Differential movement of pebbles marked by RIFD tags on two artificial coarse beaches (Marina di Pisa, Italy)

Longshore and cross-shore sediment movement is a longtime studied topic because of the importance it has on the management of a littoral area and on sediment budget assessments. Assessing this issue on coarse beaches, it is even more pressing because grave! beaches are being often utilized as a form of shore protection against erosion processes. The coast of Marina di Pisa (Tuscany, Italy) presents two artificial pebble beaches (named Cella 7 and Barbarossa) which ha ve bee n set up to counteract the effect of both the erosion and the major storms that periodically struck and endangered the littoral area. They are made of 30-to-70mm diameter pebbles and bounded by groynes at both ends. The biggest difference between these two beaches is represented by the absence of a submerged breakwater 60m off the coast-line of Barbarossa beach. This study is aimed to better define the movement of pebbles in those two particular schemes in relation with seaweather conditions. The RFID technology, which is one of the most common automatic identification technologies, has been used to carry out the research. This technique allows the detection of pebbles marked with transponders. A specific technical solution has been studied in order to make the pebbles detectable also under water: in particular the realized system works at the operative frequency of 125kHz, which provides good performances and few interferences from the surrounding environment. A particular structure has been studied in order to realize a waterproof reader and many tests have been performed, with different kinds of transponders, to ensure the efficiency of the system. About two hundred pebbles have been drilled and a transponder has been inserted and glued in each one of them. Those pebbles have been accurately weighed and a picture 78 of each one has been taken. Then, about one-hundred pebbles have been placed within the ordinary berm and the step along both beaches: the position of each pebble has been recorded with a highly accurate GPS instrument. The retrieving campaign, which represents the second phase of the research, is performed after any storm occurred on the Marina di Pisa coast. The marked pebbles are found back using the reader and their position recorded with the GPS instrument. Once retrieved, they are weighed and pictured again to enable evaluation of te x turai parameters like abrasion and roundness rates in the future. The retrieving phase willlast up to the end of May 2009, just before the opening of the summer recreational activities of the beaches. A t the end of the spring the storm berm is levelled in order to make the beach more attractive to tourists, causing the end of the natura] observation though. Therefore, the marked pebbles will be retrieved at the end of May, but not thrown again immediately. The experiment will resume for further tests in early October, when human activities will not affect the beach. First campaigns ha ve shown a retrieving rate to the tune of 70% o n the Cella 7 beach. The bulk of the marked pebbles has been found back on the foreshore, which means pebbles tend to move offshore. Moreover, in particular in the northem sector of the beach, a slight northward displacement of the pebbles has been pointed out. This trend is probably due to the south-westerly provenance of the major storms occurred on the coast in this span of time. Positive outcomes of this research could mean a new and inexpensive approach to sediments movement research and sediments textural properties reckoning. Besides, the results could be spent to optirnize and improve future artificial grave! nourishments.