Effect of vegetated foredunes on wind flows and aeolian sand transport - A field study of Aeolex II
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Coastal foredunes are important for coastal safety and biodiversity. The complex interplay between wind, morphology, vegetation and aeolian sediment transport across foredunes has been widely studied. This study aims to add to existing knowledge by studying the controls of foredune growth on high, densely vegetated foredunes. Field data were collected during a period of five weeks in Egmond aan Zee, the Netherlands (AEOLEX II campaign). This site is characterised by a wave-dominated beach with flat slope (1:30), approximately 20 m high foredunes with steep slope (1:2) and dense cover of European marram grass (Ammophila arenaria). Wind velocity, direction and turbulent kinetic energy (tke) were measured across the foredune, while sand transport was recorded on five days. Moreover, vegetation surveys were done across three transects, which were complemented with sedimentation data from LIDAR elevation maps. Results show that depending on the incident wind direction, the wind flow changes in both magnitude and direction. A similar pattern was observed for all onshore wind directions: the wind was first reduced and deflected to alongshore, followed by acceleration up to 310% and turning to perpendicular onshore. Highly oblique to alongshore directed flows were hardly altered at the foot but deflected up to 38° to more onshore at the crest. In absolute values the tke was directly proportional to the wind velocity. But relatively (r), it was largest at the dune foot for perpendicular onshore winds and on the slope at highly oblique to alongshore winds. Aeolian transport decreased substantially across the foredune. Fluxes at the upper dune foot increased first to 328% and subsequently decreased. Transport fluxes varied for different days, most likely due to the wind velocity and maximum fetch size. The vegetation assays showed that the foredune was covered densely (20 – 100%), reaching its maximum at the crest. The vegetation strongly influences the foredune, since most sedimentation was related to vegetation cover. The majority of wind flows were observed, resulting in a conceptual understanding on how wind changes across the foredune. Most of the findings are in line with previous studies. The study showed that the foredune has large influences, both on wind characteristics as well as on transport.