18 April 2014
British Pathe News was a company that generated short (four minute) films summarising the latest news, which were then shown in cinemas in the UK and elsewhere. The company was active from 1910 to 1970, before being killed off by television news coverage. The archive is clearly a wonderful source of information about world events over this period. In the last few days, British Pathe have uploaded about 88,000 films onto a Youtube Channel. The films are genuinely amazing – I thoroughly recommend a browse if you have an hour or two.
Pathe News and landslides
Unsurprisingly, situated within the archive are various films showing landslides. I will highlight just a small number here:
There is some spectacular footage of the aftermath of a rockslide-induced tsunami in Norway in 1936, which the film indicates killed over 70 people:
There is also a nice piece of footage of the aftermath of a landslide on the Adriatic Coast in Italy in 1956, which knocked a steam train into the sea:
There is some quite dramatic footage of the aftermath of a large rockslide in Los Angeles, USA in 1937:
16 April 2014
The Arroumd Rock Avalanche
An interesting paper has just been published in the Bulletin of the GSA by Philip Hughes and colleagues (Hughes et al. 2014) on the Arroumd Rock Avalanche in the Atlas Mountains of Morocco. This is a very interesting feature, very clearly visible on Google Earth:
The village labelled Aremd, which seems to be more commonly known as Arroumd, is built on the toe of the landslide deposit. The landslide itself have originated from the steep slopes in the background, traveled down the side valley and been deposited in the valley floor and in part in the main valley. The resulting deposit has blocked the main valley to a degree. The image below (from here) shows the landslide deposit and the village from upstream – there appears to be the scar from a breach event on the left side; the boulders that form the avalanche are clearly visible to the right of the town:
This deposit has been discussed by geologists for over 130 years, with most interpretations indicating that this debris was the moraine from an ancient glacier (very often rock avalanche deposits have been interpreted as having a glacial origin). More recently, Hughes et al. (2011) first suggested that the deposit might have a landslide origin; this study involved detailed mapping to investigate whether that hypothesis might be correct.
A complex geomorphic history at Arroumd
The conclusion from the mapping and dating of the deposits is that this is an area with a very complex geomorphic history. Three moraines are present in the valley, but also present are two course, boulder-dominated deposits that are interpreted as having a rock avalanche origin. This suggests at least two major landslide events, which have in turn modified the older moraines. These two major rock avalanche deposits have both been dated to 4,500 years BP (+/-500 years), suggesting that they occurred quite close together in time (which is of course quite common for large rockslope failures).
Unfortunately, there is no simple geomorphic indicator of the likely trigger of rock avalanches, and many such failures occur without a trigger. Thus, any discussion of a likely trigger event for the Arroumd Rock Avalanche is speculative. In this case, Hughes et al. (2014) suggest that the proximity of the nearby, active Tizi n’Test fault may well be the culprit. I think that it would now be really interesting and worthwhile to trench the fault to see if there is a movement event with a similar date.
Finally, Hughes et al. (2014) point out that Arroumd is not the only valley-blocking rock avalanche in this part of the Atlas Mountains – indeed there is an even more spectacular example to the south at Lac d’Ifni, clearly evident on Google Earth:
In this case the landslide has blocked the valley and has not breached, allowing a lake to form. This lake is now mostly filled with sediments, leaving just the small remnant Lac d’Ifni behind.
Hughes, P.D., Fenton, C.R., and Gibbard, P.L., 2011, Quaternary glaciations of the Atlas Mountains, North Africa, in Ehlers, J., Gibbard, P.L., and Hughes, P.D., eds., Quaternary Glaciations—Extent and Chronology, Part IV: A Closer Look: Amsterdam, Elsevier, p. 1071–1080.
