continued

The problem is significant and reflected in the Italian government's declaration that the safeguarding of Venice is of "national interest." The problem, however, does not belong to Venice in isolation; it includes the surrounding lagoon and its complex interdependencies between environment, architecture, and socioeconomics.

Invasions of the First Millennium

Why was a city built on a sedimentary island in a lagoon off the coast of Italy? Blame Attila the Hun. During his invasion of Italy in 452, the population of the countryside fled to the small islands of the lagoons lining the western coast of the Adriatic.

Although many of the refugees returned to their mainland homes after Attila's withdrawal, the seed was planted for the founding of Venice. Many of the migrants remained on the islands such as Torcello, Burano and Malamocco.

It was not until the assault on the settlements of the Venetian lagoon in 810 by Pepin, the son of Charlemagne, that the foundations were firmly laid. Pepin's forces easily seized land-based towns, but his main aim was Malamocca island, the Venetian capital. The tidal channel between Palestrina and Malamocco, however, ultimately proved too much of an obstacle.

The lagoon's inhabitants decided to move the capital to the more protected group of islands in the center of the lagoon. That area was known as Rivo Alto, or "Ri'Alto" (high bank), because the islands were sedimentary banks of the Brenta River delta, whose mouth lay several kilometers to the west.

As the population grew, they expanded their island's area dramatically by landfill behind rows of pilings. The area at Ri'Alto soon became the metropolitan center of the lagoon, the city known today as Venice. "Rialto" survives as the name of the commercial area that surrounds the oldest bridge on the city's Grand Canal, which is itself the vestigial river bed of the Brenta River.

Flooding of the 20th Century

The flood of 1966 was of symbolic importance. Venice and the other historic towns and villages in the lagoon were submerged under more than three feet (one meter) of water. Inspired by this disaster was the initiative for a series of studies, experiments, projects, and remedial work.

The main parties were the Italian government, responsible for physical safeguarding and restoration of the hydrogeological balance in the lagoon; the Veneto Region, for pollution abatement; and the city councils of Venice and Chioggia, for urban conservation, maintenance, and activities aimed at promoting socioeconomic development.

Researchers from the mechanical engineering department of Swansea University in Wales, UK provided computer models of the rock strata, including joints, cracks, and faults. They have been instrumental in understanding the cause of the subsidence problems.

Solutions for the Third Millennium

In broad terms there are two types of remedial work: protecting the land mass and preventing the destructively high tidal movements. Within this there are a number of initiatives for protecting coastal zones from sea storms and restoring environmental balance to the ecosystem.

In the cities, towns, and villages in the lagoon and along the coastline, local protection initiatives involve permanently "raising" the lowest areas. However, concerns for historic architectural integrity and entry-way accessibility limit such lifting to an agreed-upon 40 inches (100 centimeters). Therefore additional means are needed to control the highest tidal waters.

It is widely accepted that modern boats contribute partially to the general malaise in Venice's waterways. Agitation caused by propellers and waves is undermining building foundations. Workers are finding larger holes in buildings and quays. Canals are occasionally closed and pumped dry to carry out remedial work.

There are three types of flooding: from water breaking over the wall, filtering through subsoil, or overflowing from drains. The plan provides a solution for all three types of flooding with solutions appropriate to the delicate architectural and construction context.

To prevent flooding caused by filtration through the subsoil without modifying the height of the square, a "horizontal defense system" will be adopted involving the laying of a clayey compound membrane under the paving in the public areas.

In addition to protecting public areas up to 40 inches (100 centimeters) above sea level, some private areas on the ground floor below sea level will also be safeguarded against seepage. In some forty properties, waterproof overflow tanks will be constructed, flooring will be raised, and waterproof membranes will be installed as appropriate.

Land is also being protected from the erosive forces of the sea by rock jetties that extend out to sea.

The Flood Barriers

One of the most ambitious projects is construction of a high-water barrier, which will protect the whole lagoon from the sea.

The Consorzio Venezia Nuova (Venice Water Authority) is responsible for implementing measures to safeguard Venice and its lagoon. Its main task is the construction of mobile flood barriers at the lagoon inlets, which will come into operation when tides exceed 40 inches (100 centimeters).

The barriers' conceptual design was completed in 1989. In 1995 the Massachusetts Institute of Technology (MIT) in Boston produced an environmental impact study of the design.

In 2001 the work on the design and planning continues its path through politics, environmental objections, economics, and other obstacles.

The mobile flood barriers will consist of box-shaped metal flap-gates built into the inlet canal beds. In normal tide conditions they are full of water and lie flat in their housings. A gate is 66 feet (20 meters) wide and varies in height 66 to 98 feet (20 to 30 meters) and thickness 13 to 16 feet (4 to 5 meters) depending on the depth of the inlet.

The flap-gates are "mobile" because when tides exceed 40 inches (100 centimeters), an emission of compressed air empties them of water and the unhinged edge rises. They temporarily isolate the lagoon from the sea, blocking the flow of the tide.

The inlets remain closed for the duration of the high water and for the time it takes to maneuver the flap-gates, a total of 4.5 hours.

The housing consists of prefabricated concrete caissons which are inset in the lagoon floor and contain service tunnels and machinery.

Eighteen flap-gates are required at the Chioggia inlet and 20 at the Malamocco inlet. Because the Lido inlet is twice as wide, a structure will be placed in the center of the entrance, with 20 gates to one side and 21 to the other.

Once the go ahead is finally given, it will take eight years to carry out the work at a cost of an estimated £2 billion.

The struggle to save this historic site continues through localized remedial work against the rise of tidal waters and with a groundbreaking engineering solution that has never been tried on this scale before.

Don Barker is a freelance writer and photographer in London, UK, who has lived and worked in Europe, Australia, Thailand, Sri Lanka, Hong Kong, and Singapore.   >>>

Fishermen's cottages on the lagoon island of Burrano. Photo: Don Barker The historic tranquil "streets" of Venice. Photo: Don Barker The San Marco Basin from Piazza San Marco. Photo: Don Barker Aerial view of the Venice Lagoon. Image: Consorzio Venezia Nuova The hole in the bottom of the door was caused by motorized boats. Note also the tell-tale signs of high tides. Photo: Don Barker Remedial work being carried out on one of Venice's waterways. Construction workers carry out their work behind the steel pilings, often working below the water line. Photo: Don Barker A blanket facade covers Ca Foscati on the Grand Canal concealing remedial work behind. Photo: Don Barker Santa Maria della Salute church on the Grand Canal. The top four steps are new, part of the local barrier defenses. Photo: Don Barker   Click on thumbnail images to view full-size pictures.