22 February 2018
River of rock: a fantastic video of a granular flow from New Zealand
From New Zealand comes a new video of a “river of rock“, a somewhat beautiful granular flow. This was triggered by Ex Cyclone Gita, which made landfall across the central part of New Zealand, bringing heavy rainfall and, of course, landslides. The “river of rock” was captured on video by Donna Field on the Rakaia River:
This is a magnificent example of a granular flow. Whilst they appear to be very exotic, granular flows are quite common and have been well-described in the literature. In essence the pebbles behave as particles, allowing behaviour that is akin to that of a fluid. Of course the density of the fluid is somewhat higher than that of a more familiar liquid, such as water, and the particles are many times larger, but the river of rock is still able to generate the sorts of flow structures that we see in water:-
I suspect that this one comprises particles that are of a comparatively even size – Simon Cox from GNS suggested to me that the material is probably greywacke. It may be that this uniformity lies behind the unusual characteristics of this flow.
Stuff.co.nz has an article about the granular flow. They note that:
“A river of shingle rock flowing down Terrible Gully has left eight farms cut off from town in Rakaia, near Canterbury’s Mt Hutt. Incredible video of the “rocky river” was captured by Donna Field of Cleardale Station as she surveyed the damage from former Cyclone Gita on Wednesday. The flow of careering shingles closed Double Hill Run Rd which services the farms in the South Island region. It’s an event that has become a regular occurrence for the farmers in the area.
I guess it may not be too hard to understand why this location was named “Terrible Gully”.
Many thanks to the several people who pointed this one out to me. Much appreciated.
20 February 2018
First satellite images of the Puerto Venus debris flows
Yesterday I highlighted the Puerto Venus debris flows that struck a rural community in Colombia last week, destroying a number of houses. Whilst loss of life was avoided, the events caused significant damage. They were also caught on some remarkable videos, which I included in my post. There was speculation yesterday that the landslides had been caused by a the breach of a valley blocking landslide upstream near to El Pinal, but of course this can only be confirmed via satellite imagery or by fieldwork. I have taken a look at the Planet Labs images of the area affected by the debris flows. Whilst we are still waiting for a cloud-free image (challenging in this area at this time of year), these are quite informative. The image below, collected by Planet Labs (and used with permission) with a 3 m resolution before the debris flows, shows the catchment above Puerto Venus, from which the debris flows originated:-
The image below, of the same area, was collected yesterday (19th February 2018), also by Planet Labs:-
Whilst the image yesterday is affected by cloud and haze, the image shows that the debris flows originated high up in the catchment, upstream of the two landslides that were triggered late in 2017. Unfortunately the source is in cloud, but the dramatic scour downstream is evident. It seems likely that there is extensive slope collapse. A closer look at this area shows the amount of material that has been removed from the channel:-
Hopefully a cloud-free image will become available in the next few days.
Planet Team (2017). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://api.planet.com
19 February 2018
Puerto Venus: massive debris flows in Colombia yesterday
The municipality of Narino in Antioquia in Colombia was affected by large debris flows, triggered by heavy rainfall, late last week. Worst affected appears to be the village of Puerto Venus, which was struck by a large debris flow, destroying 12 houses. The debris flow was captured in two remarkable videos that have been posted to Youtube. This one shows the debris flow from one side of the river:-
El Colombiano has a detailed article (In Spanish, though Google Translate does a reasonable job) about the landslide, together with some images of the events. It reports that the inhabitants of Puerto Venus were warned that the landslide was coming by the inhabitants of a village upstream at El Piñal. The suggestion is that the river had been blocked by a landslide near to El Piñal, and that the debris flow was then triggered by heavy rainfall. I am unsure as to whether this implies that the earlier landslide was a valley-blocking event, and that the debris flow was the result of a breach.
This image, from El Colombiano, shows the aftermath of the debris flows:-
Back in November, Caracol.com reported that a landslide had developed in the vicinity of Puerto Venus. The Google Translate version (lightly edited) suggests that:
The inhabitants of the village of Puerto Venus in the municipality of Nariño, east of Antioquia, are concerned because for several days they have seen how a mountain that surrounds them threatens to collapse due to the recent rains….”We are on a hillside, stuck in the mountains and we are at high risk because for example this week a very large bomb went down the river and people who live nearby no longer dawn there. In addition, the mountain has already covered a road that is nearby and the entire village is in danger; It is a very big mountain and the river grows with the surrounding streams. We are encapsulated between the mountains, “added [Nohemí Mejía, an inhabitant of the area].
