NORTH COUNTY -- If only Santa Catalina Island were bigger. The popular tourist destination off the shore of Orange County is 22 miles long and 8 miles across at its widest point. Even at its smallish size, its rocky bulk shelters beaches from Oceanside to Los Angeles from giant winter waves and helps them retain sand, according to a new study by scientists at UC San Diego's Scripps Institution of Oceanography.

Similarly, other Channel Islands get credit for the relatively wide and sandy beaches around Ventura and Santa Barbara.

Farther south, much of North County lies outside of Santa Catalina's shadow. Its beaches bear the brunt of brutal winter swells. Consequently, local beaches tend not to be as sandy as those elsewhere in Southern California, said Bill O'Reilly, a senior development engineer at Scripps who spearheaded the study, in an interview last week.

O'Reilly recently presented study findings to the San Diego Association of Governments' shoreline preservation committee, which is contemplating another sand replenishment project like the one in 2001 that beefed up a dozen scrawny beaches from North County to Imperial Beach. The committee has decided to shop around for state and federal grants to defray the estimated $25 million cost of dredging sand from the ocean floor and spreading it on six miles of beaches.

After the presentation, Steve Aceti, executive director of the Encinitas-based California Coastal Coalition and a committee member, joked that the panel's priorities might be misplaced.

"All these years, we've been talking about trying to get a grant for more sand," Aceti quipped. "We ought to be talking about getting a grant to make Catalina Island bigger."

But it's not just the big waves.

The scientists found another set of villains in North County's picturesque lagoons -- Buena Vista, Agua Hedionda, Batiquitos and San Elijo, O'Reilly said.

According to a brochure titled, "Littoral Cells, Sand Budgets and Beaches: Understanding California's Shoreline," which was published by the Institute of Marine Sciences at UC Santa Cruz in October, sand is in constant motion. During winter, storm swells erode beaches and suck sand underwater, creating sandbars just offshore. The bars act like sand reservoirs. When gentler waves take over in spring, they push sand back up on the shore.

Waves also create a current along the coast that pushes sand south a mile each year. That movement is often interrupted by human activities and structures, such as Oceanside Harbor, O'Reilly said.

"After Oceanside, really, sand is pretty free to travel down the coast," he said. "But places like North County continue to be sand starved."

That's where the lagoons enter the picture.

Scripps scientists believe that the southerly sand flow is blocked by huge sandbars built up just offshore of the lagoons.

"Sand is essentially stalling in this area and then getting chewed on by the big waves," O'Reilly said.

Scientists aren't sure where the sand goes.

"We think that it may be moving away from the beach, out into the ocean," O'Reilly said. "And there is no mechanism for bringing it back onto the beach farther down the coast because of the nature of the waves around the lagoon."

O'Reilly said the bulky presence of the sandbars diverts the storm waves around the blocks of sediment, and focuses them onto spots south of the lagoons. There, just below the lagoons, erosion hot spots are created and beaches become particularly narrow, he said.

Below hot spots and in between lagoons, beaches are more robust. But the disrupted southerly flow prevents them from reaching their potential widths, he said.

"It is a new hypothesis," O'Reilly said. "We don't really have the information yet to prove it or disprove it. That's the next step. But it is consistent with the data that we have collected to date. It could be that this has been going on for thousands of years."

The theory, which stems from observations taken over the last few years, came out of something called the "Southern California Beach Processes Study," a $1 million project funded by the U.S. Army Corps of Engineers and California Department of Boating and Waterways.

O'Reilly said the project team plans to test its hypothesis and conduct an in-depth, follow-up study focusing on a particular lagoon in 2008.

The preliminary findings help explain why half the sand placed on North County beaches through the association's $17 million replenishment project in 2001 has disappeared. But O'Reilly said the theory does not suggest that such projects are a lost cause.

"If anything, I think it reinforces the general philosophy that this is a sand-starved region, and that if you want to have sandy beaches, it's likely that you're going to have to place sand on them from time to time," he said. "Waiting for sand to come from Oceanside is going to be a long wait."

Contact staff writer Dave Downey at (760) 740-5442 or ddowney@nctimes.com.