GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 12, 1562, 10.1029/2001GL013211, 2002
Evidence of the present relative land stability of Venice, Italy,
from land, sea, and space observations
Luigi Tosi and Laura Carbognin
National Research Council, Istituto per lo Studio della Dinamica delle Grandi Masse, Venezia, Italy
Pietro Teatini
University of Padova, Department of Mathematical Methods and Models for Scientific Applications, Padova, Italy
Tazio Strozzi and Urs Wegmüller
Gamma Remote Sensing, Muri BE, Switzerland
Received 23 March 2001; revised 6 December 2001; accepted 8 December 2001; published 18 June 2002.
[1] The city of Venice is very vulnerable to loss in
surface elevation as a result of subsidence and eustacy
because of its small elevation above sea level. The alarm
of the scientific community related to this persistent
problem is increasing because of the forecasts of sea level
rise caused by global warming. In order to evaluate the
present relative subsidence of Venice, a study has been
performed by combining high precision leveling, Satellite
Radar Interferometry (SRI) and tide gauge measurements.
The analysis of the most recent data points out with an
unprecedented detail the present ground stability of the
INDEX TERMS: 1208 Geodesy and Gravity: Crustal
city.
alta’’ (a local idiom meaning flooding) events. Studies
demonstrated the cause and effect relationship between
aquifer system depletion and land settlement, and humaninduced subsidence ceased after the closure of the artesian
wells started in 1970 [Gambolati et al., 1974; Carbognin
et al., 1977; Carbognin et al., 1995]. However, since the
past loss in elevation is mostly irrecoverable and mean sea
level is forecast to rise, the need to control the vertical
movement of the city still persists. But is subsidence in
Venice still a problem?
movements—intraplate (8110); 1204 Geodesy and Gravity:
Control surveys; 4556 Oceanography: Physical: Sea level
variations; 6924 Radio Science: Interferometry
2. Data analysis and discussion
1. Introduction
[2] Land subsidence is the result of a variety of subsurface displacement mechanisms, both natural and maninduced. Natural subsidence develops on geological time
scales at rates of a few millimeters per year. Human related
processes occur on much shorter time scales with higher
rates of up to tens of centimeters per year.
[3] The subsidence of Venice, one of the most beautiful
and famous cities in the world, is well known not for the
magnitude of its occurrence but because it has seriously
compromised the ground safety level of the city in relation
of its small elevation above mean sea level [Gatto and
Carbognin, 1981]. Historical evidence of the human
response to natural subsidence, mainly attributable to
sediment compaction, is the repeated raising of building
ground floors and street pavements required over the
centuries to combat sea floods [Bonardi et al., 1998].
The sinking of Venice began to worsen in the 1930s and
more during the 1950s and 1960s as a result of groundwater over-exploitation for industrial use on the nearby
mainland. During the last century, the relative elevation
loss of Venice has totaled 23 cm, consisting of about 12
cm of land sinking, both natural and anthropogenic, and
11 cm of sea level rise. The most immediate consequence
was the increase in frequency and degree of the ‘‘acqua
Copyright 2002 by the American Geophysical Union.
0094-8276/02/2001GL013211
[4] The ground elevation of Venice and surroundings has
been determined over time by repeated high precision
leveling. The overall leveling network is connected to a
stable area in the Alpine foothills and is composed of about
400 benchmarks, 100 of which are located in Venice.
Spacing of benchmarks is usually 1 km and on the order
of 200 m in Venice [Tosi et al., 2000].
[5] Focusing on the situation in Venice, the evolution of
the subsidence during the last 40 years is given in Plate 1.
Plate 1a shows the settlement trend of Venice between
1961 and 1969, when Venice was affected by high subsidence rates of up to 7 mm/year. The city was affected
landward by the subsidence cone of the industrial area and
eastward by the sinking of the coastland due to groundwater withdrawals for agricultural, tourist and domestic
needs. The maximum yearly subsidence rate of 14 mm/
year occurred between 1968 and 1969. Soon after, drastic
measures to close the industrial pumping wells and
strongly curtail other artesian extractions were taken and
a pressure recovery in the aquifer system was rapidly
obtained. Subsequent anthropogenic subsidence ceased as
evidenced by the leveling surveys carried out in 1973 and
in the following years.
