EVOLUTION OF BRAIDED MORPHOLOGY IN RESPONSE TO VEGETATION FORCING IN A LABORATORY CHANNEL

The familiar patterns of braided and meandering rivers can be thought of as the net result of self-organization within a “three-part” system comprising fluid, sediment, and vegetation. We investigate these dynamic interactions in a set of controlled experiments at the St. Anthony Falls Laboratory consisting of an unvegetated braided morphology continuously forced with vegetation and cycled high and low discharges. 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. Key elements of self organization within the system 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. Vegetation that is not removed while young will become stronger and increasingly resistant to erosion and removal by the flow. Vegetation that is subjected to a high flow before it is firmly rooted will be removed from the system and never become established. 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 mobility time. The experiments demonstrate how vegetation can alter a braided morphology into a system with a well-established vegetated floodplain and a single dominant channel with a sinuous thalweg. This transition is associated with increases in mean channel depth, and decreases in total wetted width, velocity variability, and depth variability. The transition from unvegetated braided to vegetated meandering is characterized by sediment storage in the system, reflected in an imbalance between the amount of sediment being fed and the amount exiting the system.