Airflow dynamics and aeolian sand transport across a beach-climbing dune-clifftop dune system

This study presents an analysis of wind flow and sediment transport from the beach, up a 50m high, long (130m), steep (mean slope 26°) climbing dune and across a 1.5 m high max, 85 m long and 17.5 m wide clifftop dune 30km south of Dakhla in Morocco, NW Africa during highly oblique incident wind conditions. Multiple 2D sonic and cup/vane anemometers and sand traps were utilised for measurements. Flow steering was significant on the upper climbing dune. Flow deceleration occurred near the dune toe, and topographic forcing of flow was considerable on the upper slopes of the climbing dune. Near-surface flow steadiness (CVU1, CVU0.25) on the climbing dune straight slope segment was low and constant The distance upslope over which the airflow reached the speed comparable to that on the beach increases as the incident wind speed increases. The greatest flow acceleration and speed-up was observed at the cliff edge reaching 250% at 1m height and 220% at 0.25m height for the lowest incident wind speed class (4-5 m/s). The sand transport rate declined from the beach to the climbing dune toe and lower slope, but at the uppermost section of the climbing dune was 4 times higher than at the beach for the 7-8m/s incident wind speed. Sand in aeolian transport was generally finer than surface sand with mean grain size increasing up the slope. A comparison of the sand transport data collected with sand transport models, and the effects of slope on aeolian transport are also examined.

本研究对摩洛哥西北非达克拉的30公里以南地区，一个高50米、长130米、坡度陡峭（平均坡度26°）的上升沙丘及其上方的1.5米高、长85米、宽17.5米的悬崖顶部沙丘，在高度斜入射风条件下的风流和泥沙运输进行了分析。本研究采用了多种二维声学风速计和杯叶风速计以及沙子收集器进行测量。在上升沙丘的顶部，风向偏转作用显著。在沙丘脚附近出现流速减慢现象，且在上升沙丘的上坡处，地形对流速的强迫作用相当显著。在上升沙丘直坡段，近地表流体的稳定性（CVU1, CVU0.25）较低且保持恒定。随着入射风速的增加，气流达到与海滩上相同速度的上升距离也在增加。在悬崖边缘观察到的最大流速加速和增速最为显著，对于最低入射风速等级（4-5米/秒），在1米高度达到250%，在0.25米高度达到220%。从海滩到上升沙丘脚和下坡的沙运输率逐渐降低，但在上升沙丘最顶部，当入射风速为7-8米/秒时，沙运输率比海滩高出4倍。在风成沙运输中，沙粒通常比地表沙粒更细，平均粒径随着坡度的上升而增大。此外，还对使用沙运输模型收集的沙运输数据进行了比较，并探讨了坡度对风成沙运输的影响。