Re-attachment zone characterisation under offshore winds blowing over complex foredune topography

Studies of the role of secondary airflow effects demonstrate the importance of offshore flows in dune growth and maintenance. Turbulent processes at the lee side of aeolian dunes have previously been only qualitatively described. The recent incorporation of ultrasonic anemometers, capable of measuring the three components of the wind vector, allows quantification of flow patterns in complex areas such as the lee side of dunes. This paper presents measurements taken with an array of ultrasonic anemometers during an offshore wind event at Magilligan Point, Northern Ireland, where flow separation and reversal associated to offshore winds has been previously reported. A simple analysis using the raw u and w components of the wind was conducted to extract quantitative information on the location of turbulent zones along a dune-beach profile. Results indicate sharp differences between the relation of u and w with distance downwind from the dune crest, which in turn can be used to identify turbulent zones. Variations in wind velocity and direction at the dune crest did not result in changes in the location of turbulent zones at the beach surface, suggesting that turbulent structures are significantly constant in time. A quantitative model based on actual field data and using previous conceptual descriptions as a guide is presented to identify turbulent zones at the beach surface under offshore winds.