Computing the relative land subsidence at Venice, Italy, over the last fifty years
C. Da Lio, L. Tosi
Institute of Marine Sciences, CNR, Venezia,
Italy
T. Strozzi
Gamma Remote Sensing, Gümligen, Switzerland
P. Teatini
Dept. of Civil, Environmental and Architectural Engineering, University of Padova, Padova, Italy
ABSTRACT
Land subsidence causes various damages to the infrastructures and cultural heritage in many
cities worldwide. Urban flooding is one of the main consequences of land subsidence in coastal cities, where
it is exacerbated by sea-level rise accompanying global climate change, but also in inland metropolitan areas
such as Mexico City, where subsidence zones are increasingly flooded following intense rainstorms.
The subsidence of Venice, one of the most beautiful and famous cities in the world, is well known not for the
magnitude of subsidence but because subsidence has seriously compromised the heritage and the safety of
the city in relation to its small elevation above the sea. The storm that flooded the historical center of Venice
on November 4, 1966 dramatically revealed its fragility with respect to land subsidence and sea-level rise, or
the Relative Land Subsidence (RLS), i.e. land movement with respect to sea-level changes. That event
signaled the beginning of a systematic monitoring of the loss in elevation of the ground surface of Venice
with respect to the mean level of the Northern Adriatic (NA) Sea. Tide gauge measurements, available from
the beginning of the last century, have been supplied historically by levelling and more recently by Synthetic
Aperture Radar (SAR)-based Interferometry. On the occasion of the 50th
anniversary of the 1966 flood event, we quantify the RLS experienced by the city
over these last five decades with a detail never achieved before. The computation of the loss of elevation has
been obtained by processing and superposing the results of levelling surveys carried out in 1961, 1969, 1973,
and 1993, together with the results of Interferometric processing of SAR images acquired from satellites:
1993 to 2002 by ERS-1/2, 2003 and 2010 by ENVISAT, 2008 to 2013 by TerraSAR-X, and 2012 to 2016 by
COSMO-SkyMED. The records from the tide gauge in Trieste, which is a city on the coast of the NA Sea
close to the Alps and known to be stable, are used to evaluate the sea-level rise over the targeted time
interval.
The mean land velocity for each analyzed period has been obtained by interpolating the original
measurements using the Kriging method on a same regular 50-m grid covering the entire city. Then,
cumulative land subsidence from 1966 to 2016 has been simulated in a GIS environment by summing
the partial land subsidence over the various periods covered by the levelling and SAR surveys.
The results point out that in the Venice historical center between 1966 and 2016:
- Land subsidence rate has been more variable in space but less variable over time than the changes of the
NA mean sea level;
- average subsidence has amounted to 0.8 mm/yr and the average NA msl rise to 1.9 mm/yr;
- minimum and maximum cumulative subsidence has totalled 8 mm and 93 mm, respectively; and
- maximum loss of elevation with respect to the NA msl (i.e. RLS) has been 190 mm.
RLS has produced a tangible effect on the Venice historical center revealed by the continuous increase in
frequency of the flooding events, locally called "acqua alta". In the next years, any further loss of elevation
with respect to the mean sea level, even a few mm, will threaten the city survival with severe social and
environmental impacts. Considering the present average land subsidence of Venice and sea level rise of the
NA (i.e. both about 1.2 mm/yr), an additional loss of elevation of about 190 mm will likely occur by 2100.
Actually, according to conservative and pessimistic IPCC scenarios, the sea-level accompanying global
climate change is expected to rise from 32 to 56 cm. Therefore, the outcomes from this study should be
properly taken in account for the planning of effective interventions for the mitigation of climate changes to
maintain the historical center of this unique city.
