The Growth of Dunes in Rivers

Dune response to variable flow has been well documented, but there is no universal model to predict dune dimensions as they respond to imposed flows. Here we use a series of flume experiments to explore dune growth in response to constant flow to better understand the form of dune growth curves. Observations of dune growth from a flat sand bed were made at three flow depths, under five different constant transport stages in a laboratory flume. The transport stages ranged from the sediment entrainment threshold to suspension conditions. The bed was flattened before each run and topography was continually mapped, providing observations of dune growth and morphology at each distinct transport stage condition. The results show that dune growth curves exhibited three different behaviors: (1) exponential growth; (2) punctuated growth, when a period of initially linear growth was abruptly interrupted by exponential growth; and (3) instantaneous growth, when bed evolution happened so quickly that we were unable to take measurements of the phenomenon. Growth behavior is dependent on the applied transport stage, and the time for a growing dune field to reach equilibrium decreases nonlinearly with transport stage. Observations of evolving dunes and the time required to achieve an equilibrium bed state are used to propose a series of relations that can predict dune dimensions through time.