Secondary airflow and sediment transport in the lee of a reversing dune

Lee-side windspeed and sediment transport were measured over a small (1.2 m) transverse ridge in the Silver Peak dunefield, west-central Nevada, USA, using an intensive array of 25 cup anemometers and seven total flux traps. During crest-transverse and transporting flow conditions (u(0.3crest) ~ 8.4 m s^-1), windspeed near the surface of the lee slope averaged half (48 per cent) that of crest speeds. Dimensionless speeds in the separation zone ranged from 0.2 to 0.8 that of the outer flow (u₁₂). Along the boundary of the separation cell, windspeed increased by 10 per cent of the crest speed before separation. Equilibrium of upper and lower wake regions was not observed by the documented eight dune heights, suggesting that wake recovery may not occur over closely spaced dunes. Sediment transport measured directly on both the lee slope and interdune surfaces averaged approximately 15 per cent of crest inputs. This suggests that a significant amount (c. 70-95 per cent) of sediment transported over the crest moved as fallout. For this data set, flux was approximately proportional to the cube of the near-surface windspeed (u_0.3) and in general there was an order of magnitude difference between flux measured at the crest and that measured within the separation zone. Transport direction in the separation zone was acutely oblique to the incident direction owing to secondary flow deflection. Beyond the interdune, transport direction progressed from oblique to crest-transverse. This indicates that an appreciable amount of sediment may move laterally along the lee slope and interdune corridor under crest-transverse flows. Regarding the grain size and sorting properties of transported sediment, there was no significant difference in mean grain size over the dune, although in general particles were finer and more poorly sorted in the lee.