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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:-

1806 landslide near Rossberg

Vue du vallon entre le Rossberg e le Rigi apres la terrible catastrophe du 2e Septembre 1806.

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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.

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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.

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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.

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10 June 2020

The role of earthquake and rainstorm induced landslides in shaping mountain chains

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.

The role of landslides in mountain chains

The role of landslides in mountain chains: the landscape of the New Zealand.

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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.

Reference

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

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9 June 2020

Landslides triggered by the Mw = 7.8 14 November 2016 Kaikoura earthquake: an update

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:-

Kaikoura earthquake landslides

Map of the distribution of landslides triggered by the 2016 Kaikoura earthquake in New Zealand. Map from Massey et al. (2020).

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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.

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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.

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On reflection 2: Landslides in Space 2

A global map of rockfalls on the moon, produced by a team from ETH.

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Reference

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

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8 June 2020

A further significant failure at the site of the Alta quick clay landslide

A further significant failure at the site of the Alta quick clay landslide

In Friday night a further significant failure occurred at the site of the Alta Quick Clay landslide.  The best image that I have found of this new failure is on the nettavisen.no website:-

Alta quick clay landslide

The new failure, on Friday 5 June 2020, at the site of the Alta Quick Clay landslide. Image from www.nettavisen.no, Photo: Anders Bjordal / NVE / NTB scanpix Foto: (NTB scanpix).

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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:-

Alta quick clay landslide

A view of the scar of the Friday 5 June 2020 Alta quick clay landslide. Image from iTromos, Photo: Anders Bjordal / NVE / NTB scanpix.

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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.

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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.

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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.

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5 June 2020

The Alta Quick Clay Landslide: further footage

The Alta Quick Clay Landslide:- further footage

Further footage has been posted to Youtube showing the Alta Quick Clay landslide on 3 June 2020, which I featured yesterday:-

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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 Alta Quick Clay landslide

A still from new footage capturing the Alta Quick Clay landslide. This image shows the aftermath of an earlier failure. Footage by Jan Egil Bakkeby, posted to Youtube.

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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:-

Alta Quick Clay landslide

A still from new footage capturing the Alta Quick Clay landslide. This drone image shows the extensive area affected by the multiple failure events. Footage by posted to Youtube.

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Second, at about 7 minutes 30 the footage captures a small retrogressive landslide in the rear scarp of the landslide:-

Alta Quick Clay landslide

A still from new footage capturing the Alta Quick Clay landslide. This drone image shows a small secondary failure in the headscarp. Footage posted to Youtube.

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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).

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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.”

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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.

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4 June 2020

Alta: a truly remarkable video of a quick clay landslide in Norway

Alta: a truly remarkable video of a quick clay landslide in Norway

One of the top ten landslide videos of all time was captured yesterday at Kråknes in Alta, in the north of Norway. This video shows what is almost certain to be a quick clay landslide.  It occurred in the morning of Wednesday 3 June 2020; the video has been posted to Youtube. If this video ceases working then it can also be viewed here.

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VG.no has an article (in Norwegian) that explains the context of the video. It was collected by the owner of one of the houses, Jan Egil Bakkeby. He notes that they saw a tension crack downslope from their cabin on Tuesday evening, but chose to stay overnight.  It rained on Wednesday morning, after which the landslide occurred. He fled from the house and climbed the hill, and then shot the video.

The landslide appears to have occurred as a coherent raft in the first instance:-

Alta quick clay landslide

The initial phases of the Alta quick clay landslide. Still from a Youtube video collected by Jan Egil Bakkeby.

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There is a change in gradient of the underlying bedrock towards the sea, which caused the raft to start to fragment in the latter stages:-

Alta quick clay landslide

The latter phases of the Alta quick clay landslide. Still from a Youtube video collected by Jan Egil Bakkeby.

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There is a great deal of valuable information in the video about the behaviour of the landslide itself, and the way that it generated a displacement wave.

I suspect that this was the second phase of the landslide. The very early shots seem to show considerable existing damage at the front of the slide, on the right side of this screenshot:-

Alta quick clay landslide

The very early phases of the Alta quick clay landslide. Still from a Youtube video collected by Jan Egil Bakkeby.

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Quick clays are glaciomarine materials that have strange properties.  When disturbed they are very weak – indeed their behaviour is similar to that of a fluid.  But undisturbed they are much stronger, primarily because of the role of salt, which glues the particle structure together.  When this structure is disturbed, the quick clay rapidly weakens, allowing these spectacular landslides to form.

