HR: 0830h
AN: NG31A-0597    [PDF]
TI: Channel Patterns as the Result of Self-Organization Within the Flow-Sediment-Vegetation System
AU: * Tal, M
EM: talx0001@umn.edu
AF: University of Minnesota, St. Anthony Falls Laboratory, 2 3rd Ave. SE, Minneapolis, MN 55414 United States
AU: Paola, C
EM: cpaola@umn.edu
AF: University of Minnesota, St. Anthony Falls Laboratory, 2 3rd Ave. SE, Minneapolis, MN 55414 United States
AB: The familiar patterns of braided and meandering rivers can be thought of as the result of self-organization within a "three-phase" system comprising fluid, sediment, and vegetation. Interactions between these three components are also largely responsible for the organization of river systems into separate and distinguishable channels and floodplains. Key elements of the self organization include the space and time characteristics of seed dispersal and plant growth as well as the statistics of occupation, abandonment, and reworking of the bed by the flow. Seeds are transported and dispersed readily by wind and water and opportunistically colonize areas of the channel that are abandoned or exposed at low flows. Vegetation increases bank stability through root reinforcement of the sediment and increases the threshold shear stress needed for erosion. In addition, vegetation offers resistance to the flow by increasing the drag and reducing the velocity, thus decreasing the stream power available for erosion and transport. Vegetation that is not removed while young will become stronger and increasingly resistant to erosion and removal by the flow. Thus a key organizing parameter in the flow-sediment-vegetation system is the time scale for establishment of the vegetation relative to a characteristic channel or bed mobility time. Experiments at the St. Anthony Falls Laboratory demonstrate how repeated cycling of vegetation seeding and water discharge changes an unvegetated braided channel morphology: the flow is gradually corralled into a single sinuous channel that largely tracks the thread of maximum velocity in the original braided network. The experiments are carried out in a large unconsolidated sand bed flume in which alfalfa sprouts are used to simulate riparian vegetation and offer the only form of cohesion in the system. An initial braided pattern is allowed to evolve freely in conjunction with alternating high and low discharges and repeated seedings. As the vegetation density and age increase with time, smaller and weaker channels are choked off leaving a single relatively narrow channel with a sinuous thalweg. This channel develops its own internal bar forms with smaller length scales than the original braid bars.
UR: http://www.geo.umn.edu/orgs/seds/
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1851 Plant ecology
SC: Nonlinear Geophysics [NG]
MN: 2003 Fall Meeting