HR: 0800h
AN: H51A-0173    [Abstracts]
TI: Formation and maintenance of bifurcations by avulsion
AU: * Jerolmack, D J
EM: sediment@sas.upenn.edu
AF: University of Pennsylvania, Department of Earth and Environmental Science, Philadelphia, PA 19104, United States
AU: Mohrig, D
EM: mohrig@mail.utexas.edu
AF: University of Texas at Austin, Jackson School of Geosciences, Austin, TX 78712, United States
AU: Heller, P
EM: heller@uwyo.edu
AF: University of Wyoming, Department of Geology and Geophysics, Laramie, WY 82071, United States
AB: In order to better understand branching distributaries in anastomosed rivers and deltas, recent research has focused on the formation and stability of bifurcations from a hydrodynamic perspective. Such formulations begin with some kind of diverging flow inducing bar deposition, leading to enhanced flow divergence around the bar and formation of a bifurcation. Increasingly sophisticated transport models have attempted to determine which bifurcation geometries are stable, where stable means no significant erosion or deposition. However, there is a class of channel bifurcations - avulsions- that are created by a process that is independent of the details of in- channel transport . A scaling analysis employing field and laboratory data shows that avulsion is a large-scale gravitational instability arising from the interaction of a channel with its floodplain. The Niobrara River, Nebraska, is in transition to a branching pattern due to rapid aggradation forced by a dam built downstream. Historical data demonstrate that bifurcations have formed by avulsion and partial abandonment of channels during the last 30 years. Aerial photographs show that individual bifurcations persist for many decades. This apparent stability in planform, however, belies the dynamic nature of these bifurcations. Data collected over the past four years show repeated cutting and filling of individual branches. Bifurcations of the Niobrara River over this time do not appear to achieve stability, but rather oscillate over a wide range of configurations as the channel avulses during aggradation. A simple theoretical analysis suggests that many anastomosed and deltaic bifurcations are created and maintained by avulsion. Development of simplified, large-scale models of avulsion-driven bifurcations is needed to complement the growing body of hydrodynamic models.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1834 Human impacts
DE: 1856 River channels (0483, 0744)
DE: 1861 Sedimentation (4863)
SC: Hydrology [H]
MN: 2007 Fall Meeting