For years, surfers, sun worshipers, and coastal experts have assumed that the dominant source of southern California's golden beach sands is river sediment. But new research indicates that erosion from coastal bluffs and cliffs--previously considered a minor contributor--appears to account for about half of the region's beach sand.
Pounded by major storms and big waves, California's 600-miles of beachfront are actively eroding. Previous studies indicated that rivers were the main source of beach sand, and thus beach management strategies have largely ignored sea cliffs as a significant source.
Suspecting that the river model was incorrect, University of California, San Diego (UCSD) structural engineer Scott Ashford and graduate student Adam Young spent six years measuring the cliffs on a 50-mile stretch of coast just north of San Diego using LIDAR, a laser scanning technique more precise than the aerial photograph analyses used in previous studies. Based on a comparison of the volume of sand that fell from the cliffs and the volume of sand estimated to come from rivers, roughly half of the beach sand originated from the cliffs, according to Young, who presented the findings last week at the American Shore and Beach Preservation Association meeting in San Francisco.
UCSD geologist Neal Driscoll and graduate student Jennifer Haas came to similar conclusions using a mineralogical fingerprint technique. Haas spent three years examining thousands of sand grains along the same stretch of beach north of San Diego. The beach sand contained a large amount of clear quartz grains. Haas also found these in the coastal cliffs, but at river and dredge sites she found mainly frosted sand grains. Overall, the team's analysis, which is currently being prepared for publication, indicates that at least half of the beach sand in this area comes from the cliffs.
"This is a huge finding supported by two different methods," says Cheryl Hapke a United States Geological Survey geologist based at the University of Rhode Island in Kingston. These studies will force a surprising revision in how we think about the origin of beach sand and help scientists develop a better understanding of beach dynamics, she says.