For decades, coastal areas in California like Rancho Palos Verdes didn’t improve drainage systems or reinforce slopes with deep-rooted vegetation to slow down landslide risk. Is it too late?
Any Spaniard over 40 remembers the name of one tiny mountain village in the Aragonese Pyrenees: Biescas. They don’t recall the name because the place is picturesque or pleasant (it is) but because a lack of modern permitting allowed locals to open a popular camping in a flash-flood-prone area.
And, sure enough, a bigger-than-normal storm arrived one day, creating the conditions close-by mountain causeways to direct water and debris at a force and speed that surprised hundreds of families sleeping on tents and RVs. The 1996 campsite disaster killed eighty-seven people and traumatized villages on both sides of the Pyrenees for years.
Whoever had installed a camping site on the alluvial fan of a mountain range (concealed for decades, as nothing had happened recently) didn’t think the place was dangerous, nor did the thousands of people who visited every year. It took a mammoth storm—over 170 liters per square meter, or 6.7 inches of rain, in less than 8 minutes—to cause chaos: a summer storm so powerful that even the oldest people didn’t remember anything equal to take the entire camping in minutes.
The event was bound to happen, yet the odds had been so residual that nobody in the area had established adequate building codes to actively forbid anybody from putting people at potential risk. That it had “never” happened (in recent history, that is) was good enough to enough people to cause the death toll.
There’s bad luck, and then there’s something different. Let’s call it intergenerational recklessness, or not being aware of the risk an area has because your own experience, or the experience of the immediate cohorts preceding you, haven’t found dangerous precedents to prevent you from making a dangerous mistake.
More one-in-a-century events uncovered by insurers
An event like Biescas also involves uncertainty but could have been prevented. Summer is the only moment of the year when storms form on the southern downslopes of the Central Pyrenees—and hence, coming from the Mediterranean and not from the Atlantic—can cause flash floods, as the phenomenon requires many inches of rain in a narrow amount of time, for alluvial areas to become a death trap due to flash debris floods.
Alluvial fans are common in the Pyrenees, but people forget about them—see, for example, the one in the Cirque d’Estaubé, right in the frontier between France and Spain. In a way, that’s what different generations tend to do: they forget about the good times when they experience bad times, and they forget the bad times during times of joy and stability.
How are technologies to detect, measure, and prevent natural catastrophes evolving? Can technology learn from tradition in at least some aspects? As we leverage data science, AI, satellite imaging, and remote sensing, scientists are building better models to assess risk.
As insurance companies try to avoid paying for the consequences of more natural and extreme-weather catastrophes, writing an ever-finer small print, technologists are finally coming to terms with current technical limitations and using other tools —from existent historical data to indigenous knowledge covered by anthropological studies, to understand the long-term environmental change that behaves to slow for technology to capture.
For example, traditional knowledge can complement satellite data to explain localized transformations: Arctic Indigenous peoples assess sea ice behavior in ways that even sensors miss, understanding the effect between phenomena like ice melting and local animal behavior, migration patterns, etc.
A short memory span
Thanks to AI algorithms, current seismic sensors keep improving to detect severe shakes as early as possible. Traditional knowledge helped areas like Jōmon people—inhabitants of Japan during the Neolithic—and Mesoamerican civilizations to build with techniques—wooden framing in Japan, bahareque structures or interwoven flexible sticks in Central America—to reduce structural shock during cyclical “big ones.”
Most relatively modern vernaculars, however, keep falling in the fallacy that extraordinary things won’t happen early enough, and won’t be big enough, to disrupt our lives.
In his book about a family saga in California’s Salinas Valley, East of Eden, John Steinbeck describes the bias of humans to give more importance to current events as a curse of tradition (no wonder that the San Jose Mercury News used it to open one editorial during the very-wet winter and early Spring weeks of 2023):
“During the dry years, the people forgot about the rich years, and when the wet years returned, they lost all memory of the dry years. It was always that way.”