16 February 2018
Ramban: massive landslides cut-off the Kashmir Valley for four days
Heavy rainfall in the last few days has triggered landslides in northern India. Most notably, the key highway that links Srinagar and Jammu has been blocked, meaning that the Kashmir Valley has in effect been cut off for four days. Landslides have occurred on multiple roads, including the highway to Ladakh and the Mughal road. Many vehicles are stranded on the roads, causing significant hardship to many people.
The Kashmir Monitor has a good image that illustrates the scale of the problem:
Most impressive is the video below, which was posted onto Youtube yesterday, showing a very major slope collapse. The associated text indicates only that this occurred at Ramban:
This appears to show a really very large slide. Google Earth suggests that Ramban is an area with a very high density of landslides.
It is not clear to me as to why this zone is so landslide prone. Fortunately the highway is on the east side of the river whilst most of the landslides are on the west. Nonetheless this must be an extremely challenging section of road to maintain. The image above shows the very high level of risk to the Border Roads Organisation (BRO) in the clearance of these landslides.
I have noted serious landslides in Ramban previously. For example, in March 2016 there were reports of landslides in the Ramsu area of Ramban, whilst in June 2011 there was a fatal landslide in the Banihal area of Ramban. And finally, in September 2014 a major landslide was captured on video in the Ramban area that was causing substantial disruption on the road.
15 February 2018
The evolution of post-seismic debris flows
One of the most important legacies of large earthquakes in mountain chains is the increased occurrence of landslides, which continue to cause damage and disruption for years after the mainshock. This issue very much came to the fore in the aftermath of the 1999 Chi-Chi earthquake in Taiwan, in particular in relation to the important Central Cross Island Highway. Repeated attempts were made to reopen the highly damaged sections of road after the earthquake, thwarted in each case by landslides that destroyed the infrastructure. This section of the road remains closed almost 20 years later, and there are no plans to reopen it, with traffic being diverted to throther branch to the south.
However, the defining event in terms of post-seismic debris flows is undoubtedly the 2008 Wenchuan earthquake in China, which was the most efficient earthquake in terms of landslide generation that I have known. The area affected by the mainshock has been dogged by post-seismic debris flows, with terrible consequences. This has highlighted the lack of understanding of the fundamental mechanisms that underpin these continued events, and the processes that lead to reductions in their occurrence. In a paper just published in the journal Engineering Geology, Fan et al. (2018) explore post-seismic debris flows in a major gully in the Gaojiagou Ravine, in the heart of the area affected by the Wenchuan earthquake. This area was heavily affected by landslides in the mainshock, and thereafter was struck by four major debris flows, in 2010, 2011, 2013 and 2016. The effects of the earthquake remain as shocking today as they did at the time. This is a Google Earth image of the study site of Fan et al. (2018), collected in 2005 (note there is no imagery for the portion that is blanked out on the left hand side):
Whilst this is the same area after the earthquake:
Fan et al. (2018) studied the four debris flows that have occurred in the ravine since the earthquake. They found that through time there was a change in the initiation mechanisms of the debris flows, transitioning from simple landslides through to channel-bed failure and then to channel-bank erosion. At the same time the location of the initiation of the post-seismic debris flows migrated to lower positions on the landscape. They also observed changes in the runout characteristics of the post-seismic debris flows, with mobility reducing with time. As the mechanisms of the debris flows changed, the triggering threshold rainfall also increased.
With time the volume of loose material stored in the ravine reduced, with successive debris flows flushing sediment out of the system. Fan et al. (2018) include a really nice series of maps of the areas within the ravine that were covered in loose materials:-
This really interesting study provides real insight into the evolution of post-seismic debris flows, and the changes that occur with time in the aftermath of a large earthquake in mountains. This process is now playing out in Nepal. The study indicates that those being affected by post-seismic debris flows in that area have some years of increased levels of risk to come.
R.L. Fan, L.M. Zhang, H.J. Wang, , X.M. Fan 2018. Evolution of debris flow activities in Gaojiagou Ravine during 2008–2016 after the Wenchuan earthquake. Engineering Geology, 235, 1-10. doi: https://doi.org/10.1016/j.enggeo.2018.01.017
13 February 2018
Diatom ooze: the weak link in submarine landslides?
