A new project to monitor land subsidence in the Northern Venice coastland
(Italy)
L. Tosi, L. Carbognin, G. Brancolini
Institute of Marine Sciences, CNR, Venezia, Italy
P. Teatini
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
ABSTRACT
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 mbelow 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.
