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17 November 2017

Detecting landslide precursors from space

Detecting landslide precursors from space

A holy grail of landslide studies is the ability to anticipate future behaviour by reliably detecting precursors.  This is a highly fraught activity at any level, with the complexity of landslide behaviour rendering the interpretation of patterns of movement challenging.  However, in situ detailed monitoring has yielded some interesting results over the years, and efforts continue.  Meanwhile, developments in satellite technologies mean that detecting deformation from space is becoming increasingly possible.  In particular, the InSAR technique, which uses techniques to compare repeated radar images, is very promising. The latest generation of ESA radar satellites, Sentinel 1A and 1B provide high quality imagery every six days in some cases.  Thus, intuitively it feels that there is the potential to use these techniques to detect precursory landslide activity that might indicate that a collapse is likely, providing mechanisms to anticipate failure events and thus make communities safer.

But for this to be possible, it is first necessary to use the archive of images that is now building up to look at the precursory activity for large failures that have occurred.  This should start to indicate what might be viable, and what we should be looking at.  In a paper just published in the journal Landslides, Intrieri et al. (2017) have examined an archive of 45 satellite images using the InSAR technique for the area that was affected by the 24th June 2017 Maoxian landslide, which killed over 100 people in the village on Xinmo in Sichuan Province.  I featured this large landslide at the time.  Sentinel 1A and 1B imagery is available for this site from 9th October 2014, with the last image before the collapse being captured on 19 June 2017, five days before the landslide.

Xinmo landslide

Recovery operations from the Xinmo landslide in China, showing the scale of the challenge. Image via Xinhua

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The image below, from the paper, shows the deformation map extracted from the images, and provides a plot of three reference points.  It is clear that in the source area of the landslide there was an area of active movement over the three years prior to the collapse.  The rates of movement were not very high, 40 – 60 mm over a three year year period prior to the acceleration to failure, but the technique is able to resolve them clearly.

precursors

Precursor movement of the Maoxian landslide, from Intrieri et al. (2017)

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Interestingly, for all three data points there is a marked acceleration in the Spring of 2017, showing that the slope was evolving towards collapse.  Intrieri et al. (2017) have been able to extract additional monitoring points from the data for this area of active deformation, which clearly demonstrate this accelerating pattern.  As I have noted previously, first time collapses in brittle rocks often show a hyperbolic increase in movement rate (or perhaps seismicity) prior to failure – the so-called Saito effect.   The Maoxian landslide shows this as well, according to Intrieri et al. (2017), starting in April 2017.

This is a really important and interesting paper.  It demonstrates that InSAR can be used to detect landslide precursors at a scale that is useful, and that the technique can extract the rate of movement to the extent that acceleration to failure can be detected.  That opens up the possibility – albeit with a great deal of additional work – of using these new satellite tools as a warning system.  That is fundamentally exciting.  The study also provides another detailed archive of the actual nature of these precursory movements.  And, importantly, it demonstrates how a first time failure develops across a slope.

Reference

Intrieri, E., Raspini, F., Fumagalli, A. et al. 2017. The Maoxian landslide as seen from space: detecting precursors of failure with Sentinel-1 data. Landslides. https://doi.org/10.1007/s10346-017-0915-7

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13 November 2017

The M=7.3 Halabjah earthquake in Iraq and Iran: potential landslides

The M=7.3 Halabjah earthquake in Iraq and Iran: potential landslides

The M=7.3 Halabjah earthquake in Iraq and Iran yesterday occurred at an estimated depth of 23.2 km, according to the USGS.  An event of this magnitude and at this depth has the potential to generate significant numbers of landslides if the topography is available.  As the region affected by the most intense shaking is very remote, reports will take some time to emerge, but some early reports indicate that they have occurred.  Thus, for example, Xinhua reports that:

Four people were killed and nearly 50 others injured by the damage of the houses, in addition to a landslide in a mountain adjacent to the Darbandikhan Dam, according to the report.

Whilst Sky News reports that:

The quake triggered landslides in the mountainous region along the Iran-Iraq border, also destroying buildings, shattering windows and sending people running for safety.

