Bedform Disequilibria in Unidirectional and Oscillatory Flows

Abstract:

Laboratory studies of ripples and dunes show robust relations between bedform geometry and both fluid flow conditions and grain size. These empirical relations are derived at equilibrium – defined by the achievement of stationary bedform statistics under a given flow – and are utilized as both targets for theoretical models and for reconstructing paleoflow conditions from sedimentary deposits. In natural environments, however, the flow of water and wind is continuously varying, and bedform patterns adjust in response by redistribution of mass. Disequilibrium may arise whenever the timescale of changing flow is rapid compared to the timescale of bedform adjustment. We review and synthesize recent findings on bedform disequilibrium in three different systems: fluvial bedforms, wave and combined wave-current ripples, and eolian dunes. For all cases we argue that the bedform turnover time, which is the timescale to displace the sediment volume of a bedform, is the relevant timescale governing the adjustment of bed morphology to a changing flow. The mechanism of adjustment, however, is not universal. Formation and annihilation of defects occurs in all systems, and is the dominant adjustment mode for oscillatory flows where ripples are stationary. However, migrating bedforms in unidirectional river and eolian settings change their geometry primarily through collisional merger and calving. Further, the mechanisms of river bedform growth and decay are different, causing hysteresis in the response of bedform size to a flood pulse. Since the bedform turnover time is proportional to size, it appears likely that the largest dunes in rivers and deserts are always in disequilibrium – and hence their size does not reflect the present flow conditions. We discuss the consequences of fluid flow depth and sediment supply on the ability of bedforms to achieve equilibrium, and propose that there can be no equilibrium relation between flow and bedform size for eolian dunes. Finally, we outline potential signatures of bedform disequilibrium in the sedimentary record, and how this information may be used to provide a richer interpretation of paleoflows.