I have featured a number of other quick clay landslides over the years, including examples from Sweden, Norway and Canada, and a similar type of landslide from Brazil.  The most famous example is the Rissa landslide, also in Norway, for which another excellent video is available.

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On reflection 1: Further landslides at Eastchurch in Kent

Yesterday further failures occurred at Eastchurch in Kent, the site of the coastal landslide that destroyed a house over the weekend.  Further properties are now at risk.

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On reflection 2: saving lives at Brumadinho

An interesting paper is available for online review in the journal NHESS looking at ways in which lives could have been saved during the Brumadinho tailings dam failure last year.

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Acknowledgement

Many thanks to the various people who highlighted this to me via this site, by email and on Twitter. Your help is much appreciated.

 

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2 June 2020

The evolution of co-seismic landslides: the 2005 Kashmir earthquake

The evolution of co-seismic landslides: the 2005 Kashmir earthquake

As I have documented on this site on many occasions, large earthquakes are a highly efficient way to generate landslides in mountainous areas. Landslides kill people directly, but also act to increase the losses from building collapses by preventing access to rescue personnel and impeding the provision of medical assistance.  Landslides also prolong the impacts of the earthquake, often for years. Typically landslides occur extensively in heavy rainfall events in the years after the earthquake, causing high levels of damage and loss of life.

The latter point means that understanding the evolution of slopes after large earthquakes is important.  There are many unanswered questions – for how long would we expect to see an elevated level of landsliding? What controls this length of time? And how (and indeed why) does the decline in landslide activity actually occur?

A new paper in the journal Geomorphology (Shafique 2020) explores this issue for the 2005 Mw=7.6 Kashmir earthquake in Pakistan and India. This is one of the worst landslide disasters of the last two decades, with thousands of landslide related fatalities. The author has used archive SPOT satellite imagery to map the landslides in the earthquake affected area of Pakistan, in the vicinity of the towns of Balakot and Muzaffarabad, before and after the earthquake.

The results of the study are summarised in this graph:-

Kashmir landslides

The evolution of the surface area of landslides associated with the 2005 Kashmir earthauke. Image by Shafique (2020).

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Prior to the earthquake Shafique (2020) mapped 37 landslides in the study area. The first mapping exercise after the earthquake used images dated 20 October 2005, 12 days after the main shock.  The number of landslides had increased to 219, an increase of about six times.  This image, which I took in early 2006, shows some of the landslides triggered by the earthquake:-

Kashmir landslides

Landslides triggered by the 2005 Kashmir earthquake.

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The satellite mapping is only able to identify the larger landslides, so the inventory should not be used to determine the total number of landslides triggered.  As the graph shows, by the time of the next mapping exercise, in 2010, the number of landslides had started to decline.  This reduction accelerated, such that by 2014 the number had reduced markedly. By 2018, the number of landslides was close to the number prior to the earthquake.

Thus, in the Kashmir earthquake case it appears that it has taken in the order of 13 years or so for the level of landslide activity to decline back to pre-earthquake levels.  Shafique (2020) rightly highlights that there is an extra factor in play.   Other studies have indicated that the number of landslides increased by about 250% in the first summer monsoon (in 2006) after the earthquake. This would certainly accord with my own observations – I was there in May 2006, shortly before the monsoon began, and was deeply concerned by the number of partially failed slopes:

Kashmir landslide

A partially failed slope in Kashmir, taken before the 2006 monsoon season

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Thus, the peak landslide area was probably higher than the graph from Shafique (2020) might indicate.

The decline in landslide activity from this study has taken longer than has been the case for some other large earthquakes.  Shafique (2020) suggests that a major factor in the reduction might be the regrowth of vegetation.  Perhaps the climate of Kashmir, which is arid outside of the short monsoon season, means that this process takes longer than in many other mountain chains.

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On reflection 1: A lucky escape!

Climbers and hikers in Vermont had a lucky escape when boulders  tumbled down the mountainside at Smugglers Notch.

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On reflection 2:

The fall-out from the cliff collapse at Eastchurch in Kent continues. Ironically, the Daily Mirror reports that the house damaged by the landslide was called Cliffhanger.

