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.

本研究针对非洲西北部摩洛哥达赫拉以南30公里处的沙丘系统开展气流与输沙过程分析，该系统包含一座高50米、长130米、平均坡度26°的爬升沙丘，以及一座最大高度1.5米、长85米、宽17.5米的崖顶沙丘，实验期间入射风为强斜向风况。研究采用多台二维声学风速仪、杯式/叶片式风速仪以及集沙仪开展野外观测。爬升沙丘上部存在显著的气流转向现象；沙丘坡脚附近出现气流减速，而其上部坡面的地形强迫效应十分显著。爬升沙丘直坡段的近地表气流稳定性（CVU1、CVU0.25）较低且保持恒定。气流从坡脚加速至海滩水平风速所需的上坡距离，随入射风速的升高而增大。在崖顶边缘观测到最大的气流加速效应：当入射风速处于最低区间（4-5 m/s）时，1米高度处风速可达海滩基准水平的250%，0.25米高度处可达220%。输沙率从海滩至爬升沙丘坡脚及下部坡面逐渐降低，但在爬升沙丘最上部区域，当入射风速处于7-8m/s区间时，输沙率较海滩处提升4倍。风成搬运沉积物的平均粒径普遍小于地表表层沙，且沿上坡方向平均粒径逐渐增大。本研究还对比了野外实测输沙数据与风成输沙模型的结果，并探讨了坡度对风成输沙过程的影响。