[1] Many large, sand bed alluvial channels are dominated by dunes that possess lowangle lee sides, often <10°, which play a critical role in the transportation of sediment and generation of significant bed form roughness. Despite the fact that these low-angle dunes are very common in such channels many current models of dune flow dynamics are based on bed forms with an angle of repose slip face that generates a zone of permanent separated flow in the dune lee. Study of flow associated with low-angle dunes in the field is inherently difficult since it is usually both hard to measure very near the bed and hard to quantify the nature of turbulence over these bed forms. Results from a detailed scale model experimental study of flow over a low-angle dune, which is based on a prototype dune from the Fraser River, Canada, present a necessary link between flume and field studies and document the origins of macroturbulence associated with these bed forms. Two-dimensional laser Doppler anemometer measurements over a low-angle dune (maximum lower lee side slope = 14°) show that dune morphology exerts a dominant control on the turbulent flow, causing flow deceleration in the lower lee and development of an intermittent layer of shear at the interface with the higher velocity flow above. The scale model confirms that permanent flow separation does not occur over low-angle dunes but, instead, is replaced by a small region (here $7% of the dune wavelength in length) of intermittent flow reversal, which may be present for up to 4% of the time. Shear layers generated along this small zone of decelerated and/or separated flow in the lower lee have a much smaller velocity differential than is characteristic of shear layers generated by flow separation in the lee of angle of repose dunes. Turbulence production associated with lowangle dunes is dominated by eddies generated along this shear layer, which produce highly variable horizontal and vertical velocities and large Reynolds stresses in this region. These results show that macroturbulence associated with low-angle dunes is generated by intermittent separation or shear layer generation due to velocity gradients established in the zone of lee side flow expansion. Velocity profiles and maps of turbulence structure from the scale model are in reasonable agreement with field measurements from low-angle dunes in natural sand bed rivers. These results highlight the need to consider the temporal evolution and intermittency of shear layer behavior, often very near the bed, when interpreting the generation of macroturbulence and dispersal of sediment associated with low-angle dunes.INDEX TERMS: 1824 Hydrology: Geomorphology (1625); 4235 Oceanography: General: Estuarine processes; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; KEYWORDS: dune, low-angle leeside, intermittent flow separation Citation: Best, J., and R. Kostaschuk, An experimental study of turbulent flow over a low-angle dune,

Macroturbulence, which may advect through the entire water depth, dominates the flow field associated with alluvial sand dunes and has long been regarded as the principal mechanism for suspending bedload sediment over dunes. The origin of this macroturbulence has been linked to shear layer development in the dune lee, often associated with flow separation, and the form of these coherent flow structures has been noted as 'boils' that erupt onto the water surface. Although past work has quantified the mean and turbulent flow characteristics of flow over dunes using at-a-point measurements, these studies have not been able to trace the evolution of such macroturbulent events over a dune-covered bed. Additionally, the topology of the dune-related macroturbulence has not been explained in relation to the structure of the developing surface boils. This paper tackles both of these issues using a twofold approach: (i) use of whole flow field quantification using particle imaging velocimetry (PIV) over a series of fixed laboratory dunes; (ii) observations of the water surface over large sand dunes in the Jamuna River, Bangladesh.Particle imaging velocimetry results are in good agreement with past work detailing the mean flow field over sand dunes. The PIV images over the lee and stoss sides of an experimental dune, and observation of the water surface above natural dunes, reveal four key dynamic attributes to flow. 1 The shear layer and separation zone associated with dunes are spatially and temporally dynamic, and 'flapping' of the shear layer may be modulated by turbulent coherent flow structures generated upstream. 2 Reynolds stresses in the lee side are dominated by the free shear layer associated with the separation zone. 3 Ejections of low downstream momentum fluid away from the bed dominate the instantaneous flow field over the crestal regions of the dune. These ejections, in turn, however, create return flows towards the bed both in front of, and behind, the ejection. The highest instantaneous Reynolds stresses are associated with these ejections and inrushes. 4 'Boils' on the water surface over a natural dune field often consist, firstly, of a central upwelling that has a spanwise axis of rotation and, secondly, later secondary vortices that possess a vertical axis of rotation. This pattern of flow can be explained by the interaction of a vortex loop with the free surface.These results provide a mechanism that links the flow fields of adjacent dunes and highlight how dune-related macroturbulence may dominate the entrainment of sediment into both suspended and bedload transport. Additionally, the

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