The Google Earth image below displays the USGS intensity contours on the terrain, and I have also highlighted the location of the Darbandikhan Dam:

Halabjah earthquake

Google Earth image that overlays the USGS intensity contours over the terrain for the M=7.3 Halabjah earthquake

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The Google Earth imagery certainly suggests that landslides are possible for this event, and it would be unsurprising to find multiple slips on cut slopes, especially on poorly engineered roads in the mountains.  The area to the east of the epicentre, shown below, looks particularly susceptible:-

Halabjah earthquake

Google Earth imagery of the area east of the epicentre of the Halabjah earthquake

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However, at this stage it is very difficult to know what will have happened in terms of landslides.  The first is that the landslide consequence of a an oblique-thrust faulting event is not obvious to me; much will depend on the peak ground accelerations observed across the landscape.  The second is that we have few records of the ways in which earthquakes in very arid mountain areas trigger landslides.  In some previous events, such as the Sierra Cucapah earthquake in Mexico in 2010, landslide initiation was surprisingly low. On the other hand, the event yesterday was only 250 km from the extraordinary Saidmareh landslide, one of the largest known failures on the planet, and which is likely to have been triggered by a previous earthquake (see image below).  The likelihood of such a landslide in this earthquake is very low, but it does illustrate the uncertainties in terms of the landslide distribution that we may observe.

Halabjah earthquake

Google Earth image showing the location of the Saidmareh landslide in Iraq relative to the Halabjah earthquake in Iraq

The first satellite images of the area affected by the earthquake are likely to be collected today, so within a couple of days we should start to get an idea of the impacts.

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10 November 2017

The Mocoa debris flow disaster: satellite imagery reveals the cause

The Mocoa debris flow disaster: satellite imagery reveals the cause

The Mocoa debris flow disaster, which occurred in Colombia on 1st April 2017, is believed to have killed 399 people, and left a further 329 people injured.  Triggered by heavy rainfall, the debris flow swept through the town at 3 am.  I discussed the landslide at the time, and speculated on possible causes, but at that stage it was hard to know what had happened upstream of the town to cause the disaster.  In May GFZ Potsdam provided an initial analysis of satellite images of the site.

Seven months on there is an archive of satellite imagery of the area available.  This is not an easy place to image because of the high prevalence of cloudy conditions in the mountains, but in the PlanetLabs archive it is now possible to find before and after images.  This is a RapidEye Ortho image, with 5 m resolution, collected before the disaster on 22nd November 2017:-

Mocoa debris flow disaster

Rapideye Ortho image of the Mocoa area, collected on 22 November 2016, via PlanetLabs.

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This is a further RapidEye Ortho image of the same area, collected on 10th April 2017, nine days after the disaster:

Mocoa debris flow disaster

RapidEye ortho image of the Mocoa debris flow disaster collected on 10th April 2017, via PlanetLabs

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The aftermath of the disaster is clear, with extensive destruction to the western part of the town.  To the west and northwest of the town very large numbers of landslide are visible.  It is worth taking a look at that area in a little more detail – this is a zoomed in version of the image from 10th April showing the area to the immediate northwest of the town, which can just be seen in the bottom right hand corner:-

Mocoa debris flow disaster

The aftermath of the Mocoa debris flow disaster. RapidEye Ortho image collected on 10th April 2017 via PlanetLabs

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It is clear that upstream of the town the rainfall triggered hundreds of small, shallow landslides on the eastern-facing slopes.  Most of these landslides connected directly into the channels, and thus almost simultaneously would have fed debris into the drainage system.  It is this water and debris, that combined to create the catastrophic debris flow that destroyed the town.

A really interesting aspect of this is that it appears that this may have been generated by a very isolated, presumably extremely intense rainfall event.  As the secon image above shows, the slopes to the east of the town did not undergo intense landsliding. This is also true of the slopes to the southwest of Mocoa (as the second image shows).  Indeed the area of very intense landslides is no more than 10 km from north to south and 3 km from eat to west.  Mocoa was incredibly unfortunate to lie downstream of this area of rainfall and resultant landslides.

