永利皇宫手机版:Turning the tide

 作者:欧垭     |      日期:2019-03-08 02:10:04
By Fred Pearce EVER since floods engulfed most of Venice in 1966, wrecking hundreds of buildings and drowning works of art, engineers have insisted that building a metal shield against the waves of the Adriatic is the only sure way to protect one of the world’s most loved cities. With $150 million already spent, a scheme developed by a consortium of engineering companies seemed certain to enter its construction phase—despite strong opposition from environmentalists. Then, last month, the Italian environment minister Edo Ronchi pulled the plug. His decision, which followed a critical assessment of the ecological impact of the barriers on Venice’s lagoon, was characterised in the international press as politically motivated. Ronchi is a member of the Green Party, which is currently a partner in the national coalition government. Many engineers view the decision as disastrous. Jack Lewin, hydraulics professor at City University London, who worked on both the Venice barrier plan and the Thames Barrier, says: “Unquestionably, barriers are essential to save Venice. I think the ecologists have not sufficiently considered the impact of future sea level rise.” But both Venice’s mayor and the national heritage minister backed Ronchi’s decision, and John Millerchip, who represents the UN’s cultural body UNESCO in the city, calls those who support the barrier scheme “arrogant and ignorant”. Millerchip believes that the barriers would provide only partial and extremely expensive flood protection, while draining cash from more immediate flood measures. Ronchi announced that he would spend $78 million on these over the next three years. Millerchip is backed by leading flood researchers. Edmund Penning-Rowsell of the Flood Hazard Research Centre of Middlesex University in London believes that the Italian government has taken a far-sighted decision. The fundamental problem, he says, is the breakdown of the lagoon’s natural equilibrium with the sea. To save Venice, you have first to save the lagoon. The Venice that a million tourists visit each year is built on a small cluster of islands set in a large lagoon of tidal mud flats, measuring 50 kilometres by 10 kilometres and separated from the sea by a further barrier of islands. But humans are transforming the lagoon from a brackish marshland that coddles the city into an open bay that is being swamped by the sea. The transformation began 600 years ago when engineers diverted three rivers, the Brenta, Piave and Sile, that flowed into the lagoon. The engineers feared that the rivers were depositing so much sediment into the lagoon that it would silt up and allow marauding mainlanders to invade the city state’s island fastness. But they overdid it. They starved the lagoon of so much sediment that the mud banks and barrier islands shrank and the sea began to invade. The process accelerated in the 19th century, when the entrances to the lagoon were widened and channels dredged to allow in large ships. These changes encouraged tidal surges and waves that eroded banks. This century, engineers compounded the problem by draining a third of the tidal mud flats to create land for farming, the city’s airport and an industrial centre and port at Marghera. Another third has been barricaded off for fish farms or lost to erosion, eliminating another cushion against the turbulent sea. The net result is that in a typical day a third of the water in the lagoon has been flushed there by the sea, and high tides that once dissipated on the mud flats now invade St Mark’s Square instead. Faced with this crisis, most engineers claim that barriers are essential to save Venice. Their plan, which could yet be revived with a change of government, is to install 79 vast, hinged flaps at the three main entrances to the lagoon. Each would be anchored to the seabed by 300 tonnes of concrete. During major floods—the half dozen or so times a year when 10 per cent or more of the city disappears under water—the hollow flaps would be filled with air and rise up to block the entrances. The scheme would cost $2.6 billion to implement, plus annual maintenance of some $2 million. But it would not protect against the dozens of minor floods that occur each year, because to use the barriers that often would escalate maintenance costs and intensify the build-up of pollution in the lagoon. The pollution comes from the city’s sewage, which is dumped raw into the canals, and fertiliser from surrounding farmland. Despite the scouring of the tides, this pollution already creates a stinking, algae-infested lagoon each summer, killing fish and disgusting tourists. The ecological alternative now being proposed, which largely follows proposals that Penning-Rowsell made to the Italian government, will still require a lot of engineering. And the core aim will still be to isolate the lagoon once more from the sea. But while the engineers want to do it with steel and concrete, Ronchi proposes doing it with silt. He will work with the forces of nature, rather than against them. There will be two stages, says Penning-Rowsell. The first is to do the many small, simple things to help protect the city: flood-proofing buildings, raising banks and walkways, fitting drains with valves to stop them back-washing, barricading the most vulnerable islands and keeping the canals between the islands free of debris. The second, long-term stage involves reversing the damage done to the lagoon by human activity, returning it to its former state as marshland. Key to this latter stage is narrowing the wide entrances to the lagoon created a century ago, and halting dredging of the shipping channels so that they refill to half their current depth of 20 metres. Together, these measures should reduce the sea’s scouring action and prevent high tides racing into the lagoon, says Penning-Rowsell. They will take time to arrange. The shipping lanes are one of the lifelines of the city’s economy, bringing oil tankers, container ships and cruise liners. But the tankers will soon stop coming anyway when a new pipeline opens, the industry that sustains the container port is in decline, and the tourists might just be persuaded to forego their grand entrance in return for saving what they came to see. To prevent the lagoon clogging with pollution, Venice will need self-contained sewers—which could easily be laid down the existing canals—and a sewage treatment works. It will be more difficult to stop the fertiliser running off from farms on the mainland. There are also plans to revive the tidal mud banks by returning reclaimed land to the lagoon. This would reduce water levels in the lagoon. Altogether, Penning-Rowsell calculates that these measures could solve more than 90 per cent of the flood problem. And ultimately, he says, the rivers that once flowed through the lagoon could be brought back to increase sediment. Some think these ideas laudable in the long term, but they ask: why not have the barriers as well? It is a good question, admits Penning-Rowsell, but in practical terms the barriers would so drain the coffers that there would be nothing left for the rest of the work. But even he has to concede that barriers may one day be needed. The greatest unknown is how much global warming will raise sea levels and defeat efforts to regenerate the lagoon’s natural defences against the sea. Both sides agree that something has to be done urgently. At the beginning of this century, St Mark’s Square flooded six times a year for a day or so at a time. In 1996 it was awash on 101 days. By the middle of the next century, if nothing is done,