Climate change and landslides: the slippery slope towards disaster?

“In some places the whole hill comes out. When we see these places, you would never know there was a settlement there before,” says Jampa Tsering Lama, who is a Nepalese emergency coordinator for humanitarian aid organisation People in Need (PIN). The charity has helped deliver aid to people who have been hit by some of the 300 or so landslides that struck the country in 2020 alone.

“When the landslide is minor, people can recover their livelihood,” Lama explains, “but if the impact is high, their livelihood and property are wrecked.” This displaces people and means “communities are not able to return to their place of origin”.

When landslides strike, the effects can be devastating. In 2021 there were several major slope collapses around the world that have caused extensive damage. On 7 February, a landslide in Uttarakhand in the Indian Himalayas killed at least 70 people. Just days later a major landslide in Ecuador displaced over 500 people (happily with no loss of life). In November, at least four people were killed in landslides in British Columbia, Canada, following exceptional rain. Closer to home, two landslips struck a railway line near Glasgow on 6 February, halting services. The human toll of landslides is severe. Between 2004 and 2016, close to 60,000 people were killed in such events according to one study.

Researchers have detected a rise in the frequency of landslides in recent decades and this can, at least in part, be linked to climate change. To understand this link, and the technologies that might help address the problem, we first need to understand why landslides happen.

“Think about it like sand,” says Professor Jon Chambers, a geophysicist at the British Geological Survey (BGS). “When sand is damp, the moisture between the grains creates suctions,” which lets the material hold a shape. However, as anyone who’s built a sandcastle knows, if you add too much water that suction dissipates, and the shape cannot hold.

At a basic level, the same thing happens with landslides – whether the slope is naturally occurring or engineered. Most landslides occur when too much water enters a slope and it becomes saturated. If the water cannot leave the slope, or doesn’t leave fast enough, the land begins to move.

The main trigger event for landslides is therefore heavy precipitation and waterlogging. That said, they can be helped along by earthquakes. And of course, human activity increases the risk too. Cutting slopes to build roads or settlements can weaken them. Mining and quarrying have the same effect. Deforestation, too, is a well-known contributor to landslides.

“The landscape in which we currently live has evolved to be in equilibrium with meteorological conditions,” explains Professor Dave Petley, a landslide expert at the University of Sheffield. For millennia, global weather patterns have remained relatively stable. However, those patterns are now changing fast. This in turn impacts on the landscape.

There are several ways this works. For example, rising temperatures in mountainous regions melts the permafrost. That permafrost ‘glues’ the terrain together. So, if it begins to melt, there is less to hold the rocks in place. Similarly, in places where glaciers have disappeared or shrunk, there is now no barrier against rockfalls.

We are also seeing more coastal cliff collapses. Rising sea levels and heavier storms mean that once-stable sea cliffs are being undermined. “We have robust data from coastal environments in the UK which demonstrate a significant increase in these kinds of landslides,” Petley notes.

Yet it is the change in rainfall patterns which is likely to have the most significant impact, at least on human settlements and infrastructure. Petley notes that we are increasingly seeing spells of very dry then very wet weather. That intense rainfall “drives up the groundwater levels and leads to greater instability, so some slopes collapse”. We are seeing a global increase of especially intense rainfall events too. Storms that were once expected once every 100 years, for instance, are now happening once per decade.

This combination of factors makes it much harder to change how we manage slopes and prevent landslides. More slopes will be at risk of collapsing, at times and in ways we might not have expected before. So engineers and scientists are working to predict and prevent these disasters.

On 12 August 2020, three people were killed when a train travelling between Aberdeen and Dundee was derailed near the town of Stonehaven. Following unusually heavy rain the night before, a landslip left debris on the track that caused the train to go off course.

This kind of disaster is emblematic of the landslide risks that climate change introduces. Britain’s railways pass beside thousands of miles of both natural and manmade embankments. Many lines were built over 150 years ago and need upgrade and repair. They were also designed for an era with more stable weather patterns – not the kind of long dry spells and torrential rain that are becoming more common.

To avoid these kinds of disasters, Network Rail, the company that operates Britain’s rail infrastructure, uses a variety of techniques to monitor slopes that might be at risk. The most common approach continues to be visual walkover inspections, says Chambers of the BGS. However, other techniques provide engineers with more information.

For example, drilling boreholes into a slope can tell us about changes in the subsurface that might lead to failures. “Moisture is the critical factor,” he says. “We want to get an understanding of water levels in the slopes.” By drilling boreholes, you can collect a certain amount of information about what’s happening beneath the surface.

However, the limitation of this technique is that it is manual, requiring engineers to physically visit all slopes to drill the boreholes. With tens of thousands of miles of track, this is not always viable. Another issue is that such techniques can only tell us about a few square inches of a slope that could contain millions of cubic feet of material. “You only monitor a fraction of a percentage of the whole slope,” he says.

Chambers and his colleagues at the BGS have worked with Network Rail to investigate the potential of electrical resistivity imaging to learn what is happening below the ground near tracks. The team places sensors along a slope which send a pulse of electricity into the ground and then record the ‘feedback’ from below. Where there is more water, there is lower electrical resistance. When the ground is drier or consists of materials that don’t conduct electricity well, resistance is higher.

