Dutch Delta Works: in dam vs. clam, both win

Superdike, it might be called, except that it will usually have tides flowing through it, will have oysters instead of tulips growing on the inland side of it , and will depend on 63 giant sluice-gates rather than the little boy with his finger in the dike to keep the North Sea waves at bay.

So it's a dam rather than a dike that is being built here, and an unusual ''open'' dam at that. Its purpose, like that of Holland's age-old dikes, will be to keep gales from inundating the majority of Dutch territory that is below sea level. But its methods are the most modern -- literally. The pioneers who are constructing this largest hydraulic engineering project in the world -- a 25 -year, $5 billion effort -- are having to invent their technology as they go along.

The whole thing is called the Delta Works. The barrier here at the mouth of the Eastern Scheldt estuary will be the culmination of a complex of four coastal ''closing'' dams and two inland ''compartmentalization'' dams. It's a feat to top all the engineering feats of a remarkable nation that for a millennium has battled the sea for its land and livelihood.

On its completion in 1986 the project will turn the entire Eastern Scheldt inlet into a tidal, saltwater lake and shorten the Dutch coastline (and dike system) by 700 km (nearly 435 miles).Most important, it will also protect the southwest corner of the Netherlands from the kind of storm that in 1953 claimed almost 2,000 lives, destroyed 47,000 homes, breached the dikes in 89 places, and flooded and salted up nearly half a million acres of land.

That storm triggered the delta project. The southwest region of Zeeland had long been the lowest and most vulnerable area, with some of its polders (fields on reclaimed land) going down to almost 20 feet below sea level. A large part of the 65 percent of the Dutch population that lives below sea level resides here. Over the years the northern parts of the Netherlands had gradually been defended by dike and dam projects, like the one that turned the turbulent Zuider Zee into the freshwater IJssel Meer back in 1932. The 1953 flood in the south made it clear that it was now time to do something similar here.

The Dutch parliament drew up plans in 1958 that would still the waters of the whole area southwest of Rotterdam. First, the smallest closing dam was erected south of the Eastern Scheldt at the Veerse inlet. Then the Haringvliet inlet just south of Rotterdam was blocked off from the sea -- by a dam with the world's largest locks -- and turned into a freshwater lake by flushing the Maas and Waal Rivers through it.

Next, the neighboring inlet of Grevelingen was closed off from the sea, with the intention of converting it, too, into a freshwater lake. Then, in the most ambitious leap of all, work began on building a ''storm-surge barrier'' across the nine-kilometer (51/2-mile) mouth of the largest of the inlets, the Eastern Scheldt.

In the original plan the Eastern Scheldt was to be totally dammed off on the pattern of the Zuider Zee. As the project wore on, however, the world, and Dutchmen, woke up to environmental concerns. It was all well and good to protect the Zeelanders and turn the Eastern Scheldt into a sailor's paradise. But what would happen to the unique salt-water ecology of the wetlands? Where would the thousands of birds that stop off here in their migrations from South Africa to Scandinavia find food and rest? And what -- added disgruntled fishermen -- would Zeelanders do without their oysters?

After considerable debate and discussion of the environment, parliament revised the plans in 1976 (and added a third to the total cost). The Eastern Scheldt barrier would be an open, rather than impervious, dam; the inlet would remain salt water, with the tide diminished as little as possible (down from its present 111/2 feet to about 9 feet); indigenous flora and fauna would be preserved as much as would be compatible with the overriding goal of defending Zeeland against future floods.

Construction in the Eastern Scheldt estuary is now proceeding on this basis. In brief, the crew here will be fabricating and setting in place over the next four years 66 concrete piers that resemble the Egyptian pyramids. The piers will be connected with concrete beams to form three open dams in the Eastern Scheldt's three main channels. Sixty-three steel gates in the dams will normally stay in a raised position to let water through - but in case of a gale warning, they will be lowered hydraulically within one hour to make an impervious dam.

Innovative technology is required at every stage of the project: for transporting the piers to their final destination from their construction pits on the estuary's artificial work island of Neeltje Jans; for calibrating the piers' placement with an accuracy of one centimeter (.3937 inches) at sea bottom; and for providing a solid foundation for the dams with a carrying capacity of 80 tons per square meter (71/2 tons per square foot).

The problem of Transporting the 18,000-ton hollow piers (26,000 tons after they have been filled with sand and water) is being solved by producing the piers almost 50 feet below sea level on the work island, then flooding the construction area, raising the piers a few meters with a specially designed vessel, and sinking them at low tide to their final resting places.

The piers will be placed with great accuracy on the ocean floor with the aid of a series of overlapping measuring systems. These employ sonar, infrared, radio, and laser beams, a $1 million positioning system for airplane landing (used horizontally rather than vertically), and remote-control seabed crawlers that resemble lunar rovers.

The stable foundation is being laid by dredging silt off the bottom, spreading a bed of coarse sand, and compacting the sand with a barge called ''Mussel,'' equipped with vibrating tubes. A mat 138 feet wide and one foot high -- consisting of layers of sand, gravel, and rocks sewn into a nylon and steel mesh -- will then be laid on top of the sand by the vessel ''Cockle.''

The queen of the trio of unique rigs, the pier-hoisting ship, is named, inevitably, ''Oyster.''

Originally, the Zeeland oysters were programmed for obsolescence in the Delta Project. Under a concept of dynamic ecology, the Dutch acknowledge that some species are probably not going to survive the shifts in water regime, even when these are kept to a minimum. As long as the overall environment isn't violently disrupted, the Dutch accept this and calculate that nature will introduce new species to replace those that do disappear.

To everyone's surprise this theory has already been vindicated more than they could have hoped by the oysters of Grevelingen.

Over the years the plans for Grevelingen (as for the Eastern Scheldt) have been modified for ecological reasons, and officials are still undecided whether to complete the conversion of the inlet to a full-fledged freshwater lake. In the meantime Grevelingen is being kept as a salt lake - without tides, to be sure, but renewed with ocean water often enough to prevent it from turning brackish.

The oysters, it was assumed, would still fall victim to the changes, since they normally depend on tides to wash their food past them. This was an accepted sacrifice; oyster fishing was duly suspended in Grevelingen a decade ago.

Last year, however, it was suddenly discovered that the clever Grevelingen oysters have in fact adapted, and for the first time in the species' evolution are now working for their supper, oscillating their shells to create their own minicurrents. So successfully have they done this that in their years of respite from the fishermen they have multiplied to an estimated 50 million.