[6] The survey of the overall city network was repeated
in 1993. The analysis of the land elevation between 1973
and 1993 is suitable for estimating the recent natural sinking
component. During this period land subsidence was close to
zero almost everywhere (Plate 1b). Occasional bowls of
subsidence up to 1.4 mm/year rate are limited to small
western and eastern spots, where recent land reclamation
has been done and consolidation process may still be
continuing (Figure 1), and in few sites bordering the
3-1
3-2
TOSI ET AL.: RELATIVE LAND STABILITY OF VENICE
Plate 1. Rates of ground vertical displacement (in mm/year) at Venice superimposed on an aerial photograph of the city:
(a) from 1961 to 1969 (redrawn after Bergamasco et al. [1993]), (b) from 1973 to 1993 (redrawn after Carbognin et al.
[1995]), and (c) from 1992 to 1996. The first two maps are obtained by interpolating scattered values provided by
differential leveling; the location of benchmarks is indicated by yellow dots. The contour interval is 0.5 and 0.3 mm/year in
(a) and (b), respectively. The 1992 –1996 raster map is generated by SRI, with a pixel resolution of 25 m. Subsiding areas
are colored from violet to red and stable areas from yellow to blue. Vertical movements between ±0.5 mm/year are in the
range of accuracy of the measuring techniques. The black star in (c) shows the position of the Venice tide gauge.
principal canals, where the speed of currents may induce
serious erosion.
[7] The latest leveling carried out in 2000 includes only a
few selected benchmarks in the historical center because its
aim has been to verify the present subsidence within the
most critical zones pointed out by the 1993 study, i.e. the
southern and northern lagoon edges and portion of
the littoral stretches [Carbognin et al., 1995]. To complete
the survey, an upgrade of the ground-level evolution of the
overall city has been derived by using Satellite Radar
Interferometry (SRI) [Bamler and Hartl, 1998]. With regard
to land subsidence, SRI exhibits complementary characteristics to the leveling surveys, because it has the capability to
map large urban areas at low cost and high spatial resolution
[Amelung et al., 1999; Strozzi et al., 2001]. On the other
hand, high precision leveling surveys are used outside of
cities (where the lack of stable structures prevents the
formation of a coherent phase signal over time) and to set
up a reference point for the SRI subsidence values. Interferometric radar images from the European Remote Sensing
Satellites ERS-1 and ERS-2 from 1992 to 1996 have been
used in this study. In order to generate a single subsidence
map with reduced errors, 12 interferometric radar images
have been combined using the stacking technique [Strozzi et
al., 2001]. The interferograms have perpendicular baselines
of less than 42 m and acquisition time intervals between 210
and 1388 days. The high accuracy of the 1992 – 1996
displacement rates from SRI is confirmed by a quantitative
validation with the 1993 –2000 leveling data (Figure 2). For
the 17 benchmarks where values from both surveying
techniques are available, a standard deviation of 0.7 mm/
year of the vertical displacement velocity is found; the
minimum and maximum differences are 1.0 mm/year
and +1.0 mm/year, respectively. From the SRI picture of
the movement of the entire city (Plate 1c), it can be
concluded that, in general, Venice is stable. The vertical
displacement rates are between +1.0 and 2.0 mm/year,
significantly smaller than the subsidence rates measured
during the critical period 1961 – 1969 (Plate 1, Figure 2).
Moreover, the SRI 1992 – 1996 displacement rates are in
good agreement with those resulting from the 1973 and
1993 leveling surveys, even if in the former period the
sinking eastern part of the city has a larger extent. SRI is
able to see the ‘‘in-between’’ benchmark elevation changes
not obtainable from leveling.
[8] The other issue related to the relative elevation loss of
the city is the eustatic increase in the mean sea level as a
consequence of climate change. The most reliable estimate
of the sea level rise in the northern Adriatic Sea over the last
100 years is provided by historic series of tide gauge
measurements in Venice and Trieste [Carbognin and Taroni,
1996]. Trieste, a coastal city located 200 km to the North of
TOSI ET AL.: RELATIVE LAND STABILITY OF VENICE
3-3
resulting from the temporary influence of anthropogenic
subsidence.
