Controls on the Leeside Angle of Dunes in Shallow Unidirectional Flows

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Dunes are ubiquitous features in alluvial channels, serve as major agents of sediment transport and contribute significantly to flow resistance. Research in the past decade has illustrated the complexity of dune geometry and widespread occurrence of dunes that have a low leeside angle. However, debate exists concerning the occurrence of such dunes and their formative processes. This paper seeks to further our understanding of low-angle dunes by utilizing data from a robust set of shallow flow laboratory experiments detailing equilibrium bedform morphology across a range of sediment transport conditions. Analysis of bedform morphology demonstrates that dunes with low-angle leesides are generated in shallow laboratory flows, and hence are not restricted to deep rivers. Of the possible processes that have been proposed to explain the formation of low-angle dunes, this finding unequivocally shows that liquefied leeside avalanches, which rely on deep flows for their generation, are not a controlling mechanism. In addition, dunes formed under suspension-dominated conditions possess lower leeside angles compared to those formed in bedload-dominated conditions. However, where bedload transport dominates and sediment suspension is likely of lesser importance, low-angle dunes are still present, and preliminary analysis shows that bedform superimposition can result in lowering of the dune leeside angle. Low and intermediate angle dunes formed in these various conditions also have a lower potential for large-scale, permanent, leeside flow separation compared to angle-of-repose dunes, confirming the need to account for these differences in predictions of flow resistance associated with dune form roughness.