Submarine landslides are the largest, and probably the least well-understood mass movements, on Earth. As I have noted previously, they have the potential to move hundreds of cubic kilometres of materials for hundreds of kilometres on slopes with gradients of less than 3º. The mechanics of these slides have represented a substantial challenge – the low slope gradients suggest extremely weak layers must control the deformation, but the origin of these low strengths has not been clear.
In a new paper just published in the journal Geology (Urlaub et al. 2018), Morelia Urlaub and colleagues have examined ocean drilling data from the periphery of the Cap Blanc slide, which is a a 149,000 years old, large submarine landslide situated off the coast of NW Africa. The paper includes this seismic refraction line across the slide area, which provides details of the key features of the failure:-
By correlating the ocean drilling data with seismic refraction data, Urlaub et al. (2018) found thick layers of diatom ooze located at the base of the submarine landslide. Importantly, these layers were capped with a layer of clay-rich sediment. Urlaub et al. (2018) suggest that the saturated diatom layers are highly compressible, generating high pore water pressures that are trapped beneath the clay layers, inducing failure in the clay or at the interface between the clay and the diatom ooze. The authors note that diatom ooze itself has high frictional resistance, so is unlikely to form the sliding surface. Thus, it is the combination of the diatom ooze and the clay cap that is key to the generation of these slope failures.
Urlaub et al. (2018) have proposed an intriguing hypothesis that would explain the mechanisms behind at least some submarine landslides. They note that many continental margins have layers of diatom ooze. The challenge now is to recreate the mechanisms experimentally – this looks to be a task for the dynamic back pressured shearbox apparatus that we developed with GDS Instruments. This machine is ideal for exploring the behaviour of submarine landslide systems, and is the subject of our ongoing work in New Zealand.
Morelia Urlaub, Jacob Geersen, Sebastian Krastel, Tilmann Schwenk. 2018. Diatom ooze: Crucial for the generation of submarine mega-slides? Geology; DOI: 10.1130/G39892.1
12 February 2018
Juneau: a very large rockslope failure in Alaska in December 2016
KTOO has a very interesting article about a large rockslope failure that occurred at Cowee Creek to the north of Juneau on 30th December 2016. This rockslope collapse has been investigated by Rick Edwards of the U.S. Forest Service’s Pacific Northwest Research Station after it was detected as a M=3.4 seismic event. It had a volume of about 540,000 m³ according to their analysis, so was a substantial event. The rockfall descended into a lake at the foot of the slope, displacing 460,000 m³ of water. This forced a 9 m high wave down the valley, cutting a 90 m wide swathe as it went. The wave was recorded to be almost 2 m high some 13 km downstream. The wave destroyed 1,500 trees. KTOO have this image of the site, including the rockfall scar in the background and the path of the displacement wave in the foreground:-
This is a Planet Labs image of the aftermath of the rockslope failure, collected on 9th August 2017. The slope that failed is difficult to see, but the path of the displacement wave is clear in the imagery:-
The timing of the slope collapse is quite interesting – this would have been the middle of winter, when the slopes were frozen. Analyses suggest that most of the very large rockslope failures in Alaska occur in the spring and early summer. For comparison, this is a 5 m resolution RapidEye image of the same area, collected on 25th August 2015, before the rockslope collapse:
Planet Team (2017). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://api.planet.com
8 February 2018
Predicting failure using ground-based radar and INSAR
A new paper just published in the journal Engineering Geology (Carla et al. 2018) explores the use of ground-based radar and INSAR to predict landslide failure. The case study is based on events in an unspecified copper mine in November 2016, when an unexpected failure with a volume of 410,000 m³ occurred in the excavated and benched walls of the mine. The landslide was large – about 400 m in length and up to 300 m in width. Clearly such an event represents a substantial risk to mine operations. The slope was being monitored with ground-based radar, but the development of the failure was not detected. This is of course quite disconcerting.
Carla et al. (2018) have investigated this failure in detail. They found that, as a result of line-of-sight issues, the ground-based radar could detect deformation in only two benches, with the rest of the developing landslide being out of view of the system. Most of the landslide comprised failure of a natural slope above the mine. To understand the development of the failure, they examined INSAR data over the months leading up to the landslide, based upon Sentinel-1 data. The results are fascinating. The developing landslide is clearly evident on the INSAR data, and the acceleration of the slope to failure is clearly evident, as the graph below from the paper shows:-
The authors suggest that monitoring using INSAR would have allowed the landslide to be detected, and in turn this would have allowed the ground-based radar to be used to detect the deformation in the benches within line of sight as the landslide accelerated to failure on the day of the collapse (the graph on the right). Note that the period of accelerating creep started about two months before the collapse, so plenty of warning of impending problems would have been available. The authors then used the inverse velocity approach (sometimes called the Saito technique) to determine whether the time of failure could have been predicted. The data below speaks for itself:-
The paper demonstrates beautifully both the incredible opportunities that INSAR derived from Sentinel provides for the monitoring of slope deformation, and the ways in which this data can be combined with ground-based radar to provide high quality warning systems. It is an excellent piece of work.
