Cape Cod town faces relentless cycles of pounding Atlantic. Tense residents watch as the sea slowly destroys protecting barrier beach

``Couple coming down beach from the north!'' Like soldiers under siege here, young policemen, connected to one another by walkie-talkies, have been fending off curious crowds, as ocean tides from a year-and-a-half-old break in the barrier beach still threaten to dump a whole row of houses into the sea. One slipped halfway in late last winter and was subsequently dismantled and hauled away.

This strip of Chatham shore - between Andrew Hardings Lane and Holway Street, one block to the north - is under greatest attack from the ocean.

There are a lot of drawn faces here. Many are on property owners, harried by the sea and feeling harried by government agencies, which refuse to come to their aid.

Chatham perches on the elbow of America overlooking Pleasant Bay, a long north-south body of water created by a narrow strip of barrier sand, called North Beach.

Periodically, in an about 140-year cycle, the ocean storms through the barrier beach as it did January a year ago, destroying the barrier - and pounding the town of Chatham on the west bank of the bay. What started as a 150-foot break in the barrier is now about a mile and a half. Eventually, North Beach will extend itself south again, as sand is carried down from farther north, according to a report by Graham Giese, a coastal geologist at the Woods Hole Oceanographic Institution nearby.

But now what used to be a placid, sandbar-laced recreational water resource, protected by a barrier strip, surges with treacherous cross currents, crashing breakers - and menace.

During the winter, a handful of residents hauled in behemoth boulders to arrest the sea and protect the rest of the homes from tilting in. By a quirk of environmental zoning, only about half of the residents here will be allowed to leave the rocks in place. Summer tides tend to be lower than spring and fall ones, but each summer storm forecast does nothing to ease the concern of these residents.

Generally, environmental agencies - like the Chatham Conservation Commission, the state Departments of Environmental Quality Engineering and Environmental Management, the Federal Emergency Management Agency, and others - say interfering with the natural processes in Pleasant Bay by building sea walls or sand reefs to protect the inhabited area is useless. The power of nature in this case is just too strong, and the money would be wasted - or other areas would be deleteriously affected.

No face on the Chatham shore has been more stoical than John McGrath's, owner and operator of Art Gould's Boat Livery. (Gould is his grandfather, who started the business.)

Young Mr. McGrath used to ferry swimmers and nature lovers from the town landing at Andrew Hardings Lane across the bay to North Beach and South Beach Island, as the portion below the breach is called. But he has had to move his business several blocks north to Claflins Landing.

It isn't clear yet how the rough seas and erosion will ultimately affect McGrath's business. But there's a tough energy and excitement in his voice.

``I started right out of high school, '72-'73, running the water taxi,'' a boat licensed to carry six passengers, he says. ``The record I set going over to the beach: I counted 193 people I took over one day - and back.''

With the tone of a Greek chorus, McGrath, like a Homer, orally chronicles Chatham's epic erosion.

``Jan. 2 of last year the beach broke open. During the previous summer, I saw jeeps on the beach on that part over there that's barren,'' he gestures toward a low section of North Beach across the turbulent bay. ``I saw waves hit the sides of jeeps, climb right up and over the jeeps and keep going. But it was still a barrier beach.

``The storm blew it open, and for the first few weeks at low tide - it was only 150 feet across - you could still walk it without getting your feet wet. High tide was a different story.''

For years, Chatham residents have been warned of the possibility of a break (through the report by Mr. Giese, which was commissioned by the town 10 years ago).

The report also explored various preservation plans - primarily ``soft solutions,'' like sandbagging.

For several years on the lower part of the bay at Morris Island, visitors to the national wildlife sanctuary have walked past sand-filled plastic bags placed to help control beach erosion - apparently with some success.

Ironically, sand brought by the breach is beginning to collect in this formerly threatened area - and may possibly reconnect it with Monomoy Island, a long sand spit reaching southwest from Morris.

But residents further north, opposite the breach - unthreatened until last winter - did not take advantage of that option.

The first (and, so far, only) house to go was Benjamin Galanti's, built in 1938, when memories of a wipe-out 100 years earlier had been all but forgotten. At that time the Chatham lighthouse had to be taken apart and moved back from the shore. Its site is again in jeopardy.

As of this writing, it isn't certain how much the Galantis and others who might lose their homes will be recompensed, if anything, under governmental flood or erosion insurance benefits.

John McGrath continues, ``My grandfather, who's 84, said, `Oh, this is the way it was when I started!' Grandfather started taking people over in 1944. Nauset Beach ended just about opposite to here.

``Everything you see, including the break, right down to the southerly tip near Monomoy Island grew in 40 years. So the beach grew a mile every 10 years, and then we had the break last year.''

Meanwhile, the quality of the water itself in Pleasant Bay appears to be improving, because the heavier tides are washing out pollutants and boosting aeration and salt content.

Thus the scallop and soft-shell clam beds are becoming more productive, striped bass are returning - and some formerly endangered beaches are building. On South Beach Island, the piping plovers are nesting, threatened now more by a pair of foxes than the steps of man.

Over a thousand years ago, Britain's King Canute called the bluff of flatterers, who said he as king could do anything. To prove he could not, Canute slyly stood on the verge of England's southern coast during an incoming tide - and failed to reverse the tide. Efforts of Chatham homeowners to stop the sea with boulders seem scarcely more likely to succeed.

On a map of Cape Cod in yesterday's Monitor the scale was incorrect. The distance represented by the scale should have been three miles.