A few decades from this Memorial Day, you will have disappeared, or you will have become a fortress with little or no beach and high waves smashing your seawalls on a daily basis, costing you millions of dollars a year for repair and expansion.

There are no alternatives.

A war has erupted at the American shoreline. One of the belligerents is the rising sea level. There is controversy over what is causing the rise, but it is indisputable that the increase of one foot per century is accelerating. That sounds like little until you realize that a one-foot vertical rise translates into a 1,000- to 2,000-foot horizontal retreat of the shore line.

Facing this formidable enemy are thousands of well-to-do Americans who want to live right on the seashore. Their rush for the shore is aided by the federal government's mortgage-interest tax deduction on vacation homes. Yet there is no question that the ocean will win the war. Less clear is exactly how the battles will shape up over the next few decades. One absolute certainty is that Ocean City will be in the center of the combat.

The various coastal states are responding to the dilemma in different ways. Louisiana, whose shores are receding at a faster rate than any other state, now is embarking on a $40-million-plus program to armor her beaches. They are following this course (which will certainly fail) because for every one foot of shoreline retreat, a one-foot loss of continental-shelf oil land results.

North Carolina and Maine stand at the other end of the spectrum. Both have made it illigal to construct new seawalls or other types of hard objects for shoreline stabilization. Other states seem sure to follow their lead in what really amounts to a declaration of retreat from the shoreline.

The story of our retrat from the beach starts at the New Jersey shore -- particularly those long-used portions near metropolitan New York and Philadelphia.

In the early years, storm and retreating beaches took their toll, and from time to time who rows of houses fell into the sea. In the 20th century, New Jersey citizens increasingly began to respond to shoreline erosion by building solid walls either perpendicular (groins and jetties) or parallel (seawalls) to the shore.

At rates that seemed directly proportional to the magnitude of the structure, the beaches in front of the walls disappeared, to be replaced by the rubble of previous generations of walls damaged or destroyed in storms. Even the underwater part of the beach (down to a depth of 30 feet or so) eroded and steepened. With deeper water, the waves got higher, the storm damage got worse and tax expenditures on walls got bigger. This vicious cycle of protecting buildings owned by a very few individuals, leading eventually to taxpayer-supported beach destruction, is now referred to by geologists as ''New Jerseyization.''

Recently, a New Jerseyization endpoint of sorts was achieved by Sea Bright, the community nearest New York City. In March 1984, a nor'easter struck the town, causing tens of millions of dollars in damage to the beach and seawalls -- which begins to approach the value of all buildings in the town, although none of them was seriously damaged. It is apparent that a small community such as Sea Bright cannot afford many more storms. Unless the taxpayers want to repeat the folly of King Canute, Sea Bright must be abandoned in the next few decades. A point of no return has been reached.

Ocean City is heading down the New Jerseyization route at a rate that is probably unequaled anywhere in the United States. And it less forgiveable because Ocean City development is almost entirely post-World War II -- after past lessons from heavy beach-front development in places like Atlantic City and Miami Beach should have been clear to all who cared to look and listen. Nonetheless, Ocean City, with a year-round voting population of just over 5,000, still packs 250,000 people into is confines at peak season.

Ocean City occupies Fenwick Island, a more-or-less typical Mid-Atlantic barrier island. It consists of a ridge of sand that was created by sand moving north to south from the eroding Delaware shoreline over the last 3,000 or 4,000 years. According to Robert Dolan, University of Virginia coastal geologist, psot-1940 erosion rates at Fenwick Island range from 1 1/2 to 3 feet per year except at the south end (below 21st Street). The beach has built up there, for reasons that cannot be repeated in other parts of the island. A jetty was built there that is so large as to be financially impossible to reproduce repeatedly. But, more important, the sand has been trapped over the last 50 years. And even if you wanted to wait another 50 years to build more beach, the sand would not be available because of the very groins that the city is now building that are supposed to ''protect'' the beach.

If there wre no buildings on Fenwick Island, there would be broad sandy scenic beaches there forever. They would be imperceptibly moving landward, because of the sea-level rising, and the constant attack by storms. But the beaches would even retain their original shape and appearance as they moved. However, buildings do jam the shore, so the beaches can't move. Instead, they narrow, steepen and eventually disappear. In other words, the sea-level rise on Fenwick Island is no threat to the beaches, only man is.

