Variabilita' spaziale della subsidenza attuale nell'area veneziana
(Spatial variability of the present land subsidence in the Venice area)
L. Tosi, L. Carbognin, G. Brancolini, F. Rizzetto
Institute of Marine Sciences, National Research Council, Venezia, Italy
P. Teatini
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
ABSTRACT
A recent research has provided a high resolution
map of the present land subsidence of the Venice
coastland (Italy). The map, which refers to the 1992-2002
decade and covers an area of about 100x100 km2, has been
obtained by the innovative "Subsidence Integrated Monitoring
System" (SIMS) that efficiently merges the different
displacement measurements obtained by spirit leveling, differential
and continuous Global Positing System (GPS),
Synthetic Aperture Radar (SAR)-based interferometry.
The investigation points out a significant spatial variability
of the ground vertical movements, both at regional
and local scales, and displacement rates ranging from a
slight (1-2 mm/yr) uplift to a serious subsidence of more
than 10 mm/yr.
Tectonics, differential consolidation of the Pleistocene
and Holocene deposits, and human activities, such as
groundwater withdrawals, land reclamation of marshes and
swamp areas, and farmland conversion into urban areas,
superimpose to produce the observed displacements. In
this work we have distinguished the displacement components
on the basis of the depth of their occurrence.
Deep causes, acting at a depth generally greater than
400-600 m below m.s.l., refer to downward movements of
the pre-Quaternary basement and land uplift (up to 2
mm/yr) most likely related to neo-tectonic activity connected
with the Alpine thrust belts and a NW-SE fault system.
The displacement factors located in the medium depth
interval, i.e. between 400 and 50 m below m.s.l., are of both
natural and anthropogenic origin. The former refers to the
Medium-Late Pleistocene deposits that exhibit a larger cumulative
thickness of clayey compressible layers at the lagoon
extremities with respect to the central lagoon area where
stiffer sandy formations prevail. Land subsidence due to
aquifer exploitation mainly occurs in the north-eastern sector
of the coastland where thousands of active wells are located.
In a 10-15 km wide coastal strip thickness, texture, and
sedimentation environment of the Holocene deposits play
a significant role in controlling shallow causes of land subsidence.
Other factors that contribute in increasing land
sinking at a smaller areal extent are the salinization of clay
deposits due to saltwater intrusion and biochemical oxidation
of outcropping peat soils. Even the load of buildings
and structures after the conversion of farmland into urbanized
areas cause local shallow compaction.