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Reference

Shafique, M. 2020. Spatial and temporal evolution of co-seismic landslides after the 2005 Kashmir earthquake. Geomorphology, 362, 107228. https://doi.org/10.1016/j.geomorph.2020.107228

 

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1 June 2020

Eastchurch – a coastal landslide in Kent, SE England

Eastchurch – a coastal landslide in Kent, SE England

Over the weekend the newspapers in the UK have been reporting two coastal landslides that occurred at Eastchurch on the Isle of Sheppey in Kent.  These two landslides, which occurred at the same site, have damaged a house.  The first failure occurred at 10 pm on Friday 29 May 2020:-

Eastchurch landslide

The first landslide at Eastchurch on the Isle of Sheppey in Kent. Image via the Independent.

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This is quite an interesting landslide, with an unusual linear geometry in the rear scarp, similar to a wedge.  The Isle of Sheppey consists mainly of London Clay, which is well known for the presence of fissures and joints.

Over the weekend the situation deteriorated substantially, and on Sunday 1 June 2020 a further substantial landslide occurred, causing damage to the house closest to the cliff:-

Eastchurch landslide

The second landslide at Eastchurch on the Isle of Sheppey in Kent. Image via the Independent.

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The site of the landslide is 51.418, 0.873, as shown below.  I have marked the rear scarp of the second landslide at Eastchurch:-

Eastchurch landslide

Google Earth image of the site of the Eastchurch landslide.

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As the Google Earth image shows, this section of coast is highly landslide-prone.  Indeed, the Isle of Sheppey is well-known for its coastal landslides in the London Clay.    However, there are two aspects of this event that are quite surprising to me.  The first is that the slide has occurred in the middle of a spell of exceptionally dry weather – indeed this has been the sunniest Spring on record and parts of the UK have recorded their driest Spring on record too.  Perhaps this slide was associated with dessication rather than high pore water pressures?  But second, the images after the landslide yesterday seem to show a great deal of water in the rear scarp of the landslide:-

Eastchurch landslide

Image showing the aftermath of the second Eastchurch landslide. Note the large volume of water. Image from Kent Live.

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It is not clear to me as to the source of this water, although it could be that the failure has ruptured a pipe?

It goes without saying that this is a devastating landslide for the occupants of the house.  In general, UK house insurance does not cover damage caused by landslides or by coastal erosion.

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On reflection 1: too good to be true?

Various news outlets report a miracle recovery of a newborn baby buried in a landslide in Northern India.  The baby appears to be thriving in hospital, but the idea that a baby could survive burial in a landslide without any protection is surprising.

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On reflection 2: 50 years after Yungay

Yesterday was the 50th anniversary of the devastating landslide at Yungay and Ranrahirca in Peru, caused by a seismically triggered landslide from Mount Huasaran.

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28 May 2020

The July 2018 Xe Nammoy hydropower complex dam failure: a new paper

The July 2018 Xe Nammoy hydropower complex dam failure: a new paper

On 23 July 2018 a saddle dam at the Xe Nammoy hydropower complex in Laos failed and breached, releasing 350 million cubic metres of water. The resultant flood inundated an area of about 46 square kilometres along the Vang Ngao River, a tributary of the Mekong River basin, causing massive damage. I featured a detailed review of that event by Richard Meehan and Douglas Hamilton in 2019.  They considered the cause of the failure:

An initial review of this failure by the first author was presented in late 2018, and was followed six months later by a review by an independent expert panel drawn by the Lao government from the International Committee on Large Dams (ICOLD). Both reviews concur in finding that the failure was caused by a foundation failure beneath one of the project saddle dams.

A paper has recently been published in the journal Geomorphology (Latrubesse et al. 2020) that also consider carefully the causes and impacts of this event.  Whilst the paper is focused mainly on modelling and understanding the flood that resulted from the breach of the dam, it also considers the failure mechanism of the dam itself.  Interestingly the authors have examined the materials from which the dam was constructed.

What is not in doubt is that heavy rainfall prior to the failure induced the breach event.  However, the dam did not overtop – indeed analysis in the paper suggests that the water was at least 15 m below the crest of the saddle dam when failure occurred.  This suggests that the problem was a structural problem within the dam or within its foundation. Latrubesse et al. (2020) provide this illustration of the aftermath of the failure at the saddle dam.  This is the clearest picture of the failure site that I have seen:-

The Xe Nammoy hydropower complex

The site of the saddle dam breach at the Xe Nammoy hydropower complex in Laos. Image from Latrubesse et al. (2020), using a drone video published at https://news.v.daum.net/v/20180803103600946?f=m.

