This sensor network could help predict bluff collapse on the California coast

Researchers have found a way to improve hurricane warnings, tsunami warnings, and even earthquake early warning systems. But when it comes to predicting the collapse of an iconic California beach cliff, science has been tricky to pin down.

But with all the dramatic landslides and tragic deaths along the coast, officials have turned to scientists for help. Is it possible, many wondered, to foresee when and where the bluff will fall – and perhaps turn these predictions into early warnings?

Now, after a new experimental study backed by decades of specialized research, a team at UC San Diego's Scripps Institution of Oceanography says they may have cracked the code. In a report released this month, scientists shared a proof of concept and found that there are surprisingly reliable ways to accurately detect coastal landslides before they occur.

Experimental research has been able to predict five collapses at least hours – sometimes even days – before they happen.

“It was surprising how good the data turned out, and how well it matched the types of landslides we were monitoring,” said Adam Young, a coastal geomorphologist at Scripps who led the study. “We have high hopes that what we learned in San Diego can be applied to other areas throughout the region.”

Bluff fall forecasts have been in high demand along the California coast, and the stakes are high with sea level rise and a stronger El Niño headed your way. In Southern California, cliffs could erode more than 130 feet by the end of the century, and the effects of erosion are already devastating highways, railroads and other critical infrastructure.

The results are also deadly. At least 25 people have died on California beaches in landslides, and many in San Diego's North County are still reeling from the 2019 bluff collapse that killed three women in Encinitas. That same year, a similar collapse in San Francisco killed a woman walking her dog at Fort Funston.

The collapse worried state Assembly member Tasha Boerner (D-Encinitas), who first floated the idea of ​​an early warning system.

He envisioned an admittedly unscientific invention (“in my mind it was a stick with light and sound,” he said) but when he approached Scripps about the possibility, scientists took his question seriously. It wasn't going to be easy, but they agreed to try.

They explained that coastal cliffs are particularly challenging to study. The height of the cliffs, the movement of the waves, the type of rocks and the slope of the sea can all affect the stability of the cliff. How rain seeps into the cracks can also create pressure and lead to collapse.

Man's desire to build near the coast — whether it's a coastal highway or a bluff-top home — also affects erosion by changing water flow and adding weight to the cliff.

It also doesn't help that when it comes to cliffs, scientists tend to talk in terms of averages. Stretched out over a long period of time, the rate of soil erosion — say, a few inches or a foot per year — may not sound like much.

But the cliffs tend to collapse gradually over time, marked by sudden falls. A cliff measuring one foot a year may actually do nothing spectacular for 20 years, and then a 20-foot slide will suddenly come down in one fell swoop.

Boerner listened to the research needs and coordinated with the state Legislature. Through Assembly Bill 66, he received a $2.5 million grant for Scripps to begin with a pilot study.

Young, considered one of the world's leading experts on coastal bluff collapse, teamed up with Mark Zumberge, a geophysicist at Scripps who has spent decades developing advanced sensors that can take precise measurements of earthquakes.

They chose three well-known hot spots to study: San Elijo County Beach, a popular beach with a clifftop campground; Beacon's Beach, a popular beach in Encinitas with public access to a rolling breakwater; and an important railroad in Del Mar that runs along unstable bluffs. At each research site, they install a variety of sensors to see what works.

Sensor technology includes seismometers, wave pressure sensors and a special sensor called a tiltmeter, which is often used to measure the movement of earthquake faults and can detect how much the ground tilts towards the ocean to an accuracy of 1/8 the width of a human hair. They also installed advanced sensors that can detect movements up to one million meters through fiber optic cables that can expand or contract in the event of any landslide.

Their team also installed rain gauges and goes into the field every week with advanced laser-imaging technology, known as lidar, to measure and track the cliffs before and after they fall.

Patterns soon emerged. In the hours, sometimes days, leading up to the collapse, the sensors could clearly detect a rapid increase in tilting motion.

Their most notable prediction occurred on April 21, 2024, in Del Mar. During a maintenance visit earlier that month, researchers noticed a new crack in the stone that was about 0.1 inches in diameter. Over the next few weeks, their sensors noted that the crack was expanding at about 0.015 inches per day, a rate that is invisible to the naked eye.

It then rained on April 7, and again on April 14. By April 19, tilt sensor measurements were increasing rapidly leading scientists to conclude that a fall was imminent. They notified the coast guard, and two days later, at about 5 am, more than 200 tons of rock fell onto the beach. Fortunately, it didn't happen until dawn.

On April 21, 2024, approximately 200 tons of rock fell onto the beach in Del Mar. A tiltmeter that sends data in real time had warned Scripps researchers two days in advance that a landslide was very likely.

(Adam Young/Coastal Processes Group at Scripps Institution of Oceanography)

“The implications of AB 66 and this first phase go beyond my imagination,” said Boerner, who hopes this proof of concept lays the foundation for a future statewide warning system. “I'm very grateful that Adam Young and his team took a chance on this idea. … If they had said no, there would be no science.”

The next important step is to make sure people know what to do with this information – and to make agreements with lifeguards, emergency managers, transport agencies and other officials who decide when a beach or road is closed.

Dozens of legislators, staff and agency workers have already been briefed on the latest study, and Boerner said he is putting together a task force. He plans to seek more federal funding to expand research and explore ways to develop an emergency response system. He also envisions working with weather apps so that surfers can eventually get a geo-fenced bluff warning, the same way high surf advisories might be issued.

Patrick Barnard, who spent years leading coastal erosion research for the US Geological Survey, said the latest Scripps data is promising and the next big question is scalability. The crumbling cliffs line more than 530 miles of California's coast, and the cost of establishing a statewide sensor network would be enormous.

But Barnard, who left the USGS last year and now serves as research director of UC Santa Cruz's Center for Coastal Climate Resilience, noted that the Scripps pilot project is an encouraging example of science and government working together to solve a problem that could save lives.

“It's great that the state has invested in this issue, and they've invested in one of the best cliff experts we have,” Barnard said. “It's comforting to hear that these things are moving forward, and that science here is playing a role in policy making. …It's not like that everywhere, but that's how those relationships should be in an ideal world.”