The impact of coarse sediment tracing experiments for an in-depth characterization (management; planning) of artificial pebble beaches

In this paper the results of a series of innovative tracing experiments about coarse sediment transport have been put together and compared in order to discern which are the major factors affecting the morphodynamics of coarse-clastic beaches. Gravel beaches are now frequently used as a form of coastal protection against erosion processes, mainly because coarse sediments are not entrained and displaced as easily as finer particles such as sand. Several aspects about the behaviour of these beaches are yet to be fully understood though. The fieldwork activities were carried out at three artificial pebble beaches located in Marina di Pisa (Tuscany, Italy). Pebbles collected from those beaches were marked by means of the radio frequency identification technology, which allows to couple each pebble with a transponder. This technique guaranteed high recovery rates both in the subaerial and underwater environment. The displacement trajectories showed that the movement patterns of the tracers is tightly connected to the sea floor topography, which affects wave motion and how swash flows approach the shoreline. Experiments aimed at analyzing the abrasion rate of the pebbles also showed that mass loss is noteworthy in limited time frames. As it was a somewhat unexpected resultance, subsequent experiments demonstrated that pebbles are subjected to relevant shifting also during short fair-weather periods. All these novel findings suggest that the use of artificial coarse-clastic beaches to replace sandy shores in order to increase the protection of coastal areas is not just a matter of unloading impressive volumes of particles coarser than those naturally present, because also a coarse grain-size may be subjected to high rate of reworking and transport. In particular, mass loss is a factor that must be taken into proper consideration, even for the possible repercussions to water quality due to the significant amount of calcium carbonate released by the abraded marble pebbles.