John Steinbeck, East of Eden, 1952
If we were to analyze the impact of cyclical natural disasters on civilizations, every society, modern or traditional, seems doomed to forget that one area experiences catastrophic events. Our recency bias reminds us of our tendency to give greater importance to recent events. In contrast, past occurrences or long-term patterns remain overlooked—at the expense of those who will experience natural events they could have been more prepared to tackle and adapt to.
The abnormality of hoping for things to get back to “normal”
Insurance premiums are already skyrocketing due to increased fires in temperate areas (the Mediterranean basin, coastal California) and vast forested areas (Canada, the Pacific Northwest in North America, or Siberia in Eurasia). Despite this reality, which is prompting residents of wealthy enclaves in the Bay Area like Orinda to rely on more expensive state-backed home insurance as private insurers leave areas that become riskier, we are also affected by the normalcy bias. This cognitive impairment makes us assume that things will continue to function the way they “always” have, underestimating the likelihood of a disaster or significant change.
When such occurrences happen in relatively short intervals—more than once per lifetime—societies invest and innovate to adapt in the best way possible. However, when catastrophe hits sparsely enough, a new occurrence catches people by surprise.
Ancient civilizations and traditional societies developed intergenerational ways of information transmission to let know their remote descendants that, every so often, an estuary may flood completely, a place may suffer gigantic fires, tsunamis, earthquakes, flash floods, hurricanes, landslides, etc.
A growing body of research studies how ancient civilizations integrated warnings of very sparse disaster cycles in cultural encodings such as taboos (designing certain forests or animals to prevent deforestation and pandemics, respectively), beliefs, and even holy texts to prevent societal collapse at different scales, locally or at the level of one entire civilization.
Plant cultivation in the Middle East during the Younger Dryas (a period between 12,900 to 11,700 years ago) responded to the adaptation of hunter-gatherers towards the cultivation of wild plants as a response to the return of the ice ages and worsening climate, which reduced their ability to feed themselves without establishing long term strategies.
Short memory span vs. long-term maintenance
In the Pacific Ocean, old stories repeated generationally preserve a collective memory about tsunamis, as well as warming enclosed as a call to action within a tale: a volcano eruption or an earthquake can trigger a god to send a big wave, and therefore people need to tie their boats with long ropes and run for high ground.
Did Indigenous knowledge prove an effective tool for managing long-term risk? Examples from around the world illustrate this. Climate adaptation of Pacific Islanders included risk management of cyclical phenomena like the climatic patterns to the Pacific Ocean currents, known as El Niño and La Niña, which they rightly linked to long-term effects on rainfall and ocean conditions.
Due to heavy cyclical rains, for example, they created taro pits (swamp taro) in flooded areas. The plant, they realized, didn’t only sustain communities but also sequestered water during flood events. Planting taro would become a barrier to guide water and avoid flood destruction.
In India and West Africa, several communities recognize specific groves as sacred, virtually protecting them from deforestation; such ecosystems preserve the regions’ biodiversity and water sources. Closer to us, the ancient Zuni people from the US Southwest developed a water management system to capture rainwater and avoid water runoff during the monsoons.
Ancient knowledge can be a tool for managing long-term risk. That said, some issues communities face nowadays seem detached from any such reference. Or do they? Let’s consider, for example, the slow-motion landslide that has affected Rancho Palos Verdes since the fifties. Developed as an exclusive community on the bluffs of the Palos Verdes Peninsula south of Los Angeles, it had defied evacuation due to the effects of a landslide active since at least 1956.
The city was incorporated in 1973, and the land has been shifting, though very slowly, for decades… until the last 12 months. Now, some areas are moving up to 10 inches a week.
How traditional communities prevented soil erosion
Is there any example of traditional communities dealing with landslides or preventing them with remediation techniques that have proven effective over the centuries? It turns out there are many examples.
In the Andes, one of the most arid high-altitude areas of the world, the Inca and earlier Andean civilizations created terrace farms on steep slopes, which retained rain and moisture and minimized soil erosion. This adaptation was crucial to explore the variety of crops like potatoes, the region’s contribution to the world. They learned to manage droughts and climate variability by building a system (terraces) to regulate water and farm during years when there was virtually no rain.
