Controls on dune dimensions in rivers

Dunes are bedforms commonly found in sand-bedded rivers. They are important sources of flow resistance and mechanisms for sediment transport, so there is a practical need to directly predict dune dimensions. Migrating dunes also leave signatures of their presence and evolution as cross-strata in sand deposits, which can be used to indirectly hindcast paleo-flows. The most common method to predict dimensions in modern flows or hindcast paleo-flows is depth-scaling relations, which assume a depth control on dunes. There is substantial scatter about depth-scaling relations, suggesting that other physical processes may control dimensions. This study identifies primary controls on dune dimensions in rivers through a meta-analysis of published dune data, and a series of flume experiments. The data compilation shows approximately two orders of magnitude variation in dune height and length at any given flow depth. Dune heights in shallow flows (< 2.5 m), where asymmetric dunes with high lee angles are common, grow relatively higher in the flow than symmetrical low lee angle dunes in deeper channels (> 2.5 m). The data set is used to provide depth-scaling relations with added statistical uncertainty. Scatter about the scaling relations is attributed to natural variability in dune dimensions and transport stage effects. Flume experiments are used to better identify controls on dune dimensions. Results confirm dune-depth scaling is weak and that transport stage is a fundamental control. The experimental data are used to derive new non-linear scaling relations between equilibrium dune dimensions and transport stage. The relations provide a physically-sound method to guide predictions of dune dimensions in rivers and paleo-reconstructions from estimated dune dimensions in the rock record. A series of experimental observations of dune growth from a flat bed were also made. The results show that growth behaviour also depends on transport stage, and the time to equilibrium dimensions decreases non-linearly with transport stage. The observations are used to propose a series of relations that can predict dune dimensions through time as they respond to an imposed flow.