23 June 2020
The Gjerrild Klint landslide on the east coast of Jutland, Denmark
Guest post by Gregor Luetzenburg, Kristian Svennevig & Marie Keiding
Researchers from the University of Copenhagen and the Geological Survey of Denmark and Greenland were recently informed about a landslide at Gjerrild Klint at the Danish Kattegat coast north of the City of Aarhus (56.513 N, 10.865 E). Based on observations made in PlanetScope and Sentinel 2 scenes, the landslide most probably occurred at the beginning of March 2020, but was first discovered and reported by locals about three months later. A period of unusually low precipitation from March to June 2020 preserved most of the distinct young morphological features in the soft glacial till, such as pinnacles on the main body of the slide.
Landslides occur regularly in Denmark, but fortunately most occur as earth slides and flows along the sparsely populated coastline, and thus do not lead to fatalities. Most damage reported is to holiday homes, which are common along the coast. This specific landslide is a good opportunity for us to investigate the complex interplay of geology, hydrology, climate, wave erosion and land use leading to slope failure.
The area covered by the landslide is around 2,500 m² (70 x 35 m). The height of the cliffs on the coast is around 25 m. Having the shape of a textbook landslide, the Gjerrild Klint landslide features the main characteristics of a rotational landslide. A clearly definable crown at the top of the cliff with smaller crown cracks covered by the surrounding crops can be observed. The head of the slide, beneath the main scarp, is characterized by parallel displaced and rotated blocks with minor scarps in between. At these blocks the topsoil is still intact allowing the remaining crops to continue to grow.
Water supply to the slide is increased by the broken artificial drainage system of the overlying cultivated field. However, drainage of the slide’s surface is poor leading to ponding in the back-tilted areas. Further down, the slide shows minor scarps and transverse cracks. At the foot of the slide, waves have already started to erode the toe. Uplifted beach deposits can be seen demonstrating the rotational process of movement of the slide. Several minor toe collapse slides can be observed as well.
The cliff at Gjerrild Klint consists of alternating layers of glacial till and clay and several older vegetated slides can be seen next to the new one. This indicates that the Gjerrild Klint slide is one of many in a long sequence of rotational slides, forming the local coastline by occasional but consecutive mass movements. The main conditioning factor here is probably a local glaciolacustrine clay unit located just at sea level and observed in the slide toe on several places, probably acting as the sliding plane. Intense agricultural land use and manmade drainage systems may further precondition landslide processes, by redirecting the naturally vertical flow of water laterally towards the cliff. Moreover, wave erosion at the toe of the cliff is constantly removing material, destabilising the prevailing equilibrium. February 2020 provided optimal conditions for triggering the landslide with 103 mm of precipitation during the month in the area of the Gjerrild Klint landslide – a record for the highest amount of rainfall since the beginning of the measurements in 1874. The slide was thus most probably activated by water infiltration into the glaciolacustrine clay reducing the friction. Shifting towards a surge in weather extremes and rising sea levels under a warming climate, Denmark is likely to experience an increase in this type of landslide activity in the next years and decades.
On reflection 1: Recent landslides in Darjeeling
The wonderful Save the Hills blog has a good post about landslides triggered by heavy rainfall on 11 – 13 June 2020 in Darjeeling, highlighting the role of humans in increasing landslide susceptibility in this area.
On reflection 2: The summer monsoon is rapidly developing across South Asia
As the rainy season gets under way across South Asia, landslides are starting to inflict losses. In the last day or so three people (including two children) were killed in a landslide in Sikkim in India, three people were killed in a landslide in Palpa in Nepal and cross-border trade between Tibet and Nepal has been halted by landslides near to Tatopani.
22 June 2020
The Old Fort landslide has reactivated – about 100 m of new movement
Back in October 2018 I wrote about the Old Fort landslide, a large earthflow that has affected access to a subdivision close to the town of Fort St John in British Columbia, western Canada. In recent days a period of heavy rainfall has reactivated the landslide, which has shown an impressive amount of movement. Energetic City, a local news outlet, reports that the landslide has moved about 100 metres. Their report includes this impressive image of the disruption to the access road to the Old Fort subdivision:-
Note the very impressive displacement of the road shown in the image, which also captures the extensive landslide activity on the margins of the main slide.