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Note the weathered material that formed the saddle dam, sitting on top of basaltic bedrock.  Images B and C show slumping in the aftermath of the breach.

The core of the dam used weathered materials quarried locally.  The research team examined the characteristics of these materials.  They concluded that the dam materials may have had a lower clay content than the designers had anticipated, which in turn provided a higher level of permeability than had been expected.  Thus, Latrubesse et al. (2020) suggest that water penetrated into the core of the dam, driving piping and, ultimately, triggering a rotational failure in the dam itself, which then allowed the breach to occur.

This mechanism of failure is a hypothesis rather than a definitive analysis.  But of course it is interesting at this point because of the similarity in mechanism to the failure of the Edenville Dam earlier this month.

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On reflection 1: an official report on a collision between a train and a landslide in the UK in 2019

The UK Rail Accident Investigation Branch has published a report into a collision between a train and landslide debris at Corby in Northamptonshire on 13 June 2019.  Key finding:

The investigation found that the cutting slope had failed because it was not designed to cope with a large volume of water that had accumulated at its crest. Flood water had accumulated at the crest because two adjacent flood storage ponds had overfilled with water from a nearby brook.

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On reflection 2:  Coastal rockfalls in Sidmouth, Devon

The coastal cliffs of Sidmouth in Devon, in the Southwest of the UK, have undergone three significant collapses in a 24 hour period. Many parts of the UK are undergoing an exceptionally dry Spring, so the failures are generating large plumes of dust.

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Reference

Latrubesse, E.M, Park, E., Sieh, K. et al. 2020. Dam failure and a catastrophic flood in the Mekong basin (Bolaven Plateau), southern Laos 2018. Geomorphology, 352, 107221.

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27 May 2020

The aftermath of the Sanford dam failure in Michigan

The aftermath of the Sanford dam failure in Michigan

The catastrophic failure of the Edenville Dam in Michigan last week was not the only dam failure that day.  About 16 km downstream from Edenville lay the smaller Sanford Dam, which also failed. In a sense this is an understandable collapse – the dam would not have been constructed to withstand the flows associated with the failure of the Edenville Dam. It would perhaps have been surprising if the structure had withstood such an event.

Planet Labs have collected images of the aftermath of the failure of the Sanford Dam.  This is a Google Earth image, collected in November 2018, which shows the site of the dam:-

Sandford Dam

Google Earth image of the site of the Sanford Dam, collected in November 2018.

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Note that the fuse plug, designed to allow an emergency increase in flow to prevent dam failure, is clearly visible.  This is the Planet Labs high resolution SkySat image of the aftermath of the failure:-

Sandford dam breach

Planet Labs image of the aftermath of the Sanford dam breach. Image copyright of Planet Labs, used with permission.

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The terrible flood damage downstream of the breach is all too evident.  The dam itself has been almost completely removed.

Rebuilding these sites is going to be a long and expensive process.

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An early failure similar to the Edenville Dam?

Meanwhile, in the comments on my earlier posts about the Edenville Dam, Bruce Feinberg has noted the similarity with the failure of the Kelly Barnes Dam on 6 November 1977 at Toccoa in Georgia, USA.  This was another earthen dam that breached during heavy rainfall, killing 39 people.  The USGS investigated this failure, and the report is online.  The report makes shocking reading – the dam was poorly documented and in a very poor state of repair at the time of failure. Photographs from 1973 show that a slope failure in the face of the dam had already occurred. The USGS report hypothesises that the breach may have been caused by a further slope failure in the downstream face of the dam:

[Slope failure] appears to be a distinct possibility, particularly on the downstream slope when the previous slope failure is considered along with the possibility of the development of tension cracks upslope of the previous failure together with a computed factor of safety that is marginal. The long period of rain would have saturated tension cracks, if they existed, and the entire downstream slope would have become essentially saturated and even more susceptible to failure. A local downstream slope failure similar to that observed in 1973 could have caused limited breaching allowing localized overtopping. This concept would corroborate the hydraulic computations.

This proposed mechanism of failure is indeed similar to that of the Edenville Dam.

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On reflection 1: no resting place

A landslide has disturbed Vicksburg National Cemetery in Mississippi, requiring archeologists to relocate the remains of the Civil War Union soldiers.  Work will now be undertaken to stabilise the slope.

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On reflection 2: A landslide video from Colombia

A nice landslide video was captured on the Florencia – Neiva highway in Colombia:-

 

 

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