Tools

FIVE months of tests on the structure of the barrage that will protect Venice from floods show that so far the project's engineers have made all the right choices. The prototype steel canister which will form a key component of the barrier has performed well. If Venice's water authority approves the design, then by 1995 the city should see rows of these canisters in place as barriers to the lagoon that surrounds Venice.

Although Venice is no longer sinking, it needs to be protected from freak tides and the gradual rise in sea levels caused by changes in the Earth's climate. The Venice water authority and a consortium of engineers have just completed tests on a life-size model of sections of the barrier, as the final stage in an extensive programme of simulation to find the best design for the barrier ('Between Venice and the deep blue sea', New Scientist, 4 August 1988). Earlier tests using computerised mathematical models and scaled-down physical models enabled the engineers to determine the shape of the canister, and the best position for the barrier and sea walls at the entrances to the lagoon.

According to an independent civil engineer, Ennio Gallo, speaking at the Italian Institute in London last month, the authorities have to take account of pressures from local shipping interests, who demand access to the lagoon, and environmentalists who fear that an insensitive design will further damage the already fragile ecology of the lagoon's waters.

Each canister is rectangular and sits in a 1300-tonne steel frame called Moses (an Italian acronym for 'experimental electromechanical module'). It measures 20 metres long by 17 metres deep and 4 metres thick. It will sit on the seabed filled with water. When air displaces the water, the canister rises, turns underwater hinges and creates a barrier.

The critical component in the canister is the device which joins it to air compressors, water pipes and the electric cabling which drives its hydraulic valves. This mechanism pumps air into and out of the canisters, to control them. The engineers have just completed tests on a British connecting device, which is modified from its function in an oil wellhead. This allows all the necessary cabling to pass through the hinge itself, rather than dragging on the seabed where it might get damaged or fouled.

The engineers tested this connector, designed by Vetco Gray, a company based in Aberdeen in Scotland, with two driving mechanisms. One is based on compressed air and the other on an electric pump. 'Both systems worked well,' says Franco de Siervo, technical operations manager for the project, 'but, we have selected the compressed air system because it is much more simple and reliable.'

This month, the engineers will start tests on an alternative Italian connector. These tests will last until the summer, then the canister itself will undergo further tests. These will look at the feasibility of creating a waterproof channel to run along the seabed between the hinge and the foundations of the barrier. The engineers hope to be able to use this channel for access for maintaining the barrier.