March 7, 1996 Contact: Erik Stokstad or Robert Irion (408/459-2495)

UC SANTA CRUZ GEOLOGISTS TRACK THE RESTLESS MIGRATION OF SAND

FOR IMMEDIATE RELEASE

SANTA CRUZ, CA--The beaches may look stable, but California's sandy shores are moving. Waves and currents drive sand down the coast, affecting the fortunes of seaside communities. Sandy beaches bring in tourist dollars and help prevent sea cliffs from eroding. But sand can be a nuisance when it fills up harbors or disappears from towns that depend on the grainy recreational resource.

Led by coastal geology expert Gary Griggs, a team of geologists at the University of California, Santa Cruz, is clarifying the big picture of sand movement just offshore. Sand grains, it seems, travel along certain submarine routes, like cars following highways. With nimble research boats, the team has mapped major thoroughfares and parking lots for sand along the central California coast. They are also using computers and a mathematical bag of tricks. Two researchers have developed a novel way to decipher the pathways of sand along the coast. Others think they know why a popular beach appeared in Santa Cruz last year, only to vanish within months.

Most of the sand near Santa Cruz comes down from the north, swept by the oblique attack of waves in a process called littoral drift. From streams all along the coast--and perhaps from huge shoals outside San Francisco Bay--sand enters the offshore highway and arrives at the northern end of Monterey Bay decades later. Griggs estimates some 300,000 cubic yards pass by every year. "That's about one dump truck full every 17 minutes," he says.

But beaches are interrupted by rocky headlands--roadblocks for migrating sand. Postdoctoral researcher James Tait wanted to know how sand moves around such obstacles. For three years, he and doctoral candidate Roberto Anima darted in and out of rocky coves and treacherous straits from Pacifica to Santa Cruz in the 43-foot R/V David Johnston. With help from the U.S. Geological Survey, they mapped the shallow coastal seafloor with sonar, a global positioning system, and a depth recorder. "As a kid I wondered what the seafloor would look like if you could let the water out for a while," says Tait, who has sketched the underwater landscapes with pulses of sound.

Tait's team discovered large offshore areas devoid of sand. Most sediment collects in submerged stream valleys carved out when the sea level was lower, 10,000 to 20,000 years ago. Tait suspects these valleys trap sediment until storms churn the water enough to carry sand over the ancient stream banks. This happens rapidly, but infrequently; big storms reach down 100 feet or more to swirl sediments on the seafloor only about once every decade.

With UCSC seismologist Justin Revenaugh, Tait has also adapted a mathematical tool from seismology to investigate the transport of marine sand. Called "source-transport inversion," the method separates complicated jumbles into simpler components. The method is perfect for analyzing samples of sand grains--usually a hodgepodge of shapes and colors, like a bowl of mixed nuts.

Tait and Revenaugh tested their technique by tracing the sources and movement of sand in Monterey Bay. They based their detective work on the unique blends of dense mineral grains, including red garnet and dark green hornblende, that come from the offshore highway north of Santa Cruz and the rivers flowing into the bay. Into Revenaugh's computer went data from thousands of samples analyzed by UC Berkeley scientists in the 1960s. For each sample the source-transport inversion sorted out the mix of different ingredients, indicating how much sand came from each source. The results matched existing ideas of sediment transport: Most sand in Monterey Bay comes from the northern coast, not local rivers.

Information about the natural movement of sediment also helps geologists understand the delicate balance between erosion and beach formation. Something upset this balance in 1995 when a new beach, 50 feet wide and 1,000 feet long, appeared in Santa Cruz between the Dream Inn and Lighthouse Point. The beach, instantly popular with Santa Cruz residents, arrived during storms in January but washed away by October. Doctoral candidate Laura Moore worked with Carrie Randolph, an undergraduate, to uncover the source of the sand and how it came to Santa Cruz.

The type of sand hints that the sediment originally came from the north, as Tait's research would suggest. Once the sand rounded Lighthouse Point, it parked just offshore. Moore and Randolph think the beach formed when big storm waves, marching parallel to shore, shoved the sand into the protected cove while eroding less sheltered beaches. "That's very counterintuitive," says Moore. "The general rule is that more energetic waves are more erosive." In other words, storm waves should not create beaches. But the combination of a protected cove within Monterey Bay, a source of sand offshore, and powerful waves allowed this ideal surfing spot to form. During calmer times, littoral drift gradually swept away the ephemeral beach.

Looking at historic aerial photographs and weather records, Moore and Randolph determined that only severe winter storms deliver beaches to this area. The sandy attraction showed up last in 1983, but without knowing when the next big coastal storm will arrive, says Moore, "it's tricky to predict when the beach will return."

Editor's note: You may contact the researchers as follows: Gary Griggs--(408) 459-5006 or griggs@cats.ucsc.edu Jim Tait--(408) 459-2403 or jtait@earthsci.ucsc.edu Laura Moore--(408) 459-2403 or lmoore@earthsci.ucsc.edu

This release is also available on the World Wide Web at UCSC's "Services for Journalists" site (http://www.ucsc.edu/news/journalist.html) or via modem from UC NewsWire (209/244-6971).

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