A computer program then creates an ‘image’ of the ground below the sensors showing where there are rocks, clay, soil, and perhaps pooling of water. Chambers points out that this doesn’t replace other techniques but can certainly complement them. By spreading sensors right across a slope, it’s possible to get a far more comprehensive picture of the whole area, rather than just peppering it with boreholes. The method is potentially quicker and cheaper, too.

These sensors can be hooked up to the internet and send back regular reports on the state of a specific slope. If it is starting to show signs of saturation, Network Rail would use this information to perform remedial actions early. “It means you don’t have to wait until you’ve got an emergency before acting,” Chambers says.

This kind of technology is useful for monitoring individual slopes, but over larger areas a different approach is needed.

Hong Kong is widely recognised as a world leader in landslide detection. Its early warning system is invaluable in mitigating the risk of landslides in the densely populated and hilly city.

Stuart Millis, a landslide expert at the offices of engineering firm Arup in Hong Kong, explains that the territory has a long history of landslides. “From the late 1800s until today there has been a lot of land reclamation around the toe of hills sticking out into the sea,” he explains. But prior to the 1960s, “many were not engineered to today’s standards, which has left a legacy of slopes retaining water”. In the 1970s, Hong Kong experienced several major landslides that left hundreds dead. Towards the end of that decade a geotechnical control unit was established to address this situation.

A key part of Hong Kong’s landslide prevention and mitigation programme is the collection of data. Millis explains that the city has a catalogue of where landslides have happened, and this is correlated to precise data on precipitation on every single slope. “They have a landslide potential index of when landslides are likely to happen,” and the authorities then send out warnings to the public when the index passes a certain level.

Arup has been working with the authorities for around 40 years to support Hong Kong’s landslide monitoring and prevention efforts. One project has been to install a variety of sensors on at-risk slopes. These sensors include things like GPS monitors that show if a slope is moving and by how much, or how much moisture is collecting beneath the surface. That data is beamed to a central hub in near real-time.

Another technique is the remote monitoring of landslide barriers found at the toe of hills in Hong Kong. The authorities have built barriers in many places which are designed to prevent debris sliding onto main roads and residential areas. However, without sending someone round to inspect all these barriers it’s impossible to tell if landslides have in fact struck them. So, Arup has installed remote sensors on the barriers which send a notification to tell the authorities when a slide seems to have happened. The systems can even take and send photographs so the authorities can assess the level of the impact.

Hong Kong is a compact territory, which makes managing and controlling for landslides somewhat easier. But what about measuring for landslides at a bigger scale?

Mountainous Italy is one country where landslides are a problem. Dr Davide Tiranti, a hydrologist at the Regional Agency for Environmental Protection of Piemonte, has been working on the design of regional Landslide Early Warning Systems.

Dr Tiranti’s research focuses on “the relationship between weather and climate conditions and the triggering of landslides of different typologies”. To do so, he has collected datasets on historical weather reports over long periods (of up to 100 years) and statistically correlates them to recorded landslides. This historical data is then connected to a system of environmental gauge networks distributed across the territory. These gauges feed back data about precipitation and temperature in different locations to a central processing system.

After crunching the numbers, the software can raise an alarm when a landslide looks likely to happen. Such alerts could be used by the authorities to take protective actions such as the evacuation of entire small, inhabited centres, or temporary closure of wide road networks.

The kinds of advanced landslide monitoring and prevention systems exhibited in places like Hong Kong or Italy rely on access to large amounts of historical weather and landslide data, as well as accurate sensors on the ground. However, most landslides and related deaths happen in poorer countries which simply lack this information.

Take the 2017 landslide in Sierra Leone’s capital Freetown. Following days of torrential rain, a huge mudslide swept down a steep hill overlooking the city, destroying homes, and burying neighbourhoods. Over 1,000 people were killed.

The problem for authorities in places like Freetown is that they have relatively little data that can inform them and their citizens of when such disasters are likely to happen.

Elisa Bozzolan is a researcher at the University of Bristol who studies landslides in informal settlements in developing countries. She explains that in many such settlements, construction techniques make landslides more likely. Residents often cut into hillsides to build their homes, but this can weaken the slope. At the same time, water pipes may leak, helping to saturate the ground. The problem is that in informal settlements “there’s no one responsible for recording small landslides so they get overlooked” in any official data.

Bozzolan’s current research aims to address this problem by using a new methodology in places where there is very little data available about local conditions. She uses a computer program to generate thousands of virtual hillslopes with properties like those observed in the study site. The stability of these hillslopes is evaluated using CHASM, modelling software developed at Bristol University.

Once models are built, Bozzolan can then begin testing what the addition or removal of different variables will have on the slope – things like hill cutting, vegetation, rain intensity or slope angle.

“If we have lots of data, we can be more confident in linking cause and effect,” she explains. “But in developing countries, data is generally scarce, so with our methodology we effectively create our own dataset.”

These models could help planners in various ways. They could be used to figure out how much rain would be ‘needed’ to trigger a landslide, or they could identify what effect slope cutting will have on a number of hills’ propensity to collapse. This information could then be used to inform interventions.

“Landslides typically kill several thousand people each year around the world, far more than other disasters like volcanoes, which typically kill no one on an average year,” says the University of Sheffield’s Petley. Landslides are pervasive yet are often relatively small in scale and less visibly dramatic than other ‘natural’ disasters. “It’s easier to raise awareness for things that are big and spectacular,” he points out.

With a close correlation between climate change and an increase in landslides, it seems reasonable to expect more in future. “It’s in all of our interests to get on top of the problem now,” Petley warns.