Monitoring of two artificial gravel beaches from Marina di Pisa (Tuscany, Italy): preliminary data

Preliminary results of a monitoring carried out on the shore of Marina di Pisa (Tuscany, ltaly) are presented in this work: Marina di Pisa has been the centre of an impressive coastal defence project. This project has got started in the autumn of 2006 and will come to an end in the spring of 2008: it has been necessary due to the poor conditions of this area, stili struck by lingering episodes of erosion . In fact, dating back to the late XIXth century the Pisan shore experienced first erosive phenomena (due t o a strong decrease of River Arno sediments discharge caused, amongst others, by an uncontrolled excavation of the riverbed), progressively worsened up to seriously menace the city centre of Marina di Pisa. The defence of buildings and houses had become a pressing priority and it was realized by the accomplishment of hard protection systems like sea walls (rock armouring in particular): these sea walls solved the problem they were built for, but they caused the complete erosion of the fronting sand beaches as well. Subsequent forms of defence (breakwaters 80 m off the coast-line and grave l replenishments) were carried out, but with no particular gain even by an environmental impact standpoint. The above mentioned defence project has been put into execution in such a worrying situation: i t consists of the enlargement of few pre-existent groynes (subsequently linked at the head by submerged breakwaters) and of a huge gravel replenishment. A series of big cells will form that way, characterized by better water ctrculattOn, lessened envtronmental tmpact and wider gravel beaches. The monitoring ai m is to contro l the shore evolution in order to better define the processes acting on that sector of the littoral zone and, moreover, the need of integration and/or variations to the project. Besides, the monitoring has been extended to a beach (named Gorgona) one kilometre south of the cells where larger works h ave bee n executed: gravel replenishments have been realized a Iso on this beach, but the difference lies in the absence of offshore breakwaters. Therefore, it w ili be possible verifying the evolution of both these beaches given the sa me sea weather and w ave motion conditions. Thus beach profile and coast-line surveys have been accomplished at Gorgona beach using GPS technologies during various climatic conditions: so it has been possible keeping under contrai the beach evolution without the interference of any offshore structures parallel to the shoreline. At the same time sampling along transects going from the backshore to the shoreface have been made to define the grain-size variations normal to the coast-line. This observation has been particularly interesting because further shows that gravel and pebbles assume a dimensionai zonation on the backshore, while their diffusion under water is probably limited to the step, that is the limit foreshore-upper shoreface. Grain-size analysis has been conducted on every sample; the coarsest ones have undergone morphometric analysis as well j ust to better locate the mentioned zonation. Considering the remarkable peculiarity of the area in which this project has been rea lized (so much peculiar to probably make Marina di Pisa the only case of such a type), the obtained resu lts will be very useful not only to better understand processes acting on a gravel beach, but even to sort of deal with this kind of coastal defence works by improving the approach with which they are studied and at the same time to optimize them in the future .