Turning back the tide

Engineers and ecologists are divided on how to save Venice. Fred Pearce reports The next disastrous flood could come later this decade, when lunar cycles give an extra push to local tides

LONG before this winter's floods in northern Europe, St Mark's Square in Venice was under water. Last November, flood waters in the lagoon that surrounds the city backed up through drains, reviving memories of the great flood of 1966 which damaged hundreds of historic buildings. The latest inundation comes as the Italian government is considering a trillion-lira plan to barricade the lagoon's last three links with the sea.

But with closure of the city's industrial port in prospect, Venice has the opportunity to protect one of Europe's greatest architectural treasure houses the ecological way, by recreating the old lagoon.

The engineers' solution, developed by a consortium appointed by the Italian government, would place hinged barriers on the sea floor at each inlet to the lagoon. There they would lie, out of sight, ready to be raised to hold back exceptional tides.

The barrier would be the latest in a centuries-long series of efforts to tame the lagoon. But environmentalists say that it too is doomed to fail because engineers have got their hydrology wrong. Rather than confronting nature with concrete and metal, ecologists want to recreate the lagoon's natural equilibrium with the sea.

"The engineers still see the lagoon as the enemy," says Paolo Rosa Salva, local spokesman for the Italy's largest environment group, Italia Nostra. "It is not. It is Venice's best defence against the sea." As an architect based in Venice, Rosa Salva has spent his professional career repairing the buildings damaged by the 1966 floods. But he believes that the city can only be saved from future floods by saving the lagoon.

The Venice that a million tourists visit each year is a small island in a large crescent-shaped lagoon, an expanse of tidal mud flats 50 kilometres long and more than 10 kilometres across, separated from the Adriatic Sea by islands. The dank island was originally colonised as a refuge from the barbarians ravaging Italy after the collapse of the Roman Empire 15 centuries ago. Its isolation, surrounded by a large natural moat, ensured that it thrived as a great port, trading with the Orient.

Efforts to remodel the lagoon have a long history. Three rivers build up a constant supply of silt, brought down from the Alps, in the lagoon. So in the 14th century engineers diverted the Brenta from the central lagoon. Later they did the same with the Piave and Sile. But without the silt to hold it back, the sea increased its influence, and by the 18th century engineers were locked in a battle against tides flowing in from the Adriatic that continues today.

They reduced the number of inlets from the sea to the lagoon from a dozen to the present three, and began transforming the sandy islands that separate the lagoon from the sea into fortresses. The walls built 250 years ago along the seaward face of the island of Pellestrini still stand, though the beach in front has eroded.

But engineers have carried out many other public works that have made flooding much more likely. During this century, they have drained nearly a third of the tidal mud flats that occupy most of the lagoon. Some have become farmland. More have been taken for a road, the city's airport, and for the industrial centre and port at Marghera, at the landward end of the causeway to Venice. Other flats have been barricaded off from the lagoon and turned into fish farms.

Today, tides reach only two-thirds of the area that they flooded at the beginning of the century. They call all this "reclaimed" land, says Rosa Salva. "But it is land lost to the lagoon, and hence to the city's flood defences. Once flood waters could spread slowly across the mud flats. Now they wash into St Marks's Square."

Meanwhile, the harbour authorities have dredged deep channels through the shallow lagoon to bring large ships, including oil tankers, to the industrial port. These channels have encouraged the sea to invade the lagoon, eroding the channels further. Mala-mocco canal, dredged to 14 metres, is now more than 20 metres deep in places. The high tides flow in quickly, and there are waves now in the lagoon.

According to Giuliano Sestini, an adviser to the UN Environment Programme on flood threats, "The degradation of the Venice lagoon started with the alteration of the tidal flats for the harbour at Marghera." Now, he says, "the islands have become unsettled and the remaining tidal flats eroded."

As the sea has invaded, the land has sunk, partly naturally and partly because the city authorities have pumped water from sediments beneath the lagoon to fill the city's taps and supply the factories at Marghera. Venice has sunk by between 10 and 14 centimetres in the past century. Today, it is on average only 80cm above the mean high tide. When high tides invade the lagoon, Venice is in peril.

On 4 November 1966, the tide was almost two metres above normal and most of the city disappeared beneath water. St Mark's Square was waist-deep in water filled with garbage, sewage and drowned rats.

Though there has been nothing so bad since - not even during last month's floods - exceptional tides are becoming more frequent. Since 1972, the average high tide has risen by 40cm. During the 1970s, two-thirds of the city was beneath water on five occasions. In the 1980s it happened eight times. The next disastrous flood could come later this decade when, as in 1966, lunar cycles give an extra push to the local tides.

It is time, everyone agrees, to reverse the process. Land re-claimed for industry but never used is now being given back to the lagoon. Some farmland is being reflooded and mud flats conserved. But environmentalists want to go further. The Marghera industrial zone is on its last legs; coke and glass factories have shut; oil tankers will soon be banned because of the risk of pollution.

"The industrial area is close to extinction, and when tankers are excluded that could be the end," says John Millerchip, who works for UNESCO liaising with the organisations saving Venice's buildings. "A lagoon without crude oil and chemical transport will be a great chance," says Rosa Salva. "We have to get rid of the idea of Venice as a port."

Banishing large ships would help the buildings, too. The wash of vessels heading through the Giudecca Canal for Marghera eats away at buildings around St Mark's Square. "You can feel the ships' vibrations destroying the mosaics in the Basilica," says Rosa Salva.

Groups outside Venice concerned with conserving the city's architecture have mostly supported the barrier plan. But in Venice opposition to the scheme is growing. "A very big question is how often they would be allowed to shut the gates," says Millerchip. "You certainly couldn't shut them every time St Mark's Square looked like being flooded."

Now, in a bizarre alliance, industrialists and greens have joined forces against the barrier scheme. But the alliance is purely tactical. Industrialists fear it will disrupt shipping. Greens want an end to the industrial port and want the shipping channels filled in. "This will stop the sea water coming in and provide flood protection that way," says Rosa Salva. Hydro- logists warn that reducing the size of the inlets would have little impact on exceptional surges. Greens say the hydrological research has been paid for by construction companies, but argue that in combination with reviving the mud flats, filling the channels will turn the tide.

Greens want the farmland flooded and the high-tech fish farms returned to traditional methods, which allow the tides to wash through them. And, ultimately, Rosa Salva wants the diverted rivers brought back into the lagoon to resume their job of holding back the sea and to recreate the natural dynamic balance that over thousands of years created the lagoon and which, he argues, can sustain it again. "Without the rivers we don't have proper marshes, bringing down silt to replace what is lost through erosion by the sea," says Rosa Salva. "To return the rivers to the lagoon, that is the ecological solution." !