, it appears that flow separation processes dominate subsequent growth. A Kelvin-Helmholtz model of interface instability between two superimposed fluids of different densities is introduced to explain this organization. In the model, the water column is the upper, less dense but faster moving fluid and the active sediment transport layer is the lower, more dense and slower moving fluid. Measurements of near-bed fluid velocities, sediment velocities and bedload transport rates allow for calculations of a predicted wavelength for the initial 2D bedforms. At all flow strengths with intense and widespread sediment transport the predicted instability wavelengths are nearly identical to observed initial bedform lengths. This suggests that the origin of instantaneously developed bedforms is a Kelvin-Helmholtz instability formed at the interface between the fluid flow and the active transport layer.
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 4558 Sediment transport
SC: Nonlinear Geophysics [NG]
MN: 2003 Fall Meeting