Tommaso Carlà, Paolo Farina, Emanuele Intrieri, Hakki Ketizmen and Nicola Casagli 2018. Integration of ground-based radar and satellite InSAR data for the analysis of an unexpected slope failure in an open-pit mine, Engineering Geology, 235, 39-52. https://doi.org/10.1016/j.enggeo.2018.01.02
7 February 2018
Taroko Gorge: the potential effects of the M=6.4 Hualien earthquake yesterday
The M=6.4 earthquake near to Hualien in Taiwan yesterday is known to have caused some building collapses in the city. Rescue operations are ongoing; hopefully the impacts will not be too severe. The epicentre of the earthquake was very close to the mouth of the Li Wu River to the north of Hualien. This is the USGS Shakemap data for the earthquake:-
The course of the Li Wu river is the alignment of the Central Cross Island Highway, the most important road linking the east and west coasts of Taiwan. The west end of this road was profoundly damaged during the 1999 Chi Chi Earthquake, and indeed the northern branch on the western side of the Central Mountains has not reopened. On the east side of the island the highway runs along the base of Taroko Gorge through a National Park, truly one of the great natural wonders of the world. This area is very special to me as I spent a large part of the early stages of my academic career undertaking fieldwork in Taroko Gorge – it is an area that I know well and love.
The effects of this earthquake will be fascinating as the Central Cross Island Highway is very prone to landslides and rockfalls. The Taroko Gorge section has been carved into the base of what are usually described as marble cliffs, although in reality the geology is a mixture of marble and schist in many places. This is an area that suffers landslides on a regular basis; its performance during significant shaking has always been a concern.
To date I have found one image of the impact of the earthquake, via Twitter, which suggests multiple small detachments:
In recent years a huge amount of effort has been put into making this road more resilient to rockfalls. If it has come through this earthquake relatively unscathed (and reports suggest that it is open, although we await confirmation of this) then it will be a major triumph for the engineers. The image above shows however that it was very fortunate that the earthquake occurred late at night, when there were no tourists, and little traffic, on the road.
6 February 2018
The first submarine sackungen: a new paper
A new paper published Geo-Marine Letters (Conway and Barrie 2018) describes the first known submarine sackungen. The Colorado Geological Survey has a nice page about the sackungen phenomenon, which includes this definition:
A sackung structure can be a trench (small-scale graben) or an upslope-facing scarp. They are found most commonly in Alpine glaciated regions near the crest of a range. Their origin is interpreted to be a result of post-glacial, gravitational spreading of the ridge crest and over-steepened ridge flanks. Plural of this German word is sackungen.
And they have this example on their website:-
These features are formed by slow, very deep-seated creep in a mountain flank. They are common across high mountain areas. However, submarine sackungen had not been observed to date, although technically there is no reason why they should not exist. The paper by Conway and Barrie (2018) describes deformation in the wall of central Douglas Channel in British Columbia. Two large, creeping blocks are evident, as the image below (from the Researchgate version of the article) shows:
These are landslides on a giant scale – the blocks have moved over 350 m in each case and the blocks are 62 million and 70 million cubic metres in volume respectively. Because this is an area that was heavily glaciated, the movements must have occurred since retreat of the ice. The authors suggest that movement probably happened after retreat of the glaciers – i.e. between 15,800 and 13,400 calendar years ago – when the slopes would have been debuttressed. Evidence from the younger sediments that drape the submarine environment suggests that they are not currently moving in any substantial manner, although smaller-scale slope failures may still be happening.
It is not every day that a new type of landslide feature is identified. The discovery of these submarine sackungen feels like an important advance in our understanding of slope processes.
Conway, K.W. and Barrie, J. 2018. Large bedrock slope failures in a British Columbia, Canada fjord: first documented submarine sackungen. Geo-Marine Letters. https://doi.org/10.1007/s00367-018-0533-y