The Interaction of Bedforms and Changing Discharge

In a fluvial environment, the response of the flow properties such as mean velocity and water depth (stage) to changes in discharge are vital to predict, for example, the extent of flooding. Bedforms such as dunes play a crucial role in determining how changes in discharge are partitioned between changes in mean velocity and stage. Most past flume experiments on bedforms use steady, unidirectional flow in order to establish relations incorporating the interactions between flow, sediment transport and bedforms. Under these conditions the bedforms grow and propagate until they approach equilibrium. Applying these relations to natural flows such as rivers often exposes their inability to predict bedform morphology, sediment transport, and water levels in flows which are constantly changing. Conversely, studies conducted in the field suffer from difficulty in taking measurements, unknown history of bedform development and the lack of ability to control conditions, making untangling the dominant physical processes impossible with current models. To bridge the gap between lab studies of bedforms under steady flow conditions and field studies a series of experiments with changing flow conditions is currently being carried out. This study establishes two equilibrium conditions, low (stage A) and high (stage B) flow, each with corresponding equilibrium dunes. In order to better understand the processes which occur in nature, flow conditions are systematically changed to get a better handle on the key processes controlling the evolution of bedforms. The experiments focus on the response of bedforms to step changes in properties, both from low to high and from high to low discharge, but also more realistic gradual changes from Stage A to Stage B and vice versa. In addition, changing depth while maintaining mean velocity and changing mean velocity while maintaining depth are investigated. Data is collected using a Multi-Transducer Array, an Acoustic Doppler Velocimeter, and Acoustic Doppler Profiler. Relatively two-dimensional conditions are achieved so that the Exner equation can be used to estimate sediment flux as a function of position over the dunes in changing flows. The goal is a robust transfer function between flow and sediment response in a bedform environment.