Effects of ridge height and spacing on the near-surface airflow field and on wind erosion of a sandy soil: Results of a wind tunnel study

Abstract

In wind tunnel experiments, we measured the airflow field near the ground surface and the resulting sand transport rate over a sand bed that contained widely but uniformly spaced non-erodible ridges. We found that when airflow passed over the bed, the wind velocity changed drastically around the first ridge, but the magnitude of the change decreased gradually moving downwind until a relatively stable airflow developed, characterized by regular fluctuation over the uniformly spaced ridges. The distance from the first ridge to the zone of stable airflow increased with increasing ridge height or spacing between ridges. This airflow field led to a non-uniform distribution of wind erosion that affected total sediment transport from the sand bed. Using our wind tunnel simulation results as inputs for existing sediment transport equations, we developed a model to predict sediment transport (q) over a sand bed that contained widely spaced ridges: q=C'dDρgu*2u*-u*texp⁡(-aHu*3n). The model accounts for the ridge height (H), number of ridges per unit length (n), and the wind’s free-stream friction velocity (u*) and threshold friction velocity (u*t). We discussed the different mechanisms by which ridges influence the near-surface airflow at large spacing (> 5H) and small spacing (≤ 5H). The ridges appear to affect the airflow as roughness elements when the spacing is small, but act as barriers to the wind when the spacing is large.