SECONDARY AIRFLOW AND SEDIMENT TRANSPORT IN THE LEE OF A REVERSING DUNE IAN J. WALKER Department of Geography, University of Guelph, Ontario, N1G 2W1 CANADA Received 6 July 1998; Revised 6 October 1998; Accepted 10 October 1998 ABSTRACT 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4ms 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 (u 12 ). 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. Copyright # 1999 John Wiley & Sons, Ltd. KEY WORDS: dune; lee; airflow; sediment transport INTRODUCTION Sedimentary bedforms project into the airstream altering primary near-surface boundary layer flows. Resultant flow patterns over the bedform are secondary in that their vertical velocity distributions, or profiles, deviate from the expected log-linear form (i.e. the Law of the Wall). Secondary flows generate spatial differences in the distribution of fluid velocity, pressure and shearing stress over the form. In response, variations in sediment flux occur resulting in regions of localized transport and deposition. As such, sedimentary bedforms reflect the complex interactions and continuous feedbacks between flow, form and sediment transport. Although secondary flow patterns such as lee flow separation and reversal are often referred to in the aeolian literature, the physical properties and sedimentological implications of such patterns remain poorly understood. This is not to say that research into dune lee airflow is lacking. Rather, considerable work has been done on describing lee flow and sediment transport, particularly for longitudinal dunes (e.g. Bagnold, 1953; Folk, 1970, 1976; Wilson, 1972; Tsoar, 1978, 1982, 1983; Tsoar et al., 1985; Livingstone, 1986; Tseo, 1993). However, research on lee flow for transverse dunes is less extensive (e.g. Sweet and Kocurek, 1990; Sweet, 1992; Frank, 1994; Frank and Kocurek, 1996a) and is particularly sparse regarding implications for sediment transport. Earth Surface Processes and Landforms Earth Surf. Process. Landforms 24, 437±448 (1999) CCC 0197-9337/99/050437±12 $17.50 Copyright # 1999 John Wiley & Sons, Ltd. * Correspondence to: I. J. Walker, Department of Geography, University of Guelph, Ontario, N1G 2W1 Canada. Email: iwalker@uoguelph.ca Contract/grant sponsor: NSERC