| dc.description.abstract | More accurate modelling of aeolian sediment transport in coastal environments is crucial to improving the planning and efficiency around soft coastal protection measures, such as sand nourishments, and a full understanding of the underlying transport mode is key to achieving it. This study aims to contribute to that widely shared purpose by trying to crack the code of the highly dynamically complex aeolian sand streamers. A wide range of different aspects of streamers have been investigated by exploiting an at least as wide a range of different both qualitative and quantitative analytical techniques. Amongst others, this report presents a series of spatiotemporal saltation transport maps as well as one of the first known applications of the non-globalised wavelet transform to time series of transport intensity. In October 2017, saltation intensity records were collected at the beach nearby the Dutch town of Egmond aan Zee using three replicas of the newly developed Saltation Detection System (SDS), while co-located ultrasonic anemometers provided synchronised records of 3-D wind measurements. In addition, time-averaged water levels were measured just off the beach and in two cases surface moisture levels were monitored along a transect parallel to the wind field upwind of the SDS. Four sequential streamer patterns were recognised under intensifying transport conditions, in which spatiotemporal variability in saltation intensity is systematically reduced. Individual streamers were found to to have a typical head-tail structure, coming up quickly, then slowly dwindle. High-intensity streamers were found having lengths of up to 0.5 m, widths of the order of 0.2 m and centre-to-centre spacings of up to 0.5 m. Neither shear velocity nor turbulence kinetic energy provide deterministic relationships for saltation intensity, but a clear, strong response is recognised of saltation intensity to increasing shear velocity. Coupling between the latter two variables is evident on temporal scales of the order of tens of seconds to minutes, which reflects the sizes of the turbulent eddies associated with streamer formation. Surface moisture greatly inhibits streamer formation if the moisture threshold is exceeded, but given that shear velocities are high enough, just a 10-m length of beach with (slightly) below-threshold moisture levels is sufficient for streamer formation to rapidly recover. Most (intense) streamers were seen on the mid beach, with less forming higher up the beach until some distance close to the dune foot, from where an opposite downwind trend was observed. The SDS was instrumental in gaining a better understanding of the saltation system on narrow beaches under all sorts of circumstances, but really comes into its own in low-energetic environments and under fair-weather conditions. The results of this study can hopefully be used as one more step towards unravelling the mysteries behind streamers, and finding the best methods and analytical techniques for future research to achieve this. | |
