At the shore, it's retreat - not fight. As the sea level rises, beach developments are in danger of being washed away. In one Texas resort, three blocks have already been lost to the Gulf. But rather than fight the waves with concrete, some states say the answer is to move inland.

The tale is in the rubble in the sand - once foundations and walls - in a washout area called the Follies. The beach has moved in. Elsewhere, where sea walls armor private property against the waves, the beach has all but disappeared.

The rise of the oceans appears to be accelerating this century. The beaches are marching inland, on a collision course with beach front cottages and hotels. From Cape Cod to the Gulf coast of Texas, the rising sea means beaches are creeping landward at an average rate of two to four feet per year. (Why the rise, Page 6.)

The traditional response, barricading the shore with sea walls and rock revetments, is losing public favor to the notion of retreating before the surf. But retreat is meeting hard resistance from people losing property to the ocean.

Sargent Beach, Texas, is losing 50 to 60 feet a year. Three blocks have already been swallowed by the Gulf of Mexico. Along 15 miles of coast, notes Texas Bureau of Economic Geology scientist Bob Morton, ``there have been huge chunks of real estate gobbled up.''

In Westhampton Beach, N.Y., the beach is steadily moving under 220 cottages lining the shore. Nags Head, N.C., has even more houses meeting the high tide line.

The largest symbol of retreat may turn out to be Cape Hatteras lighthouse, a 208-foot beacon built in 1870, 1,500 feet from high tide. The National Park Service proposed fortifying the landmark - now within 160 feet of the shore and losing ground fast - with a 23-foot seawall.

The National Research Council has instead recommended moving the 208-foot, brick tower inland. The park service is expected to make a decision this fall.

Americans are moving to the coasts in droves, building condominium towers, resort hotels, and expensive houses. Catastrophes, coastal geologists say, happen because people build fixed structures on a moving shoreline. In South Carolina, at least half the population is projected to live on the coast in 20 years. Until last month, the state exerted no control on building above the high-tide line.

As a result, even in luxurious, well-planned resort developments as on Hilton Head, ``there is nothing near the beach front that isn't going to be threatened in the next decade,'' says Duke University geologist Orrin Pilkey.

Now a new law in South Carolina restricts building. A projection of where the dune line will be in 40 years has been made. Anything built seaward of this line must be small enough so that it can be moved. The law also bars building new sea walls, which eventually result in less beach, not more (see diagram). The South Carolina law remains far less restrictive than in Florida, North Carolina, or New Jersey.

These laws are expensive to developers and property owners. The South Carolina savings and loan association fought the bill tooth and nail. But losing beaches to sea walls is also expensive.

Tourism is the state's second largest industry, and beaches are its greatest draw - even at Hilton Head with its famous golf courses, says Doug Woodward, a University of South Carolina economist.

The political battle over coastal management here, Dr. Woodward says, ``became an issue of the social good vs. some private developers that were going to be hurt.''

Sea walls stop the beach retreat temporarily, but the ocean keeps coming. Once high-tide or storm-driven waves reach it, the barricade intensifies erosion by bouncing waves back onto each other. The seawall becomes increasingly vulnerable to a storm breach.

At Hilton Head, 81 percent of the beaches with sea walls or rock revetments disappear at high tide, according to Dr. Pilkey.

There is a middle ground, gaining in popularity - beach replenishment. Pumping new sand onto beaches is expensive and not yet well understood geologically, but it saves both property and beach for a time.

Most of the new sand pumped onto an Ocean City, N.J., beach in 1982 disappeared in a storm two months later. On the other hand, Miami Beach created miles of 100-yard wide beach a decade ago that shows little erosion. Boca Raton to the north now wants to follow suit, creating a wide beach over what is now a rocky offshore bottom favored by snorkelers.

Much of the coastal development along the Eastern seaboard is on long, thin barrier islands. These islands are migrating up the coastal plain with the rising ocean. The slope is so gentle that for every foot in rise, the islands move a hundred to a thousand times that distance horizontally.

Man has a heavy hand in the system. Groins erected across the beach to trap sand moving along the shore, and jetties protecting inlets rob down current beaches of sand. Dredged inlets also shoot sand out of the beach system with the tides, either deep inland or out into deep water. At St. Marys inlet in Georgia, enough sand has been drawn out to deep water by currents in the dredged channel to create 30 miles of beach 100 yards wide, notes Pilkey.

A few inlets now have man-made systems to pass sand from one side of a channel's mouth, over a deep-dredged area or around jetties, to the other side - simulating the natural migration of the sand.

West Coast beaches face a different kind of problem. Beaches are usually thin strips of sand against sea cliffs. The sand comes directly from flooding Western rivers. Extensive damming has cut off some 50 percent of this sand, according to Doug Inman of Scripps Institute of Oceanography. Dr. Inman and colleagues are developing ideas to allow sediment to bypass dams.

Most beach erosion is dwarfed by the loss of 50 square miles a year of south Louisiana wetlands to the Gulf of Mexico.

On the Nile, the Aswan dam has cut off sediment flow resulting in miles of delta land washing into the sea every year. ``It makes our problems look trivial,'' says Inman. (Drought on the Nile, Page 9.)