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Fighting the flood

Plans for a submersible barrage to protect Venice from floods have to date been thwarted by the ministry for the enviornment. Last month its objections were overturned. Andrew Mylius visited the city to find out what engineers hope to build

Tourists absolutely love it when Venice floods, ' remarks hotel owner Alberta as an international corps of camera-toting visitors mill excitedly outside her door. 'It makes their trip if they get wet feet, ' she says.

The regular dunking of St Mark's Square clearly does Venice's thriving tourist industry little harm, but its urban fabric is suffering. Water levels are now regularly lapping brickwork above the impermeable stone course designed into most of the city's old building stock.

Gradually the splendid facades are crumbling from canal-level up. And of immediate concern, the city's waterborne emergency services are increasingly thwarted by bridges made impassable by high tides.

Venice was originally founded on a group a mudbanks within a shallow lagoon connected to the sea by three channels, from north to south - the Lido, Malamocco and Chioggia inlets. Com2. Lido is more than 840m wide with a bottom depth of 6m to 11m, Malamocco is 400m wide and 15m deep and Chioggia 360m wide by 11m deep. Seven or eight times a year tides and wind combine to produce 1m plus surges through these channels - and it is these surges that have to be restrained if Venice is to be saved.

Venetians have historically combated flooding by raising pavement levels - but 230mm subsidence has occurred since 1900, while global warming is fuelling fears that the sea could rise by up to 1m over the next 100 years. The most optimistic predictions are for a minimum of 100mm change by 2050. Action is clearly needed.

Following the legal ruling in July that ministry for the environment objections to a flood defence scheme on the grounds that it would damage the lagoon's ecology were 'illogical', hopes are now high that the government will unlock funds for construction of the Venice barrage.

A barrage was first mooted in the 1970s. In 1984 a private sector consortium, Consorzio Venezia Nuova, was created under the ministry of public works to develop schemes for the protection and rehabilitation of Venice and its surrounding lagoon. Since 1987 it has carried out nearly $2.25bn worth of work regenerating beaches, jetties and sea defences on the ribbon of islands that separate the lagoon from the Adriatic sea outside, and on reinstating areas of salt marsh within the lagoon eroded by shipping. With consulting engineer Technital it has also developed designs for the $2bn flood defence.

Protection of the Venetian skyline has been a major factor in evolving the barrage design, points out Consorzio Venezia Nuova assistant director Maria Teresa Brotto. The municipal authority opposed any form of obtrusive above ground structure for the support or operation of a barrier system. As a result Technital has developed a system that is wholly submerged, bar for discreet control and plant buildings on the adjacent islands that will merge with the jumble of structures already there.

The barrage, or rather barrages, will be formed from a row of submersible steel barriers, docked when there is no tidal threat in giant seabed concrete caissons. To combat flooding compressed air will be pumped into the barriers to raise them. Rotating in the horizontal plane around a hinge on the seaward edge of the caisson, the bright yellow barriers are designed to float at 45degrees.

'What we are proposing is a fleet of yellow submarines, ' jokes Brotto. In all there will be 79 barriers measuring 20m wide and from 20m to 30m high by 3.6m to 5m deep, depending on the depth of water in which they operate.

Combined tide and wind can create inward flows to the lagoon of up to 7m/s while the Adriatic is often whipped into impressive, choppy waves. When activated, an 80mm gap between each barrier will allow slight inflow from the sea to the lagoon. Each individual barrier, meanwhile, will be able to bob and sway independently to absorb and dissipate wave energy.

The barrage will be able to hold back up to 2m height difference in water level.

Water will be circulated through the cofferdams to flush out sediments that may jam the barriers, but the simplicity of design means there is little scope for mechanical failure. Brotto says the danger of a s ing le gate fa i l ing is less than 1 in 1,000 years; the risk of the entire structure failing is 1 in 10,000.

Providing the government gives the project the green light, construction is likely to take eight years. All three navigation channels have to remain operational while work proceeds, starting with dredging to flatten and widen the bottom profile. This will enable work at each in let to con t inue in two stages - half the channel will be taken over by contractors and half left free for traffic.

At Lido, an artificial island will be built midway across the inlet; it will effectively be closed by two discrete barriers, to be completed in a single hit one after the other.