Rhehoboth Beach and other Delaware shore areas are in somewhat better shape. Delaware has allowed itself more flexibility for future response to the sea-level rise, in that buildings there are farther away from the ocean than in Maryland, and the buildings that exist are not a skyscraper jungle like in Ocean City. Nonetheless, the same underlying dynamics at work in Ocean City will ultimately threaten those resorts, too.

Dr. Steve Leatherman of the University of Maryland has observed that, just as in the classical New jersey examples, underwater beach (called the shoreface) off Ocean City is now rapidly steepening. For example, leatherman has shown that between 20th and 80th streets the shoreline retreated 32 feet in the 16-year interval from 1962 to 1978. At the same time, the 20-foot-depth contour line retreated landward 120 feet.

As a result of both streepened shoreface and narrowed beaches, Ocean City is more vulnerable to storm damage that at any time in its history. Two New England environmental scientists working for the state of Maryland reported in 1984 that there are three areas of particularly hazardous living. Between 74th and 87th Streets, the erosion hazard is particularly high. For 20 blocks south of 132nd Street, the potential for flooding in the next moderate storm is great, and between 32nd and 57th Streets is a zone where a new inlet may well pop through in the next big storm.

Hazardous living isn't the only problem. The beach, that priceless stretch of snad that was the raison d'etre for Ocean City to begin with, is slowly disappearing. At high tide in a few areas, there no longer is a beach. Furthermore, the risk of beach sand removal by storms is now greater that ever.

Some of these unhappy scenarios could be avoided now if the city would take action, but the past record of Ocean City officialdom provides little reason for comfort. Among those familiar with the city and town governments on American barrier islands, Ocean City's reputation is legendary. Its failure to recognize development hazards is awesome. Its political skill in extracting money from higher government levels for beach erosion control is downright awe-inspiring. In fact, the latter may be responsible for the former.

Examples include:

* The city had no plans for hurricane evacuation until this year. Evacuation may be an impossible task anyway. A Maryland state consultant suggested that hurricane preparedness has been lagging because of concern that it might hurt tourism.

* After the 1962 Ash Wednesday storm, the Corps of Engineers built a 12-foot-high continuous dune, 150 feet back from the high-tide line. This federally funded storm barrier was accompanied by appeals from the Corps to the city government to protect and maintain it. Subsequent construction has completely removed it, largely because it obstructed the view of the sea.

* After storms, the city bulldozes sand from the lower beach to the upper beach to protect buildings. While it is an acceptable measure for a one-time storm contingency, it has for years been the only citywide strategy for combating beach erosion. In fact, bulldozing actually increases erosion in the long run.

* The state forced the city to establish a construction set-back line in the early 1970s. The line was intended to prevent construction in front of the natural dune line. As natural shoreline erosion has occurred, the setback line, in some cases, is in the surf zone and the city, trampling on the spirit of the setback concept, has recently allowed construction in extremely hazardous areas. They now use the vulnerability of those buildings to justify additional state aid to protect their shore.

* ''Too close's construction has not only been allowed, it has been encouraged; for example, the 1983 extension of Wight Avenue, a beach-front road, seaward of beach cottages already thre; for example, in 1984 permitting costruction of two condos with underground parking (in violation of federal flood-insurance regulations) on the beach at 81st and 82nd Streets, a block away from Oceano condo which has already had its ground-level parking lot washed away.

* Ocean City continues its practice of siting high-density development projects in such a way as to increase erosion problems (i.e., assuring the need for seawalls) even as the city successfully extracts money from state taxpayers for beach-erosion control projects.

Ocean City is also causing problems for the neighboring uninhabited island of Assateague. Those jetties built more than 50 years ago have widened Ocean City's beach from about 20th Street south, but they have also caused sand starvation of northern Assateague Island, resulting in its very rapid migration landward.

The island migration provides a spectacular example of barrier-iosland dynamics. It will, within 20 years, fill in the waterway behind Assateague as well as expose Ocean City's southern flank to storms. Plans are afoot to spend a lot of federal money to prevent the filling in of that intracoastal waterway.

What is the solution? A while back the Corps of Engineers proposed a $20-million-plus beach-nourishment project for Ocean City. It was supposed to provide both a recreational beach and storm protection. However, in 1984 the project was shelved because the benefits were largely recreational, which is out of the federal bailiwick. So the ball is back in the court of state and city governments.

Sooner or later, as the city comes to grip with the beach-loss problem and more fundamentally, the sea-level-rise problem, they will have to begin a massive beach-replenishment program.

