September 19, 2021

‘It’s certain that more landslides will happen in Freetown’

7 min read
The problem is that people more and more are building up onto those slopes and living within those catchments.

James Gray and Alejandra Garcia in conversation with Kathryn Goodenough and David Boon

Photo courtesy: British Geological Survey. Cover graphic: Sunil Krishnan

Kathryn Goodenough and David Boon of the British Geological Survey (BGS) are perhaps more intimately familiar with the lay of the land in Freetown than most Freetownians.

Boon, an engineering geologist, had just completed a review of landslide hazard and risk in the Freetown Peninsula for the World Bank-ARUP when the catastrophic mudslides of 14 August 2017 occurred. Goodenough, a principal geologist, flew in a few days later on a planned trip for a UK Aid-funded capacity-building project. The presence in the country of these experienced geologists (Goodenough and Boon have clocked 20 and 15 years at the BGS, respectively) at this critical time was fortuitous, and their skills were quickly put to use. 

“We were asked by the UN team to review satellite imagery to actually map out where landslides had occurred across the Freetown peninsula, and then, feed that information back to the people on the ground because they may not have discovered all the landslides that had occurred in the more remote areas,” Boon said. Following this, the scientists were also part of on-the-ground efforts led by the United Nations Office for Project Services (UNOPS) to gauge the stability of the slope and to give recommendations to pre-empt future disasters in the area.

On the fourth anniversary of the mudslide, Goodenough and Boon spoke to Tie u Orja’s James Gray and Alejandra Garcia on how landslides develop, the risks of geological disasters in Freetown, and the complexities of finding solutions.  Edited excepts:

On the causes of mudslides

David: The main causes are generally a combination of a steep slope, weak geology, and water. We often think of rocks as kind of hard. But in reality, a lot of the rocks we have under our feet are actually quite soft and weak.

Water plays a very important part for causing landslides. As it falls on the slope, some of it will run over the slope, some will drain into the slope. That water makes the soil heavier. When they’re water saturated, your trousers get heavy. The same is true of a slope. So that increases the stress and the pressure and makes that slope want to break apart and fail.

You also have a trigger, the straw that broke the camel’s back. Sometimes it can be quite a minor thing like water level in the ground rises, or often it’s a rainstorm like it was in Sierra Leone. It can be earthquakes. Sometimes the cause can be humans, even someone digging a hole in the wrong place on the slope, or cutting into the slope to build a road or railway. A river eroding away at the bottom of the slope can also destabilise the soil.

On why the death toll of 2017 mudslide was high

David: The main reason was that people were living directly in the firing line and it was early in the morning, when people were still at home. The landslide was not enormous, but it was several hundreds of metres wide, and the source area––the area that initially moved––was several hundreds of metres long. Then what happened, we think, was that the soil that got initially moved broke up on the slope. It got mixed with the water that was pounding down the valley, which was bowl-shaped. It created a very mobile flow of soil during a rainstorm. The landslide then went into a channelised river or a gully system and ran out seven kilometres to the sea. And anyone living within that area was impacted. The other thing was the vulnerability of the buildings… soil moving in a landslide is incredibly destructive and abrasive, and was full of very large boulders. Some of them were the size of a bus.

Kathryn :  The nature of the peninsula, the topography, the geology means that there will always be landslides. The difference with this one was that there were a lot more people in the path of the landslide.

On deforestation, urban planning, and landslides

Kathryn: On that day, there were several landslides across the Freetown peninsula and some of them were in forested areas, including up at the Tacugama Chimpanzee Sanctuary. Landslides have occurred around Freetown peninsula in the past too, when it was all covered in forest. So, you have to be careful about saying that deforestation is the cause. It perhaps means that there is less natural protection, but it’s not necessarily a cause…

On poor urban planning, I think it is important to remember Freetown topographically is very steep. It has quite particular geologies that we know have specific plains of weakness in them, but there’s no immaculate geological map to tell you exactly where the problem areas are.  I think you can’t necessarily say that it’s poor urban planning, and that’s that—because it’s not necessarily the case that the information is all there to inform the urban planners. So, there is undoubtedly a need for more sound data. Is that fair Dave?

David: Yeah, we do see evidence that landslides were occurring in areas that had virgin rainforest on them. But I would say that they’re more likely to occur on deforested slopes… Having forest there reduces the amount of water that gets through. It’s a buffer, so it slows down the transport of water to the ground, and if you can slow that down, it would reduce the triggering effect.

The problem is that people more and more are building up onto those slopes and living within those catchments. The actual danger of the landslides themselves, what we call the hazard, is probably about the same as it was 50 years ago. But the exposure is higher because people are living in the way now.

Kathryn: It’s a complex system of understanding where the hazards are––which isn’t fully mapped in Freetown. It’s being able to do the planning better, and then it’s ensuring that people can actually [afford to] live in the areas considered to be a lower risk.

On the risks faced by Freetown communities

David: I think it’s certain that more landslides will happen again. The slope that failed is still potentially unstable. A landslide may not happen in exactly the same place, but the worry is that it might happen just to the side of that, where the landslides created a new cliff face, [which] will be unstable because it’s a free face. There’s actually no way of knowing when and where the next landslide will occur. People constructing buildings on slopes need to be educated not to do anything that might destabilise the slopes, such as cutting deeply into the side without some geotechnical advice, or changing drainage provision.

Whatever the hazard is today, it’s probably worse tomorrow, because we have these compounded effects of increased population and climate change bringing more intense rainstorms.

On risk-reduction

David: Learning from the slides that already occurred in 2017 and 1945 you could develop a forecasting and early-warning system, where we say, so much rain has fallen, so today there is a green, amber, red alert for landslides. But that should be underpinned by as good as possible meteorological, geological, and topographical data bound together in computer models. Even then, the risk will never be zero––the best approach is to work with nature, and avoid living and building critical assets in the potential path(s) of destructive landslides.

Kathryn: We’re not talking about things that can be easily done in a short time with relatively small amounts of money. We are talking about long-term research that would need capacity that doesn’t exist in Sierra Leone at the moment.

David: There needs to be an integrated land-use policy and management, and education schemes right across the society. And the funding needed to ultimately make it happen.

On the need for long-term planning and global partnerships

Kathryn: I think the key message is that in Freetown there will always be landslides. The more data you can acquire, whether that is geological, topographical, related to weather, forest cover, where people are living, how their buildings are built—all that kind of information is really important. In Freetown, a lot of that data is not available. So, it’s really important to appreciate that there is long-term problem here. It can’t be fixed quickly, and it can’t be fixed easily.

David: It needs political will over a long period. It needs institutional structures and funding and systems and expertise, and the capacity to run and maintain [all these systems].

Kathryn: It needs political will. Not only in Sierra Leone, but we need global partnerships to really be able to make a difference in this sort of situation. This is not something that Sierra Leone can do on its own.

NOTE: This article was first published on 14 August 2021 on www.tieuorja.org, which works to strengthen disaster communication in Sierra Leone.

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