This blog provides a commentary on landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings. The blog is written on a personal basis by Dave Petley, who is the Wilson Professor of Hazard and Risk in the Department of Geography at Durham University in the United Kingdom.

This blog is a personal project that does not seek to represent Durham University.

Wednesday, 25 February 2009

A new mechanism for landslide initiation

ResearchBlogging.orgIt is not often that one reads a paper and finds a new and exotic landslide mechanism being suggested. I was somewhat surprised yesterday to find that in a paper just published in Geomorphology, Steve Evans and his co-authors have done just that. Although it requires further research, the mechanism is intriguing and undoubtedly has some very interesting implications for glacial hazard management as well.

The origin of the theory is the extraordinary Kolka Glacier landslide of 22nd September 2002. There is quite a nice basic description of this event at here. In a nutshell, this was a catastrophic collapse of the Kolka Glacier in the Genaldon Valley, which is located in the Caucasus Mountains in the Republic of North Ossetia, part of the Russian Federation. A catastrophic debris avalanche swept down the valley (Fig. 1) at velocities of up to 65 m/sec. The flow travelled a total of about 19 km, transitioning to a debris flow when it hit a narrowing of the valley. In all, about 125 people were killed.

Figure 1: This pair of ASTER images, from NASA, taken before and after the collapse, shows the vast extent of the disaster. Debris and ice filled the Genaldon Valley from the Kolka Glacier Cirque to the Gates of Karmadon—a distance of about 18 kilometers . (Images by Robert Simmon)

The controversy about this event comes in trying to determine what initiated this massive landslide. Two theories have been postulated:
  1. The simple one is that the flow started because a large rockfall detached from the mountain behind and fell onto the glacier. This instantaneously loaded the ice, inducing massive pore pressure increases through the mechanism known as undrained loading. In consequence the resistance to movement rapidly decreased, and the flow started. This is the model proposed by Huggel et al. (2005).
  2. The alternative, rather minority view, is that the event was a conventional glacier surge that rapidly developed into a full failure (see Kotlyakov et al. 2004). Whilst glacial surges do occur, the rates are usually much lower than was observed here.

Figure 2: NASA image, hosted by Wikimedia, showing the source area of the glacial surge

This paper suggests a third model. Most importantly, they propose that there is no evidence that the flow was initiated by a large rockfall event. The authors have looked at satellite imagery immediately before the event, which suggests that there were smaller scale failures occurring almost continually in the summer months of 2002. This is supported by observations by mountaineers at the site. Perhaps most importantly, they have obtained a Landsat ETM+ image that was collected on 20th September 2002, 8.5 hours before the failure event. Comparison with post event imagery suggests no major differences (Fig. 3), suggesting that a large failure did not occur.

Fig. 3: This is Fig. 4 in Evans et al. 2009, captioned: Fig. 4. (A). Landsat ETM+ satellite image obtained 20 September 2002, 11:31 am (local time); Kolka Glacier (1) is covered by new snow, with a very fresh and large (0.17 km2) debris trail (2). Also note exposed bed (3) of the former hanging glacier that entirely collapsed between 19 August and 20 September 2002, a pronounced shadow (4) indicating a 50-m-high margin of a northward glacier surface rise, and another shadow (5) of a high ice cliff where Kolka Glacier has already started to deform 8.5 h before the catastrophic detachment at about 20:05 h local time. (B) QuickBird image taken on 25 September 2002 (©2007 Google™, 2008 DigitalGlobe). Note that there is very little difference in morphology of the mountain slope above the Kolka glacier (arrow), compared to the image of 20 September 2002 in (A).

However, it is clear that the glacier itself shows signs of extensive deformation in the pre-failure image. Most notably, they observe that a 50 m high ice cliff had develped on the glacier (at point 4 on Fig. 3A), suggesting that extensive movement was occurring. Evans et al. (2009) therefore suggest that
the Kolka Glacier started to deform in response to loading from ice and debris. This disrupted the internal drainage of the glacier, triggering the development of excess water pressures at the base of the ice body, which in turn triggered a catastrophic decrease in effective stress and thus an almost complete loss of frictional resistance at the base of the glacier. As a result the glacier detached from its bed (85-175 m below the surface). As the upper part of the ice slide it loaded the lower portion of the glacier, which also began to move through undrained loading.

The theory is supported by seismic data, which do not show a signal that could be interpreted as the impact of a large rockfall (these are usually picked up by seismic stations).

Analysis
So how likely is this? Well, technically it is undoubtedly feasible. However, two things should be noted. First, the authors have not provided any numerical analysis to support the hypothesis that very high basal fluid pressures can be generated in this way. It would be interesting to try to model this to see if it can happen in reality, although this modelling is far from easy. Second, the analysis is based upon post-event reconstructions and the use of pre-event satellite imagery whose resolution is really not good enough. It is still possible that a rockfall occurred that can't be resolved on the imagery - vertical images with comparatively low resolutions are not good at observing vertical rock faces. It could be that a hybrid model is appropriate - i.e. that the massive glacial deformations observed created the conditions that allowed a much smaller rockfall to initiate failure.

If the authors are right, and they may well be, then this will change the way that we view hazards associated with surging glaciers. Given the changing dynamics of glaciers associated with anthropogenic climate change (global warming) (see here for example), this will become increasingly important.

Main Reference
Stephen G. Evans, Olga V. Tutubalina, Valery N. Drobyshev, Sergey S. Chernomorets, Scott McDougall, Dmitry A. Petrakov, Oldrich Hungr (2009). Catastrophic detachment and high-velocity long-runout flow of Kolka Glacier, Caucasus Mountains, Russia in 2002 Geomorphology, 105 (3-4), 314-321 DOI: 10.1016/j.geomorph.2008.10.008

Other references
C. Huggel, S. Zgraggen-Oswald, W. Haeberli, A. Kaab, A. Polkvoi, I. Galushkin and S.G. Evans, 2005. The 2002 rock/ice avalanche at Kolka/Karmadon, Russian Caucasus: assessment of extraordinary avalanche formation and mobility, and application of QuickBird satellite imagery, Natural Hazards and Earth System Sciences 5 (2005), pp. 173–187. This paper is avilable online for free here.

V.M. Kotlyakov, O.V. Rototaeva and G.A. Nosenko, 2004. The September 2002 Kolka Glacier catastrophe in North Ossetia, Russian Federation: evidence and analysis, Mountain Research and Development 24, pp. 78–83.

Monday, 23 February 2009

A few items of interest

You may have noticed that the number of posts on this blog has dipped of late. My apologies for this - we learnt just before Christmas that my seven year old son Adam needed open heart surgery, which was undertaken earlier this month. I am glad to say that it was a complete success and he has today returned to school, so normal service will hopefully be returned.

So, I thought I would start off with a short post highlighting some interesting stories that have emerged over the last few weeks:

The aftermath of the Gonaives mudslides in Haiti
There is a short but very interesting (great for teaching) video on the World Focus website that describes the legacy of the mudslides last summer in Gonaives in Haiti. An earlier post on this problem is available here.

Wenchuan (Sichuan) Earthquake: 18,000 households to be relocated to avoid geological disasters
China Daily reported a couple of weeks ago that 18,000 households are having to be relocated over the next three years to avoid geologically-related (i.e. landslide and sediment) disasters, at a cost of $166 million. The article also gives some interesting data on the scale of the problem., noting that the province "will establish a monitoring network for some 3,695 at-risk sites". They note that "8,061 sites with potential geological disasters were found in 39 county-level areas in Sichuan after the quake, threatening the safety of 158,000 households with a population of 640,000 people, and assets worth 30 billion yuan." Finally, the article notes that "the effects of potential geological disasters triggered by the quake will last for ten years".

What happens when a small rockfall hits a car
Meanwhile, the Manawatu Standard in New Zealand (I am sure that you never miss a copy....) carried an interesting story about a car being hit by a rock in Manawatu Gorge on North Island, with the following picture of the damage:

It is fortunate that there were no passengers in the car at the time.

Increasing rates of erosion in northern India
There is also quite a nice article on an Indian news site, Samay Live, about increased rates of erosion in Himachal Pradesh in N. India as a consequence of landslides. The article notes that:
"Expressing concern over large-scale soil erosion in Himachal Pradesh mainly due to landslides, Chief Minister Prem Kumar Dhumal said today said the government has initiated several measures for soil treatment. Dhumal said according to scientific surveys, the soil of the state has the capacity to bear erosion of only 10 tonnes of soil per hectare but this erosion has unfortunately risen to between 16 tonnes to 40 tonnes per hectare at present...The state's about 53.80 per cent soil is affected by erosion, out of which the impact on 34 per cent soil is so immense that it was on the verge of losing its fertility."

Thursday, 19 February 2009

A new landslide video from Bolivia

I have come across another video of a landslide from Bolivia. I suspect that this is the recent slide in La Paz, as per the following report from The News:

"At least 300 people have been injured in the Bolivian city of La Paz following a serious mudslide, Mayor Juan del Granado said Tuesday. "The (movements) are still ongoing but we are talking about more than 60 families affected and 300 people injured," del Granado said. Until now we are talking about more or less 50 houses that are affected," he added. The landslide affected an area of four hectares and was caused by "the presence of subterranean water, which lubricated a level of earth" according to Johnny Bernal, a municipal expert. No fatalities have been reported"

The video is a news report Carried on Live Leak. It is available here, or you should be able to view it below:



Some pretty dramatic imagery of the impact of landslides in urban areas.

