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Accounting for changes in landslide patterns with time is very difficult. Collecting the underlying datasets is problematic in itself (still requiring mapping by hand in most cases) and, of course, landslides result from a combination of a whole range of natural and human factors, all of which change with time. It has been frequently postulated that one of the underlying causes of the increase in landslides in mountain areas in less developed countries is road building – indeed in a paper that I wrote with some colleagues a few years ago (Petley et al. 2007 – drop me a line if you want a copy) we proposed that inappropriately engineered road construction might account in large part for the increase in landslide impacts in Nepal over the last 20 years or so.
As part of the Earthquakes Without Frontiers project we will once again be working in Nepal over the next few years. I am genuinely delighted about this – Nepal is a quite wonderful country, but the landslide problem there is severe. The main aspect of our work will be to think about the threat posed by earthquake-induced landslides, and a major issue here is the way that road networks will be disrupted when a large earthquake occurs. So, this morning I was looking at the ways that road building has changed the landscape in Nepal in recent years, and came across this really interesting example from Central Nepal. The area in question lies close to the Prithvi Highway, which is the main access road from the southern plains through the mountains to Kathmandu. The rural road in question, which is in the very southeast of Gorkha District, is a small road that links to the main Prithvi Highway and passes through a settlement called Ghyalchok before climbing over the ridge, ultimately joining another road near a settlement called Darbung Phant (see Figure 1 below). The mountains here are large, but they are not the huge snow-capped high Himalaya to the north. The road, which starts at about 300 m above sea level and climbs to about 1300 m, was presumably constructed to link the many ridge-top communities to the main road network, and as such is typical of thousands of roads being built in Nepal. The road, which is almost certainly unsurfaced, can be seen as a thin brown line on Fig 1.
This post from Durham Geography BSc student Amy Wright tells the story of a unique student field trip to the Upper Bhote-Khosi River catchment in Central Nepal to investigate landslides in the region and the communities affected by them.
For many of you who were privy to the widespread September flooding in North East England this year, which according to the BBC was the most intense September storm in the UK for 30 years, it may be hard to imagine areas that were experiencing more turbulent weather. However, for myself and a cohort of 30 fellow Durham University BSc Geography students in their final year of study and various staff members, intense rainfall interrupted by blazing sunshine were frequently experienced on a two-week field trip in the Upper Bhote-Khosi, Nepal.
Due to heavy monsoon rains, combined with high rates of tectonic uplift, Nepal experiences a wide range of mountain hazards. Slope failure is particularly common due to monsoonal rainstorms between June and September and annual rainfall totalling 3500 mm a year in the Upper Bhote-Koshi Valley. This makes Nepal the perfect place to gain both theoretical and practical training of hazards in a dynamic environment and speaking on behalf of the group the field trip certainly did just that. In particular, being able to observe the remnants of mass movements first hand was truly breathtaking and a once in a lifetime experience. Not to mention the fact that one of the mini-buses containing some members of the group actually got stuck in a small debris flow!
For the last decade I have maintained a database of landslides that cause loss of life in Nepal. This work was started as part of a DfID project on landslide risk assessment for rural roads in that country and in Bhutan. As part of that project we tried to extend the database back to 1968, although the older data is less robust. This work is now part of the larger project that I undertake on landslide-induced fatalities, but I retain a particular interest in Nepal because it is both highly landslide-prone and subject to rapid changes in both climate and social setting. This data is written up properly in Petley et al. (2006) – this can be downloaded for free from here, with an update in Petley (2009) and a write-up of the role of climate on landslide occurrence across Asia in Petley (2010).
At the request of a few people, the graph below shows the data for the period 1980 to 2010 inclusive. The solid black line shows the numbers of recorded deaths due to landslides in Nepal for each year for the period 1980 to 2010, whilst the dashed line shows the number of recorded landslides that caused one or more deaths: