HR: 14:40h
AN: NG12A-05    [PDF]
TI: Dynamic Interaction of Bedforms at low Transport Stage
AU: * Jerolmack, D J
EM: douglasj@mit.edu
AF: Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Mohrig, D
EM: mohrig@mit.edu
AF: Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AB: Results are presented examining the control of initial spacing between two sandy bedforms on their subsequent behaviors when subjected to flow of Froude number = 0.2 (mean velocity = 0.30m/s) in a channel 10m long by 0.16m wide by 0.20m deep. Temporal evolution of bedforms was monitored using time-lapse photography to characterize the pathways by which bedforms adjusted to the given flow field. Medium and coarse-grained bedforms had median grain sizes of 0.35 and 0.77mm, respectively, and initial dimensions: length = 0.40m, height = 0.07m, stoss angle = 13 degrees and lee angle = 33 degrees. Cases were examined of (1) one bedform alone, and two bedforms with crests spaced (2) 0.40, (3) 0.67 and (4) 1.40m apart. Flow conditions were chosen such that time-averaged sediment flux was nearly zero for the first case of one bedform and bed deformation was minimal, allowing the kinematic effects caused by the presence of additional bedforms to be unambiguously quantified. The presence of another bedform with crest 0.40m downstream, placed mainly in the recirculation zone of the upstream bedform, induced deformation of both bedforms, and increased sediment flux by an order of magnitude. Changing initial spacing to 0.67m decreased the activity of the upstream bedform but enhanced deformation of the downstream bedform, which was placed mainly in the growing internal boundary layer downstream of the point of reattachment. At a spacing of 1.40m, with the downstream bedform placed beyond the zone of flow disturbance from the upstream bedform, sediment flux was reduced and deformation of both bedforms was small. Sediment fluxes were comparable for medium and coarse sand trials. For medium sand, small reductions of lee-face slopes and the filling of bedform troughs with sand often preceded rapid adjustments in bed geometry, with turbulent bursts and sweeps visibly important in ejecting and advecting sediment grains. Evolution of course grained bedforms was more uniform with little or no bedform splitting, probably because turbulent bursts and sweeps could not effectively lift larger sediments so bed evolution was dominated by grain rolling and sliding. Velocity measurements taken over rigid bedforms using an Acoustic Doppler Profiler were used to characterize velocity vectors and turbulence production for several bed configurations, simulating snapshots in time of bed evolution. Results show that placement of a solid plate in the bedform trough enhances turbulence production by a factor of 5, by forcing the recirculation zone in the trough and the free shear zone at the crest to merge. Increased turbulence caused by changes in trough geometry suggests a pathway by which small-scale topography can induce larger-scale structural adjustment of bedforms. It is likely that local topography, turbulent bursting and sensitivity to boundary conditions play an important role in determining the geometry of bedform systems through dynamic bedform interaction. Experiments challenge the commonly held assumption that bedform geometry asymptotically approaches equilibrium with flow and sediment transport fields. Comparison with field studies shows the mechanisms described herein are likely active in natural systems, suggesting laboratory results are relevant to understanding bedform adjustment in real systems, where flow is neither steady nor uniform.
DE: 1824 Geomorphology (1625)
DE: 3240 Chaos
DE: 3379 Turbulence
DE: 4558 Sediment transport
SC: Nonlinear Geophysics [NG]
MN: 2003 Fall Meeting