If we assume the sea level will rise at the rate predicted by the National Academy of Sciences, Leatherman predicts 13 million cubic yards of sand will be needed between now the year 2025 to maintain the beaches at their present width. That figure rises to 34 million cubic yards by the year 2075.

The Corps of Engineers and others have determined that a large body of sand, perhaps 10 million cubic yards, exists on the ocean shelf just seaward of the jetties. Much of this would have flowed to Assateague Island if the jetties weren't there. The Corps estimates that the first 4 million cubic yards of sand will cost $26 million to put on Ocean City's beach. The next 5 million cubic yards will cost $35 million. And so it goes.

There is almost no likelihood of getting sufficient sand to maintain the beaches economically beyond 30 or 40 years from the present. Even if ocean City begins replenishment immediately, the future points to an armored, beachless island. Adding to the problem is the fact that the federal government also covets the offshore sand to pump on Assateague's beach to halt the relentness march of that island into the intracoastal waterway.

At present the city's plan is to install a series of 47 groins spaced along the entire length of the island. Each groin will cost on the order of $400,000. So far only the 7th and 9th Street groins are in place, and the 12th Street groin is under construction. The state of Maryland will furnish $1.5 million this coming year for groin construction, a figure that will not begin to implement the city's entire plan. Ocean City will contribute little if anything to the groin construction costs.

The original plan for these groins devised by engineering consultants had two aspects. First, the groins were supposed to be built sequentially, starting in the south, and then heading north. Second, the area between the groins was supposed to be filled with sand pumped up, by man, at great expense, from the ocean bottom.

If these aspects had been retained, they would have had the combined effect of increasing the quality of the recreational beach and providing some protection against minor storms.

Incredibly, both aspects have dropped. Not only is no sand being placed between the groins, but groins are now scheduled to be built this fall of 74th and 77th Streets -- far to the north. These will trap sand at the top of the island, and starve the beaches to the south, where the greatest erosion problems are today. Thus, the project will neither increase recreational beach quality nor provide protection -- in fact, it will cause erosion. Because of the expected sea-level rise it can be safely assumed that the present program ofgroin construction is a waste of state taxpayers' money.

Many observers, including me, believe that if Ocean City itself is made to pay more for its beach-erosion control program, irresponsible construction practices will halt.

When asking for state money, Ocean City argues that it puts much more money into state tax offers than it takes out. This logic seems to have worked better in Maryland than in other coastal states where, almost without exception, some local funding is required before beach erosion mitigation measures are carried out. Even if one agrees with the city's contention, it seems reasonable to expect that the local government that accepts state tax funds should also accept responsibility for insuring that the money is well spent. More and more states are beginning to view their beaches as though they national parks -- to be preserved for all to enjoy now and in the future.

Why haven't the taxpayers of Maryland revolted? The state's only stretch of open-ocean beach that can legally be built on is being destroyed before their very eyes for the benefit of a very few people who built imprudently. The number of beneficiaries is particularly small compared to the number of Marylanders who use the beach.

Ironically, a survey of Ocean City property owners might reveal that many beach-front owners are from out of state. In Maine, the discovery that only 5 percent of beach-front property owners voted in the state elections was a major impetus in passing anti-seawall regulations. Federal taxpayers stand to lose by Ocean City policies too. There is an ever-increasing likelihood of a large federal flood-insurance payment after the next storm to locations like Ocean City that are still grandfathered under this program.

It is my prediction that unless fundamental changes in philosophy occurs, Ocean City will be walled from one end to the other within 10 or 20 years. It will be like Miami Beach during the 1960s and '70s in that it will be known for its ocean breeze rather than its ocean beach. The tax rate for property owners will be high because of the need for maintenance of walls and for continuous beach nourishment. Twenty years from now, sunbathing will mostly be carried out in small pocket beaches on the north side of the massive groin field.

Bars and restaurants and motels will lose a lot of business. People will go elsewhere because swimming will be mostly during brief periods of low tide when beach remnants are exposed. When a big storm strikes, Ocean City flooding and property damage will extensive and many people will die, especially if the storm strikes during the ''season.'' The inter-tidal zone -- the stretch of beach belonging to the public and providing public access, will have disappeared.

It is not too late to learn from Ocean City's mistakes. For Rehoboth, for Virginia Beach, and for the Outer Banks, there is still some time. There is time to understand that you cannot build unmovable high-rises only yards from the waves. There is time to hire professionals who understand beach geology who can try to work with the ocean, not against it.

But for Ocean City, the only lesson left is that in a war of attrition, the ocean always wins.