The Peace River Regional District (PRRD) has an emergency response website, providing updates to the community affected by the landslide. About 150 people live in the Old Fort Subdivision. The PRRD has provided the following map that shows the main features of the landslide:-
Many thanks to Susan DeSandoli, who highlighted that this really interesting landslide has reactivated.
On reflection 1: a valley-blocking landslide in Nepal
The Kathmandu Post reports that a large landslide on Sunday 21 June 2020 partially blocked the Arun River in Makalu Rural Municipality, Sankhuwasabha District. A lake has formed, although this does not appear to be particularly large. Communities downstream have been put on alert.
On reflection 2: A fatal landslide in Côte d’Ivoire
On 18 June 2020 a large landslide at Anyama, a community located north of Abidjan in Côte d’Ivoire, killed 13 people. A news report on the Medafrica website suggests that this was the failure of a railway embankment, possibly caused by blocked drainage.
18 June 2020
Quick clay landslides: an online documentary about Rissa
The recent interest in quick clay landslides, triggered by the remarkable video from Alta in Norway, has led to a number of questions as to the mechanisms of these strange failures. Loyal reader George Haeh kindly pointed out that there is a wonderful video, produced by NGI, that seeks to explain the famous 1978 Rissa landslide in Norway. This was the first quick clay landslide to be caught on video; the recording remains a classic. The NGI documentary has been posted to Youtube. It includes the famous footage of the landslide in action.
The video is most definitely of its time, complete with haunting music, and it’s highly instructive in terms of explaining the sequence of events at Rissa, and the underlying processes. I especially recommend the sequence from about 2 minutes 34 seconds into the recording, which seeks to explain the mechanics of quick clays. The sequence includes an initially intact block of quick clay being subjected to a load that exceeds its strength:-
During failure the video shows the rapid change in the properties of the clay:-
The video then goes on to show that remoulding the clay, without adding any fluid, leads to a complete change in the material properties, that now behave as a liquid:-
Perhaps the most interesting part of the video is the response to the addition of a small amount of table salt. The quick clay quickly regains much of its strength, to the extent that the technician can stand a knife up in the material:-
I have not seen such a clear explanation of the mechanics of quick clays.
On reflection 1: A valley blocking landslide in Sichuan
A large valley blocking landslide in Danba County in Sichuan, China has caused extensive flooding. 20,000 people have been evacuated.
On reflection 2: A mudslide-induced house collapse in Nigeria
16 June 2020
New landslide videos: Russia and California
A couple of new landslide videos have emerged that are worth a look. Whilst they are not as dramatic as the quick clay landslide video from Norway, they are nonetheless interesting. The first was highlighted to me by the famous Russian landslide scientist, Alexander Strom. The location is uncertain – it is probably somewhere in Russia. Unfortunately I cannot embed this one, so you’ll need to follow this link.
This is clearly a failure in a large mine. The magnitude is large, although it is hard to get a sense of the depth of the view. Interestingly, early in the video a large quarry truck lumbers up the haul road on the right even though the foot of the slope was actively failing:-
To me this suggests that the landslide had not been anticipated, or that it is larger than expected.
It would be good to get more information about this failure, and about the outcome of the collapse (which appears to still be in progress at the time of the video).
Meanwhile, on Youtube there is an interesting video taken on Highway 101 in Sausalito, California. The video is from a dashcam, and captures the moment when a small rockfall impacts on the road directly in front of the car. Be careful as the video captures an understandable profanity from the driver, who was definitely in the wrong place at the wrong time:-
The video captures the fragmentation of the rock blocks rather beautifully:-
According to the text accompanying the video, the damage to the car cost $21,000 to repair. It is interesting that this failure occurred on a sunny day. This one was highlighted to me by loyal reader Fabien, many thanks.
On reflection 1: Another railway landslide in the UK
A landslide has blocked the line between my home town Sheffield and Scunthorpe. The disruption is expected to last 10 days.
