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 (CV_U1, CV_U0.25) 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.