More El Salvador lahar satellite imagery

In an earlier post today I highlighted the availability of satellite imagery covering the El Salvador lahar disaster, triggered by Hurricane Ida earlier this month. Whilst sitting in a conference session this morning it occurred to me that the Disaster Charter, which provides satellite images for disaster relief and recovery operation, was also triggered by this event. So I had a quick look at the web page for this triggering of the charter, and sure enough there are some good materials there.

Perhaps the best image is this interpretation of the lahar tracks, produce using the Taiwanese Formosat2 satellite (famous for its images of Beichuan and Tangjiashan after the Wenchuan Earthquake) - do click on the image to see this properly, it is worth it!:

There is also a more detailed look at the town of Guadeloupe using the same satellite, showing before and after images of the town:

Viewed in conjunction with the NASA images in my earlier post, this is a very useful resource.

Satellite imagery of the El Salvador lahars

NASA has released before and after satellite images of the lahars (volcanic debris flows) triggered by Hurricane Ida in El Salvador earlier this month. The final toll of the landslides remains unclear, over 60 is the widely reported statistic.

The images are false colour composites collected by the ASTER instrument. These images differentiate between vegetation, which appears as red colours, and bare ground, which appear as a blue-grey colour. Such images are ideal for delineating new landslides, which usually strip away vegetation to leave bare soil.

This is the before image of the area affected by the landslides:

As usual, click on the image for a better view in a new window. Here is the image after the hurricanes passed through:


The lahar tracks are pretty clear. Note that they start high on the volcano, mostly with small, translational landslides. These slides then enter the channel, entraining (eroding and incorporating) material along the channel. The towns are quite clear on the images - the debris flows have hit the settlements in several locations, leaving this type of damage (Image from here):


When the landslides reach the plains below the volcano they spread out and stop, leaving a large area covered in debris. This is also clear on the image.

The ten greatest landslide papers?

Whilst pushing my five year old daughter on the swing the other day (a task that leaves plenty of time to think!), I was pondering upon the greatest landslide papers of all time. I thought that it would be interesting to compile a list and invite suggestions of alternatives. So, here is my list, in no particular order:

1. Terzaghi on the principle of effective stress
In 1936 Karl von Terzaghi laid the foundation for modern soil mechanics, and the basis of our understanding of how landslides move, by stating the principle of effective stress. The recognition that pore water pressures control the frictional resistance of slopes remains the most important concept in understanding landslide behaviour.
Reference: Terzaghi, K., 1936. "The Shear Resistance of Saturated Soils". Proceedings of the first International Conference on Soil Mechanics and Foundation Engineering, Cambridge, U. S. A., 1, 54-56.

2. Keefer on earthquakes caused by landslides
Dave Keefer's 1984 paper on landslides triggered by earthquakes was a remarkable analysis. Taking a huge dataset of earthquake-triggered landslide inventories, Keefer demonstrated both the simplicity and the complexity of the relationship between numbers and areas of landslides and the earthquake parameters. The approach taken in this paper has been much reproduced since; remarkably it the core conclusions are essentially unaltered.
Reference: Keefer, D.K., 1984, Landslides caused by earthquakes. Geological Society of America Bulletin, 95, 406-421.

3. Hutchinson and Bhandari on undrained loading
Undrained loading is one of those concepts that makes you go "of sourse" when it is explained to you. The idea is that the sudden application of a load (such as for example a rockfall from a cliff onto an mudslide below) drives a dramatic increase in pore pressures, which can't dissipate quickly. This reduces the effective normal stress, allowing the slope to move. It explains landslides in many settings - a crucial step forward.
Reference: Hutchinson, J. N. & Bhandari, R. K. 1972. Undrained loading, a fundamental mechanism of mudflows and other mass movements. Geotechnique 21, 353-358

4. Caine on thresholds associated with landslide initiation
Almost all of the working large area landslide warning systems are based on this paper. Caine set out to understand the rainfall thresholds at which landslides occur, recognising that it is a combination of medium term low intensity rainfall (to get the ground wet) and short duration rainfall (to get initiate movement) that is the key. This is literally the paper that launched a thousand studies - and more appear every year. There is even a website dedicated to this type of work: http://rainfallthresholds.irpi.cnr.it/threshold_info.htm
Reference: Caine, N., 1980. The rainfall intensity-duration control of shallow landslides and debris flows. Geografiska Annaler, 62A, 23-27.

5. Skempton on the principle of residual strength
Residual strength is a key idea within landslide science. Skempton's work was prompted by the difficulties of explaining why low gradient slopes were failing. The idea that materials have a residual strength that is less than the peak strength is key - slopes that have in previous times been reduced to this lower strength will fail much more easily when disturbed. This concept really made Skempton's career, and he was ultimately knighted for his scientific contributions.
Reference: Skempton, A.W. 1964. Long term stability of clay slopes. Geotechnique, 14, 77-101 (the fourth Rankine lecture).

6. Carson and Petley on the existence of threshold slopes
In case you are wondering, this Petley is not me, but shall we say that when I was a child I spent a great deal of time with him (and continue to do so, but more rarely now). In many ways the dataset in this paper is not comprehensive, but the ideas that it introduced are fundamental. The paper suggested that in any environment a slope will relax to a constant gradient that reflects its stength and the conditions to which it is exposed. The idea was spot on, and is just being rediscovered by geophysicists!
Reference: Carson, M.A., and Petley, D.J., 1970, Existence of threshold hillslopes in denudation of
landscape. Transactions of the Institute of British Geographers, 49, 71–95.