On reflection 2: Images of the Norway quick clay landslide
Louise Vick of UiT The Arctic University of Norway has tweeted some images of the aftermath of the Norway quick clay landslide, collected during a field visit:
Nipped up to Alta today to collect some data at the Kråknes #landslide. I learned some surprising facts about the sequence of events (eg the slide comprised many(!) smaller failures) and saw some crazy features (eg natural pipes in the soil profile and exposed sliding surfaces). pic.twitter.com/g6ZfcMQTQT
— Louise Vick (@LandslideLouise) June 15, 2020
Her observations about the site are interesting. A follow up tweet included a further set:-
Hope these are of interest pic.twitter.com/OCwBGNyPOk
— Louise Vick (@LandslideLouise) June 15, 2020
15 June 2020
The 15 March 2019 Xiangning landslide in Shanxi Province, China
On 15 March 2019 a large landslide occurred in Xiangning County in Shanxi Province in northern China. The landslide, which occurred in loess, destroyed a number of residential buildings and the local health centre. In total 20 people were killed and a further 13 were injured. A paper has recently been published in Natural Hazards (Zhao and Zhao 2020) that provides some more detail of this disaster.
The authors provide this highly informative montage of photographs and drawing to illustrate the event:-
This is quite an unusual loess landslide as it had the form of a slump with limited runout. As the image above shows, the main slump block did not fragment, and one of the larger buildings remained remarkably intact. The landslide had a volume of 73,000 cubic metres and the displaced block was 125 x 80 metres.
Interestingly, Zhao and Zhao (2020) could find no external triggering event, such as an earthquake or intense rainfall. They concluded that the landslide was the result of natural degradation of the loess, perhaps accelerated by human activity such as deforestation.
An unusual aspect of this paper is that it provides some detail about the rescue efforts for the Xiangning landslide:-
Judging from the rescue situation, the on-site rescue environment was highly complicated, challenging, and high risk. First, the landslide had a large volume and deeply buried pressure; those factors considerably influenced the location of rescue site searches. Second, the location of the landslide was on the hillside and caused a complicated situation, while the mountain road was meandering and narrow. Thus, the work surfaces of the rescue operation were small, large machinery could not be used or even come close to the scene, and the efforts of small rescue equipment were limited. Third, the soil of the landslide body was flaccid and the structure was unstable. During the rescue, landslides occurred several times, cracks became larger, and rockfall transpired. Many large building wreckages were scattered on the collapsed mountain; the height from the top to the bottom of the mountain measured dozens of meters, and the slope was almost vertical. The wreckages of the buildings fell off, likely causing harm to the rescuers below who were not evacuated in time.
This is a good illustration of the multiple challenges faced in the aftermath of such disasters.
On reflection: The first large landslide disaster of 2020 in Nepal
A large rainfall-induced landslide at Kushma in Durlung, Parbatin Nepal on Saturday night killed nine people. The summer monsoon is becoming active in South Asia.
Zhao, B. and Zhao, Y.Q. 2020. Investigation and analysis of the Xiangning landslide in Shanxi Province, China. Natural Hazards https://doi.org/10.1007/s11069-020-04109-2
11 June 2020
Landslides in Art Part 33: Vue du vallon entre le Rossberg e le Rigi apres la terrible catastrophe du 2e Septembre 1806
Landslides in Art Part 33: Vue du vallon entre le Rossberg e le Rigi apres la terrible catastrophe du 2e Septembre 1806
The British Museum collection includes a print of a painting by Gaspar Rahn entitled Vue du vallon entre le Rossberg e le Rigi apres la terrible catastrophe du 2e Septembre 1806. This translates as View of the valley between Rossberg and Rigi after the terrible disaster of 2nd September 1806.
This is the print:-
This painting clearly shows a very large landslide with a long runout. The material involved in the failure is clearly mostly large rock blocks. Note in the foreground clear damage to buildings, and considerable strewn debris, suggesting that the landslide generated a displacement wave in Lake Lauerz.
This painting depicts of the aftermath of the so-called Goldau Landslide in Switzerland, which has featured in this series previously thanks to a painting by Joseph Mallord William (JMW) Turner. As I noted then, the Goldau landslide was triggered by heavy rain, with an estimated volume of 120 million cubic metres, covering an area of about 20 square kilometres. The landslide, and the tsunami it created on Lake Lauerz, destroyed 111 houses, 220 farm buildings and two churches, resulting in the deaths of 457 people. There is a brief write up of the Goldau landslide on the Scientific American blog, whilst another article on the same site notes that this was the first landslide to be investigated in depth by geologists.
On reflection 1: Landslides at the start of the rainy season in China
Xinhua is reporting multiple landslide fatalities triggered by heavy rainfall in China. For example, in Baojing County in Hunan Province, heavy rainfall has triggered landslides and floods that have destroyed several village houses, killing six people, with a further person missing and three others injured.
