Decades ago, about 40 percent of California's beaches were eroding while the rest were actually growing a little bit over time.

But in the past 25 years, the number of beaches along the state's 1,100-mile coastline that have been losing ground has swelled to two-thirds of the total, according to a new study.

And many of those eroding beaches are in San Diego County.

That's largely because much of the region's shore is armored with man-made structures designed to block damaging waves, said Cheryl Hapke, a coastal geologist with the U.S. Geological Survey's Woods Hole Coastal and Marine Science Center in Massachusetts, in a telephone interview Friday.

The man-made structures are slowing the retreat of cliffs and bluffs by shielding them from powerful waves, Hapke said. But the devices are accelerating the erosion of beaches by deflecting waves back onto the sand -- in many cases, beaches that had been gradually expanding.

The findings were highlighted in a report authored by Hapke and scientists Dave Reid and Bruce Richmond and published in the Journal of Coastal Research. The article is titled "Rates and trends of coastal change in California and the regional behavior of the beach and cliff system."

Hapke said the scientists were trying to figure out whether cliff erosion was a good indicator of beach erosion, as they suspected. That proved to be the case for the sparsely developed central coast, but not for Southern California.

"We don't see the system responding in a way that we would expect because of human manipulations to the system," Hapke said. "Because the system has been so dramatically altered, we can no longer say whether there is a relationship between cliff retreat and beach erosion."

And it is most likely because of the presence of those man-made structures, she said.

The report termed the shore along San Diego and other urban Southern California counties "the most engineered coastline in the state, consisting of numerous harbors, ports, breakwaters, jetties, and groins."

Some of the larger structures -- such as those associated with Oceanside Harbor in North County -- have also altered the natural flow of sand along the shore and accelerated erosion.

"You actually shut off the river of sand that naturally wants to move down the coast," Hapke said.

Many beaches in Southern California have been losing ground over the last 25 years after gradually widening during the century before that. The most dramatic example of that is in southern San Diego County, between La Jolla and the U.S.-Mexico border, where the shoreline is retreating at an average of 1 meter, or more than 3 feet, per year after previously expanding by nearly that amount, the study said.

"That is a very dramatic change," Hapke said.

The trend is less pronounced in North County. There, the shoreline is treating by one-tenth of a meter per year after expanding by two-tenths of a meter over the longer term.

But those are averages over wide areas, and rates vary widely by beach.

One of the hot spots is Del Mar City Beach, where the sand is eroding at a rate of more than 3.5 meters per year, Hapke said. Mission Beach in San Diego is retreating at about the same rate.

Another hot spot is Torrey Pines City Beach, with an erosion rate of 2.2 meters per year.

And San Onofre Beach is eroding at an average rate of close to 2 meters per year.

Other North County beaches are eroding at slower rates, generally at 1 meter per year or less, according to a 2006 study Hapke worked on. In the case of beaches in the vicinity of Oceanside Harbor, they are actually growing as the structures there collect and spread the sand flowing down the coast.

Because of the proliferation of man-made structures, Southern California generally has slower rates of bluff erosion than do the cliffs of Northern and Central California.

"Many of the portions of the coast that are backed by cliffs have coastal protection structures, which have likely affected the rates of cliff retreat and thus contribute to Southern California having the lowest average retreat rates in the state," the study concluded.

But Southern California also has some of the highest beach retreat rates, and the 1-meter-per-year average rate for southern San Diego County is the highest in the state, the report shows.

"There is no question that a sea wall, for some period of time, is going to stop or slow the erosion of a cliff or bluff," Hapke said. "But you impact the beach at the base of that cliff and accelerate erosion of that beach. That sea wall reflects the wave energy and scours the beach more quickly."

And, she said, structures can only hold back the sea for so long.

Call staff writer Dave Downey at 760-745-6611, ext. 2623.