The dangers of coastal erosion are an all-too-familiar reality for the modern residents of California’s iconic mountainous coastal communities. With a new tool, researchers are now bringing historical perspective to the topic of how to manage these disappearing coastlines.
Using a model that incorporates measurements of the amount of time coastal cliffs and their remnant deposits were exposed at the Earth’s surface, Stanford University and Lawrence Livermore National Laboratory (LLNL) researchers found that the rate of cliff erosion in the past 100 years is similar to that of the past 2,000 years. One of the causes of the erosion was found to be rising sea level.
The proof-of-concept, published in the Journal of Geophysical Research: Earth Surface opens the possibility of using this new approach to understand the long-term history of coastal cliff erosion, or retreat, in other parts of the state. The work was conducted in Del Mar, California, a beach town in San Diego County with infrastructure atop its coastal bluffs.
“We were able to quantify long-term coastal cliff retreat over timescales relevant to sea level rise, and this was a pilot study to see if we could make the necessary measurements on short shore platforms like those prevalent in California,” said LLNL scientist Alan Hidy.
Doing this required an ultra-low beryllium-10 isotope measurement capability, which is only possible at LLNL’s Center for Accelerator Mass Spectrometry (CAMS).
“This pilot study sets the stage for future studies presently being proposed in California and elsewhere to quantify sea cliff retreat in coastal areas with at-risk critical infrastructure,” Hidy said.
Del Mar is among locations that are critically important for understanding cliff retreat. Homes are situated up to 70 feet above its beach, in addition to public infrastructure. A major railroad between Los Angeles and San Diego runs atop the coastal bluffs, where cliff failures have resulted in several derailments in modern history, as well as rock fall events that led to closures in recent years.
"I think this study bolsters the thinking that we should do something about cliff retreat sooner rather than later," said lead study author Travis Clow, a Stanford graduate who took part in the CAMS Training Program back in 2019 when he was a Ph.D. student (then at UC San Diego). Clow came to CAMS specifically to learn the analytical techniques used for this work and to take part in the analysis of these samples.
Del Mar beach features a narrow shore platform, the bedrock where tidepools are typically found. Using nine samples of bedrock, the researchers measured concentrations of the chemical isotope beryllium-10 that tracks landform exposure to cosmic radiation from space. The data were compared with cliff retreat rates from recent studies based on aerial photography, showing that coastal erosion rates have remained relatively constant over the past two millennia — at about 2 to 5 inches per year.
The researchers' approach explores the influence of different factors, including wave impacts and weathering that occur at the shore platform and the cliff interface.
"It does more than just spit out a retreat rate," Clow said. "It also allows us to have a relative assessment of what might be driving cliff retreat over longer periods of time."
The research found that over half of all coastlines on Earth are eroding like California's. The scope of the problem, which will be exacerbated by sea-level rise in the next century, presents an opportunity for using this new technique in other areas.
"There are plenty of other places in California and the Pacific Northwest where active erosion of coastal rocky cliffs is happening, and we hope to use this technique in a wide variety of environments," Clow said.
Stanford’s Danielle Tucker contributed to this report.
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