On reflection 2: How medieval Europe recovered from earthquakes
The Conversaton has a very nice article on the ways in which societies in medieval Europe recovered from destruction earthquakes. The impact of coseismic landslides and rockfalls features heavily.
10 June 2020
The role of earthquake and rainstorm induced landslides in shaping mountain chains
Whilst in this blog I tend to focus on landslides that are caused by humans, or that cause harm to people or and/or property, it is important to remember that they mainly occur naturally in the landscape. The role of landslides is particularly important in high mountain chains, where tectonic processes are driving uplift. There is a limit to how high mountains can become, in the first order set by the strength of the rocks of which they are composed (although actually this is somewhat complex). Thus, on the scale of the mountain chain (in time and space), landslides are the process that enables a balance to be reached between uplift and erosion.
It is well-established that in high, active mountain chains landslides are the dominant process of erosion. But in a paper published in Science Advances (Wang et al. 2020), with a superstar list of authors, the role of different types of landslide has been explored in more detail. In particular, active mountain chains typically experience landslides triggered by two major processes, which we can classify as being meteorological (primarily rainfall) and seismic (primarily high magnitude earthquakes). In the paper, the research team have examined sediments deposited in Lake Paringa in New Zealand, which is located close to the Alpine Fault in the Southern Alps. This is an area that suffers frequent large rainfall events and large earthquakes. The timing of historic earthquakes on the Alpine Fault is very well-constrained.
The research team have looked at organic matter stored in the sediments of the lake, They have shown that the organic matter in soils from high elevations has a different geochemistry from that lower down in the mountain chain. By tracing that geochemical signature in the sediment core they can identify the source of the the material through time. And of course the dominant process that has released that material is landsliding.
The results are really interesting. In the period between earthquakes they found that most of the sediments came from soils at lower elevations in the mountain chain. These are the landslides driven by rainfall. But in the 1,000 year period of the study there were four major Alpine Fault earthquakes, which will have caused extensive landsliding. Immediately after the earthquakes the dominant source of the sediment was landslides at higher elevations. In the post-seismic period the mean elevation from which the sediment was sourced declined with time, and in the inter-seismic periods the dominant source was landsliding at lower elevations. It remained this way until the next earthquake, when the focus source returned to higher elevations in the mountain chain.
This is really neat. The data suggest that earthquakes shape the highest elevations in mountain chains, which rainstorms shape the lower elevations. So the landscape of different parts of the mountain chain are the result of different geomorphological processes occurring at different times.
The results do fit with our understanding of landslide mechanisms. It is well known that earthquake induced landslides tend to extend to the ridge crests because of the process known as topographic amplification – basically the seismic shaking is more intense at high elevations. On the other hand, rainfall induced landslides are primarily the result of high pore water pressures, which of course tends to occur lower in the slope.
This is a clever and insightful study that greatly clarifies our understanding of how mountain processes operate. It is an important contribution. It will be interesting to see if the same observation can be made in other mountain chains.
Wang, J. et al. 2020. Long-term patterns of hillslope erosion by earthquake-induced landslides shape mountain landscapes, Science Advances DOI: 10.1126/sciadv.aaz6446
9 June 2020
Landslides triggered by the Mw = 7.8 14 November 2016 Kaikoura earthquake: an update
The 14 November 2016 Kaikoura earthquake in New Zealand was the result of a complex fault rupturing process in a mountainous environment. Unsurprisingly, an earthquake of this size generated a large number of landslides. I have blogged about these landslides previously, and have also posted images of some of them.
Mapping the resultant landslides has been a huge task led by a team from GNS Science in New Zealand. Earlier papers had been based on initial mapping, including about 10,000 earthquake triggered landslides. In a paper just published in the journal Landslides (Massey et al. 2020) the team have provided an update based on a full inventory. This may well be the most detailed earthquake induced landslide inventory compiled to date. There is fantastic data on both the fault ruptures at Kaikoura and the resultant ground motion, meaning that this dataset can provide great insight on earthquake – landslide interactions.
The headline figure is remarkable. The earthquake (and its aftershocks) triggered 29,557 landslides, all of which have been mapped and digitised by hand. This is the resultant map of the landslides, with the faults that ruptured to generate the earthquake marked in red:-
There are some interesting take home messages from this study. The most important is from the analysis of the factors controlling the distribution of the mapped landslides triggered by the Kaikoura earthquake. Having assessed various factors, Massey et al. (2020) conclude that the key variable is distance to the surface fault rupture – indeed this predicts the density of landslides better than does the measured and modelled peak ground accelerations. This is not the first study to conclude that proximity to the fault is important, but the quality of this dataset gives the results additional weight. The debate continues as to why this is the case – distance to fault must in reality be a proxy for a physical parameter (or set of physical parameters) that are controlling slope behaviour. What are these parameters?
