Predicting When the Next Bluff Will Fall
Researchers in Southern California are using lidar to improve scientists’ understanding of the erosional forces that cause bluffs to collapse.
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In August 2019, three women were relaxing on* the beach of Encinitas, California, north of San Diego, when the oceanfront bluff unexpectedly crumbled, showering them with tonnes of sandstone. One of the women, who had been celebrating her recovery from breast cancer, was killed instantly, while her sister and niece later died in the hospital.
That tragic event was neither the first nor the last bluff collapse in a scenic and densely populated, yet precarious, coastal region. Just a few kilometers to the south in Del Mar, a bluff collapsed following a rainstorm in 2016, undermining a busy coastal roadway. Sections of beachside cliffs came crashing down in the area in 2018, too, though no injuries were reported. In February this year, another bluff collapsed—along with the aging sea wall intended to hold it back—about 10 meters from the rail line that links San Diego and Los Angeles and serves nearly eight million passengers and numerous freight trains annually.
Collapsing coastal bluffs are a threat wherever waves, earthquakes, and intense rainstorms can destabilize steep seaside terrain, and with sea levels rising, this risk is increasing. It is a pronounced risk throughout many areas along the Pacific coast of North America, especially in Southern California. Considering that many lives, homes, and vital infrastructure are at stake, scientists have been trying to figure out exactly what causes such cliffs to fall.
Adam Young, a marine geologist at Scripps Institution of Oceanography at the University of California San Diego, is developing a tool that could eventually be used to predict bluff collapses in order to better protect lives and property. He and his team have spent three years driving up and down a 2.5-kilometer stretch of the coast near Del Mar, firing a sensitive lidar laser mounted atop their research truck at the cliffsides. Through repeat measurements, the equipment can track tiny shifts in the ground, and by taking measurements over years the team is giving a warning of potentially vulnerable coastal areas.
Young and his team have focused on two main processes as they map the coastal bluffs: the relentless erosion of the lower layers of rock by the crashing waves, and the gradual wearing away of the upper layers of soil by rainstorms and seeping groundwater. Both can undermine, sometimes subtly, the stability of a cliff.
While using lidar is a common approach to studying unstable terrain, with measurements often taken from research airplanes once or twice a year, Young’s efforts have added a new twist. “The main thing new here is doing the high-resolution survey every week, which allows us to isolate time periods of when waves are hitting the cliff, or when there’s rainfall, giving us a better idea of how these different processes are acting on the cliff,” he says.
It’s important to understand the particular qualities of rainstorms, waves, and groundwater that result in erosion and trigger landslides, especially in the context of the coastal changes scientists anticipate as sea levels rise farther, Young says.
While lidar observations can’t indicate exactly when a cliff is going to collapse, Gary Griggs, a geologist at the University of California, Santa Cruz, says it can be used to pinpoint particularly vulnerable spots and make general predictions, such as that a section of cliff will collapse within the next 50 years. He’s unsure, however, whether one can make more precise predictions. Young agrees that it’s difficult, but he expects his new approach will ultimately help improve predictions.
The data from Young’s research and similar surveys is informing local officials, state authorities, and property owners, many of whom are looking for strategies for adapting to a future with more erosion, and erosion at higher elevations as the ocean rises.
In some places, erosion will eventually make it uneconomical to stay, says Charles Colgan, the research director at the Center for the Blue Economy at the Middlebury Institute of International Studies at Monterey in California. Long before one’s property topples into the ocean, the cost and time of annual repairs and maintenance to clean up will become unmanageable, he says.
According to a 2018 study led by Colgan, the economic threat to Southern California stems less from big storms than frequent small ones, which are becoming more common. “The combination of erosion and sea level rise is what’s going to do in much of the San Diego County coast. It makes sense considering you have a lot of high-value property sitting up on those cliffs.”
Ultimately, infrastructure, including houses, roads and rail lines, entire city blocks, and possibly even a desalination plant and a decommissioned nuclear power plant, will have to be moved. Homeowners in the region have balked at the term managed retreat, but regardless of what it’s called, some kind of community relocation will have to happen, he argues.
Historically, says Griggs, people along the California coastline have used armor, such as wooden, concrete, or riprap sea walls and other structures, to fortify eroding cliffs, while adding sand to beaches that are being washed away. This armor is costly to build and requires periodic maintenance, and in many areas, even the densest armoring won’t be sufficient. “I think those days are over in California, and in some other states as well,” he says.
That means people now have to decide between retreating or continually rolling the dice with coastal landslides. “In the long run, there’s no way to hold back the Pacific Ocean,” Griggs says. It comes down to the question, he says, “What risk are you willing to accept?”
*Correction: This sentence originally said that the three women were strolling on the beach when the bluff collapsed, when they were actually seated.