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ES&T News
California’s shifting sands
Southern California’s golden beaches, immortalized by the musical group the Beach Boys, may not last forever thanks to erosion. Even the widely held assumption that rivers are the main source of the beach sands appears to be crumbling away. Instead, two new studies indicate that the dramatic sea cliffs, which Californians fight to save, supply about half of the beach sand.
In southern California, beach cities have spent millions of dollars on “beach nourishment” projects that dredge and replace sand that has been swept away. At the same time, seawalls, rock piles, and other protective structures keep coastal properties and highways from tumbling into the sea in some places. The “armoring” of the coastline has pitted private property owners against environmental groups who argue that the barriers are ugly and damage the beaches.
To better understand these processes, University of California, San Diego (UCSD) structural engineer Scott Ashford and his Ph.D. student Adam Young spent 6 years measuring the cliffs on a 50-mile stretch of coast north of San Diego. They collected data by using LIDAR (laser imaging detection and ranging), a laser scanning technique more precise than the analyses of aerial photographs that researchers had used previously. By comparing the volume of sand that fell from the cliffs with estimates from other studies of the volume deposited by river sand, they concluded that roughly half of the beach sand probably originated from the cliffs. LIDAR has been used previously to map beaches, but this is the first time it’s been used to monitor beach erosion, says Young, who presented the study results at the American Shore and Beach Preservation Association meeting in October.
UCSD geologist Neal Driscoll and graduate student Jennifer Haas came to the same conclusion by using a mineralogical fingerprint technique, according to Driscoll. Haas spent three years examining thousands of sand grains along the same stretch of beach. She found that the beach sand is dominated by clear quartz grains—the same kind of grains that are abundant in coastal cliffs. In river and dredge sands, she found mainly frosted grains. Haas estimates at least 50% of the beach sand comes from the cliffs.
Mineralogical fingerprinting is a “great way” to estimate sources, says U.S. Geological Survey sedimentologist Jonathan Warrick, because it is a simple method that measures what’s actually in place. It does not rely on any calculated estimates.
The researchers from both studies hastened to note that their work doesn’t address the value of armoring, although it does show that cliffs are important in saving beaches. Their findings do highlight the current scientific ignorance about California’s beaches in particular and coastal zone processes in general. “If we have been overestimating the contribution of river sand, then we don’t understand beaches as well as we think we do,” says Ashford.
The new work also casts doubt on the validity of river sediment estimates, Warrick says. These estimates assume that stream-gauge measurements of sediment load reliably reflect the amount of sand deposited in the coastal zone. This is an enormous amount of sediment, but it may not get to the beaches. Because river water thick with sediment is often denser than seawater, it may sink to the bottom and escape the coastal zone that replenishes the beaches. “This is our hypothesis—but we don’t have any observations,” he says.
Ashford and Driscoll plan to map sediments offshore by using sonar profiling in an effort to answer this question.
