''It is all so quiet and sad and faded, yet all so brilliant and living.'' Henry James's description of St. Mark's Cathedral in Venice could be as much a description of the city itself as of its magnificent Byzantine basilica. Once the center of a flourishing trading republic of the 14th and 15th centuries, with a population of several hundred thousand, this island city, dissected by canals, now holds 90,000 inhabitants, whose numbers swell to almost a million in the high summer tourist season. Most of the ancient palaces and churches of Venice were built on wooden pilings driven into the muddy sea floor. Centuries of periodic flooding caused by winds and tides, with the added affliction of 20 th-century pollution, have turned this museum city into a mecca for art restorers and hydraulic engineers as well as for tourists.''
''Venetians born and bred believe their city will last forever,'' says Prof. Roberto Frassetto. ''It's difficult to convince them that it is in desperate need of consolidation.''
A physicist who is turning his attention to research on flood control, Professor Frassetto has long been concerned with two of this city's most pervasive problems: the water and air pollution that are eating away the city walls and foundations.
The city's problems came to worldwide attention after Nov. 4, 1966, when, driven by a strong wind blowing up the Adriatic, the tide flooded into Venice at six feet above the normal high-tide mark, drowning St. Mark's Square in murky water. Since then, flooding has become a more frequent winter phenomenon.
Now, however, new government measures are addressing the problem of trying to stop Venice from sinking farther into its surrounding lagoon. Last month the Italian Senate approved a law allocating 600 billion lire ($324 million) over the next two years for the problems assailing Venice. Of these funds, the largest portion (more than 340 billion lire) is to be used for flood control. (Some 145 billion more is earmarked for the restoration of houses already damaged by flooding, and 80 billion is to be spent on pollution control.)
The main project - defending this island city against high floodwaters - is made difficult by the need to maintain the delicate balance of tidal waters, which must continue to sweep through the lagoon. Each tide flushes between 150 and 350 tons of water in and out of the wall-bound lagoon - the only flushing system of this waterlogged area.
The other sections of the law provide for the cleansing of waters that flow from the mainland into the Adriatic Sea via the Venice lagoon, and for the restoration of buildings damaged by pollution.
''We feel at least we are on the right road,'' says Paolo Cacciari, deputy mayor of Venice, from his office inside the 12th-century Byzantine palace that is the city hall - and is itself one of the buildings under restoration.
''If all goes well,'' he adds, ''the government will renew automatically the allocation of funds every three years, until we reach our goal of $1.3 billion, which is what we calculate we need for comprehensive water control and restoration here.''
The first question to be confronted is flood control. ''We have come up with an approved project, but it still needs a year or so of further study,'' explains Lamberto Sortino, president of the state-controlled Venice waterways authority (Magistrato Alle Acque) responsible for the technical application of flood control. The dike control project, he explains, consists of a new set of sea walls running outward on each side of the three mouths that lead from the Venice lagoon to the open sea.
In the plans put forth by this project, the sea walls would curve, like rock-filled jetties, for about half a mile along either edge of each opening into the lagoon. On the side that faces the open sea, these walls would be constructed of loose boulders or huge blocks of stone cut from nearby quarries. On the lagoon side the walls would be cemented and made of the white Yugoslavian stone (pietra d'Istria), the same material that was used to build the foundations of many Venetian houses. The walls would also have walkways along their upper surfaces.
Across the openings of the three mouths would stretch mobile dams made of 60 -foot steel cylinders. Under normal conditions, these pipes would lie filled with water on the bed of the lagoon, one end tethered to the sea floor. But in response to a man-controlled signal warning of the approach of an unusually high tide or flood, the water in the pipes would be expelled and the pipes would fill with air and stand up - rather like a row of organ pipes - not perpendicular but slightly inclined in the direction of the current. The length of each of these steel tubed dams, which would form a temporary barrier for the duration of the floodtide, is to be about about 830 feet.
There are, however, objections to this plan from Italy's main ecological association, Italia Nostra, which claims the project could upset the balance of the water flowing into the lagoon area. The group's attention, instead, is focused on the huge Montedison chemical works at Porto Marghera, two miles behind Venice on the mainland, said to be responsible for several thousand tons of chemical waste that contribute to the 3,230 tons of polluting phosphates flowing into the lagoon every year.
One problem, says Giuseppe Rosa Salva, president of Italia Nostra, is that ''nobody talks about reducing the flow of water into the shipping canal, which was purposely deepened to allow large oil tankers to reach the Porto Marghera works.''
''Instead of building inflatable dams,'' adds Riccardo Rabagliati, an engineer who works with Italia Nostra, ''they ought to construct a pipeline to ship oil to the factory, open up several shallower canals that have been closed over the years, and so naturally reduce the height of the water which spreads through the lagoon every tide.''
With the government approval of the recent law, however, Italia Nostra recognizes its defeat over the dam project. On the other hand, Venetian authorities are, if not jubilant, at least optimistic, and experiments are being carried out on many fronts. On the tiny island of Povegliano, near Venice, Professor Frassetto describes successful attempts at ''mudjacking'' - the injection of neutral materials into the lower strata of mud on which the houses were built centuries ago, in order to lift entire areas of the city out of danger of being engulfed by water. Experiments with this technology, he says, have already proved successful in the Netherlands and in California.
And in the city itself, where sewage has been piped directly from houses into the nearest canal since the 8th century, experiments are going on in tiny areas to unite individual sewage pipes (where they can be found) into one large waste pipe so that the sewage can be treated before being discharged.
Also playing a part are the restoration groups. After the 1966 flood, several foreign organizations mushroomed up to defend the city's crumbling architectural treasures. Of those that remain, the British Venice in Peril Fund and the International Fund for Monuments are in the forefront.
''What the situation in Venice has done is attract art restorers from all over the world,'' says Sally Spector, who directs the New York-based International Fund for Monuments, whose latest project has been the restoration of Venice's Jewish ghetto, recognized as the oldest such ghetto in the world.
''We have done some breakthrough work in stone conservation,'' she says. ''This is just one example of Venice-based research which has helped art cities all over the world to keep their monuments intact.
''After all,'' she adds, ''if something can survive in Venice, it can survive anywhere.''