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The Landslide Blog is written by Dave Petley, who is widely recognized as a world leader in the study and management of landslides.
As I have noted previously, between 26 and 28 September 2024 Nepal suffered an extraordinary rainfall event that triggered thousands of landslides, killing over 200 people. The cause was a heavy rainstorm that was well-forecasted.
Estimated losses from this rainstorm are now US$340 million (which is about 0.7% of GDP), including damage to 41 roads, 26 hydroelectric facilities and 54 bridges. Almost 6,000 houses were destroyed, such that almost 11,000 families have been displaced.
One of the most seriously affected districts was Kavre. The Nepali Times has a very interesting article that highlights the role that quarrying has played in the initiation of debris flows in the area. This quote, from the article, sums up the situation:-
“When it rained, the debris that had piled up in the quarries was swept down in a wall of mud that took away my house and everything I own,” says [Rajesh] Humagain, who now rents a single room in Panauti town.
I have taken a quick look at the Planet Labs imagery of this area before and after the rainstorm. The data is absolutely fascinating.
First, I think it is right to say that the quarries have played a key role. This is another part of the Panauti area, with multiple quarries:-
The main channel flows west to east across the image, with at least four large quarries on the north side. Note the huge amount of sediment in the channel to the east of the first (westmost) quarry in the channel, and thereafter to the east. This appears to be sediment input directly from the works, and it has caused immense damage downstream.
But this is not the only story. There are huge numbers of landslides on other slopes unaffected by quarrying.
The image below, centred on about [27.5542, 85.5131], shows an area close to Panauti in Kavre before the rainstorm:-
The image below shows the same area after the rainstorm:-
And below is an image compare:-
First, and foremost, the landsliding is clearly absolutely devastating in this area. There are literally hundreds of failures in this area alone, causing immense damage to farmland, roads and houses. It is rare to see such landslide intensity in a populated area.
But there is a very interesting pattern to these landslides. In the image above, many areas that are populated have high landslide densities, but many areas with forest have very few failures. Compare the middle part of the image – to the west the number of landslides is almost negligible, to the east there are many.
There are two obvious possible explanations for this.
The first is that some areas are unsuitable for habitation (too steep or too dry, for example), which also makes them less prone to landslides.
The second is that human activity has greatly increased the likelihood of landslides occurring.
The true explanation may be a combination of both, but it is interesting to look in more detail at the failures. Let’s take just this small area:-
Running east – west across the image is a low cost road, following the ridge top. There are multiple failures extending from the road (and from others in the area) – this implies that this set of landslides had an anthropogenic origin. The role of low cost roads in landslide initiation in Nepal is well documented.
Elsewhere, the landslides have destroyed, and/or started from, areas of agriculture. Again, this implies an anthropogenic origin.
There are also a quarry in the image, which appears to have some associated landslides too.
So, it appears that the extreme rainfall has led to multiple failures in Kavre in which humans have also played a role. This needs much more detailed investigation – a good Phd topic for someone, or even an undergraduate disstertation.
Given that these extreme rainfall events are going to be more likely in the future, this spells deep trouble for Nepal.
Reference
Planet Team, 2024. Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/
Text © 2023. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
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