HR: 17:30h
AN: H54B-07 INVITED    [Abstracts]
TI: Scaling Relationships and Evolution of Distributary Networks on Wave Influenced Deltas
AU: * Swenson, J B
EM: jswenso2@d.umn.edu
AF: University of Minnesota Duluth, 1110 Kirby Dr., Duluth, MN 55812, United States
AU: Jerolmack, D J
EM: sediment@sas.upenn.edu
AF: University of Pennsylvania, 240 S. 33rd Street, Philadelphia, PA 19104, United States
AB: Relatively little is known about the temporal and spatial scales of processes that evolve distributary networks, and no genetic model can explain the variability in distributary network pattern on modern deltas. We derived scaling relationships for two fundamentally different processes known to create distributary channels and, with these laws, constructed a simple model for distributary network evolution on wave-influenced deltas. The first process is mouth-bar deposition at the shoreline and subsequent channel bifurcation; the second is avulsion—the wholesale abandonment of a channel in favor of a new path. The former, which is generated by a fluid- mechanical instability at the shoreline and is fueled by local progradation of the delta front, creates relatively small networks with power-law distributions of channel length. In contrast, avulsion arises from a gravitational instability between the superelevated channel and its surrounding floodplain and, in principle, can operate anywhere on the delta surface. The avulsion process generates relatively few but larger distributaries that are bounded in scale by the backwater length. We collected channel statistics from a representative set of modern deltas. Frequency-magnitude plots of channel length agree with our theoretical predictions and show a clear separation in scale that reflects the two channel-forming processes: Mouth-bar distributary lengths scale with the width of the parent channel, and avulsive distributary lengths scale with the backwater length; interestingly, intermediate channel lengths are relatively rare. Wave energy controls network topology by suppressing mouth- bar development, thereby preferentially eliminating smaller-scale, mouth-bar distributaries. Hence, the absence of fine-scale channel structure, as quantified by deviations from expected fractal scaling, provides a metric of the relative importance of waves.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
DE: 1861 Sedimentation (4863)
DE: 1862 Sediment transport (4558)
DE: 3022 Marine sediments: processes and transport
SC: Hydrology [H]
MN: 2007 Fall Meeting