Reference

Planet Team (2017). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://api.planet.com

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9 November 2017

Corinto in Colombia: first satellite images of the mudflow that has killed 4 and left 18 people missing

Corinto in Colombia: first satellite images of the mudflow that has killed 4 and left 18 people missing

On Tuesday afternoon a devastating mudflow struck the town of Corinto in Colombia.  At the time of writing the confirmed death toll is four people, but a further 18 are reported to be missing.  A total of 29 people were injured, and at least 22 houses were destroyed,.  The mudflow struck during a period of heavy rainfall, but fortunately it appears that a warning system allowed many residents to escape the flowEl Pais has an excellent gallery of images of the aftermath of the mudflow, including this overview taken from the air:-

Corinto

The aftermath of the mudflow in Corinto, Colombia. Image via El Pais.

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This is a Planet Labs image of the town collected on 2nd November 2017, the last cloud free occasion when one of their satellites passed over the site:-

Corinto

Planet Labs image of the town of Corinto, collected on 2nd November 2017. Used with permission of Planet Labs.

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Whilst this image, taken on 8th November 2017, shows the aftermath of the mudflow:

Corinto

Planet Labs image of the town of Corinto, collected on 8th November 2017, showing the aftermath of the mudflow the day before. Used with permission of Planet Labs

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Unfortunately there is too much cloud covering the hills upstream of the town to see what has happened there as yet.  This is just the most recent in a long sequence of landslides in Colombia, with the most recent large event, at Mocoa in April, having caused at least 273 deaths.  In May 2015 a mudflow killed at least 63 people at Salgar in Antioquia. There are notable similarities between those previous landslide events and this one.  In each case the community was built in proximity to a channel fed from an upland area.

Reference

Planet Team (2017). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://api.planet.com

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8 November 2017

Evidence of catastrophic landsliding from medieval earthquakes in Nepal

Evidence of catastrophic landsliding from medieval earthquakes in Nepal

Recent earthquakes in high mountain areas have generated catastrophic landsliding – in recent years I have featured examples from China, Nepal and New Zealand on this blog for example.  Patterns of landsliding vary greatly though, and remain quite poorly understood.  For some years there has been evidence that far more catastrophic landsliding than this might have been associated with past earthquakes in Nepal, as evidenced by huge valley fills in the Pokhara basin, part of the Seti Khola gorge.  This is the site of the huge landslide and debris flow of a few years ago that we analysed here, but these landslides would have been on a far greater scale.  In recent years this record of historic landsliding has slowly been unpicked and analysed, and a paper just published in Quaternary Science Reviews (Stolle et al. 2017) provides new insight.

The basis of the work is a huge (148 km²) fan in the vicinity of the town of Pokhara below the Annapurna Massif, which has inundated 15 tributary valleys.  This fan can be seen in the Google Earth image below.  There are older valley fills in the Pokhara basin, so the events uncovered by this research may not be unique.  The team have mapped and logged the stratigraphy of these deposits, and they have dated them extensively.  They have found that the deposits represent three pulses of sediment delivery from the mountains, dated at around 1100 AD (providing sediments that are up to 25 m thick, representing a volume of about 1.9-3.3 km³ of sediment), 1255 AD (2.9-3.7 km³) and 1344 AD (0.3-0.5 km³).

catastrophic landsliding

Google Earth image of the Pokhara area, with the Annapurna massif behind. The town is built on the huge fan deposit that resulted from catastrophic landsliding generated by three medieval earthquakes.

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Stolle et al. (2017) interpret these deposits as being the geomorphic legacy of earthquake-driven sedimentation.  They consider that each of these three deposits was the result of a large earthquake, each of which generated huge debris flows that, as they put it, invaded and plugged the tributary valleys, allowing lake deposits to form.  There is evidence from archive sources of two of these large earthquake events.  What is not clear as yet is the source of these huge debris flows – were they massive rockslope failures that transitioned into channelised debris flows on an epic scale, or were they triggered by the collapse of valley-blacking landslide dams or suchlike?  This will require fieldwork in the high mountains, an environment in which it is very hard, and very expensive, to work.

Stolle et al. (2017) note that these huge deposits are essentially unique to this area of Nepal at present – similar scales of deposits have not been identified elsewhere as yet.  But they do suggest that earthquakes have the potential to generate catastrophic landsliding on a scale far greater than we have observed to date, which of course has important implications for our analysis of hazard.

