Environmental Engineering Reference
In-Depth Information
Figure 5.1
.
Cross-section of the Po Valley between Ravenna and Venice
The total effects of the three different subsidence mechanisms for the period
1908-1996 are shown in Figure 5.2. The effects of water pumping account for about
47% of the total lowering (taking the rebound into account). The joint effect of the
three causes is a 24 cm rise in sea level. This (quite limited) value has to be
compared with the mean elevation of the city, which is only 1 m above the mean sea
level. Further, the mean spring high tide is 55 cm over tide gauge zero, i.e. 24 cm
represents 53% of the safety margin for ordinary high tides. As a result, flooding of
Venice occurs much more frequently than in the past.
Figure 5.3 presents the cross-section of the Po Valley that Venice is built on.
The six principal aquifers to a depth of 350 m can clearly be seen. The water
pumping from the aquifer-aquitard system underlying Venice and the surrounding
mainland was particularly heavy during the 1960s and early 1970s. A research
group has been active in Venice, collecting and processing a large amount of
experimental and field data regarding the local and regional stratigraphy since the
time land subsidence became very serious. This information includes the material
properties obtained from the analysis of a core sample from test bore holes, and the
withdrawal rates of water in the industrial zone (Marghera), Venice and Lido (area
between the lagoon and the sea).
