An experimental study of turbulent flow over a low‐angle dune

Abstract

Many large, sand bed alluvial channels are dominated by dunes that possess low‐angle 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 low‐angle 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.