Radar interferometry-based mapping of the present land subsidence along the low-lying northern Adriatic coast of Italy

The northern Adriatic coastal area, Italy (from the Veneto region northward, and to the Romagna region to the south), is characterized by low-lying environments such as lagoons, e.g. those of Venice and of the Valli di Comacchio, wetlands and deltas, such as that of the Po River delta, and reclaimed farmland and beaches subjected to marked anthropogenic pressure. The coastal area is characterized by an elevation generally well below the mean sea level (down to 4 m m.s.l.) and never exceeds 2 m above m.s.l. Man-induced land subsidence has greatly affected the whole coastal area over the 20th century, and especially from the 1950s and the 1970s, when over-exploitation of subsurface fluids was responsible for the occurrence of general lowering. Although the sinking rates have significantly decreased over the last decades, land subsidence is still a process threatening the entire coastal environment. In this study we report a recent investigation aimed at mapping the vertical displacements recorded in the period 1992-2000 on the 20-30 km wide and 250-km long coastal area from the Tagliamento River to the north, to the town of Rimini to the south. Measurements have been carried out by Persistent Scatterer Interferometry (PSI) using levelling and GPS records to reference the interferometric solution. The results identify the variability of the ground movements that are presently occurring along the northern Adriatic coastline. Substantially stable areas, the most important of which are the cities of Venice and Ravenna and their surroundings, contrast with subsidence rates of more than 10 mm/year recorded in some parts of the Po River delta and to the south. The observed land displacements have been associated with the geological features of the study region, i.e. tectonics and differential consolidation of the Middle Upper Pleistocene and Holocene deposits, and to anthropogenic activities, mainly groundwater withdrawal from the Upper Pleistocene-Holocene alluvial deposits and more locally peatland oxidation in reclaimed areas and gas exploitation from Plio-Pleistocene reservoirs.