Reference

Stolle, A., Bernhardt, A., Schwanghart, W., Hoelzmann, P.,  Adhikari, B.R., Fort, M. and Korup, O. 2017  Catastrophic valley fills record large Himalayan earthquakes, Pokhara, NepalQuaternary Science Reviews, 177,  88–10

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6 November 2017

The Nuugaatsiaq landslide in Greenland: understanding the failure processes

The Nuugaatsiaq landslide in Greenland: understanding the failure processes

An interesting paper has just been published in the journal Geophysical Research Letters (Poli 2017) that examines the seismic record leading up to the failure of the Nuugaatsiaq landslide in Greenland in June.  This is the event that triggered a significant local tsunami that killed four people.  The landslide was located close a set of seismic stations that meant that good data were collected leading up to the failure; as I noted previously, a series of small seismic events prior to the main collapse had been observed in the record by Jackie Caplan-Auerbach; this new paper seeks to analyse and interpret them.

For me the most interesting aspect of this paper is that the event rate (i.e. the number of mini-earthquakes) increases with time leading up to the final failure event.  Poli (2017) provides this illustration of this effect (there are other data in the original that I have removed for clarity):

uugaatsiaq landslide

Increasing seismic event rate for the Nuugaatsiaq landslide, after Poli (2017).

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In the paper, the author describes this increasing event rate as being exponential, but in fact this is not correct.  The actual function in hyperbolic; the distinction is important.  Work that I did with Chris Kilburn some years ago (Kilburn and Petley 2003) and then re-examined using field and lab data (e.g. Petley et a. 2005) showed that hyperbolic increases in event rate are associated with brittle processes, representing the first time failure of the landslide in brittle materials, driven by the growth of the shear surface as a fracture.  In this interpretation, the seismic events represent the failure of the rock bridges as the fracture grows.  The failure of each successive rock bridge increases the stress on the remaining intact rock bridges, which in turn drives this increasing event rate.

Non-brittle processes also show an increasing event rate, but in this case the characteristic function is exponential.

A fascinating aspect of this is that, in cases in which this processes is operating, it is possible to predict the time of final failure from the event rate data.  This is the so-called Saito effect.  The trick is to plot 1/event rate or 1/velocity (depending on which data are available – Saito used velocity).  In a hyperbolic acceleration to failure this plots as a straight line.  I have extracted the data from the Poli (2017) paper, and plotted this graph:

Nuugaatsiaq landslide

The “Saito effect” graph for the Nuugaatsiaq landslide. Data from Poli (2017)

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The time of failure can be predicted as the point at which the trend reached 1/event rate = 0 (i.e. a hypothetical infinitely high event rate).  This is characteristic of first time failures; we do not see this effect in reactivations of landslides.

The analysis of the seismic signals from the landslide by Poli (2017) is fascinating, but needs to be seen in the context of the understanding of the mechanics of first time failures in slopes rather than in the behaviour of earthquake faults (which are usually ruptures on an existing failure planes).

References

Kilburn, C.J. and Petley, D.N. 2003.  Forecasting giant, catastrophic slope collapse: lessons from Vajont, Northern Italy.  Geomorphology 54, 1-2, 21-32.

Petley, D.N., Higuchi, T., Petley, D.J., Bulmer, M.H., and Carey, J.  2005. The development of progressive landslide failure in cohesive materials.  Geology, 33, 3, 201-204.

Poli, P. 2017. Creep and slip: Seismic precursors to the Nuugaatsiaq landslide (Greenland). Geophysical Research Letters 44 (17), 8832-8836.

 

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5 November 2017

George Town: another serious landslide in Penang

George Town: another serious landslide in Penang

Heavy rainfall in George Town, Penang overnight has triggered another significant landslide in Malaysia.  This new landslide, which fortunately did not kill anyone, occurred close to the Tanjung Bungah landslide last month.  Once again the slide has occurred at a site under construction.  The New Straits Times has an article about the landslide that includes this image:-

George Town

New Straits Times image of the landslide at George Town in Penang, Malaysia yesterday. Image by RAMDZAN MASIAM.

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The houses upslope from the failure are newly constructed and are reportedly empty at the moment. This image, also from the New Straits Times, shows the damage to them and to the road:-

George Town

Damage to the road and houses caused by the landslide in George Town, Penang. Image via the New Straits Times / RAMDZAN MASIAM.

