One of the most fundamental processes in the creation and evolution of fluvial systems is the process of channel bifurcation wherein a single channel splits into two (or more) smaller channels. Bifurcations traditionally have been grouped into two types based on whether they arise within a channel (confined), or within a standing body of water (unconfined). The former case includes braid and alluvial fan bifurcations; the latter includes deltas and crevasse splays building into bays and flooded alluvial plains. Numerous questions remain concerning the origin of both types of bifurcations: 1) What are the statistics of hydraulic geometries and water discharges for bifurcating channels? 2) Are the bifurcate channels usually equal in discharge or do they show the ratios predicted by stability theory for braid bifurcates noted above? 3) What are the statistics of angles of bifurcation in various geomorphologic settings and do they show statistically significant correlations with such basic parameters as water discharge and sediment type? 4) What factors set the angle of bifurcation in the unconfined case? 5) What factors set the longitudinal spacing of unconfined bifurcations? 6) What physical processes are responsible for the growth of the sub-aqueous levees that form the outer walls of the emerging bifurcate channels? 7) Is secondary circulation within the expanding jet a necessary condition for central bar growth as it is in braided streams? 8) What conditions are necessary to keep both arms of a bifurcation open in suspended load systems? To answer these questions a physical- and model-based study of the bifurcation process will be conducted with the purpose of understanding the roles that three-dimensional flow velocity fields, secondary circulation, and sediment transport play in causing unconfined channel bifurcations. Although the emphasis is on the unconfined type, the ultimate fluid dynamical causes in both types may be similar. To answer the above questions the research group will: 1) conduct physical experiments in a basin with sediment feed capability suitable for studying delta deposition from sediment-water flows; 2) collect data from the literature and maps and photos on the geometries and hydraulics of both types of bifurcating channels (while emphasizing the unconfined type); 3) construct a three-dimensional, large eddy simulation (LES) model of turbulent flow and bed- and suspended-load sediment transport based on the conceptual model outlined below, and 4) use the model to conduct numerical experiments. LES results will help understand the hydrodynamics that are giving rise to the flume and field observations. Comparisons of LES results with observations will tell us whether our conceptual model for unconfined bifurcation genesis is capable of predicting the basic geometries and behaviors of unconfined channel bifurcations.
The Intellectual Merit of this Work: A better understanding and predictive capability for channel bifurcations would improve the planning and development of floodplain and channel structures, channel designs, and the success of stream restoration efforts. The contributions of this study include advancing our understanding of stream stability and channel morphology.
Broader Impacts Resulting from This Study: This work constitutes the dissertation topic and financial support for one Ph. D. candidate (already in residence). Two undergraduates will be trained in the scientific methods of morphodynamic modeling of sedimentary systems. This project will form the core of senior theses for the undergraduates. All students will benefit from exposure to a problem requiring the integration of geomorphology, sediment transport, and hydrodynamics.
