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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):

image1

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):

rainfall

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:

image5 Read the rest of this entry »

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

Read the rest of this entry »

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 »

Regular readers will know that the lack of attention that is paid to potential and actual landslide impacts during earthquakes in upland areas is a real hobby-horse of mine.  Time and again we see the situation in which there is a lack of preparedness for landslides, causing huge disruption to the response and recovery operations, even though the threat was entirely forseeable.  It is pleasing to see increased interest in the science of this issue in recent years, with a succession of good papers exploring both the mechanics of the landslide process (which is a very complex problem) and the likely occurrence of landslides. 

This week, a paper has been electronically released on the BSSA website, to appear in a forthcoming edition of the journal, which examines the likely impact of landslides in the event of a Mw=7.0 earthquake of a “Seattle earthquake” – i.e. a quake on the Seattle Fault in Seattle, Washington.  The paper, Allstadt et al. (2013) uses synthetic broadband seismograms to model shallow landslides in the area likely toi be affected by such an earthquake.  Such an analysis is complex and computationally extremely intensive.  I should also note that the technique uses the so-called Newmark method to model the slope behaviour.  Newmark is basically the best technique that we have at our disposal at present, and so the team were right to do this, but in my view it is somewhat deficient in terms of the ways in which it models slope behaviour.  We need a better technique; the trouble is that at the moment we do not have one. Read the rest of this entry »

vajont

Wednesday of next week will mark the 50th anniversary of the Vajont disaster, which occurred on 9th October 1963.  This was of course the worst landslide disaster in European history.  In addition it marked a watershed in a number of areas, not least landslide management and the development of large dams.  On Thursday Michele and I will set off from the UK to attend the conference to mark the anniversary – the organisers were kind enough to invite me to give one of the keynote addresses, which will be on the topic of fatal landslides and large dams over the last decade.  I will make it available online in due course.

In the meantime, I thought it would be useful to provide a list of some of the key resources that are available on the dam and the landslide disaster: Read the rest of this entry »

The M=6.6 earthquake in Gansu province this week killed about 100 people in a poor area of China.  Gansu is earthquake-triggered landslide country – the great M=8.5 earthquake of 1920 killed between 70,000 and 200,000 people, many of them in huge seismically triggered flowslides that buried whole towns.  So, even though the event this week was by comparison a small earthquake, it is unsurprising to find that landslides have been a significant issue.  News from the earthquake-affected area is scarce, but the Big Picture, the photo section of Boston.com, has a wonderful gallery of images from the area.  The best of these shows two spectacular flowslide failures:

13_07-Big-Picture-2

Courtesy of the Big Picture.

Other images show the aftermath of the landslides; this one for example has blocked a road: 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:

china

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 »

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