24 March 2017
The Mitchell Creek landslide in British Columbia, Canada
The Mitchell Creek landslide is a very large, complex failure that has developed in response to the retreat of the Mitchell Valley Glacier in British Columbia, Canada. In a paper just published in the journal Landslides, Clayton et al. (2017) describe how this very complex and large (more than 1 km wide and 1 km long) landslide started to develop sometime between 1956 and 1972 as the slope was debuttressed due to the loss of ice at the toe. Since then parts of the landslide have moved by 40 metres or more.
The Google Earth image below, collected in 2010, shows the landslide. The location is 56.525, -130.233 if you want to take a look. Note I have had to change the contrast in the image to make it easier to see. The Mitchell Glacier is at the toe of the slide, whilst tension cracks are clearly evident on the slope behind.:-
In many ways though this landslide is easier to understand from the side, as this Google Earth image shows:
The key to this landslide has been the retreat of the Mitchell Glacier, which can be seen in the image above. As the ice has melted the slope has been debuttressed – a well-known mechanism – and has started to deform. Clayton et al. (2017) have identified three major zones in the landslide – at the rear a sliding zone (in the images above the huge tension cracks are clearly visible), which is moving at about 20 cm per year on average. At the front there is a zone of toppling, which is moving much faster – about 80 cm per year. Between the two is a transition zone, in which movement rates are about 33 cm per year and in which the blocks are showing evidence of subsidence. Clayton et al. (2017) have used this to generate a really nice conceptual model of the landslide, below, which demonstrates in a very straightforward way the manner in which this landslide is developing:
If you compare this model with the Google Earth images above then the behaviour of the landslide becomes clear. It will be fascinating to see how this behaves in the next few years.
Clayton, A., Stead, D., Kinakin, D. et al. 2017. Engineering geomorphological interpretation of the Mitchell Creek Landslide, British Columbia, Canada. Landslides doi:10.1007/s10346-017-0811-1
22 March 2017
Comet 67P: a large landslide on another world
In a paper published in the journal Nature Astronomy (Pajola et al. 2017), Maurizio Pajola and a cost of dozens have recorded a large rockfall event occurring on Comet 67P / Churyumov-Gerasimenko. This event was recorded by the European Space Agency’s Rosetta mission over the period between 2014 and 2015.
The event started when, in September 2014, the team spotted a large (70 m long) fracture at the top of a cliff now known as Aswan:-
Pajola et al. (2017) report that on 10th July 2015 Rosetta captured a large plume of dust emanating from this area; and on 15th July 2015 a new image of Comet 67P showed that the rock slope had collapsed.
The volume of the rockfall is estimated to have been about 22,000 cubic metres, from a cliff that is 134 metres high. Before and after images of the rockfall site reveal an arcuate scar and a boulder field below:
Pajola et al. (2017) suggest that that collapse was driven by thermal changes in the rock mass, which is considered to be deeply fractured, as Comet 67P moved closer to the sun. However, the actual failure occurred at night, when the calculated thermal gradients were low. I wonder therefore if this is the first time that progressive failure has been observed on another body in the solar system?
In astronomical terms, the main interest here is that the rockfall mechanism provides an explanation for the outbursts of dust that have been observed to issue from the surface of Comet 67P.
Pajola, M. et al. 2017. The pristine interior of comet 67P revealed by the combined Aswan outburst and cliff collapse. Nature Astrononmy 1, 0092 (2017).
21 March 2017
Longdendale: dam building in an area of ancient landslides
Last week I was in a taxi being driven from Manchester airport back to Sheffield along the A628 road across the Pennine hills in northern England. As we were passing through the hills I was admiring the impressive landslides on the hillside above the Longdendale dams. These landslides can be seen on the southern (right) side of the valley in the Google Earth image below:
These dams were constructed between 1848 and 1877 to provide water to the city of Manchester to the the west. These are early Victorian puddle core dams. With hindsight building such structures in an area of such obvious landsliding was courageous (though of course that was not known at the time); unsurprisingly the project hit major problems (which explains the very long construction time). To provide an illustration of the scale of the landslides, this is a Google Earth image of the Woodhead dam and reservoir, which is the highest of the sequence, with the spectacular ancient Lawrence Edge landslide on the slope above:
A book published by E.A. Labrum entitled “Civil Engineering Heritage: Eastern and Central England” provides a lovely description of the problems encountered with these dams. This book is available online. The first of the Longdendale dams to be constructed was Woodhead, shown above, which in the words of E.A. Labrun:
“…had geological problems in the form of the Lawrence Edge landslide on the south slope of the valley and faulted rock, heavily fissured, on the north side”
Whilst the dam was completed in 1851, it suffered severe leakage problems, requiring a new dam to be constructed. Downstream further problems with landslides were encountered during the construction of the Rhodeswood dam (this can be seen on the first Google Earth image above, but in this case I suspect the landslide is on the north side of the valley). The landslide was ultimately stabilised with drainage adits.