Confronted with similar issues, Nepal peoples like the Gurung and Tamang combined traditional engineering with systems maintenance to avoid the formation of gigantic alluvial fans in the Himalayas. As people from the Andes, they combine terracing, gardening, and drainage systems to stabilize steep slopes, planting deep-rooted bamboo on marginal terrain and keeping as much forested areas as possible.
Various groups in Southeast Asia (often living in the upland areas of Indochina, the Philippines, and Indonesia) tested and improved agroforestry techniques that helped benefit from the monsoons and reduced landslides. By cultivating trees and crops together, they stabilized soil on steep slopes by simple observation and trial-and-error of conjectures over the centuries: the deep roots of trees helped bind the soil during heavy rains and earthquakes.
They also realized that forest cover reduces water runoff and reinforces the soil’s stability and porosity. Instead of soaking uneven areas, the water penetrates the ground and feeds subterranean aquifers instead of liquefying the soil’s superficial layers. Their combination of terracing, forest management, and drainage systems stabilized steep slopes using deep-rooted plants such as bamboo.
Rancho Palos Verdes: a coastal community slowly falling into the sea
Such ancient, commonsensical techniques don’t seem to have anything to do with phenomena such as the soil instability that makes life very difficult for the residents 30 miles south of downtown Los Angeles at Rancho Palos Verdes. It has been under a local state of emergency since October 2023, unable to rely on utilities since July 29, 2024.
Companies are cutting homes from electricity and gas connections because the soil keeps shifting near the oceanside cliffs. According to Southern California Edison and SoCalGas, 170 homes in the area were already without network power and gas, and 54 more have been added to the list in September. Some have agreed with leaving their properties at risk but want reinforced measures to prevent looting from the vacant houses, whereas many others—some very aged—refuse to abandon their place.
Residents can’t argue they didn’t know about the risk. The complex of landslides was detected and surveyed in 1956, which preserved the area from further development despite the allure of picturesque and relatively remote Portuguese Bend to early Hollywood stars and business moguls. Many homeowners bought their properties for far less than their listed worth; in many cases, however, the constant renovations and uncertainties have turned their idyllic corner in the Palos Verdes Peninsula into a struggle.
It took two consecutive years, with above-normal rain seasons in 2022 and 2023, to see an acceleration of the landslide. Last winter, the damage was too big to declare the issue controllable. Now, many residents who live outside the expensive shoreline enclaves and define themselves as a self-sufficient community of artists and creatives can’t afford to pay for the constant renovations and don’t know where to go. So, an overwhelming majority has decided to stay.
Up until recently, the landslide had been somehow manageable. Now, with the utilities cut, only houses capable of maintaining their sewage system up and working live in some sort of restless normality. Despite the area’s proximity to Los Angeles, many families are willing to invest in a fully off-grid system and hope for the better despite the pervasive cracks in streets, lots, concrete slabs, and even homes.
Sallie Reeves, 82, a retired school psychologist, has explained how the landslide advanced through her house, tearing the property apart: the visible damage started with a crack in her bedroom’s closet, which grew bigger fast. When the house started tilting on one side, it became unsafe for her husband—who has a disability—to walk. They had to move into their garage but didn’t want to leave their property.
Reforestation strategies to prevent/diminish landslides
Moreover, homeowners at Portuguese Bend can’t call their insurance for help, and no policy covers the costs associated with landslide damage.
It may be too late for some people to try to prevent further damage to their homes. The problem, however, could have been prevented by creating remediation systems to improve water drainage and avoid movement near the surface.
If current residents were to ask the early residents of the peninsula, the Tongva (also Gabrielinos), they’d learn that natives were very aware of landslides, which have been active in the area for thousands of years. The current landslide, which spans 260 acres and has an average thickness of 135 feet (41 meters), occurs mainly around 30 meters below the surface, as strata get pushed towards the sea due to coastal erosion.
