Sand transport and deposition over nonerodible elements
This study examines the role of non-erodible elements in inducing sand deposition and their influence on sand transport rates. Experiments were conducted in a wind tunnel in two stages. The first series of wind tunnel tests were carried out in a sand-free environment to investigate the change in the flow regime in relation to the non-erodible element concentration. Wood dowels of a uniform diameter, 23 mm, and of two different heights, 23 and 46 mm, were placed along the length of the wind tunnel working section (2.5m) in a diagonal array and in rows normal to the flow using two different spacings, 34.5 and 92 mm. Then sand flux and deposition around these elements were investigated using a well-mixed, flat sand bed, of length 5.7 m and the sand feeding system at the upstream edge of the wind tunnel. The elements were exposed to three preselected air velocities all above the threshold (22.8cm/s). Finally, a numerical model was developed to simulate the pattern of sand flux and deposition over fixed elements. The model was developed on the basis of the following main experimental findings and observations: i) the reduction in the flow condition near the rough bed reduced the sand flux rate. ii) the non-erodible elements prevented some saltating grains from colliding with part of the erodible bed by sheltering a portion of it. iii) the decrease in the roughness height of the non-erodible elements, due to sand accumulation, resulted in an increase in local splashed grains and therefore increased the sand flux. The numerical model used in this study succeeded in providing a qualitative rather than a quantitative result and the differences between the model prediction and experimental results were generally within 50%.