These dams and slopes are of course actively monitored now, and show no sign of instability. Further up the valley, the Millstone Rocks landslide is considered to be active but not dangerous.
20 March 2017
La Pintada landslide in Mexico: a new paper that links the landslide and the cultural history
In September 2013 a large landslide occurred at La Pintada in Mexico, killing 86 people. This landslide, which was triggered by Hurricane Manuel, is considered to be the worst landslide event in recent times in Mexico. I blogged about this landslide at the time, highlighting both the initial reports and the more detailed information. In a paper recently published in the journal Landslides, provide a detailed description of the landslide. They have found that the landslide had a volume of about 125,000 m3 and occurred on a slope with a long history of instability. The local community is considered to have a high level of social exclusion; the majority of the population had not completed primary school and had no access to health services. Almost 12% had no toilet facilities. Thus, this was a community with a high level of vulnerability to natural hazards. et al. (2017)
Google Earth has a decent image of the aftermath of the landslide and the extensive work that has been undertaken on both the slope and to rebuild the community:
But the most interesting aspect of the paper is an analysis of the cultural history of the area in relation to landslides. In the paper, note that the La Pintada slide was a reactivation of an ancient landslide complex, with a likely movement event in the 1940s but a history that extends back much further. The name La Pintada means graffiti, deriving from the presence in the village of a large boulder (which itself is probably derived from an earlier landslide) that bears several petroglyths (images carved on the rock). et al. (2017) describe these carvings as follows: et al. (2017)
The repertoire of representations of the petroglyphs is reduced to a set of repetitive geometric forms mainly circles with different designs (concentric, connected with lines and divided with internal radial lines), rectangles with inner divisions, joined ellipses, networks and parallel wavy lines. There is only one figurative element, a zoomorph motive seen in profile: Its body is formed by wavy lines that connect to an ovoid head containing an eye, mouth and a pair of antennas topped with a circle, apparently a horned snake.
This for example is the animal described above, taken from the paper:
et al. (2017) note that the circles and wavy lines are generally considered to represent water, whilst the horned snake is likely to represent an element of the landscape. In petroglyths in Mexico the snake is known to represent water, or to represent the processes that cause catastrophic flows in steep catchments (i.e. debris flows). There is extensive documentation that in Latin America the snake can represent landslides as well, for example:
For the Chortí people of Guatemala, chicchanes are divinities who can appear as humans, huge snakes or in a form that is half human and half feathered serpent. At the beginning of the rainy season, the chicchanes, in the form of snakes, descend from the hills and enter watercourses, driving up their volume by water displacement. If many chicchanes swim downstream at the same time, the waters overflow and cause flooding. If a chicchan leaves a river and moves by land to reach another stream, her long body, snaking down the slopes of the hills, rolls away rocks and mud downslope, causing landslides (Wisdom 1961).
Alcántara-Ayala, I., Garnica-Peña, R.J., Domínguez-Morales, L. et al. (2017). The La Pintada landslide, Guerrero, Mexico: hints from the Pre-Classic to the disasters of modern times. Landslides. doi:10.1007/s10346-017-0808-9
17 March 2017
Koshe, Ethiopia: the worst garbage dump landslide in recent years
The death toll from the terrible garbage dump landslide at Koshe in Ethiopia on Saturday is now known to have reached 115, with fears that more bodies may remain buried. The landslide occurred on Saturday when a section of the dump collapsed onto a slum built at the toe of the slope. The majority of fatalities are reported to be women and children. The most-informative set of images can be found in this CBS News article from a few days ago, including the image below of the crown of the landslide, which suggests that the crown of the landslide has a rotational component:-
From this image the landslide looks to be a comparatively simple failure in an oversteepened slope. The image below, also via CBS News, shows the rescue operations further down the slope, which indicates that the lower reaches of the landslide may have behaved as an earthflow. Again, this is not unexpected:
Africa Review has a good article on the possible causes of the landslide:
The people of Koshe lived in squalor of a degree that is uncommon in the city of four million people. Last year, the government tried to close the landfill and move it to a new location, but opposition from people living near the new site forced authorities to reverse their decision.
