When the Army Corps of Engineers built the Chesapeake Bay Hydraulic Model here, it rewrote the book on big.

It's the biggest model of an estuary in the world, so big it needs a treatment plant fit for a town of 5,000 just to supply fresh water. If you stand in the model's Norfolk, you can barely see Baltimore in the dim distance.

The model meticulously duplicates the Chesapeake and its tributaries on a scale of 1,000 to 1. Since the bay is 195 miles long and up to 30 miles across, the building housing the model had to be more than 1,000 feet long and 630 feet wide. The roof covers 14 acres. It's big enough to run the Super Bowl, Rose Bowl and the USFL Bowl side-by-side, with space left over.

But there are no crowds here. The corps, after a sometimes frustrating 10-year association with the model, has scaled down to minimum maintenance. There hasn't been an experiment in 14 months. Twelve employes rattle around the immense tin building by the bay, keeping gear and expensive computers from rusting and waiting until summer, when orders are due to shut down for good.

Soon the corps will close the book on an expensive government project ($24 million to $32 million, depending on whom you ask) that did only a part of what it set out to do. The building will become surplus, according to the corps. It could be sold as warehouse space if anyone wants it.

Scientists are wary of writing off experimental ventures as wastes of time or money, but even Dr. Eugene Cronin, the avuncular head of the Chesapeake Research Consortium, conceded, when asked if the model was a white elephant, "It certainly has some light gray streaks in it."

The massive model sits four miles from the eastern end of the Chesapeake Bay Bridge on the lowlands of Matapeake State Park. It was the brainchild of then-Rep. Rogers C.B. Morton (R-Md.), who sponsored legislation in the mid-1960s mandating an exhaustive study of the bay and its future and creation of the model to chronicle currents, tides and the interaction of salt- and freshwater.

The building was completed in 1974 and the concrete model was finished 1 1/2 years later. It took two years just to verify accuracy. The first experiment was run in 1978.

Several studies were completed before the concrete unexpectedly buckled in 1979. After repairs, accuracy had to be reverified. The model wasn't back on line until 1981, and two more experiments were finished before budget restraints forced the cutback to maintenance status. The model has sat like a beached whale since, burning up $55,000 a month in standby operating costs.

The model's original mandate called for four major studies. Only one, measuring effects of low freshwater flow on saltwater intrusion into the bay, was completed.

The three never undertaken were (1) measurement of the effects of high-freshwater flow, as in Hurricane Agnes, (2) analysis of low freshwater flow on specific upriver sites, and (3) analysis of ways to deal with low freshwater flow.

Along the way the corps also tested effects of deepening the Baltimore and Norfolk ship channels, looked at supplementary freshwater supplies available to Washington in the Potomac estuary and ran a test for Maryland on likely dispersal of pollutants in the Nanticoke River after PCBs were found there.

The last experiment it ran was to try to determine where flotsam and bodies might end up after the Air Florida crash in the Potomac in January 1982.

If this sounds like failure, the corps has another view.

Ted Robinson, chief of the corps' Chesapeake Study Program, believes results from the completed freshwater study can be extrapolated to answer questions posed by two of the scheduled experiments that weren't done. And he feels the extra tests conducted along the way, particularly those clearing the way for deepenng Baltimore and Norfolk harbors, could not have been done any other way.

Scientists also are regularly using information gathered and future projections made by the corps from the bay study. "The existing conditions report (1973) and future conditions report (1977) are the only comprehensive sets" of their kind, Cronin said.

"It's not a joke," said Robinson, "it's a valuable facility and the decision to close it was a difficult one."

But technology is catching up with the huge mechanical facility on the Eastern Shore. Within a few years computers and mathematical equations should be able to handle questions of water dynamics faster and more economically than the model, which Cronin points out "costs as much to operate as a ship."

One thing that will be lost when the bulldozers arrive is a magnificent teaching tool. Nowhere else can a person stand and with the naked eye take in the sweep and scope of the world's largest estuary. Some 20,000 people visit the model yearly (there are tours weekdays at 10 a.m., 1 p.m. and 3 p.m.). It leaves many with "an image not easily forgotten," said Cronin.

And there is one question only time will answer.

"I can guarantee that somewhere down the road," said Dr. Robert Ulanowicz of the Chesapeake Biological Laboratories, "some large-scale problem will arise and someone will say, 'Now if we only had the model . . .'

"The acid test will come at that point," he said. "If indeed large-scale problems can be addressed by computer models, accurately and cost-effectively, the decision to close it will have been right. If not, it will already be too late."