Land subidence monitoring service
T. Strozzi, U. Wegmuller, C. Werner
Gamma Remote Sensing, Muri (BE), Switzerland
L. Tosi, L. Carbognin
Istituto per lo Studio della Dinamica delle Grandi Masse, CNR, Venezia,
Italy
P. Teatini
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
Land subsidence due to natural and anthropogenic causes has represented one
of the most serious environmental problems for the Lagoon of Venice. Land
subsidence has increased the vulnerability and the geological hazard
(i.e. river flooding, riverbank stability, intrusion of seawater in the
aquifer system, deteriorating of the littoral sectors with a general coastline
regression and an increment of the sea bottom slope close to the shoreline)
of these areas, a large portion of which lies below the mean sea level.
After the regulation of groundwater exploitation from the Venetian
aquifer-aquitard system, a remarkable slowing down of the induced subsidence
in Marghera (industrial zone), in the historical center of Venice and along
the littorals was ascertained in the 1970's. Recent studies have shown that
land subsidence is still in progress in the southern and northern coastal
areas and in the nearby mainland. In these areas groundwater is extracted
from artesian wells, thicker and more compressible Holocene sediments are
present, and organic soil oxidation takes place in reclaimed zones.
Until 1999, levelling of the Venice region was carried out only along the
coast and the lagoon edges and the monitoring network was composed by benchmarks
along single levelling lines; a fine grid network was established only in
the city of Venice. In recent years, the levelling network has been updated
to cover with a grid of about 1000 benchmarks all the southern part of the
Lagoon, and plans exist to cover also the northern sector. The same network
used for the levelling surveys is also considered for differential GPS
measurements. In addition to these ground-based methods, differential SAR
interferometry using long series of SAR data and the interferometric point
target analysis have emerged as very promising tools for the monitoring of
land subsidence at high spatial resolution and with a more complete spatial coverage.
The objective of the Data User Programme (DUP) Small Services Project VENEZIA -
started in November 2001 and to be completed in November 2003 - is to implement
a land subsidence monitoring service in the Lagoon of Venice for regional
administrative and water authorities. An important element of the project
is the integration of SAR-based monitoring techniques (conventional differential
SAR interferometry and the interferometric point target analysis) with levelling
and GPS surveys into an overall database and information system capable to provide
the best knowledge of the subsidence process to the regional, administrative
and water authorities that manage the Po Plain area around the Lagoon of Venice.
As it can be observed in the figures for two specific areas around the Lagoon of
Venice, SAR interferometry exhibits complementary characteristics to the levelling
surveys, because it has the capability to map urban areas at high spatial resolution.
In particular, SAR interferometry is very useful to perform detailed investigations
in coastal areas: in Chioggia and Jesolo a significant seaward gradient in land
subsidence, practically impossible to be detected by other traditional monitoring
techniques like geometric leveling and differential GPS, is visible. A critical
situation is also observed in Eraclea, where the sinking velocity reaches 4 mm/year.
The high precision levelling surveys, on the other hand, are used outside of the
cities, as to the east of Chioggia, and to set up a reference point for the SAR
subsidence values. The high accuracy of the two surveying techniques is confirmed
by their cross-validation along the levelling lines.
