Beaches Slipping Off Into Sunset, One Grain of Sand at a Time : Erosion: Waves and man-made intrusions are reshaping profiles, leaving slivers of sand. : The losses have thinned the usual parade of sunbathers, strollers, swimmers, surfers and snorkelers.

In various spots along the Southern California coastline, and especially here in the Eden of tourism and surfing, the beach is slowly vanishing. Waves and man-made intrusions are interrupting the yin and yang of sand movement and eroding the depth and length of beaches.

In Oceanside, 50 miles north of San Diego, one man remembers being able to walk on the sand toward the ocean for 300 feet before getting wet. Now, he says, at high tide, he can’t even walk on the beach.

At the Seacoast Inn in Imperial Beach, 15 miles south of San Diego, steps that several weeks ago led directly onto the beach now hang five feet above the sand.

Gazing down the contour of San Diego’s beaches, toward the graceful, towering bluffs of La Jolla and the glassy tidal pools of Del Mar, a visitor might believe the sand is a permanent part of the picture.

But looks are deceiving.

“We have areas, where in 10, 20, 30 years the beaches are going to be gone,” said Steve Sachs, a senior planner at the San Diego Assn. of Governments.

Beaches and sand are as crucial to California’s postcard image as palm trees. They are part of the pitch that beckons tourists and settlers from far-flung places.

They’re also big business.

In 1990, the last year for which figures were compiled, visitors to San Diego beaches spent more than $200 million, according to Sachs.

One reason why Del Mar became Del Mar, with its elegant race-track and cocktail-party set, is its expansive, lush beaches.

But on many California beaches these days, expansive and lush are bygone terms. Every year, Sachs says, San Diego beaches lose an average of one foot to five feet in width.

Frank McDonald, who once owned property by the water in Oceanside, says beach erosion has thinned the usual parade of sunbathers, strollers, swimmers, surfers and snorkelers.

Along what locals call Restaurant Row near the Del Mar racetrack, diners once looked out picture windows onto a broad carpet of sparkling sand. These days, boulders are piled under the windows to form a buffer against the encroaching waves. During the winter, waves pound the glass as patrons dine on calamari and shrimp cocktail.

Beaches may seem simple to those who come to build sand castles or bask in the sun, but in fact they are complex. They extend into the ground about 30 feet and out into the water hundreds of feet, before dropping into underwater canyons.

Healthy beaches have an inherent balance; whatever sand is lost generally is replaced over time. Waves push sand out into the ocean so far that it can’t return to shore, but those same waves lash sea cliffs, chipping away at their core and sprinkling the beaches with new sand.

Similarly, as wind scatters sand over land, rivers and streams reclaim some of it and dump it back onto the shore.

Urbanization, says Sachs, has upset this cycle.

Dams built on the rivers have slowed the water’s force, preventing them from carrying sand to the lowlands, and sea walls protecting real estate keep the waves from lapping at the cliffs.

McDonald, 68, lives near Camp Pendleton, where soon after World War II the Army Corps of Engineers built a harbor with jetties so Marines could practice amphibious maneuvers.

He says the jetties, which project into the ocean, have blocked the sand from migrating southward along the beach and abetted beach erosion for some 20 miles of coast down to Del Mar. He writes angry letters to the U.S. Army Corps of Engineers asking that some action be taken.

“We don’t have a beach anymore,” he said. “We have a sea floor that is uncovered at low tide.”

When sand becomes depleted, there are two options--wait and hope that nature reverses itself, or reconstruct the beaches by barging sand offshore or pumping it onto the beach itself.

The Navy is offering to give away 11 million cubic yards of sand for beach rehabilitation--194 football fields worth of usable beach--from proposed dredging projects in San Diego.

Local cities are interested, though some environmentalists contend that dumping sand could harm marine life. City planners, meanwhile, worry that adding to the beach may further strain coastal streets and parking lots.