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This is clearly a rotational failure in an engineered slope with a retaining structure.  I suspect that the landslide involves fill behind the retaining wall, presumably emplaced to create the bench for the houses and road.  It seems to me that the slide surface may well extend beneath the footing of the retaining wall – it can be seen to toe out in the disturbed vegetation downslope from the wall in the first image.  It would be interesting to know how deep the foundation of the wall extends.

Rotational failure below the footing of a retaining structure is a well-known hazard, and the design of the wall should consider this problem, and ensure that it does not materialise.  In a structure so new it is possible that this is an error in the design and/or construction of the retaining wall and associated works.  A proper investigation is needed to decide.

Penang has now suffered two major construction-related landslides in less than a month.  This must be a real source of concern.

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3 November 2017

Managing the hazards in Franz Josef, New Zealand

Managing the hazards in Franz Josef, New Zealand

Franz Josef is a small town on the west side of South Island in New Zealand.  Whilst its permanent population is small (510 people), it hosts about 700,000 visitors per year, many of whom visit the Franz Josef Glacier.  Unfortunately, Franz Josef is also one of the most hazard-prone communities in New Zealand.  Located right on the range front, Franz Josef is built upon the Alpine Fault, which typically ruptures about every 300 years (last event 1717); it is next to the Waiho River, which has an increasing level of flood risk due to aggredation of the river bed; and it is below a slope that is potentially unstable  in a large earthquake (and which I described previously).  These hazards can be seen in this Google Earth image of the town and surrounding landscape:-

Franz Josef

Google Earth image showing the landscape around the town of Franz Josef in New Zealand

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To consider the pros and cons of the various options, the local authority (West Coast Regional Council) have commissioned a study by Tonkin and Tayor, which is now available online as a PDF.  This study has examined a range of options for the town, including complete relocation and a minimal intervention approach.  The conclusions are clear:

Do nothing is not an option
There is significant flood hazard at present, and due to ongoing bed aggradation that flood risk (and the cost of mitigating it) increases with time. There is already flood risk to assets below the river bed. If left unmanaged, it is likely to be less than 30 years before the Waiho River is above the level of the township. This places a level of urgency on a decision to manage the risk from this hazard.

Additionally, an Alpine Fault rupture event would cause significant disruption to Franz Josef township, including impacts caused by shaking across the wider Franz Josef area and uplift and / or lateral spread along the active fault line(s). It is also possible (if not probable) that an Alpine Fault rupture could trigger a large landslide, which would have catastrophic consequences in terms of loss of life, building stock and tourist revenue.

Franz Josef

A stopbank on the Waiho River in New Zealand, protecting the town of Franz Josef. Note the height of the river bed on the left side, and the elevation of the buildings on the right.

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The report proposes three options in terms of interventions:-

Avoid nature’s most significant challenges: This package seeks to physically avoid the natural hazard challenges in Franz Josef by moving the township to Lake Mapourika, out of the area subject to flooding from the Waiho River and away from the Alpine Fault and the range-front landslide risk. This package may create new investment opportunities, in addition to protecting the tourism value currently generated by the township.

Live with nature’s challenges: This package involves generally decreasing stopbank management and allowing the river to fan out in its natural pattern, which will reduce flooding risk and flood management costs. It also allows for relocating township assets off the active known fault line in the short- to medium-term; however, over time the value of the land to the south of the Waiho River will be eroded due to increased flooding risk.

Defend against nature’s challenges: This package involves continuing to build stopbanks and implementing a gravel extraction programme to allow the township, and the businesses and residents in the wider Franz Josef area, to remain in their current locations with lower flooding risk. It also allows for relocating township assets off the active known fault line in the short to medium-term to reduce earthquake-related risk. The costs of gravel management will occur in perpetuity and the town will remain exposed to residual flooding risk from stopbank failure or overtopping.

Unfortunately none of the options is compelling relative to the others, and all have pros and cons.  For example, the cost of relocating the town to Lake Mapourika is estimated to be about NZ$300 million with an anticipated benefit of just NZ$120 million.

The report makes kit clear that the magnitude of the problem is so large that this is a national level problem, rather than one that can be managed regionally.  But the solution is far from clear for a country already dealing with the effects of recent large earthquakes.