Immediately after dredging cofferdams will be placed to prevent migration of silt plumes created by construction into the lagoon. Within these, sheet piled retaining walls will be installed to allow excavation of the trench in which the barrage's reinforced concrete caisson will sit.

Like the gates, the size and section of the caissons will vary according to their location, from 21.5m long by 15m wide and 9m high to 60m long, 48.7m wide and 12m high. Excavation must be carried out to depths of 28.7m at the Malamocco inlet, 24.7m at Chioggia, and 23.5m and 16.7m at the two Lido inlets. Ground will be consolidated by small diameter reinforced concrete piles up to 20m long, at 2m centres. As many as 12,000 may be needed, says Brotto.

A 1.2m course of heavy gravel will be placed in the trench to provide a loadbearing foundation for the caissons, which are to be towed from fabricating yards at Marghera and Treporti within the lagoon, at Ravenna just south of Venice on the Adriatic coast, and at Brindisi near the heel of Italy. The caissons will be sunk into position on top of 500mm corner supports and the remaining space filled with pumped cement grout. The space between the caissons and piled retaining wall will be made up with a sand, cement and bentonite mix.

Installation of the caissons will be completed with the laying of a geotextile net under 1.5m of rock scour protection extending 150m on the barrage's seaward side and up to 250m on the lagoon side.

In all, 18 different caisson types are needed to house mechanical and electronic plant, anchor the gates and abut the islands. But essentially the massive cellular structures will provide four walk-through galleries allowing access for maintenance, housing for plant, and ventilation. The remaining cells will be filled with ballast.

Fabrication of the gates will call on shipbuilding technology. They are designed in steel, with a welded reinforced sheet metal skin supported on transverse frames with compound beams. It is planned to build the gates within the lagoon at Marghera, a district dominated by heavy industry including ship yards. Though the barrage has a 100 year design life it is envisaged gates will be replaced for maintenance on a five year cycle.

A special jack-up barge is being designed to place the gates. Standing on the caisson, it will lift the gates from a transporter barge that will dock between its legs. Preliminary placement will be to within 300mm accuracy. Fine tuning will be carried out using a submerged space frame slung from the barge and fitted with lifting gear.

Guarding the ecosystem

Environmentalists are insisting that the lagoon ecosystem, polluted and eroded by decades of unfettered shipping and process industry, is not further damaged.

Technital claims flows into and out of the lagoon during and after construction will be reduced by only 2%, posing no significant risk of oxygen depletion - most barrier closures will be during the winter, when the danger of stagnation is lowest.

Studies carried out in conjunction with the Delft Hydraulic Laboratory, Danish Hydraulic Institute and Voltabarozzo Padova, Italy, show that marginally reduced flow will, in fact, slow the loss of sediments, aiding the regeneration of salt marsh.

On the spot

Age: Venice was founded in 452AD when the people of Aquileia and Padua took refuge from invading Teutonic tribes on sandy islands among the treacherous shoals of what is now Venice lagoon.

Key events: Venice has had a history of invasion and conflict, but a trade agreement following a failed Saracen campaign in 836 launched the city as an international mercantile centre. The break up of the Byzantine empire in 1204 saw Venice emerge as a Mediterranean superpower. Trade was concentrated along the banks of the Grand Canal and centred around Rialto - originally the highest part of the sedimentary land mass. Land area was increased through reclamation, with spoil taken from increasingly ambitious dredging programmes.

In the late 13th century Venice became one of four independent Italian maritime republics. The ensuing three century economic boom fuelled the Renaissance flowering that now draws droves of tourists.

In the same period, ever larger ships and the volume of shipping led hydraulic engineers to look for ways of reducing sedimentation in Venice lagoon. A series of river diversions culminated with the Po being channelled out of the lagoon in the 17th century.

That sinking feeling: Street levels have been raised - the columns fronting the Doge's palace in St Mark's Square, and St Mark's cathedral itself have lost their bases beneath the pavement.

But subsidence has accelerated in the last 100 years. Abstraction of groundwater by heavy industry surrounding the lagoon during the 1960s and 70s led to an alarming relative rise in sea level.

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