Hughes, P.D., Fink, D., Fletcher, W.J. and Hannah, G. 2014. Catastrophic rock avalanches in a glaciated valley of the High Atlas, Morocco: 10Be exposure ages reveal a 4.5 ka seismic event. Geological Society of America Bulletin, doi: 10.1130/B30894.1
14 April 2014
The Dart River (Te Horo) landslide
The Te Horo landslide on the Dart River in New Zealand is the subject of a new report (Cox et al. 2014) that has been prepared by GNS Science that has been made available online. The landslide is fascinating and unusual. It consists of a very large (56 million cubic metre, 0.9 square kilometre) compound rock and debris slide on a steep mountain side. Below the landslide is an enormous debris fan – this has an estimated volume of 100 million cubic metres – which has diverted the Dart River in the main valley below. This image, taken from Cox et al. (2014) provides a panoramic view of the landslide:
The slide caused some problems earlier this year when a period of more intense activity in the landslide led to increased sediment deposition on the debris fan. The volume of material entering the channel was sufficiently large that the fan blocked the river, allowing a large lake to form upstream:
Unusually, this type of valley blocking landslide is likely to pose a low level of hazard. Despite the large lake volume (estimated at 11-15 million cubic metres), the low gradient of the blockage and the presence of large boulders in the deposit means that a rapid breach event is unlikely. Thus, the lake is likely to persevere for some considerable time and may fill with sediment from upstream.
The report notes that the area of activity of the landslide itself has moved from an elevation of 800 – 900 m in 2010-12; 900 – 1200 m in 2013; and 1000 – 1350 m in 2014.
There is a great more detail about the landslide, the debris fan and the lake, and many interesting images, in the report, so do take a look.
Cox, S.C., McSaveney, M.J., Rattenbury, M.S., and Hamling, I.J. 2014. Activity of the landslide Te Horo and Te Koroka Fan, Dart River, New Zealand during January 2014. GNS Science Report 2014/07. 45 pp.
7 April 2014
A large landslide in Jalisco, Mexico
Liveleak has this new video of a large landslide that apparently occurred in Jalisco in Mexico over the weekend. I know no more details, but the impatient driver of the red car was lucky that the landslide did not occur 20 seconds later:
A small landslide at Hannover Point
Meanwhile there is a much better quality video of a somewhat less dramatic landslide at Hannover Point on the Isle of Wight, this time on Youtube:
The On The Wight website has a good description of how this video was collected.
Thanks also to Dr Phil Collins of Brunel University (@PhilCollins_UK) who tweeted that the start of this landslide event is available separately on Youtube:
2 April 2014
The Oso (Steelhead) landslide in Washington
The best news about the Washington landslide is that, as forecast, the number of victims is likely to be rather lower than was feared at one stage (I noted earlier that this might well be the case). The official toll has now reached 28 people known to have been killed, with a further 22 remaining missing. To have recovered 28 bodies to date is an impressive achievement, and this will have been deeply traumatic for those involved in the search and rescue operations.
So now attention, in the media at least, is focused on whether the landslide could have been foreseen. Opinion on the internet varies greatly (no surprises there!), ranging from the view that the risk was tolerably low to the view that it was completely foreseeable. I’ll nail my colours to the mast – to my mind this was foreseeable event, and as such the disaster represents a failure of hazard management.
Using the past as a guide to the future
The simplest way to understand the likely future behaviour of a landslide is to examine how similar landslides have behaved in the past. In the case of the Steelhead landslide, this was very simple. Immediately to the west of the landslide there is another large failure. This landslide is very obvious on the LIDAR imagery. Dan McShane provided an analysis of the Steelhead landslide history from Google Earth images within a day or so of the slide event, and included LIDAR imagery (from before the most recent movement of the Steelhead landslide) that showed this other failure to the west:
In this image the Steelhead landslide is highlighted on the right, and this much larger landslide is highlighted in the centre of the image. Note that there is another landslide scar between these two landslides. The key aspect of this larger landslide is the extensive area that the debris has covered. And look at the morphology of the deposit as shown in the imagery, and in particular the structure of the material that has flowed onto the valley floor, as shown in this BBC/AP image:
So this bluff has a long history of landslides, and the evidence on the ground suggests that they can be very mobile. Note also from the LIDAR imagery the shape of the land above the old Steelhead landslide scar – this promontory does not look like a stable configuration in such a weak deposit.
