Scientists have cracked one of the enduring geological mysteries of Southern California’s famed beaches: where the sand comes from.
Researchers had long assumed that coastal rivers washed the bulk of the naturally introduced sand to the beaches south of Los Angeles.
But two new research projects by scientists at the University of California at San Diego found that erosion of the area’s majestic sea cliffs is the primary source.
“We were very surprised,” said UCSD engineering professor Scott Ashford, who used a portable laser imaging system to study coastal formations as part of one study. “It’s telling us that we don’t understand the beach system as well as we think.”
Ashford’s research, presented Wednesday at a meeting of the American Shore and Beach Preservation Association, looked at six years worth of imaging data from the 50 miles (80 kilometers) of coast between Dana Point and La Jolla.
In the past, geologists had assumed that as much as 90 percent of beach sand in this sector had been introduced by deposits from coastal rivers.
But Ashford’s study showed that erosion of sea cliffs is responsible for an annual inflow of 80,000 to 100,000 cubic yards, or 67 percent, based on current estimates of total sediment input.
A separate UCSD graduate project using so-called “sand fingerprinting” techniques found similar results.
The sand movements documented by the studies are dwarfed by recent human efforts to restore coastal terrain, which include the introduction of 1.8 million cubic yards (1.38 million cubic metres) of sand to San Diego area beaches in 2001.
Still, the findings are likely to be caught up in a pitched debate over regulation of California’s expensive beachfront property.
Coastal residents, many of whom are determined to protect their slowly eroding land by building sea walls, must currently pay a government fee to compensate for the sand that would be withheld from the beach ecosystem.
One such sea wall project, slated to be reviewed by authorities this week, would lead to the imposition of a $271,000 fee on a group of Solana Beach condominium owners who have proposed a 35-foot concrete embankment.
Environmentalists have long pressured coastal regulators to increase those charges dramatically, and one activist said the new research could bolster that effort. “I think this research is great news,” Todd Cardiff, chair of the Surfrider Foundation’s San Diego chapter, told Reuters.
UCSD scientists say they are determined to learn more about the movements of sand along San Diego’s coastline and eventually hope to take measurements on a weekly basis. “We’re taking the pulse of the beach,” he said.