The Landslide Blog is written by Dave Petley, who is widely recognized as a world leader in the study and management of landslides.
Channelised debris flows are a major cause of landslide fatalities globally. These are terrifying events, occurring with huge speed and violence, sometimes at night and, of course, usually in poor weather. It is hard to portray the true impact of these events, but mobile phone videos have provided real insight in recent years. Amongst the best footage is still the 2008 debris flows from Lantau in Hong Kong:-
There’s also some remarkable footage from the Ilgraben catchment in Switzerland:-
A few days ago, I wrote about the deadly 26 August 2022 debris flows in Swat, Pakistan. The major failure at Budai Kamar destroyed 42 houses. This is the junction of the gulley with the major channel before the event:-
Image dated 1 May 2022.
And this is the same site after the disaster:-
Image dated 12 August 20223
And here is an image compare:-
The geomorphology of this site is interesting. Most of the houses are constructed on a higher terrace that slopes from right to left in the image. This was presumably formed during a much earlier debris flow event (or series of events) that must have been on a much larger scale than the disastrous debris flow in 2022. Thus, none of these houses in the image are likely to be safe.
That terrace has been incised by subsequent flows, and a series of houses were constructed near to the channel at a lower level:-
Image dated 1 May 2022.
These houses were lost in the debris flow. I can only imagine the horror being trapped in this location as one of the debris flows in the videos at the top of the post came thundering towards them.
A major challenge in such remote locations is how to protect these people. In Japan, “sabo” engineering is extensively used to mitigate these hazards, and similar approaches have been adopted elsewhere (for example in Hong Kong, South Korea and Taiwan). But such approaches are extremely expensive to build and maintain, and there are concerns about the environmental impact of these hard structures.
An alternative is to establish a warning system – indeed, in the Ilgraben video above, the flashing light on the left side is just such a system. warning systems do not protect properties, but they provide an opportunity to escape. However, warning systems can be unreliable; they require maintenance and, most importantly, they require that the population understands the risks and is able to respond. This has frequently proven to be more challenging than one might expect, especially if there is a series of false alarms.
Thus, we are often left with a perceived need to relocate the vulnerable population. This is the ultimate way to keep people, and their property, safe. But it is psychologically and socially fraught. Very often, the people concerned have a strong affinity to their place, sometimes for ancestral reasons. If the relocation is to temporary camps then the outcome for the displaced people can be very poor, most notably for children and women. Even where people are moved into permanent homes, they can lose access to their livelihoods (for example, their farmland) and to their social networks.
Thus, protecting vulnerable populations against channelised debris flows is a major issue, and one that will become more serious as climate change continues to increase the frequency and severity of these events. The approach needs to be holistic, to understand the full risk portfolio faced by the population (and not to consider this risk in isolation), to be focused of the long term needs of the people, to be sensitive to their complex needs and to be properly resourced. At a time when major countries are reducing their international development aid, this is becoming less likely.
The long term consequence is likely to be a steepening of the curve of annual landslide fatalities.