Other factors that are important in determining slope behaviour from the Kaikoura earthquake are easier to understand, and include the geology (i.e. material strength), the slope angle, the local slope relief and the peak ground velocity.
One nice aspect of this study is that it finishes with some recommendations for future research – I welcome this. Massey et al. (2020) suggest that fruitful avenues might include 1) investigations of the interaction between surface fault rupture, earthquake-induced ground shaking, and the initiation of slope failure; and 2. remodelling of the ground motions caused by the multiple fault ruptures.
On reflection 1: Landslides in Space 1
An interesting article on the Forbes website discusses the role of landslides in generating the trails of dust that flow behind comets.
On reflection 2: Landslides in Space 2
Massey, C.I., Townsend, D.T., Lukovic, B. et al. 2020. Landslides triggered by the MW7.8 14 November 2016 Kaikōura earthquake: an update. Landslides. https://doi.org/10.1007/s10346-020-01439-x
8 June 2020
A further significant failure at the site of the Alta quick clay landslide
It appears that this is another major failure that is retrogressive from the events of Wednesday. This landslide is notable for the destruction of the road. Note that the landslide scar is almost completely evacuated – i.e. there is no landslide debris in sight – suggesting that this was once again a highly mobile event. The iTromso website has an image taken from a different angle:-
This image of the landslide scar shows the thick layer of marine silts and clays that is responsible for the high mobility of these landslides.
I suspect that the focus now will be on securing the road, which is clearly an important transport link for people living in this area. I hope that in due course some bathymetry work might be undertaken offshore from the landslide. It would be extremely interesting to understand the mobility of the landslide on the seabed.
On reflection 1: Heavy rainfall causes landslides in Hong Kong
Heavy rainfall yesterday caused landslides in the Sai Kung, Shatin, and Tai Po areas of Hong Kong yesterday. News reports indicate that 24 landslides have been observed.
On reflection 2: Coseismic landslides in Kamikochi, Japan
An ongoing swarm of small earthquakes is causing landslides and rockfalls in the Kamikochi area of Japan. The Asahi Shimbun has some good images.
5 June 2020
The Alta Quick Clay Landslide:- further footage
This new film provides some additional footage of the site prior to the main slide and some drone footage of the aftermath. In both cases the imagery is very revealing.
The early footage was also filmed by Jan Egil Bakkerby, who shot the main video that went viral yesterday. It shows the site shortly before the main failure. Interestingly, it clearly captures the aftermath of the earlier failures to which I alluded:-
The footage confirms that a substantial failure occurred prior to the main Alta quick clay landslide. There is a large vacated scar in the background, whilst in the foreground slumping has developed.
The video then includes the original footage of the landslide before switching to drone footage of the aftermath. There are a couple of things to note here. First, further failures have developed even after the viral video was captured. In particular, the landslide has developed laterally, so that a larger portion of the coast has now been affected:-
Second, at about 7 minutes 30 the footage captures a small retrogressive landslide in the rear scarp of the landslide:-
Notable here is the highly mobile nature of the failed material. This material was sufficiently strong to stand in a near-vertical scarp but, once failed, flows like a fluid. This is a good illustration of the very challenging geotechnical properties of quick clay.
Finally, for today, the text accompanying the video suggests that the first failure was observed at this site on 29 May 2020 by the Norwegian Water Resources and Energy Directorate (NVE).
On reflection 1: warnings of further landslides in Norway
NVE has warned that there is the possibility of further landslides in Norway. “It’s important to stress that there’s still a lot of snow in the mountains,” Bjørn Sønju-Moltzau, a hydrologist at state waterways agency NVE, told state broadcaster NRK on Thursday. “We’ve only gotten rid of around half of it. That means we still have the rest of it in the central and northern parts of the country.”
On reflection 2: Looking for southern Appalachian rockfall scars using a high-resolution LiDAR dataset
The Field blog, also hosted by AGU, has a nice article about the use of LIDAR for mapping the scars and tracks of rockfall boulders.