Using high resolution data to reveal depth-dependent mechanisms that drive land subsidence: The Venice coast, Italy

Recent research has provided a high-resolution map that depicts the effect of land subsidence on the Venice coastal plain of Italy. The map, which covers the decade of 1992 to 2002, was obtained by an innovative “Subsidence Integrated Monitoring System” (SIMS), which efficiently merges the different displacement measurements obtained by high precision-leveling, differential and continuous Global Positing System data (GPS), and Synthetic Aperture Radar (SAR)-based interferometry. The displacement rates exhibit significant spatial variability, ranging from a slight 1 to 2 mm/yr uplift, to a serious subsidence of more than 10 mm/yr. This paper aims to describe the many natural and anthropogenic mechanisms that drive the pattern of the ground displacement. The movement sources are presented based on their depth of occurrence. Deep causes act at depths generally greater than 400 m below m.s.l. (mean sea level), and are recognizable in the movement of the pre-Quaternary basement. Medium causes act at depths between 400 and 50 m below m.s.l., and include geological features, such as a major presence of compressible clay layers in the southern and northern portions of the study area and groundwater withdrawals, mainly in the north-eastern coastland and western mainland. Shallow causes, i.e. those occurring from a depth of 50 m up to the ground surface, are related to the architecture and geomechanical properties of the Pleistocene and Holocene deposits, which are more thick and compressible approaching the littoral belt; geochemical compaction, due to the increasing salt concentration in the clayey sediments; and oxidation of the outcropping organic soils drained by land reclamation. These two latter factors primarily involve the southern portion of the Venice coast. The building loads in newly developed areas also cause local compaction of shallow deposits. We conclude that the consolidation of Holocene deposits and anthropogenic activities (groundwater withdrawal, land reclamation, and urban land use) are the major factors that contribute to the present land subsidence in the Venice coastland.