Construction of Venice’s mammoth flood defence scheme reached a crucial stage in construction this week when work began on the barrier’s vast concrete caissons.
The €4.6bn (£3.8bn) Mobile Barrier System (MOSE) in the Venice Lagoon is now 65% complete and is set to be operational in 2014.
The Consorzio Venezia Nuova, the consortium responsible for building the scheme, has started forming the large concrete caissons that will house the main components of the oscillating barriers at three lagoon inlets outside Venice.
Hinged and buoyant, the barrier gates will lie flush with the seabed and be filled with air to make them rise up when needed.
“We are at a very important point of the project, constructing the caissons and realising the hinges that will move the gates,” said a Consorzio Venezia Nuova spokesman.
This stage is crucial because any flaws in the trench, caissons or hinges could mean the barrier may not lie flush with the seabed, or the gates may not be perfectly aligned and watertight. “They will be under the water and they all have to be at the same perfect level,” said the spokesman.
Large concrete caissons are being cast at four sites built from reclaimed land at the three lagoon inlets. Once the caissons are complete, these sites will be flooded, allowing the caissons to be floated out into place and sunk with ballast.
“We are at a very important point of the project”
A contract has not yet been let for the construction of the barrier gates themselves, which will be the final phase of the project. There will be 78 gates in total –18 at Chioggia, 19 at Malamocco and two lengths of 20 and 21 gates at Lido, separated by an artificial island which is already completed.
Each gate is 20m wide, with thicknesses varying from 3.6m to 5m and lengths varying from 18.5m to 29.6m. The gates will withstand a difference of 2m in water level between the lagoon and the sea.
An average high tide event will see the barrier deployed for around 6 hours, but it can be in place for longer in extraordinary tides. High tides are expected five times a year on average, and newly built locks will allow shipping activities to continue when the barrier is raised.
Work began on the project in 2003 and is set to complete in 2014. The cost of the project, financed wholly by the Italian government, rose from €4.2bn to €4.6bn due to materials costs. Last year Italian minister professor Renato Brunetta told an Italian Embassy meeting at the ICE that the barrier would need private cash to allow it to remain viable once open (NCE 25 June 2009).