[9] Differences between tide gauge values recorded at
Venice and Trieste since 1896 are shown in Figure 3a. The
anomalous increasing difference in the period 1930 – 1970 is
related to anthropogenic land settlement of Venice, while
from 1970, the constant difference confirms the currently
negligible vertical movements of the city. As a result, the
computation of mean sea level rise based on the Venetian tide
gauge records is possible for the period 1970– 2000. A rising
rate of 0.6 mm/year is obtained (Figure 3b), which is notably
smaller with respect to the centenary value. Although the
causes of such a decreased trend are not quite clear, this trend
is consistent with the results obtained from seven other tide
gauges in the Mediterranean and a few northeastern Atlantic
stations [Tsimplis and Baker, 2000].
Figure 1. Sketch of the growth in area of Venice from
900 A.D. to present (redrawn after Carbognin et al. [1984]).
The city developed over ancient well-consolidated sandy
islands during the first millennium, and the following
expansions were done by reclaiming and filling parts of the
lagoon and channels. Stable sectors showed in Plates 1b and 1c
generally correspond with the city extension before 1500.
Venice at the foothills of the Alps (Figure 3a), is known to
be stable. There, a century-long eustatic growth rate of 1.1
mm/year has been computed by a linear regression of the
tide-gauge measurements. A statistical analysis demonstrated the validity of the linear regression model and indicated
that the extent of the historic series is sufficient to smooth
out the internal short period climate variations. For Venice
the existence of a non-unique secular trend emerged,
3. Conclusions
[10] In this research the elevation loss of Venice relative to
the mean sea level was determined with an integrated analysis
of leveling, SRI and tide-gauge measurements. Nowadays,
the Venetian soil is quasi-stable and the northern Adriatic sea
level is not significantly rising. In spite of these reassuring
results, over the last century land subsidence and eustacy
concurred to make the relative sea level in Venice 23 cm
higher. This has brought about a seven-fold increase of
flooding events with great inconvenience for the population
and enormous damage to the cultural heritage. Note that the
man-induced land subsidence, which occurred during the
1950s and 1960s, is equivalent to about a two centuries of
natural subsidence on the present day scale. Even if significant interventions have been made in the last decade, at the
beginning of the new millennium the historic and architec-
Figure 2. Quantitative validation of the 1992 – 1996 vertical displacement velocity from SRI with the measurements
provided by 1993 – 2000 leveling surveys along the line A– B shown in the inserted map. The 1961 – 1969 subsidence rates
along the same leveling transect point out the general present stability of Venice as compared with the 1960s sinking trend.
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TOSI ET AL.: RELATIVE LAND STABILITY OF VENICE
tural treasures of Venice are still deeply compromised by the
‘‘acqua alta’’ phenomenon. Moreover, the hypothesis of a
future dangerous rise in the sea level due to climate change
will have to be properly evaluated to plan and adopt the most
suitable solutions to protect the city of Venice.
[11] Acknowledgments. Authors acknowledge the Venice Water
Authority, Consorzio Venezia Nuova-Servizio Informativo, that completed
the 2000 leveling survey partially supported by the ISES Project. ERS SAR
data analysis supported by the ESA Data User Program (DUP). ERS SAR
data copyright ESA, processing Gamma. Tide gauge measurements made
available by the Servizio Idrografico of Venice and Istituto TalassograficoCNR of Trieste. Charles Werner, Adrian Luckman, and Jane Frankenfield
Zanin are acknowledged for helpful discussions.
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Figure 3. (a) Difference between the mean sea level at
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difference. (inset panel) Location of Trieste with respect
to Venice in the northern Adriatic Sea. (b) Time series of
mean sea level recorded at the Venice tide gauge during the
last 30 years (solid curve). A mean sea level trend of +0.6
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L. Carbognin and L. Tosi, ISDGM, National Research Council, San Polo
1364, 30125 Venezia, Italy. ([email protected]; [email protected])
P. Teatini, DMMMSA, University of Padova, Via Belzoni 7, 35131
Padova, Italy. ([email protected])
T. Strozzi and U. Wegmüller, Gamma Remote Sensing, Thunstrasse 130,
3074 Muri BE, Switzerland. ([email protected]; [email protected])