The defence of the emergency planners (as reported by Andrew Alden) in Washington seems to be that after the 2001 movement event at Steelhead the landslide was mitigated by protecting the toe from erosion by the river. There is little doubt that a large amount of work was undertaken in this respect. This is a Google Earth image of the foot of the landslide in 2003:
And this is the same view in 2007:
Clearly the river was relocated and the toe of the landslide was protected. Note the houses that are visible in the second image that were not there in 2003. However, for these very large landslides it is likely that the failure is driven by groundwater driven processes in the upper part of the slope. Protecting the toe will have done little to prevent this from occurring. Thus, the mitigation did not address the primary concern.
The 2001 landslide left material high on the hillside that was sitting above a scar that was far too steep (see the image in this post). The LIDAR data suggests that the runout from such a collapse could be extensive. In that context I find the decision to build new houses at the foot of the landslide to be very surprising.
28 March 2014
The Oso (Steelhead) landslide – mechanisms of movement
In the last three days 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:
For comparison, I have tried to create a pre-failure view from Google Earth – this uses a Landsat image from last July:
To me the most logical explanation that ties the images and the seismic data together is a two phase movement event, as Kate Allstadt suggested:
The first movement event would have been a very rapid and violent collapse of a lower landslide block. I have labelled the remains of this block, now mostly broken up, on the image below:
This first collapse event would have generated a very rapid mudflow as it collapsed onto the sediments below – this may have caused most of the damage. The best video I can think of to illustrate this initial collapse event is the Po Selim tin mine landslide in Malaysia – although note that in the case of the tin mine the mudflow may have been more energetic due to the possible presence of pools of water in the floor of the quarry:
The violence of the process does, fortunately, suggest that the victims will not have suffered for long.
Recovering the victims
To date 25 victims have been recovered. This might seem low when there may be as many as a further 90 people buried in the debris. However, recovering victims from a mudflow is exceptionally difficult as the remains are likely to be deeply entombed in a material that has very low permeability. For this reason, the authorities may need to decide to preserve the site as a grave. If the search is to be continued, a very detailed mapping exercise will be needed. This will require three key elements:
- Attempts will be needed to identify where on the ground each victim was located when the landslide struck. Were they in their house, in a car on the road, etc. The better the initial location information the better the chances of finding the final resting place;
- The landslide mechanisms will need to be identified in great detail. Detailed mapping of the landslide will be needed to do this – the investigators will need to ascertain whether victims were pushed ahead of the slide, were entrained within it or were buried in situ. In my experience the latter is the least likely in this type of slide, but the patterns will vary across the landslide;
- Detailed mapping of the human debris will be needed. The remains of each structure will need to be mapped out. This will give key information about the dynamics of the landslide at each point, and of course it is also likely that many of the victims would have been close to, or within, buildings.
This is a time-consuming, challenging and sometimes dangerous task, and one that will not necessarily be successful. Generally speaking this level of work has rarely been undertaken, so it would push the boundaries of our knowledge. The authorities will need to balance competing pressures in determining a way to proceed. This may not be an easy, or a popular, choice.
25 March 2014
The Steelhead landslide
The death toll from the Steelhead landslide near to Oso in Washington State is continuing to rise. Latest reports suggest that there are now 14 known fatalities, but 176 people are reported to be missing. It is quite normal in this sort of event for the number of reported missing people to exceed substantially the actual number of victims, so this maximum toll may reduce in the next few days. However, it is still likely to be the costliest landslide in terms of lives lost for many years in the USA.
