Transport trajectories of “smart” pebbles on an artificial coarse-grained beach at Marina di Pisa (Italy): implications on beach morphodynamics

The definition of pebble transport and movement along the shore has been subject of revived interest lately mainly because of significant advancement in the technologies that enable to trace such coarse sediments. The chance to improve the understanding of a topic sometimes neglected in the past has been favoured by recent refinements on the Radio Frequency Identification (RFID) technology, which has proved to be a reliable tracing technique, perfectly suitable for pebble-sized sediments. The RFID technique allows to couple the pebbles to small, discoidal transponders (tags). These “smart” pebbles are detected back by an antenna (reader), which transmits low frequency (125 KHz) radio signals. The choice to opt for the low frequency was suggested by its capability to better transmit and receive the radio signals even underwater. The research was performed on an artificial pebble beach at Marina di Pisa (Tuscany, Italy), a small village located on the Pisa coast, which has been subjected to serious erosive processes since 1850. The original sandy beaches could not be preserved, thus prompting local authorities to plan coarse replenishments. The artificial beach, named Barbarossa, was set up in its final configuration in 2007. Presently, it is 110 m long and 20-to-35 m wide. It is bound laterally by two groynes, which impede longshore sediment movement. One-hundred and two pebbles were sampled on the beach and carefully prepared for the installation of the transponders, which univocally identify the pebbles because each tag possesses an unambiguous code. The tracers were injected on the beach along cross-shore transects, three pebbles each (on the fair-weather berm, on the beachface, and on the step crest). The release position of each tracer was recorded with a total station. The recovery campaign was carried out after three storms, which all came from the same direction (SW). Fifty-three smart pebbles were found back, which is not a negative outcome considering the high rates of sediment reworking on the backshore of Barbarossa due to the intense wave energy this beach is subjected to during the storms. The recovery position of the pebbles was recorded as well. The analysis of the tracer displacement pattern highlighted a tendency of the sediments to move towards the storm berm, where 55% of the smart pebbles was detected. This movement is probably related to the energy of the incoming waves, hardly dissipating in the surf zone of Barbarossa. The waves unload their energy almost entirely on the beachface, where they force a sediment onshore movement rather than offshore. In addition, the smart pebble transport pathways outlined the presence of two distinct convergence area at Barbarossa. The tracers showed the tendency to converge to two separate sectors along the beach. A topographic survey of the sea-bottom fronting the beach was carried out prior to the recovery campaign by means of a single-beam echosounder. Data processing showed the presence of two portions of the sea-floor characterized by shallower depth. These shoals are just opposite to the two convergence areas pointed out by the pebble movement. These features of the sea-bottom might imply refraction and diffraction processes of the incoming waves, which in turn are responsible of the generation of convergent fluxes on the beachface. The convergent pattern of these fluxes leads to the transport trend of the smart pebbles. These results confirm the existence of a tight connection between the sea-bottom morphology and sediment transport on a beach. In-depth considerations about pebble movement on a coarse-grained beach might be useful to an optimization of future coarse replenishments, since they are fre - quently used as a form of coastal protection. In addition, a better definition of the dynamics acting on this environment might support studies on similar, ancient depositional settings