Geological evolution and land subsidence of the Volturno River coastal plain (northern Campania).

Most of the world’s floodplains are affected by subsidence, a phenomenon that includes, among others, the salinization of the aquifer, coastal erosion, greater vulnerability to flooding and storm surges, structural damage to infrastructure. In the Mediterranean, numerous floodplain plains are affected by subsidence due to both the rise in global sea level and other natural and anthropogenic factors. In analyzing the potential drivers of subsidence, in recent times the sedimentological evolution of modern river deltas has been considered, which began to form around 6500 years, after the fall of sea level in the last glacial phase (Last Glacial Maximum - LGM) and the subsequent aggradation and Holocene progradation. The Incised Valleys (IV) formed during the LGM were filled by fluvial-lacustrine, transitional and marine deposits, characterized by sands, silts, clays and peat. Recent studies confirm that natural compaction can cause a decrease of several millimeters for years, especially in coastal deposits rich in organic substances. The aim of the present study is to define the stratigraphic architecture of the Holocene and the correlation with the ground deformation trends referring to two decades (1992-2010), which characterize the coastal alluvial plain of the Volturno River, in northern Campania. Assessment of subsidence trends was previously based on SAR (Synthetic Aperture Radar) interferometry techniques. The reconstruction of the satellite time series shows a general tendency to the ground subsidence over time of the whole area and a strong negative subsidence in three sites between the coast and the lower valley of the Volturno. The reconstructed stratigraphic architecture has shown that the unit which is the first substrate for Holocene and Recent sedimentation is represented by the Campania Grey Tuff (CGT), produced by the explosive volcanic activity of the Phlegrean Field about 39 ky and deposited over the whole Campana Plain. The CGT was deeply eroded by the fluvial activity during the LGM and the resulting Incised Valley was subsequently filled by fluvial-lacustrine, transitional and coastal deposits. The spatial intersection of the deformation data with the geological data showed a net overlap of the subsiding areas with the paleovalley contour. Furthermore, higher subsidence rates affect areas characterized by thicker Holocene sedimentary sequences and in particular where silt, clayey silt, clay and peat are the main lithologies. The latter,from a geotechnical point of view, are classified as fine grained soils with poor mechanical properties (high compressibility and low resistance). Furthermore, the inclusion of a significant amount of peat and organic matter is reflected in high values of secondary consolidation coefficient. This suggests that if anthropic activities can be the cause of a generalized process of primary consolidation, a key role is played by sthe tratigraphic structure and in particular by distribution and thickness of Holocene deposits, whose characteristics are at the origin of secondary consolidation and of the variability observed among subsidence rates. The results of this study confirm the importance of an integrated study for the assessment of hazards to coastal areas.