Details are slowly emerging of the landslide history of this site. It is clear that major landslides have occurred here on many previous occasions; indeed so much so that the landslide is known as either the Hazel landslide or the Steelhead landslide; at this stage I am opting for the matter given that the inundated area is known as Steelhead Drive.
Better images of the Steelhead landslide
I very much appreciate the help that numerous people have given me over the last few days to put together this post – too many to name, but thanks to you all. The best graphic that gives an overview of the slide is in the Seattle Times:
The original version has a very impressive slider function that allows the user to flip from one image to the other. I can’t replicate that, but putting the two images side-by-side shows the extent of the devastation. The number of inundated houses is large, suggesting that the loss of life will be high, especially bearing in mind that the slide occurred on a Saturday.
The best set of aerial images of the slide are on the Flicker page of Governor Jay Inslee – there are some wonderful images there. This image shows the source of the landslide:
Whilst this image from the same source shows the entire landslide mass.
The landslide has been widely reported as a mudslide. In terms of the lower portion, which did the damage, this is correct, although in places it might have been more of a mudflow than a mudslide. However, the upper portion is a rotational landslide – the rotated block with the fallen trees is very clear. A working hypothesis would be that this block failed catastrophically, transferring load onto the block below, which in turn generated very high pore water pressures, causing fluidisation and a very rapid mudflow that struck the settlements across the river.
The history of the Steelhead landslide
The Yakima Herald has a very nice article that details the chronology of events on the Steelhead landslide. This includes:
- 1949: A large landslide (1000 feet long and 2600 feet wide) affected the river bank
- 1951: Another large failure of the slope; the river was partially blocked
- 1967: Seattle Times published an article that referred to this site as “Slide Hill”
- 1997 report, by Daniel Miller, for the Washington Department of Ecology and the Tualialip Tribes
- 1999: US Army Corps of Engineers report by Daniel and Lynne Rodgers Miller that warned of “the potential for a large catastrophic failure”
- 25 January 2006: large movement of the Steelhead landslide blocked the river
There is a good presentation about the 2006 landslide available online (NB pdf). This includes the following (somewhat blurry) image of the source of the 2006 landslide:
The slope that formed the scarp of the 2008 slide was undoubtedly very over-steepened, and of course was formed from weak materials. This looked like an accident waiting to happen.
24 March 2014
Oso landslip costs
Unfortunately the toll from Saturday’s Oso landslip is rapidly mounting. Latest reports suggest that at least eight people have lost their lives and that up to 18 more might be missing in the debris. Unfortunately, the site remains very dangerous, such that substantial areas have yet to be entered. This will be the worst landslide in the USA for many years. The last event on a similar scale of which I am aware was the 25th December 2003 debris flow in San Bernadino County, California, which killed 16 people. It looks likely that this landslide will be worse.
Oso landslip resources
Whilst I am referring to this as the Oso landslip, in fact it is a reactivation of an existing landslide, known as the Hazel Landslide. This landslide is known to have moved 1988, and went through a second phase of movement in 2006. It is well described in a blog post from 2009 that can be found at: https://slidingthought.wordpress.com/tag/north-fork-of-stillaguamish/
You can find a geological map of the area here: http://www.dnr.wa.gov/Publications/ger_ofr2003-12_geol_map_mounthiggins_24k.pdf
The landslide did occur in glacial sediments, as I indicated might be the case yesterday. There are some excellent resources on the landslide at the following three blog posts:
23 March 2014
A very large landslide occurred yesterday morning near to the town on Oso in Washington State, USA. Unfortunately, three people have been killed by the Oso landslide, and newspaper reports suggest that three survivors, including a six months old boy, are in a critical condition, and that two more people have been seriously injured. At present it is not clear as to whether there may be other victims in the landslide debris.