Monday, 16 February 2009

Seismic frescoes - possibly the oddest earthquake induced landslide story of all time?

There is little doubt that the Wenchuan (Sichuan) earthquake in May generated a huge number of shallow rock slides. A very odd story has now appeared in the Chinese media in which it is observed that the rock slide scars appear to form frescoes (wall-mounted murals) on the mountain sides. So, in the words of the People's Daily Online "Although the massive Wenchuan Earthquake caused tremendous damage, it has also brought new natural wonders - a large number of seismic frescoes that were formed on cliffs from Yingxiu to Luobuzhai by landslides during the earthquake."

So what do these seismic frescoes look like? This one apparently resembles "waving reeds" (images courtesy of People's Daily Online):

This one is I think meant to be a cockerell crowing (the caption in the original article is a little bizarre):

And finally, this one is apparently "a girl playing the violin" (I am struggling to see this one, I must admit).



The article implies that there are many more, consisting of "human figures, animals, plants, as well as legends of ghosts and monsters. The majority of the frescoes depict various kinds of human figures."

The article finishes by noting that this phenomenon could be developed for tourism, although I suspect that the combination of enlargement of the scars during rains and revegetation during the rest of the year may render this difficult. I have never heard of landslide scars being interpreted in this way - landslide scars as art!

Tuesday, 10 February 2009

Landslide on the Mahakali River on the Nepal / India border

Thanks to both David Hopkins and Ripendra for drawing my attention to a landslide on Friday on the Mahakali River on the border between Nepal and India. Although it has occurred in a remote area, it is interesting because the landslide appears to have partially blocked a fairly large river coming down from the high Himalayas.

The landslide occurred in the Chautuldhar (Chetalkot) area of Rephalikot, close to Tawaghat in Dharchula, right on the border of Nepal (to the east) and India (to the west) (see Google Earth image below - click on the image for a better view).

The reports suggest that cracks were first seen on Thursday, which led to a large scale failure on Friday. The location is shown on the Google Earth perspective view below (the actual location of the landslide appears to be in a small area of low resolution imagery):


Reports suggest that the river is flowing through a gap of just 2.5 metres, which has created a barrier lake that is threatening a number of villages upstream. The main road is blocked and may take as much as a month to reopen, and there is a threat of further landslides.

An interesting dimension to this is the proximity of the Dhualiganga I hydroelectic scheme. I have annotated the diagram above to show the location of the dam and the power house. This is a run of the river scheme in which the Dhauliganga River has been dammed with a 56 m high concrete faced rock fill dam to create a head of water. The water is then passed through a tunnel beneath the mountain to the powerhouse, where turbines are located. The project was completed in October 2005. Some newspapers are speculating that the landslides may have been associated with the construction of the tunnels as the failure occurred in the dry season with no obvious trigger. However, the tunnel is 6 km from the landslide site, so this is not likely. However, if a large lake were to form at the landslide site, and then was to be released rapidly, then the powerhouse would be threatened. It is not clear to me whether this is a possibility, but given that the river is now reportedly flowing again I suspect the chances at present are low.

As an aside, the dam site itself appears to be quite interesting from a slopes perspective:

Monday, 9 February 2009

Australian wildfires and risks of increased erosion rates

ResearchBlogging.org(Updated 10th Feb to include latest casualty numbers)

The extraordinary wildfires in Australia are dominating the headlines in the UK, half a world away. Wildfires are quite common events, but the number of fatalities that this particular episode has caused is really quite unusual. Below in Figure 1 I have plotted the recorded worldwide recorded number of deaths from wildfires for the period since 1980, using data from the CRED EM-DAT database . I have added the (updated to) 173 reported deaths from this event so far as an extra column, although note that reports suggest that this total may rise substantially:

Figure 1: Global numbers of reported fatalities from widlfires, based upon the CRED EM-DAT database. The 2009 value is the reported number of deaths from the Austrlian wildfires.

The average annual global total number of deaths is 59.5 fatalities per annum. Care is needed in the interpretation of the above as CRED only record events that kill ten or more people, thus these values consistently underestimate the true toll, but nonetheless the unusual impact of these events is clear.

In the context of this blog it is also interesting to think through the likely long term impact of these fires in terms of erosion and landslides. A recent paper by Smith and Dragovitch (2008) looked at the long term consequences of wildfires in SE. Australia. These two researchers have published extensively on sediment production and erosion in Australia, so have a strong pedigree.

The study focused on a fire that occurred in January 2003 during a drought in the Snowy Mountains near to Thredbo (Fig. 2), a sub-alpine environment. The study used erosion pins to monitor surface level change on both burnt and unburnt hillslopes over a period of 795 days after the fire.


