Opinione:Per una revisione del progetto italiano per salvare Venezia/en

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In spite of many scientific uncertainties and the strong negative impacts feared for the environment, the project called the Experimental Electromechanical Module (MoSE) for mobile flood barriers at the inlets of the lagoon of Venice, to protect the city from storm surges, is under construction. However, a recent study has shown that a revision of the project is becoming necessary.

The foundation stone of MoSE was set on 14 May 2003 by Prime Minister Berlusconi, The project was contested by the municipality of Venice, most local associations for the safeguard of the city and of the ecological environment of the lagoon, several scientific experts and a wide range of general people.

In 2005, the municipality of Venice proposed to the Italian government several projects alternative to the MoSE, all much less expensive and with lesser environmental impacts (http://www2.comune.venezia.it/MoSE-doc-prg/à 6). They do not seem to have been seriously considered by the Italian government, which preferred to continue the works for the MoSE.

In 2009, after spending in preparatory works (mostly for dredging and for the building of dykes) about one half of U.S.$ 6 billion initially scheduled, the construction of the huge concrete caissons to which 79 gates should be attached has not yet started. More seriously, a complete plan of execution for the whole project is still missing. According to the MoSE designers, each gate is expected to be raised by injecting compressed air and to oscillate independently with waves. However, an essential detail, the design of the hinges that should connect the gates to the caissons and control the flow of injected compressed air, has not been illustrated and seems to be still under study. One may wonder how such an important project, still undefined in certain essential parts, could have obtained the necessary administrative authorizations in Italy and even an important financial loan from the European Community.

The possibility of non uniform oscillations of the gates with waves, permitting a flow of sea water into the lagoon even when the gates are closed, was the main criticism addressed to the MoSE planners. According to a report by international experts [Collegio di Esperti di Livello Internazionale, 1998], the resulting increase in the lagoon water level has been estimated at 0.27 cm/hr if the gates do not oscillate, 0.46 cm/hr for relative rotations of 20° (case considered as possible), and of 2.09 cm/hr for rotations of 30°(case considered as unlikely).

By simulating the occurrence of certain storms of the past with the flood-gates fully operational, the very prudential estimate for an increase in the lagoon level of 1cm/hr was used, including heavy rainfall occurred over the lagoon and its hydrological basin, while persistent strong winds over the Adriatic Sea were creating strong waves in the lagoon passes. It was shown that in several cases, with a sea-level rise of only 20 to 30 cm, flooding for many consecutive hours of the lowest parts of Venice could not be avoided by the MoSE barriers. [Pirazzoli, 2002; Pirazzoli and Umgiesser, 2006]. A team of MoSE supporters [Bras et al., 2002], which have studied the project on behalf of the Italian government, disputed these assumptions. They stated that “the gates are engineered to prevent large oscillatory rotations”.

For comparison, the global sea-level rise by the year 2100 is expected to vary between 0.5 and 1.4 m (Rahmstorf, 2007; see also www.ozean-klima.de for the presentation by M. Vermeer and S. Rahmstorf at the climate conference, Copenhagen, March 2009), with a more plausible value of about 80 cm [Pfeffer et al., 2008].

More recently, a study commissioned by the municipality of Venice to the French Company Principia R.D. has compared the hydrodynamic behavior of MoSE gates to that of the gravity gates of a project that had been proposed as an alternative to MoSE. It appears (http://www2.comune.venezia.it/mose-doc-prg/ à 12) that for certain steep wave conditions with significant height >2 m, not rare in the area, an unstable behavior is obtained for the MoSE gate. In this case the chaotic response with high dynamic amplification of the oscillations of the MoSE gate does not permit a reliable design of the connection of the gate to its caisson. In these conditions, contrary to the statements of MoSE supporters [Bras et al., 2002], the flow of sea water into the lagoon through the gates would increase to a level that cannot be specified by modelling.

It is probably not too late to have the courage to carry out, instead of a patch up of the inadequate old project, a drastic revision, either towards a more diffuse solution, as proposed by the Municipality of Venice, that would make it possible to provisionally gain a few decades with limited costs; or directly towards a harder solution that would take more into account the near-future sea-level rise predicted by recent and new climatic models. In the latter case, it would be necessary for a new system of temporary closures to be converted, when flooding would become unavoidable, into a watertight separation of the lagoon from the sea.

References

Bras, R.L, D. R. F. Harleman, A. Rinaldo, and P. Rizzoli, Obsolete? No. Necessary? Yes. The Gates will Save Venice, Eos, 83, Pages 217, 224, 2002. Collegio di Esperti di Livello Internazionale. Report on the mobile gates project for the tidal flow regulation at the Venice lagoon inlets, Venice, June 1998, 48 p. Pfeffer, W.T., J.T. Harper, and S. O’Neel, Kinematic Constraints on Glacier Contributions by the 21st-Century Sea-Level Rise. Science, 321, 1340-1343, 2008. Pirazzoli, P.A., Did the Italian Government Approve an Obsolete Project to Save Venice? Eos, 83, 20, Pages 217, 223, 2002.

Pirazzoli, P.A., and G. Umgiesser, The Projected “MOSE” Barriers Against Flooding in Venice (Italy) and the Expected Global Sea-level Rise, Journal of Marine Environmental Engineering, 8, 247-261, 2006.

Rahmstorf, S., A Semi-Empirical Approach to Projecting Future Sea-Level Rise, Science, 315, 368-370, 2007

Paolo Antonio Pirazzoli is a researcher at Centre National de la Recherche Scientifique, Meudon, France. E-mail : pirazzol@cnrs-bellevue.fr

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