Addis Ababa’s construction boom didn’t leave Koshe untouched; a biogas plant is being built on top of the rubbish dump. Koshe residents who spoke to AFP blamed the landslide on the facility’s construction. They said bulldozers that packed down soil to make way for the new plant destabilised the hillside.
[Communications Minister Negeri Lencho] said he could not comment on the cause of tragedy, saying an investigation was under way. He had earlier said slum dwellers may have inadvertently caused the disaster.
Being buried in a garbage dump landslide is truly terrible. The survival rate is particularly low given the nature of the material and the potential for the waste to generate methane that can fill any spaces within the mass. Back in 2008 I wrote a blog post about the impact of garbage landslides, and our comparatively poor understanding thereof. In 2011 I posted about a terrible garbage landslide at Irisan in Baguio, Philippines.
9 March 2017
Chamba: an impressive set of landslide videos from Himachal Pradesh
A really impressive set of the three videos has been posted to Youtube showing a landslide at Chamba in Himachal Pradesh in India:
The landslide apparently occurred on the road that links Chamba with Saroo. Reports suggest that it occurred on Monday 6th March. I have seen no indications as to the trigger.
8 March 2017
Bucium Hill in Romania – an interesting landslide problem on a major road
A very interesting landslide problem has developed on the AA1 motorway at Bucium Hill in the Transylvanian hills of in Romania. This section of road, was built between April 2011 and November 2014, In includes a very substantial cutting through Bucium Hill, with then transitions into a major viaduct, as seen below (via Youtube):
Instability quickly developed in the slopes of the embankment, and the road was closed between September 2015 and October 2016. To deal with the problem the slope appears to have been successively regraded to a lower angle. The video below shows the site in September 2015 after substantial works on the north side of the road.
This appears to show that the regraded slope is undergoing substantial deformation, and when compared with the video from December 2016 it appears that there has been considerable recent movement. There appear to be multiple types of landsliding occurring here. The most substantive movement appears to be occurring in the section shown below:
This image captures the toe bulge of the landslide, driven by substantial deformation at the crest:
Further along the slope there seems to be a much larger area of instability developing, albeit at an earlier stage:
These appear to be large, shallow failures in sandy materials. It is impossible to forecast behaviour without a detailed investigation of both the site and the materials – I assume that this is ongoing. It is slightly surprising to see no obvious signs of monitoring on the slope, but perhaps I have missed this. To my way of thinking, with major capital asset at the toe of the slope this is a site that deserved detailed investigation and observation.
I would welcome views and thoughts on the hazard, especially in light of how close some of the deformation appear to be to the edge of the road:
7 March 2017
Oroville dam: multiple riverbank failures on the Feather River after the flow was abruptly stopped
The San Francisco Chronicle has an excellent article about what happened next on the Feather River when the flow down the Oroville Spillway was abruptly stopped last week to remove debris from the channel:
When state water officials scaled back their mass dumping of water from the damaged Oroville Dam this week, they knew the riverbed below would dry up enough to allow the removal of vast piles of debris from the fractured main spillway. But they apparently did not anticipate a side effect of their decision to stop feeding the gushing Feather River — a rapid drop in river level that, according to downstream landowners, caused miles of embankment to come crashing down. With high water no longer propping up the shores, the still-wet soil crashed under its own weight, sometimes dragging in trees, rural roads and farmland, they said.
“The damage is catastrophic,” said Brad Foster, who has waterfront property in Marysville (Yuba County), about 25 miles south of Lake Oroville.
The farmer not only saw 25-foot bluffs collapse, but also lost irrigation lines to his almonds. “When the bank pulled in,” he said, “it pulled the pumps in with it. It busted the steel pipes.”
In the article, farmers describe losing substantial sections of their riverbank. For example one, Philip Filter at Live Oak describes losing most of the banks in his section, which extends over a kilometre. This image shows some of the damage:
Meanwhile on Twitter, Mike Luery of KCRA3 has provided another example of a riverbank collapse on the Feather River:
Riverbank collapse during rapid draw down is a well-known phenomenon – indeed riverbank failures are most likely on the falling limb of the hydrograph. As the river level rapidly falls the banks are left in a saturated state, which means that they are weak. But more importantly, they also have a hydraulic imbalance, which drives flow of water through the soil towards the river. This provides an additional force that reduces stability.
