Search and characterization of relict sand deposits on the tuscany continental shelf for the artificial nourishment of eroding beaches

The management of the beach erosion processes is a paradigmatic example of the difficulty in combining the different aspects of social and economic development with the preservation of the environment and its resources. As a matter of fact, there is not a perfect solution to the beach erosion problem. However while during most of XX century the remediation mainly relied on building hard defense structures (jetties and breakwaters), since the 90’s in Italy beach nourishment projects became common, i.e. beaches were fed with sand quarried elsewhere to compensate the scant sedimentary input. Such remediation strategy does not downgrade the coastal landscape and does not impact negatively on adjacent beaches; on the contrary, the latter position undergo a sedimentary input increase, so avoiding the feedback spiral of erosion -> protection -> new downstream erosion -> new protection ->… that led to the cementification of large spans of the Italian coast. However it is noticeable to observe that, because of the high coast and of the exploitation of not-renewable resources, beach nourishment can only be performed on selected parts of the coast and is not a long-term solution. Sapienza University of Rome and Regione Toscana carried out a research project to search and characterize sandy deposits lying on the Tuscany continental shelf, to be used for beach nourishment purposes. Such project lasted 3 years and encompassed four phases with an increasing detail degree. Ten marine surveys (six devoted to geophysical data acquisition and four to subsurface sampling) were ruled out. A very large single-channel high-resolution seismic, multibeam and vibrocore dataset was acquired. The inner-middle shelf is mainly covered by a thick Holocene muddy blanket, emplace during the present sea-level high-stand (that started at 8- years ago); the muddy sediments were and are brought to the basin by the major rivers (Arno and Ombrone) and redistributed by the shelf currents; they lies on Plio-Pleistocene prograding depositional sequences, deposited during sea level falls, are mainly muddy and are truncated at their top by an erosional surface that was created during last sea leve low-stand and following sea level rise. As far as relict sandy deposits are concerned, the uneven morphology and wide extension of the Tuscany continental shelf allowed the formation and preservation of littoral deposits. In a few shelf areas, such deposits have not been completely buried by the Holocene muddy sediments and therefore may be potentially quarried for beach nourishment projects. The sandy deposits show a complex setting and heterogeneous nature. Some were formed during low-stand (such as submerged depositional terraces East of Capraia Island and around Giglio Island), other during early sea-level rise (south of Elba Island and between Argentario promontory and Giannutri Island), other during the advanced sea-level rise (offshore Piombino and Massa) and finally others were formed by coastal currents reworking the littoral prism such as the deposits offshore Massa. Such different genesis produced different grain size and composition, that go from mainly carbonate gravelly and (depositional terrace or deposit formed during early sea-level rise) to siliciclastic sand and sandy-mud (deposits formed during transgression on littoral environment or reworked by coastal currents). The goal of the project was thus accomplished and a complete census of the sandy deposits prone to beach nourishment has been realized. Such census will allow Regione Toscana to manage in the appropriate way such not renewable resource, in order to mitigate the erosion of sandy beaches.