Environmental Engineering Reference
In-Depth Information
5.1.4.
Case studies: comparison between observed behavior and the predictions of
numerical models
The aim of this section is to discuss some real and well documented cases of
subsidence due to water withdrawal, testing the capacity (here
a posteriori
) of the
numerical models to provide reliable forecasts. The possibility of giving reliable
predictions is of paramount importance for the correct management of the territory
and the environment.
The case studies presented here deal with two areas in the Po Valley in the
north-east of Italy (see Figure 5.1). Numerical results are compared, as far as
possible, with data measured in place.
5.1.4.1.
Subsidence of Venice: the field data
Manmade subsidence in the Venice area is caused mainly by the exploitation of
aquifers in the industrial zone of Marghera, close to Venice. The lowering of the soil
surface is quite limited, but added to the eustatism, i.e. the increase in mean sea
level, and to the effects of natural subsoil consolidation (alluvial layers), it results in
serious consequences for the very delicate equilibrium of the lagoon-city system.
During the past century, the eustatic rise has had an average value of 1.27 mm
per year, resulting in a loss of elevation of about 11 cm during the period 1908-
1996. Natural consolidation has a rate of 0.42 mm per year, producing a lowering of
3-4cm from the beginning of the 20
th
century.
This contribution is essentially due to the increase in salinity of interstitial water
in the pores of the clay sediments, which is induced by a change in the brackish
characteristics of the lagoon to a saline environment during evolution of the basin.
The subsidence caused by human activities started around 1925, with the
development of the industrial zone of Marghera (hinterland of Venice). During
1952-1969, water pumping caused a settlement of 10 cm in the center of Venice. In
the following years, the rate of land subsidence decreased due to a reduction in the
withdrawal rate.
In effect, from 1970 onward, the piezometric level in the exploited aquifers
began to recover, and this produced a small rebound in surface level (2 cm during
the period 1969-1975).