The Tongva weren’t aware of the area’s geological evolution but had observed it over generations, which forced them to create seasonal and flexible settlements instead of creating permanent villages. When they settled, they favored elevated areas and bluffs, avoiding steep slopes and other areas prone to erosion. Their dome-shaped dwellings (kish or kiitcha) made of reeds, grasses, and wood were light, flexible, and easy to rebuild or move.
There isn’t any documentation regarding landslide management by the Tongva. Other native peoples, like many groups from the Pacific Northwest, still manage landslide risk by controlling land use and respecting waterways, as well as risky areas conveniently deemed “sacred land” by their ancestors.
Talking about best practices by hunter-gatherers inhabiting the Palos Verdes Peninsula before European arrival may seem worthless, especially to those needing solutions to mitigate the effects of the speeding landslide.
Active reforestation and restoration of natural drainage, from waterways to subterranean aquifers, could make a difference in the medium term. Infiltrometers, which measure infiltration rates, could help assess the situation in the Palos Verdes Peninsula.
The things that speed erosion: conventional urbanism’s self-immolation
Big slabs of concrete, conventional roads without proper drainage, and conventional home foundations contribute to the damage in Rancho Palos Verdes, as slopes with little to no vegetation speed erosion significantly. Deep-rooted plants that grow fast, as some grasses like bamboo do, could help buy some time in certain located areas within properties, though authorities could help with a reforestation plan, as well as a new design for infrastructures capable of avoiding runoff.
A study by geology and forestry experts Roy Sidle and Hirotaka Ochiai shows that by strengthening shallow soil layers and improving drainage, deep-rooted trees and shrubs reduce the occurrence of shallow, rapidly moving landslides. Roots penetrate the entire soil mantle, functioning as anchors to anything built on the surface:
“Although surface erosion is a natural process, it is exacerbated by surface disturbance and compaction that reduce the soil hydraulic conductivity and break down soil aggregates. Management practices and attributes such as roads and trails, agricultural cultivation, fire, land clearing, and recreation all accelerate surface erosion processes due to their disturbance, compaction, and connectivity along hillslopes. Agroforestry practices in Southeast Asia that incorporate cover crops with trees reduce surface erosion by more than an order of magnitude compared to monoculture plantations with no ground cover.”
“Cleared fields tilled up and down steep slopes are highly erodible; passive conservation practices (e.g., contour tillage, strip cropping, reduced tillage; maintaining adequate ground cover) are effective in reducing surface erosion if properly implemented. Poorly designed and managed terraces are not effective in controlling surface erosion and may actually increase mass wasting if they concentrate water.”
Sidle, R.C., Ziegler, A.D., Negishi, J.N., Nik, A.R., Siew, R. & Turkelboom, F. 2006. Erosion processes in steep terrain—truths, myths, and uncertainties related to forest management in Southeast Asia. Forest Ecology and Management, 224(1-2): 199-225
Is deep-seated geological instability reversible?
These researchers also point out underestimated benefits associated with abundant deep-rooted vegetation, as transpiration from large tree canopies decreases water content in the soil. Evapotranspiration, they argue, shouldn’t be underestimated.
If trees are a solution to prevent shallow landslides, what happens in deep-seated ones? Forests may not be prevent landslides after continuous heavy rainfall, though the tree-covered steep slopes slow the speed and visible damage of such occurrences.
A report from the Food and Agriculture Organization studies the damages of landslides in the Asia Pacific region due to coastal erosion and increased heavy rainfall. As mitigation strategies for the future, they recommend planting on steep slopes “where slope materials remain unstable” as long as “complementary engineering works” such as contour trenching take place.
Back in February, California approved fast-track landslide mitigation works in Rancho Palos Verdes after an appeal by the city.
Among the plans, the city wanted to install two additional dewatering wells that extract groundwater to stop it from infiltrating the layers of clay, as well as improve drainage systems to prevent water from entering the ground. Who advises on the works, and how many forestry experts are on the team?
Those are questions that residents at Rancho Palos Verdes should quickly ask their representatives.