7. Carrara on the evaluation of landslide hazardLandslide hazard assessment is big business these days. Carrara's paper was not the very first to do this, but it was a pioneering study in terms of using the rapidly evolving technologies that were becoming available. The myriad of factor and suchlike based studies that have followed over the last 26 years all owe a great deal to this study.
Reference: Carrara, A. 1983. Multivariate models for landslide hazard evaluation. Journal of the International Association for Mathematical Geology, 15, 403-426.

8. Hoek and Bray on rock slope engineering
This is the only item on my top ten that is a book rather than a paper. The stability of rockslopes is primarily controlled by the properties, orientations and interactions of discontinuities rather than by the intact material strength. This book brought together for the first time the principles of rockslope stability and design. It has been revised and reprinted on numerous occasions, and remains the bible of rock slope engineers.
Reference: Hoek, E. and Bray, J. 1974. Rock slope engineering. Institution of Mining and Metallurgy, 309 pp.

9. Bjerrum on progressive failure
The principles and processes of progressive failure remain poorly understood, but Bjerrum's 1967 examination of this key topic remains the salient work in this area. Bjerrum proposed a model for the weakening of slopes with time as a crack grows through the base. The concepts remain fresh and valid today - there is surely a great opportunity for someone to take this issue and produce the definitive follow-up on how this process actually operates.
Reference: Bjerrum L. 1967. Progressive failure in slopes of overconsolidated plastic clay and clay shales. Journal of the Soil Mechanics and Foundations Division, ASCE, 93, 1-49.

10. Bishop and Wesley on soil testing
Without laboratory testing of laboratory materials slope engineering would still be in the dark ages. The key research machine in every geotechnical laboratory is the triaxial cell, and research purposes the stress path cell is the most important tool. This paper described just such a machine, opening the way to our fundamental understanding of soil behaviour.
Reference: Bishop, A.W. and Wesley, L.D. 1975. A hydraulic triaxial apparatus for controlled stress path testing. Geotechnique 25, 657-670.
I'm sure that you don't agree with me, so please tell me why I am wrong, and suggest alternatives. I am worried that there is nothing since 1984 - surely there must have been great papers in the intervening 25 years. What have I missed?

Landslide impacts in Turkey

The last few days have seen very heavy rain in Turkey, with a number of landslides. ITN has a short clip of home footage showing a large buikding collapsing as it is hit by a landslide:

http://itn.co.uk/9cff433ac9de157976f9b579cbe8c656.html

The distress shown by the man in the foreground - presumably an owner or resident - illustrates graphically the impact of these events on people.

A very large ancient rockslide in Chile

I am en route to Santiago in Chile to attend the Chilean Geological Congress, the organisers of which kindly invited me to give one of the keynote lectures (on Friday). I thought therefore that I would point out that Chile has an extraordinary set of very large rock avalanches. Earlier this year, Antinao and Gosse (2009) published an interesting review of a set in the Chilean Cordillera Principal. I do not intend to publish a full review of the paper here, but thought I would highlight just one of the slides, called Yeso - Meson Alto. This is a very large landslide, as the Google Earth image below shows:




I have eyeballed in the main landslide deposit (note that in places this is covered with more recent fluvial (river) deposits, so this is very approximate. I have used the supplementary information from Antinao and Gosse (2009) as a guide as this contains a map of the landslide, and another large slide downstream (which I have included below my indicative lines on the Google Earth image:

The statistics for this landslide provided by Antinao and Gosse (2009) are impressive:
Volume: 4.5 cubic kilometres
Original surface area: 22.7 square kilometres
Length (travel distance): 7.6 km

The landslide is considered to have been translational as the deposit retains the original stratigraphy. Interestingly, it is thought to have occurred in the Holocene (i.e. in the last 12,000 years).


Reference
Antinao, J., & Gosse, J. (2009). Large rockslides in the Southern Central Andes of Chile (32–34.5°S): Tectonic control and significance for Quaternary landscape evolution Geomorphology, 104 (3-4), 117-133 DOI: 10.1016/j.geomorph.2008.08.008

Extreme rainfall in NW England and SW Scotland

The Northwest of England and Southwest of Scotrland have suffered an extreme rainfall event over the last few days that has brought extensive flooding. The Met Office in the UK are suggesting that this might represent a UK rainfall record (we will see), but certainly the impacts are extraordinary (this is Cockermouth in Cumbria; there is also extensive flood damage in Dumfries and Galloway in Scotland):

Image from Skynews

Image from BBC news.

The UK Met Office have provided this graphic of the rainfall distribution:


The images above are from Cumbria, which is the area highlighted as having the highest rainfall.

Unfortunately, there is more rainfall on its way. This is the BBC Weather forecast for this area at noon tomorrow:

Fortunately, this rainfall event should not be on a par with the last 24 hours (the Met Office are forecasting up to 40 mm over higher ground), when >170 mm of rain fell in some areas of Cumbria, but it certainly won't help the clean up.

There are a few reports of landslides, such as:

• Train services disrupted because of a landslide between Carlisle and Penrith;.
• A landslide in Rossendale in Lancashire on Wednesday that damaged a packaging factory:

Image from Manchester Evening News

Surely the most over-the-top headline of the year!

Various daily newspapers in the UK are reporting on the Pennan landslide situation (more of which below). However first prize in the hysterical over-reaction category (the British tabloids pride themselves in their ability to out-do each other in this class) goes to the Mirror for this headline:

Unbelievable! Meanwhile, back in the real world, the residents of the village have now been advised to leave the village for a week whilst the landslide is remediated. The plan appears to be to remove the unstable section above the road. The BBC has a good short video of the site here.