Land subsidence in the Friuli Venezia Giulia coastal plain, Italy: 1992-2010 results from SAR-based interferometry

Land subsidence is a concern in many coastal plains worldwide, particularly in the low-lying areas already facing sea level rise due to climate change, and much still needs to be done, with respect to both mapping land subsidence and gaining a comprehensive understanding of the relevant cause-effect relationships. Land subsidence of the northern coastal plain encompassing the Friuli Venezia Giulia (FVG) region in Italy, remains, to the authors' knowledge, poorly investigated. This coastland includes low-lying agricultural and urban areas and highly valuable lagoon environments, archaeological and touristic sites, and industrial zones. Here, we resolve land subsidence in the coastal plain between the Tagliamento River delta and the Isonzo River mouth over the period 1992-2010 using Envisat ASAR and ERS1/2 interferometric datasets. We identify a large variability of the land subsidence and a spatial gradient that ranges from less than 1mm/year in the high southwestern plain toward the littoral to more than 5mm/year close to the Tagliamento River delta. A comparison between the 2003-2010 and 1992-2000 sinking rates depicts quite similar behaviors of the process over the two time spans. The analysis indicates unclear correlations between ground movements and the typical driving mechanisms acting in the north Adriatic coastal plains, such as the variability of the morphological setting, the subsoil characteristics and the land use. We reason that multi-component mechanisms contribute to the observed image of the subsidence in the FVG coastland. Specifically, anthropogenic activities, e.g., groundwater exploitations, hydraulic reclamations and the development of newly built-up areas, are superposed to natural mechanisms related to the spatial variability of the subsoil characteristics, typical of transitional coastal environments.