Sediment displacement on a corse artificial beach at Marina di Pisa: volumes shift (preliminary data)

A reckoning of sediment displacement in terms of volumes shift on a beach is paramount where onerous investments have been spent to guard an area against erosion through coarse replen ishments. This research has been carried out on the coast of Marina di Pisa (Tuscany), where a huge gravel and pebble replenishment was set up to protect the town from the dangerous effects of strong storms. The Pisa n coast has been affected by lingering erosive processes, and the area of Marina di Pisa makes no exception. The erosion sprang in the late XIXth century due to a harsh decrease in River Arno's sediment discharge: the uncontrolled excavation of the river-bed, the widespread construction of hard embankments and the rigid structures set up to reduce the hydrological hazard of mountain slopes, ali combined to dwindle the bulk of the sediments that naturally fed the beaches. The eros io n wiped out long sectors of the sandy beaches, which characterized the Pisan coast, and became a serious menace even to the centre of Marina di Pisa. Hard protection structures (g roynes, detached breakwaters and rock armour) were built to counteract the land retreat, which eventually ceased at the expense of the natura l sandy beach environment. In the late Nineties a first gravel (4-to-lOmm) replenishment was carried out to both protect the coast and give the beach some tourist appeal. The shortcoming of this grain-size was that the major storms which struck the coast literally cast the grave l onto the promenade, with any kind of risk to the population. Therefore, in 2006 a new, la rger replenishment was carried aut, th is t i me using 30-to-90mm marble pebbles. This rep lenishment was realized on three beaches along the shore of Marina di Pisa; the beach where this research has been performed on is named Cella 7. The Cella 7 is a 250m long, 30m wide pebble beach bound at both ends by groynes, and seaward by a submerged breakwater SO m off the coast· line. The sea-bottom is very steep, since the 3m depth is reached within less than 20m from the coast-line. The sea-bottom between the step and the breakwater is entirely constituted by sand. A number of su rveys has bee n made on th is beach after any significant storm since the autumn of 2007: during each survey the subaerial beach profile was registered by means of a highly accurate GPS instrument. Basically, a point every 10m was registered a long the shore on the mai n features that define the beach, i.e. the step, the beach-face, the ordinary berm, the storm berm and the base of the lee-side of the storm ber m. Moreover, an extremely accurate profile was taken every SO m, fora total of five profiles in which points were reg istered o n any significant slope change. The submerged part of the beach was surveyed with an echo-sounder Single Beam instrument. Data from these two sectors of the beach were merged afterwards with ArcGIS software. Then, the outcome of the m erge was processed yet aga in to yield meaningful indications about sediment displacement and volumes shifting. Preliminary data point out a remarkable retreat of the coast-line a long with the> formation of an impressive and steep storm berm . The measured retreat after a series of intense storms reaches the 13m mark; though not particularly worrisome, this feature forces local authorities to leve l the beach at least once per year. The bulk of the pebbles is pushed landward especially during highest energy storms, while a loss of sediments out of the celi del imited by the groynes an d the submerged breakwater is no t detected. Therefore, these pre liminary results show a slight volumetric increase of the subaerial beach after most significant high-energy events.