Aeolian processes on a natural beach, spatial variability in wind characteristics

Abstract

In order to further increase the knowledge about the transformation of wind patterns on the supra and intertidal beach during onshore wind events a field experiment was launched during October 2017 on the beach near Egmond aan Zee. Part of this field experiment was measuring wind on different (cross shore) locations on the beach using four mobile sonic anemometers capable of making three dimensional measurements of wind vectors at a rate of 10hz. Based on the data gathered in the field experiment the following research question is answered: “How do regional wind patterns compare to local wind patterns on the beach during different on-shore wind events?” Additional knowledge about onshore wind patterns is wanted in order to further improve the predictions of dune development thereby enhancing the water safety of the hinterland. The fieldwork campaign carried out in this research was highly successful due to almost perfect conditions. After the dataset gathered during the experiment was analyzed, it became clear that the wind decreases across the width of the beach during onshore wind events. The amount of decrease is highly dependent on the incident wind direction. Whereas onshore wind events show a potential decrease upwards of 60%, oblique onshore and oblique alongshore show a maximum decrease of 45% and 23% respectively. It has been observed that the turbulent kinetic energy remains constant across the beach. The wind direction changes across the beach and tend to become more dune parallel when the dune foot is approached. Compared to regional wind speed measurements, measurements performed at the back of the beach show a decrease of 48%. Almost half of this decrease however is due to the logarithmic vertical velocity profile of the wind. Furthermore, it has been established that the regional wind direction measurements could closely reassemble local wind directions when a fifth order polynomial formula is applied on the regional measurements. The new insights regarding the decrease in wind speed over distance further complicates the already complex cosine effect. Additional research regarding the interaction of the cosine effect with the new results of this study therefor is recommended.