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California’s cliffs are crumbling into the sea. This map shows the spots where they’re eroding the fastest

San Francisco Chronicle 8/9/2022 By Tara Duggan

Coastal cliffs in California’s far northern counties are eroding faster than those elsewhere in the state, according to a new study that used high-resolution data to pinpoint hot spots where cliffs are receding rapidly along the state’s entire coast.

In the Bay Area, locations with some of the highest rates of clifftop erosion include Daly City, Pacifica and Bodega Bay, according to the study published this month by researchers at Scripps Institution of Oceanography at UC San Diego. The highest erosion rates were found near Humboldt Bay and in a few remote locations in Del Norte County.

Far Northern California may have the worst erosion because it has more rainfall and larger waves, but more surveys will be needed to demonstrate a direct relationship, said Zuzanna M. Swirad, one of the authors.

Cliffs next to sandy beaches also showed the highest rates of erosion.

“This finding is counter-intuitive, because wide beaches can protect cliffs from wave action. However, waves can also use beach sand as an abrasive to erode the lower cliff,” read the study, which was published in the journal Geomorphology.

Using lidar, which takes precise measurements with laser pulses, the study analyzed 538 miles of the 1,023-mile California coast — which includes the majority of its cliffs — and found erosion on half of them. It measured both clifftops, otherwise known as the cliff edge, and the cliff faces beneath them to get the fullest picture of erosion. The results of the study, which was funded by the California Ocean Protection Council and California Department of Parks and Recreation and is based partly on previous research funded by California Sea Grant, are presented in an interactive map.

On average, the coast lost about 2 inches annually between 2009-2011 and 2016, according to the findings by Swirad and her fellow researcher, Adam Young. However, that average includes cliff failures as large as 16 feet per year in places like Martin’s Beach in San Mateo County and Big Sur.

Other areas found to have high rates of clifftop erosion in the Bay Area include El Granada, which is north of Half Moon Bay in San Mateo County, and a few stretches of Point Reyes National Seashore. But the study didn’t tease out all the reasons behind the cliffs’ retreat.

“We still need to find direct relationships between erosion and driving forces (waves, rain, rock hardness) to better predict where erosion will happen under (a) changing climate,” Swirad, who is now at the Polish Academy of Sciences in Warsaw, said by email.

The researchers say it is the first study to use high-resolution data to analyze cliff erosion all along the coast.

Sea level rise and more extreme storms that result from human-caused climate change are expected to increase coastal erosion over time. The state’s coastal region represents a $45 billion tourism and recreation economy and is also key to transportation and infrastructure, according to the California Coastal Commission, which recommended long-term planning to address the threats of sea level rise in a report last year.

The time period the study looked at — when the relatively new lidar technology became available — may not be representative of the overall picture of erosion on the coast, said Gary Griggs, professor of earth sciences at UC Santa Cruz, who was not involved in the study.

Ocean and weather conditions that cause erosion vary during La Niña or El Niño weather events and longer-term climate patterns called the Pacific Decadal Oscillation.

“One of the things we do know about coastal cliff erosion is it’s very episodic,” said Griggs, who tends to rely on less high-tech aerial photos that go back to the 1920s when measuring erosion. “It turns out the period they looked at from 2009 to 2014 was a calmer, cooler period with with less dramatic wave action and erosion,” he said, though he said storm action did pick up from 2014 to 2016.

Tara Duggan is a San Francisco Chronicle staff writer. Email: Twitter: @taraduggan

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