Wide sand beaches are a prized feature of Santa Cruz and other communities along the northern coast of Monterey Bay. But are they just a temporary aberration, destined to shrink in the years to come?
That is the hypothesis championed by Gerald Weber, lecturer emeritus in the Department of Earth Sciences at the University of California, Santa Cruz. According to Weber, northern Monterey Bay beaches have been kept abnormally wide over the past 200 to 300 years by an excess of sand drifting down the coast from a large temporary source north of Año Nuevo. That source is now exhausted. Beaches just south of Año Nuevo have already shrunk in the past 30 years, and Weber said he expects that trend to spread slowly south to Santa Cruz and beyond.
"Beaches down drift from Point Año Nuevo have shrunk drastically over the past 25 to 30 years, and sea-cliff erosion is accelerating," Weber said.
Other geologists, however, do not anticipate dramatic changes. Gary Griggs, professor of Earth sciences at UCSC and an expert on coastal erosion, said he believes that the dune fields north of Año Nuevo provided only a small fraction of the sand nourishing Santa Cruz beaches.
"Losing the Año Nuevo sand supply may not have a significant effect on beaches in the Monterey Bay because of the large amount of littoral sand supplied by other sources," Griggs said.
Weber said he is used to that kind of skepticism. "When I first presented this idea in 1981, people thought I was nuts," he said. But he said he has accumulated more evidence over the past 20 years.
Weber will make his case this week in a presentation on Friday, April 29, at a joint regional meeting of the American Geological Society and the American Association of Petroleum Geologists in San Jose.
Beaches are constantly remodeled by coastal currents that bring sand in and take it away. Along the northern California coastline, the current runs northwest to southeast. Santa Cruz beaches contain material from as far north as Half Moon Bay, Weber said.
A thin beach leaves cliffs exposed to the erosive power of waves, eventually causing the coastline to recede. As cliffs crumble, the beach widens, until an equilibrium is reached in which the beach remains wide enough to protect the cliffs from further erosion.
In Santa Cruz County, the equilibrium was tipped toward wider beaches some 300 years ago, when Año Nuevo Island separated from the mainland, perhaps as the result of an earthquake on the San Gregorio fault. A channel formed between the island and Point Año Nuevo, opening the way for sand trapped north of the point to gradually drift down the coast to more southern beaches. The extra sand made southern beaches wider, buffering the cliffs from the waves.
After releasing 13 to 20 million cubic yards of sand through the channel, the northern sand sources are now exhausted, Weber said. Southern beaches, no longer receiving an excess of sand drifting from the north, have started to shrink, and cliff erosion is poised to resume. The phenomenon is particularly pronounced at Point Año Nuevo and Greyhound Rock, three miles south of Point Año Nuevo, where beaches have receded by 60 to 75 feet over the past 30 years, Weber said.
Weber said he expects the phenomenon to spread further south within the next 20 to 100 years.
"A period of thin beaches and increased coastal erosion in northern Monterey Bay is near," Weber said.
Griggs said he does not dispute the depletion of the sand sources north of Año Nuevo. But he said the amount of sand they contributed--approximately 50,000 cubic yards per year, according to Weber's estimates--is small compared to the river of sand traveling southward along the coast--some 250,000 cubic yards per year. "What I question is the magnitude of the effect," he said.
Sand travels along the central California coast at approximately one mile per year. Santa Cruz beaches, 15 to 20 miles south of Año Nuevo, should have begun to show the effects of a decreased sand supply by now. But they have not been shrinking, Griggs said. The effect of the Año Nuevo sands might be too dilute to have a discernible impact by the time they reach Santa Cruz, he said.
Weber said he expects a decrease in beach width to take place over an extended period of time, just as the increase in width during the period of excess sand supply would have been a gradual process involving relatively small amounts of extra sand in any one year.
"This is a process that slowly built up beach width over a period of perhaps 50 to 75 years, and I anticipate that the decrease in beach width will also not occur suddenly," he said.
Both Griggs and Weber agreed that understanding beach dynamics requires long-term observations because the system is so variable. "Most of our understanding of cliff erosion is based on observations that only span the last 30 to 50 years," Weber said.
By contrast, he based his hypothesis on historic maps going back to the time of the missions and photographic records from the past 75 years. And only time will tell if his interpretation is accurate.
"I'll probably be dead by the time we know if I'm right about this," he said.
_____
Note to reporters: You may contact Weber at (831) 469-7211 or jweber@earthsci.ucsc.edu and Griggs at (831) 459-5006 or griggs@ucsc.edu.