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31 October 2017

Planet Labs imagery of the Mishor Rotem tailings dam failure

Planet Labs imagery of the Mishor Rotem tailings dam failure

Back in early July I blogged about yet another tailings dam failure, this time at the Mishor Rotem facility in Israel.  I have seen remarkably little about the Mishor Rotem tailings dam failure since – far less than for many other tailings dam events.  Lindsay Newland Bowker has an article on her blog about it that includes this photograph of the failure:

Mishor Rotem tailings dam

A photograph of the Mishor Rotem tailings dam failure, via Lindsay Newland Bowker and REUTERS/Baz Ratner

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She reports that 100,000 m³ of tailings were released in the collapse event, flowing 20 km down the Ashalim riverbed. This is a Planet Labs satellite image of the site captured the day before the failure:-

Mishor Rotem tailings dam

Planet Labs image of the Mishor Rotem tailings dam failure site,collected on 30th June 2017.

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The tailings pond that suffered the failure can be clearly seen in the lower central part of the image.  Compare that with this image, taken on 1st July:-

Mishor Rotem tailings dam

Planet Labs image of the Mishor Rotem tailings dam failure, collected on 1st July 2017.

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The breach in the dam wall is clearly visible, as is the spilt material.  Note the trail of waste in the river bed in the northeast corner of the image.  A clearer view of the breach is visible in this more recent Planet Labs image:

Mishor Rotem tailings dam

Planet Labs image of the Mishor Rotem tailings dam failure, collected on 25th October 2017

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The trail of environmental damage left by the tailings dam failure can be clearly seen in this Planet Labs image, collected on 2nd July 2017:

Mishor Rotem tailings dam

The trail of environmental damage caused by the Mishor Rotem tailings dam failure. Image by Planet Labs on 2nd July 2017.

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It will be interesting to see if there is transparency in terms of the investigation of this tailings dam failure, in the way that there has been for other recent events.  Given the frequency of these events (this listing makes for dreadful reading), it is important that each event is analysed and that the lessons are shared.

Reference

Planet Team (2017). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://api.planet.com

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30 October 2017

USGS mapping of landslide density for failures triggered by Hurricane Maria in Puerto Rico

USGS mapping of landslide density for failures triggered by Hurricane Maria in Puerto Rico

The USGS has now posted online a map of the landslide density for failures triggered by Hurricane Maria in Puerto Rico in September 2017, following on from their earlier work on landslide impacts. The mapping has been undertaken using high resolution satellite images.  At this stage the mapping has been undertaken on a grid square basis, with each square having an area of 4 km², with a simple three-fold classification of no landslides, fewer than 25 landslides per km², and more that 25 landslides per km².  The resultant map is shown below:-

landslide density

USGS mapping of landslide density in Puerto Rico as a result of Hurricane Maria

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A simple comparison with the topography of Puerto Rico suggests that, at a first order, the landscape has been a key control on the occurrence of landslides (map via Dr Jose Javier Hernandez Ayala):-

landslide density

The topography of Puerto Rico. Map by Dr Jose Javier Hernandez Ayala

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The Washington Post published a map of the precipitation deposited by Hurricane Maria in Puerto Rico:

landslide density

Total precipitation from Hurricane Maria in Puerto Rico, via the Washington Post

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This suggests that the landslide distribution can be primarily explained by the coincidence of intense precipitation and steep topography, which is of course unsurprising.  I am sure that a more detailed analysis will be undertaken in due course. The upshot is a profoundly altered landscape.  This is a Planet Labs image of the area around Lago Caonillas taken on 12th September 2017, about a week before Hurricane Maria:-

landslide density

Planet Labs image of the area around Lago Caonillas in Puerto Rico, collected on 12th September 2017

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And this is the same area, after the storm, on 4th October 2017:-

landslide density

Planet Labs image of the area around Lago Caonillas in Puerto Rico, collected on 4th October 2017

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References

Erin K. Bessette-Kirton, Jeffrey A. Coe, Jonathan W. Godt, Jason W. Kean, Francis K. Rengers, William H. Schulz, Rex L. Baum, Eric S. Jones, and Dennis M. Staley (2017).  October 25, 2017: Map data showing concentration of landslides caused by Hurricane Maria in Puerto Rico. USGS: https://landslides.usgs.gov/research/featured/2017-maria-pr/

Planet Team (2017). Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://api.planet.com

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