The use of Passive Integrated Transponder (PIT) tags to trace cobble transport in a mixed sand-and-gravel beach on the high-energy Oregon coast, USA

Abstract Cobble transport is investigated on a mixed sand-and-gravel beach on the high-energy Oregon coast using Radio Frequency Identification technology (RFID) to detect Passive Integrated Transponder (PIT) tags inserted in the gravels. PIT tags are glass-encapsulated transponders that are activated when an antenna passes near them, with each tag characterized by its own unique identification number. The tags are sealed within the cobbles, minimizing any effect on the hydrodynamic character of the particles. The tags draw power from the antenna itself, and have a potential lifespan of up to 50 years. PIT tags are simple to use and inexpensive, permitting the release and tracking of large numbers of gravel particles. As a result, PIT tags have tremendous potential for examining the complexities of particle movement and the parameters that control their distribution on the beach. Our experiments have been conducted at Cape Lookout State Park on the northern Oregon coast, on a composite beach where a seasonally variable sand beach fronts a mixed sand-and-gravel beach, with the latter impacted by waves only during the winter when the sand beach is cut back. A total of 400 cobble tracers have been released at five different locations. Subsequent monitoring of the gravel tracers revealed that they can be detected up to 1 m below the surface of the beach using a 1.0 m diameter antenna, and up to 0.5 m below the surface using a smaller 0.3 m diameter antenna, while their positions were determined using a Trimble 5700/5800 RTK-DGPS survey system. Gravel tracer recovery rates were found to vary significantly between the different release points, from as high as 90% recovery after 8 months, to a low of 18% after 17 months. The lowest recovery rates are thought to be due to the extent of aggradation of the sandy beach during the summer, partially covering the gravel berm and burying the tracer particles to depths where they could not be detected by the antennas used with our RFID system. Relocation of the gravel tracers has revealed a predominant northward migration of the cobbles, and locally demonstrate a strong cross-shore transport (both upslope and downslope movements). Initial attempts to examine the relationship between the transport distances and particle sizes and shapes have thus far revealed that the larger gravels are tending to outrun the smaller particles. This is despite the fact that both small and large particles were found to be capable of being transported both short and long distances. In contrast, our results revealed no relationships between particle shape and distance moved. In time, it is expected that the PIT tag tracers may begin to yield more definitive patterns of sorting. The results did highlight that the gravel movement occurs predominantly near the sand/gravel beach junction on the profiles, a region dominated by a higher incidence of wave breaking and strong swash activity. Finally, our analyses revealed the potential for using PIT tags to provide field assessments of the abrasion rates of gravel particles, having already found measurable degrees of weight loss to abrasion in our experiments.