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The Oso (Steelhead) landslide – mechanisms of movement
The desperate search for the up to 90 missing people at Oso has continued (see my earlier post on the landslide). Survival rates for landslide victims are very short, so this is not a rescue operation any longer. During this time some very interesting information has emerged about the landslide in the form of a seismic record of the slide. There is an excellent blog post from Kate Allstadt about this seismic signal on the PNSN blog – their understanding of this data is much better than is mine, so I won’t try to replicate it here. For me the most interesting aspect is the double seismic signal generated by the landslide, which indicates two major movement phases (followed by lots of small slips, which we would expect):
The start of the two movement events were about 4.5 minutes apart; the first lasted about 2.5 minutes, the second was somewhat shorter and less energetic (i.e. the movement rate was probably slower).
So what does this tell us about the landslide? We need to compare this with an image of the landslide after the failure – this Wikipedia image remains the best that I have seen for getting an overview of the whole landslide: Read the rest of this entry »
Extreme rainfall in southern England
Southern England is currently undergoing an extraordinary period of exceptional rainfall, especially in the south, which is causing floods on an unprecedented scale. This rainfall, which is likely to be a consequence of climate change, shows no signs of abating, with further heavy falls expected over the next few days. Not surprisingly there have been many landslides, especially on the coast and along railway lines, and more can be expected. The UK Met Office provides monthly precipitation data for Southern England . I have downloaded the data and plotted the monthly time series from 1910 (the start of the dataset):
The horizontal line in the long-term mean value (77.3 mm). The 2014 total, at 165.4 mm, is 2.8 standard deviations from the mean – a truly exceptional value. And of course it is still raining, such that since the end of January the floods have got much, much worse. Read the rest of this entry »
The Port Hills rockfall problem
The Port Hills area on the edge of Christchurch was very seriously affected by the Christchurch earthquake sequence. The Canterbury Earthquake Recovery Authority will today start the process of demolishing the rockfall affected houses in the Port Hills area. Yesterday they released a statement describing the challenges of this work; this statement is well-covered in an article in The Press, which also includes a nice video taken by a drone of some of the sites. A couple of years ago I visited many of these sites, and I have an old post that presents some of the images that I collected. As a reminder, this is typical of the state of some of the buildings at the top of the slopes on the Port Hills:
Introduction – the recent UK storms
The recent UK storms have brought exceptionally wild coastal weather, in particular to southern and western England, Wales and Northern Ireland. This has had a profound impact on the geomorphology of the British coast. The Met Office has a nice summary of these storms. The combination of strong winds, high tides, large waves and saturated ground has greatly accelerated coastal processes, promoting failure of large rock masses. The BBC has a nice article that highlights some of these changes – for example:
Porthcothan Bay in Cornwall
A little over a month ago I gave a talk at the Vajont 2013 conference on the topic of landslides and large dams. At the time I committed to making the Powerpoint file available online, so here it is:
The file is located on slideshare – you should be able to download the powerpoint file from there. The piece is also written up in an article for the conference paper – reference below. The paper can be accessed, for free, from the conference website. The other talks and papers can also be accessed from the conference website.
In this work I looked at the Durham Fatal Landslide Database to try to understand fatality-inducing landslides associated with large dams over the last decade. My analysis of the dataset suggested that in total there were exactly 500 deaths in 37 landslide events in the ten years between 2003 and 2012. Surprisingly, with one exception these were not landslides associated with the collapse of reservoir flanks (although interesting there was an event of this type in China earlier this year). Most of the landslides were either failures at the construction sites of large dams or at the sites of workers camps. Read the rest of this entry »
When I was studying Physics at school, we were shown the famous regelation experiment in which a wire is placed over a block of ice and a weight is attached to each end. Though time the pressure on the wire causes melting of the ice, and the wire slowly cuts its way through the block, and eventually the weights and wire fall to the ground. As the wire passes through, the water refreezes, such that the wire appears to pass magically through the ice.
There are various versions of this experiment on YouTube, of which this is about the best:
The interesting part of this experiment starts at about 1:22 and ends at 1:40 (in the speeded up sequence). Of course when the wire finally cuts through the ice the weights collapse to the floor with a great crash – the very last moments before this are shown below: Read the rest of this entry »
Over the last few days Central China has been experiencing exceptionally heavy monsoon rainfall, causing very high levels of damage. The TRMM data for landslide potential for the last seven days of rainfall highlights the areas considered likely to be affected by landslides (and note that there are also substantial parts of N. India and Nepal that are experiencing problems:
Unfortunately, the worst of the rainfall appears to have affected the areas struck by the 2008 Wenchuan earthquake in Sichuan Province. This is a zone that remains very vulnerable to landslides because of the legacy of the seismic even, compounded by inappropriate development in the rush to rebuild after the earthquake. The effects have been disastrous. The largest impact to date is the landslide at Sanxi in the area administered by Dujiangyan City, which is reported to have killed 12 people whilst a further 11 are missing, with very limited chances of survival. The landslide is reportedly very large, with a travel distance of 2 km and a volume of 1.5 million cubic metres. Read the rest of this entry »
Rob Parker (University of Cardiff) and Dave Petley
The Mw = 7.8 earthquake on Tuesday in Iran was the largest event in that country for about 50 years. Fortunately, the depth of the earthquake (82 km) and the low population density in the affected areas meant that loss of life was low for an event of this size. Indeed, reports suggest that only one person died in Iran, although there are reports of 40 deaths in Pakistan. This single fatality in Iran was the result of a landslide, and one of the images on the BBC reports about the earthquake also seems to show landslides:
Over the last three years or so, we have been working with our colleagues Alex Densmore and Nick Rosser, funded by the Willis Research Network, to develop a model that will allow us to make an initial assessment of landslide impacts in earthquakes. Rob recently submitted his PhD, and has now moved to a post-doctoral position at Cardiff. However, we thought that this event would be an interesting first application of the model, which has been produced through a statistical (logistic regression) analysis of spatial patterns of landslides (with areas larger than 11,000 square metres) triggered by four large earthquakes in the USA, New Zealand, Taiwan and China. The model provides a first-order prediction of the probability of hillslope failure across the region affected by seismic shaking, based on the strength of ground motions and the gradient of hillslopes. Areas likely to have experienced high levels of landslide activity are shown in red, and while areas we expect to be less affected by landslides are shown in green and then blue. Here, landslide probability has been estimated using preliminary ground motion data published by the USGS and hillslope gradients derived from the ASTER global elevation model. Read more
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.
Dr Chris Massey, a researcher with GNS Science in New Zealand is currently visiting the Institute of Hazard, Risk and Resilience. During the 2011 Christchurch earthquake large boulders were unhinged from extremely steep slopes and crashed into houses below (see Christchurch earthquake secondary hazards). In this highly informative and exciting video, Chris presents some of the work they’re doing in studying how large rocks travel down steep slopes.