The complexity of safeguarding

Georg Umgiesser

Oceanographer with the National Research Council

Because the high tides occur in autumn and the beginning of the spring, the MOSE will be closed just about every day (Bocca di Lido) with a considerable negative effect on the port of Venice (Bocca di Chioggia). To solve the problem of flooding Venice, the Italian Government approved a project to close the mouth of the harbour with buoyant oscillating mobile flood gates which are positioned so that they are hidden when not being used and are raised when the tides are high. Positioned and hinged to the sea bed in alignment with enormous cement blocks fixed and linked to each other. The project is called Modulo Sperimentale Elettromeccanico or Electro Mechanical Experimental Module, therefore the acronym: MOSE. The project has caused concern among the locals and also others and is being criticized for the procedures used for the approval of the project, the setting up of the building site as well as its maintenance and management. The enormous construction of the flood gates, with which this work is identified, is the least invasive part of the project. The environmental impact of this project compromises precious environmental areas, natural oases that are characterized by their uniqueness so it has therefore been strongly criticized by the European Community for its failure to respect the norms which regulate current legislation. From these long heated debates a different philosophy has emerged which is now in progress, fundamentally inspired by the idea of adopting two different defence modes, one for the medium to high tides and another for exceptionally high tides.

High tides are those that do not exceed a height of 140cm. on the zero point of the sea graph at Punta Salute, providing a flexible shut off system: partial for medium to high tides and total for exceptionally high tides, which would be controlled by operating movable barriers that cover much narrower sections than the current ones and of those in the previous project. This would also mean an evident reduction in construction and operating costs.

The possible alternatives to the MOSE project for closing the mouth of the harbour involve

a) reducing the section of the harbour entrance by partly raising the sea bed and introducing fixed transversal structures b) introducing transversal structures that could be moved according to the season c) extending the structures of the current wharves and diversifying the port functions of the three entrances by placing the cruiser terminal on the side of the open sea.

The mobile transversal structures which could be naval structures, that have already been tested, with the sort of ship-port or with more practical technical versions with submersible steel pontoons with gravity flood gates, complete with a self installation plant and flood gate operating systems.

They could be positioned at the mouth of the harbour, when there is a risk of flooding and then removed in the summer for maintenance to be carried out in specialized shipyards. These interventions could be implemented immediately, recuperating existing work, with a minimum amount of damage and would drastically reduce the high tides, especially the medium to high tides, which are more frequent in the city and provide the time necessary to perfect and develop, in a second phase, more suitable defence methods on a wider territorial scale deriving from the climate changes of the eustatic scenarios expected in the years to come.