The behavior of dunes and ripples under changing flow conditions: modern, experimental, and stratigraphic analyses

Laboratory studies of ripples and dunes show strong relations between bedform geometry and both fluid flow and grain size. These empirical relations are derived at equilibrium – stationary bedform statistics under a steady flow – and are used for interpretation of modern and ancient paleoflow conditions. However, the flow of water and wind is highly variable, and bedform patterns adjust in response by redistribution of mass. Disequilibrium arises whenever the flow changes faster than bedforms can adjust. I will present recent findings on bedform disequilibrium for subaqueous unidirectional flows (primarily rivers), oscillatory flows, and aeolian flows. The mechanism of adjustment is different in each case. The bedform turnover time, which is the timescale to displace the entire sediment volume of a bedform, is the relevant timescale governing the adjustment of bed morphology to a changing flow. Since the bedform turnover time is proportional to size, it is likely that the largest dunes in rivers and deserts are always out of equilibrium – their size does not generally reflect the instantaneous flow conditions. Ultimately, given the generally unsteady and nonuniform nature of natural flows, disequilibrium may be the norm in nature, and thus should be emphasized in the analysis of modern bedforms and ancient strata.  

Host: David Fike

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