Tracking clast mobility using RFID sensors on a boulder beach in Galicia, NW Spain

Abstract

Previous studies using unmanned aerial vehicles and remote sensing techniques have provided data on group sediment dynamics and overall mobility of a boulder beach at Oia in Galicia, northwestern Spain. They recorded changes in the location and disposition of hundreds of clasts but were unable to trace the intermittent movement of individual boulders. Radio Frequency IDentification (RFID) sensors were used in the present study to identify and record the long-term trajectories of boulders in this area. Sensors were installed in 80 boulders (mean intermediate axes approximately 55 cm) in September 2016 and were used to locate and record their displacement on five occasions, terminating in December 2019. Although burial and other factors prevented data being collected from every tagged boulder during these periods, recovery rates ranged from approximately 50%–75% of the original population. There was considerable variability in the displacement of the boulders, with maxima of >20 m and mean values >5 m. The distance of travel tended to decrease during the course of this study, despite a corresponding increase in storm frequency and intensity. Patterns of movement, including transport distance and direction were essentially chaotic in nature, reflecting the complex interaction and compound effect of a multitude of controls related to such factors as the location of boulders on the beach, their relationship to surrounding boulders, including the effects of sheltering, interlocking, and burial, and their size, shape, and other physical characteristics. The unpredictable behaviour and high transport thresholds of boulder beaches are in contrast to sand and gravel beaches that are characterized by more uniform and predictable patterns of sediment transport and morphological change.