Land subsidence in the Italian coastal plains: an overview

Land subsidence (LS) is the gradual or sudden sinking of the ground surface, due to a variety of processes amog which the consolidation of sediments and thus the rearrangement of earth materials in the subsurface as a result of increasing effective stress. Although LS occurs in different geographical settings, this phenomenon poses a major threat to low-lying coastal plains that are already facing the effects of climate change, i.e. sea-level rise and extreme rainfall events. LS increases the vulnerability of low-lying coastal areas to flooding and storm surges and causes severe damage to infrastructures and natural and historical heritages, which in turn harms economic production and coastal ecosystems. The causes of LS are multiple, both natural and induced. Those of anthropogenic origin are fairly well known and well documented. The natural ones, slow and silent (few mm/yr), are harmless but uncontrollable. With this contribution, we provide an overview of the LS affecting the coastal plain systems of the Italian peninsula. Coastal plains were considered by selecting the maximum land elevations of 10 m above mean sea level. For this purpose, we used TanDEM-X 30m Edited Digital Elevation Model (German TanDEM-X mission) acquired between 2010 and 2015. Then, land subsidence was investigated over the periods 2015-2021 and 2018-2022 using the European Ground Motion Service (EGMS) from Copernicus, which provides ground movement data obtained by SAR Interferometry on Sentinel-1 images. Purely vertical displacements obtained by combining ascending and descending acquisition geometries were used. Subsequently, the Italian coastal areas most subject to LS were highlighted and the potential mechanisms driving the phenomenon have been reported. Particular attention was paid to the lithological and stratigraphic characteristics of the Late Pleistocene-Holocene successions. The overlay of the main trends of subsidence with the litho-stratigraphic data suggests that the vertical ground deformation is partly controlled by the lithology and stratigraphic architecture of the geological units and, in particular, by the presence of compressible materials (such as peat and organic-rich clay), which are subject to primary and secondary consolidation. Obviously, discriminating the main driver requires more in-depth on-site analyses and the characterization of the geotechnical properties of the sediments are required, as compaction does not occur simultaneously for each layer, depending on the stratigraphic position (i.e. age and depth) and spatial distribution. In this regard, it appears of paramount importance a detailed reconstruction of the vertical stacking of the sedimentary units and their variation over short distances. While these results are still preliminary, they provide a basis for a better understanding of LS drivers in the coastal-deltaic plains and have the potential to support the management of impacts caused by this phenomenon.