Figure 2: Google Earth perspective image of the area around Thredbo, the location of the study reported by Smith and Dragovich (2008)

The study showed that after the fire the areas that had been burnt suffered a net loss of soil representing an average of 3.8 mm of material, with the most intense erosion occurring on the lower slopes (Fig. 3). On the other hand, the unburnt areas saw a net accumulation of soil of an average of 2.6 mm, again with the greatest accumulation at the lower slopes.

Figure 3: Mean net soil loss and gain for burnt and unburnt areas as reported by Smith and Dragovich (2008).

Thus, the burnt areas clearly suffered a net loss of material in the aftermath of the fires. The study showed that this loss of soil declined with time after the fire, with a slight increase again during snow melt, presumably as vegetation re-established. However, these values are perhaps surprisingly low compared with those recorded in other environments, especially in N. America, given the steep slope angles seen in Figure 2. Interestingly, Shakesby et al. (2007), who studied post-fire erosion in Eucalyptus forests in SE. Australia, came to similar conclusions, stating that "except under extreme post-fire rainfall conditions, present-day wildfires affecting south-east Australia seem to be less potent in geomorphological terms than might be expected given the severity and frequency of the wildfires". They attribute this to the rapid rate of plant growth in the aftermath of fires plus the resistance of the soil to erosion.

The conclusion is therefore that although the fires have devastated vast areas, and made thousands homeless, there should not be a serious increase in erosion in the burnt areas. This will help greatly in the post-fire recovery of the burnt areas.

References:
H SMITH, D DRAGOVICH (2008). Post-fire hillslope erosion response in a sub-alpine environment, south-eastern Australia CATENA, 73 (3), 274-285 DOI: 10.1016/j.catena.2007.11.003

R SHAKESBY, P WALLBRINK, S DOERR, P ENGLISH, C CHAFER, G HUMPHREYS, W BLAKE, K TOMKINS (2007). Distinctiveness of wildfire effects on soil erosion in south-east Australian eucalypt forests assessed in a global context Forest Ecology and Management, 238 (1-3), 347-364 DOI: 10.1016/j.foreco.2006.10.029

Friday, 6 February 2009

Name the landslide 3

Time for a bit of Friday fun, that is Name the Landslide round 3. This one has been set be Peter Weisinger, winner of round 1. The challenge is, as ever, to identify the landslide shown in the Google Earth images below and to provide a brief description, suing the comments below. Do feel free to use the comments section to work towards a team solution.

Peter has very kindly provided two perspectives:


Over to you...

Dave

Another landslide video - car hit by landslide in Japan

There has been an extraordinary flurry of landslide videos of late (e.g. here) presumably reflecting both the high occurrence of landslide events and the ubiquity video sharing sites. Another has appeared this morning - this time from Japan. The video should be embedded below or can be viewed here.

Watch more LiveLeak videos on AOL Video



I guess this is not as spectacular as for example this one, this one and this one, but is interesting nonetheless. The occupants of the car were lucky that the slide did not engulf the car or push it off the road. One interesting aspect is the way that the car was bulldozed rather than being buried, which is a useful observation when we think about the rescue of survivors.

Does anyone know where in Japan this happened? The date on the video appears to be 31st January.

Wednesday, 4 February 2009

Virtual field trip - the Marcus landslide in Arizona

The Arizona Geological Survey have created a rather nice virtual field trip and very useful virtual field trip about the Marcus landslide in Arizona. It is available here:

http://www.azgs.az.gov/MarcusLandslide_2008.shtml

The slide is a 500,000 year old rock slide that has has a length of about 2 km from scarp crown to deposit toe (see Google Earth image below) and a deposit volume of about 5.25 million cubic metres. The elevation difference is about 500 m.


Two minor criticisms from me, though:
  1. The site does not give the lat/long of its location, which makes looking at it on Google Earth rather more difficult than it ought to be. The location of the crown is: 33°40'47.27"N, 111°48'1.31"W. The deposit extends almost due east from this point;
  2. The site states that "Poised for collapse, a heavy rain, a bolt of lightning, or an earthquake could have spontaneously triggered". It is highly unlikely that lightning would trigger failure.
Finally, there is a paper in Geomorphology describing the landslide in more detail, including its dynamics, failure and age. It is available for downloading as a pdf here. There reference is:

Douglass, J., Dorn, R.I. and Gootee, B.F., 2004. A large landslide on the urban fringe of metropolitan Phoenix, Arizona. Geomorphology, 65, 321-336. doi:10.1016/j.geomorph.2004.09.022

Sunday, 1 February 2009

(updated) Name the landslide 2

Updated with a hint below

The winner of the Name the Landslide 1 hasn't yet been able to suggest round 2 (the invite is still open), but in the meantime here is one to think about. As before, the challenge is to name the landslide as shown on Google Earth and to briefly describe the main movement event. Trickier this time...

Please post your suggestions in the comments section.

Hint: the landslide was triggered in 1999, along with about 22,000 others.