SAN DIEGO — Erosion from coastal bluffs and cliffs could account for about half of the beach sand on the Central and Southern California coast, more than was previously thought, two UC San Diego scientists reported Wednesday.

Coastal geologists previously thought that sediment-laden rivers that enter the Pacific Ocean along the Central and Southern California coast were the primary supplier of sand — contributing up to 90 percent.

UCSD Jacobs School of Engineering doctoral candidate Adam Young and professor and adviser Scott Ashford presented the study at the annual meeting of the American Shore and Beach Preservation Association in San Francisco.

"Our results call into question the conventional wisdom that coastal bluffs don't contribute much to the beaches," Ashford said.

According to the research, various types of concrete surfacing and reinforcement of bluffs, as well as layering large boulders along the base of bluffs, tend to "armor" them, slowing or preventing erosion.

The scientists said determining the source of sand is the first step in any effort to preserving Southern California beaches.

Their research was based on 3-D topographical maps generated of coastal bluff erosion that occurred over the past six years.

Based on the volume of material that had fallen from the bluffs during the study period, the researchers concluded that half of the beach sand along a 50-mile stretch of coast from La Jolla to Dana Point, called the Oceanside Littoral Cell, was likely derived from the bluffs.

The study was funded by California Sea Grant, which is administered by the University of California and UCSD's Center for Earth Observations and Applications, and was submitted to the Journal of Coastal Research.