The best set of images of the slide is to be found on the Seattle Pi website, which includes this overview shot:
The landslide is complex, but appears to have occurred in weak sedimentary deposits; I would guess with a glacial origin. The landslide appears to have a large arcuate scar with a large, rotated and partly disaggregated central block:
The toe of the landslide appears to have fluidised and flowed laterally (i.e. up and down the valley), suggesting that the landslide would have been rapid and highly destructive, which accounts for the fatalities:
There can be little doubt that this is a rainfall triggered landslide, though given its size there might have been a substantial time gap between the triggering event and the slide itself whilst pore pressures built up. An interesting aspect of the landslide is that the valley is now blocked. National Weather Service Seattle tweeted the gauging station data for the north fork of the Stillaguamish River downstream from the landslide:
The very rapid decline in water depth is very clearly apparent, but note also the short but dramatic spike in water depth immediately after the landslide, presumably caused by a surge of water induced by the slip entering the river. A key management task over the next few days is likely to be the creation of a bypass channel to reopen the river.
Thanks to John Garver, Lee Allsion, Peter Weisinger and Bryan O’Sullivan for helping me to track down the material for this post.
21 March 2014
The Randa Rockslide
The Randa rockslide in Switzerland is probably one of the best known and most intensively investigated landslides in the world – so well known that it even has its own wikipedia page! The main action occurred in two collapse events in April and May 1991:
The Wikipedia page summarises the events well:
The 1991 rockslides at Randa consisted of two separate collapse events on April 18 and May 9, which released in total a cumulative volume of approximately 30 million cubic meters of rock. The elevation of the top of the scarp is 2320 m (7610 ft), while the elevation of deposit toe is 1320 m (4330 ft).
Accelerating occurrences of small rockfalls from the cliff in the decades preceding the slides gave indication of deeper movements, and fallen debris had eventually destroyed much of the forest beneath the cliff (Sartori et al., 2003). Precursory events noted immediately prior to the April, 1991 rockslide included explosive ruptures of rock slabs and new forceful water discharges from the face (Schindler et al., 1993).
April 18, 1991: This primary rockslide event occurred over the span of a few hours time, producing a large steep debris cone and a thick layer of dust over the valley. The rockslide consisted of a progressive succession of smaller collapses and block failures involving first the lower and more competent orthogneiss, followed by retrogressive collapse of the highly-jointed paragneiss above (Schindler et al., 1993). The total volume released during this rockslide phase was estimated to be 22 million cubic meters. Had this volume been released instantaneously, a devastating rock avalanche and far reaching deposit would have resulted. A lesser failure followed on April 22.
May 9, 1991: Monitoring of deformation and microseismic activity led to accurate anticipation of this follow up rockslide event. The rockslide again occurred in a progressive manner over the course of a few hours, involving many small volume collapse events mostly within the upper paragneiss material (Schindler et al., 1993). These failures resulted in retreat and reduced the inclination of the upper part of the rockslide scarp. The total volume released in this second phase was estimated to be 7 million cubic meters.
No one trigger can be conclusively assigned as responsible for the Randa rockslides of 1991. The area has experienced a long history of moderate seismicity, but no significant earthquakes immediately preceded the failures. A warm period producing ample snow melt occurred in the days prior to the April rockslide, and water could be seen emanating from springs on the rock face. Further, a period of rapid cooling occurred just one day before the April slide. However, it is unknown if this series of events combined to act as an exceptional trigger, or if they were rather part of the normal seasonal climatic and hydraulic cycles (Sartori et al., 2003).
Since the 1991 failures, the site of the rockslide has been investigated in detail by the brilliant Engineering Geology team at ETH led by Simon Loew.
A video of the April 1991 Randa landslide
The reason for posting this is that a new video has appeared on Youtube that shows the 1991 collapse event as it occurred. The quality of the video is not so good, but the content is truly amazing:
This illustrates beautifully the retrogressive nature of this type of collapse (i.e. it occurs sequentially rather than all in one go), and the way that a collapse on this scale means that the debris starts to behave in a way that at least superficially looks like a fluid.