Quantitative evidence that compaction of Holocene sediments drives the present
land subsidence of the Po Delta, Italy
P. Teatini
Dept. Mathematical Methods and Models for Scientific
Applications, University of Padova, Padova, Italy
L. Tosi
Institute of Marine Sciences, CNR, Venezia, Italy
T. Strozzi
Gamma Remote Sensing, Gumligen, Switzerland
Deltas are highly dynamic coastal systems that over the last few decades
have generally experienced a substantial area loss caused by trapping of
river sediments in upland drainage basins as well as land subsidence due
to natural and anthropogenic causes. A major example is the Po Delta in
the Mediterranean in northeastern Italy. This area has experienced as much
as 3 m of land subsidence from the 1930s to the 1970s primarily because of
the extraction of gasolsibearing waters. However, present subsidence rates
are largely unknown and the ground settlement is supposedly controlled by
natural long-term deep processes. We have combined radar Interferometric
Point Target Analysis (IPTA) with previous geomorphological investigations
on aerial/satellite images and seismic surveys, and geochronological data
from core samples and geomechanical in situ tests, to assess the current
sinking of the delta and to understand the processes controlling the vertical
movement. The high density of the measurable point targets (more than 15,000)
allows characterization of the spatial variation in the vertical land motions
(VLM), ranging from 1 to 15 mm/yr. We find that subsidence rates are
significantly correlated with the age of highly compressible Holocene
deposits that compose the shallowest 30-40 m of the sedimentary sequence.
A typical log-type consolidation equation applicable at the scale of the
entire delta has been obtained. We conclude that the consolidation of late
Holocene sediments is the major cause of the present land subsidence in the
Po River delta. This finding has significant impact on the understanding of
many other modern deltas that were formed in the lower Holocene epoch.