The problems of riverbank failure are well-described in the literature, and there is even a Wikipedia page that provides a detailed description. The tragedy is of course that riverbank failures represent a permanent loss of land, and the input of sediment into the river can have implications for the ecosystem. Drawing the river down more slowly means that the hydraulic instability is avoided, and the riverbank failures should not occur.
6 March 2017
Righa in Nepal: another landslide associated with a hydropower project
Over the weekend, various media outlets in Nepal reported a landslide near to Righa in Baglung district in Central Nepal, which briefly blocked the Daram River (although the blockage has now cleared). This landslide appears to have claimed the life of a local woman. Online Khabar reported the landslide as follows:
A landslide that occurred during the construction of a road for a hydropower project in Manewas of Righa-7, Baglung district, has partially blocked the Daram River. This has also shut the Mid-Hill Highway.
Two things to note here of course. First, that once again we see a significant landslide associated with a hydropower project (a point that I make frequently) and secondly, that this landslide was triggered by excavation. Interestingly, MyRepublica suggests that the problem may have been pipeline construction alongside the road for the HEP plant:-
According to Badri Sharma, chief division engineer at the mid-hill highway in Baglung, the ‘dry landslide’ was a result of the digging of land along the highway for laying pipes to produce energy by the Daramkhola Hydroelectricity Project Limited.
“When they asked for permission, we had told them to lay pipes toward the cliff as the edge of the highway was quite steep, ” said Sharma. He informed that clearing the rocks and land will take some time. “I have already asked the construction companies and Hydropower project working in along the highway to fix the problem and they assured to make the highway ready in about five-six days,” said Sharma.
Judging by images the landslide itself is quite significant in size (via the Gorkha Post):
There is also a collection of images of the landslide in a Youtube video that is available online:
Perhaps the most revealing element is this image of the headscarp area of the landslide at Righa:-
In common with the images of the landslide deposit, this suggests that the slide is in deeply weathered, dry soil. Such a landslide should in general be avoidable with good engineering practice, if excavation does indeed to be the cause of this landslide. There does appear to be some evidence that the landslide might be associated with a highway and associated works – this image from MyRepublica shows the crown of the landslide:
1 March 2017
New images reveal the scale of damage to the Oroville spillway
In the last few days the California DWR has shut off flow down the Oroville Spillway whilst they start to clear debris from the lower channels, which in turn should allow them to restart flow through the dam itself. This allows the level of damage to the spillway to be assessed properly – and there is no doubt that it is extremely serious. This image, from Reuters and Buzzfeed, provides an overview of the state of the Oroville Spillway now:
The damage is better illustrated in the Buzzfeed image, which shows the enormous erosion that has developed on the margin of the spillway, and the very large hole that has been created in the midsection of the spillway itself:
The challenges of rebuilding the spillway are illustrated by this image of the upper section of the spillway. The new structure will need to be properly founded onto competent rock. It appears from this image that sections of the spillway that initially appear to be undamaged are in fact weakened:
Meanwhile there is a very nice article in the Sacramento Bee that provides an overview of the crisis, and starts to probe what went wrong at the dam:
Some outside experts already have weighed in with theories as to why the chute, rated to handle 250,000 cfs, broke open Feb. 7 with a relatively modest 55,000 cfs pouring down.
Some, including Tullis, point to a phenomenon known as “cavitation,” in which the blast of tiny water bubbles gushing down the chute at 50 mph effectively jack-hammered holes in weakened sections of concrete. Others, such as Robb Moss, a professor of geotechnical engineering at Cal Poly San Luis Obispo, speculate that roots from trees growing along the chute expanded weaknesses in the concrete.
Rogers, the dam failure expert from Missouri, said the spillway may have had cracks that weren’t properly patched. He also theorized the failure may have been tied to California’s five-year drought: The aging spillway could have weakened as it underwent contractions due to the sudden heavy soaking following years of dry weather.
An investigation is underway. There is now enough spare storage capacity in the reservoir that the emergency spillway should not be needed again this year. The challenge will be to rebuild the main spillway in time for winter.