Recent research literature on secondary airflow and sediment transport patterns over flow-transverse dunes is reviewed. Various issues surrounding the behaviour, modelling and sedimentological implications of near-surface airflow dynamics over dunes are discussed, including: the Law of the Wall; the Jackson and Hunt airflow model; the effects of streamline compression, acceleration and curvature on stoss slope shear stress; and, in particular, recent efforts to characterize secondary lee-side airflow patterns. A revised conceptual model of lee-side airflow is presented and areas for further research are identified regarding the implications of such patterns for dune sedimentary dynamics, morphology, and migration.

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McKeNNn-NeuMAN, C. AND NrcKLrNG, W. G. 1989. A theoretical and wind-tunnel investigation of the effect of capillary water on the entrainment of sediment by wind.Can. J. Soil A theoretical model of the effect of small amounts of water on the threshold shear velocity of sand grains has been tested in wind-tunnel studies. The model is based upon the capillary forces developed at interparticle contacts surrounded by isolated wedges of water. These forces (F") are inversely proportional to moisture tension (P) and directly proportional to the geometric properties of the contacts (G). Given F": rT2GlP, the cohesion of the material decreases with increasing moisture tension and particle angulariry. F,:2r R2T + r R.rz T,n -*j, (4) where R1 is the radius of the air-water interface and R2 is the radius of the "waist" of the water wedge (Fig. 1) Haines (1925) and Fisher (1926) for spheres predict a sharp drop in capillary force with increasing moisture content up to l0% by mass.

Topographic interactions generate multidirectional and unsteady airflow that limits the application of velocity profile approaches for estimating sediment transport over dunes. Results are presented from a series of wind tunnel simulations using Irwin-type surface-mounted pressure sensors to measure shear stress variability directly at the surface over both isolated and closely spaced sharp-crested model dunes. Findings complement existing theories on secondary airflow effects on stoss transport dynamics and provide new information on the influence of lee-side airflow patterns on dune morphodynamics.For all speeds investigated, turbulent unsteadiness at the dune toe indicates a greater, more variable surface shear, despite a significant drop in time-averaged measurements of streamwise shear stress at this location. This effect is believed sufficient to inhibit sediment deposition at the toe and may be responsible for documented intermittency in sand transport in the toe region. On the stoss slope, streamline compression and flow acceleration cause an increase in flow steadiness and shear stress to a maximum at the crest that is double that at the toe of the isolated dune and 60-70 per cent greater than at flow reattachment on the lower stoss of closely spaced dunes. Streamwise flow accelerations, rather than turbulence, have greater influence on stress generation on the stoss and this effect increases with stoss slope distance and with incident wind speed. Reversed flow within the separation cell generates significant surface shear (30-40 per cent of maximum values) for both spacings. This supports field studies that suggest reversed flow is competent enough to return sediment to the dune directly or in a deflected direction. High variability in shear at reattachment indicates impact of a turbulent shear layer that, despite low values of time-averaged streamwise stress in this region, would inhibit sediment accumulation. Downwind of reattachment, shear stress and flow steadiness increase within 6 h (h = dune height) of reattachment and approach upwind values by 25 h. A distance of at least 30 h is suggested for full boundary layer recovery, which is comparable to fluvial estimates.The Irwin sensor used in this study provides a reliable means to measure skin friction force responsible for sand transport and its robust, simple, and cost-effective design shows promise for validating these findings in natural dune settings.

The amount of sediment transported by wind in surface creep, saltation, and suspension was measured during 15 dust storms in the Slims River Valley, Yukon Territory. Results of the investigation showed that the quantity of sediment transported in creep and saltation varied approximately with the cube of shear velocity, which supports theoretical and empirical models presented by other investigators. The suspended sediment flow rate, however, seemed to be more directly controlled by the degree of air turbulence than by shear velocity. The total sediment transport rate was also shown to be directly affected by the surface moisture content and the presence of soluble salts at or near the surface. Both these factors tend to stabilize the surface by holding individual grains in place. Although eolian sediment transport is common in the Slims River Valley, high saltation–creep flow rates and major dust storms appear to be associated with a distinct set of atmospheric conditions, which are best developed on warm clear days following periods of heavy or extended rainfall.