The cliffs that comprise much of California's coastline are constantly being reshaped by rain, waves, and other geologic forces. The erosion of cliffs threatens critical infrastructure such as highways, railways, homes, park lands, public access points, and more.
Therefore, it is important to monitor and track change occurring along the coast. Previous cliff erosion rates were a decade old. To address this issue, researchers at UC San Diego's Scripps Institution of Oceanography collected new airborne lidar in 2023 and developed new cutting-edge approaches to map cliff erosion and retreat. Using these approaches, they recently measured how California's coastal cliffs changed between 2016 and 2023 and found that change was highly uneven. In some cliff retreat hotspots, the area of land loss over the seven-year period exceeded 800 dump-truck loads for every yard of shoreline.
Their findings , published in the journal International Journal of Applied Earth Observation and Geoinformation, improve the understanding of California's coastal geology and provide a framework for systematic statewide cliff monitoring. And with a very strong El Niño expected to threaten the California coastline this winter , understanding the health of California's cliffs and erosion hot spots can help with coastal management and preparation.
California's coastal cliffs are of great interest to both scientists and the public. Roughly 70% of the California coast is made up of coastal cliffs . In addition to threatening coastal infrastructure and public resources, cliff collapse poses a serious threat to beachgoers, so monitoring erosion has become a priority for the State. That's why Senate President pro Tempore Emeritus Toni Atkins led the effort to secure funding, with support from the California State Legislature, to advance research aimed at better understanding California's coastal cliffs.
"The cliffs are not stable. They erode and collapse over time," said Danghan Xie, an assistant professor and coastal geomorphologist at the University of Maryland and lead author of the study, who did the research while a postdoctoral scholar at Scripps. "We've seen many sad news stories about fatal cliff landslides. It's unpredictable and a significant hazard to the coastal community."
Addressing these safety and infrastructure issues requires a better understanding of how California's coastline is changing and how geologic processes affect it. California's coastal cliffs had previously been surveyed from airplanes using lidar in 2016. Lidar, which stands for Light Detection and Ranging, is a remote sensing technology that uses a laser scanner to create precise, three-dimensional models of the Earth's surface. However, limited statewide cliff information has been available since then. To provide a new statewide snapshot, Scripps scientists collected new airborne lidar along the California coast in 2023.
The rates of cliff top retreat vary alongshore with retreat hotspots mostly located in Central and Northern California, with some areas experiencing retreat rates exceeding 70 feet per year, such as the Lost Coast Headlands in Humboldt County where a landslide occurred involving over 1.4 million cubic yards of rock and soil.
"70 feet per year is a lot, over the seven-year study period that is more than the length of a football field," said Adam Young, a coastal geomorphologist at Scripps and co-author of the new study and Principal Investigator of the project. Cliff top retreat rates exceeding 10 feet per year were also observed in the King Range, Redwood National Park, Point Reyes National Seashore, Big Sur, near Martins Beach, Vandenberg Space Force Base, El Capitan State Beach, and Palos Verdes. Statewide, 65% of cliffs showed a measurable change above the study thresholds.
Knowing where and the rate at which cliffs are crumbling is crucial for coastal planning and management. That's why Young and his colleagues have created a California Coastal Cliff Erosion Viewer website where all of this information is available to the public – one they plan to update as more information becomes available.
"This monitoring program is collecting lidar twice a year throughout the state of California, and we are using the data to update the rates on an annual and seasonal level," said Young. "A detailed time series of cliff changes does not exist in many areas, and this makes it hard to build better predictive models. This project creates updated routine systematic data for improved coastal planning."
While plans are in place to track changes related to El Niño this winter, funding for the project ends in 2027.
Additional study authors include Luc Lenain, who was co-Principal Investigator, and Hironori Matsumoto of Scripps, and Zuzanna Swirad from the Polish Academy of Sciences.
