12 posts · 10,587 views
This blog provides a commentary on landslide events occurring worldwide, including the landslides themselves, latest research and conferences and meetings.
Dr Dave
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- October 25, 2010
- 04:28 PM
- 480 views
Landslides, forests and pandas - conservation and the Wenchuan earthquake
by Dr Dave in Dave's Landslide Blog
The vast number of landslides triggered by the May 2008 Wenchuan earthquake, and in its aftermath has been extensively described, not least on this blog. One of the documented impacts of these landslides was the well-documented loss of habitat of the giant panda (A. melanoleuca) due to extensive forest loss. However, there is a great deal more to that story than meets the eye, as a newly-published paper by Vina et al (2010) describes. The research is very interesting, and has some quite substantial implications for landslide management in earthquake-prone areas.The study examines forested areas affected by the earthquake in Wenchuan County, in the main area affected by the earthquake. Wenchuan County includes Wolong, the famous giant Panda conservation area. The context for the study is important though - that is that contrary to popular understanding, the forested area of China has been increasing, not decreasing over the last decade or so. There are two key reasons for this:Economic growth has led to a huge migration of people to urban areas, reducing the pressure on rural resources;In the aftermath of the devastating floods in 1998, China started a huge programme of forest conservation and restoration, most notably incentivising farmers to return hillside croplands to forest. So, the key question that this research has addressed is how the dynamics of forests in Wenchuan County. The research team mapped forest cover using Landsat TM satellite images acquired in 1994 and 2001 (i.e. before the conservation effort) and 2007. Augmented with higher resolution IKONOS and Quickbird images from 2000 and 2007. These images were used to map forest cover.The results are fascinating. Between 1994 and 1991 the forest cover declined from just under 44% in 1994 to about 39.5% in 2001. However, by 2007 the forest cover was back to about 44%, showing that the conservation programmes had generated real results. The earthquake of course reversed this trend. In the aftermath of the seismic event the forest cover was again mapped using Landsat TM images. The results suggest that 192.6 square kilometres of forest were lost in Wenchuan County, representing just under 11% of the forest cover in 2007. The consequence was that forest cover declined to about 39%. However, the authors estimate that without the conservation programmes this would have been as low 33.5%. Thus, although the occurrence of landslides in the earthquake was truly dreadful, the ecological picture would have been far worse if it hadn't been for the forestry programmes. One aspect that the paper does not cover though is that the loss of forestry increasing seismic landslide susceptibility, and thus the loss might have been greater againThe authors finish by speculating on the long term recovery of the forest resources in the aftermath of the earthquake. They note that the construction of new housing, such as that shown to the left (from my recent trip to the area), is likely to have the effect of moving people away from the more remote rural areas, which may well result in the abandonment of more farmland, and thus increased forest recovery. This may well have substantial benefits for conservation efforts, albeit at potentially high cost to those people most affected by the earthquake. ReferenceViña, A., Chen, X., McConnell, W., Liu, W., Xu, W., Ouyang, Z., Zhang, H., & Liu, J. (2010). Effects of Natural Disasters on Conservation Policies: The Case of the 2008 Wenchuan Earthquake, China. AMBIO DOI: 10.1007/s13280-010-0098-0... Read more »
Viña, A., Chen, X., McConnell, W., Liu, W., Xu, W., Ouyang, Z., Zhang, H., & Liu, J. (2010) Effects of Natural Disasters on Conservation Policies: The Case of the 2008 Wenchuan Earthquake, China. AMBIO. DOI: 10.1007/s13280-010-0098-0
- April 23, 2010
- 04:47 AM
- 578 views
On morbidity and mortality in landslide disasters
by Dr Dave in Dave's Landslide Blog
Landslides kill thousands of people each year across the world but, strangely, there are very few studies of the causes of the injuries and deaths that people suffer when they are affected by a landslide. This contrasts with avalanches, which has a long track publication history of causes of mortality. This is important in the context of treatment of victims - in particular, where a rescue is ongoing, the medical practitioners need to be able to prepare for the likely state of those who might be recovered.In this context, a fascinating article, Sanchez et al. (2009), was published in the journal Disasters late last year in which a group of medical and well-being experts examined the causes of morbidity and mortality associated with a landslide disaster that occurred in Micronesia in 2002. The event was a series of rainfall induced debris flows that occurred on the islands of Chuuk on 2nd July 2002 as a result of the passage of a tropical storm named Chata'an. The storm reportedly triggered 265 landslides, 12 of which caused a total of 43 deaths and 48 injuries that required hospital treatment. There is an nice pdf of a presentation by Herman Semes Jr on the background to this disaster here, from which the following images are taken:The research team interviewed 52 survivors of the disaster, plus a number of eye-witnesses and also reviewed the death certificates of those who lost their lives. The landslides themselves were slumps that transitioned into debris flows, the largest of which had a volume of 1.5 million cubic metres.There are some really interesting results of the study, which I will summarise below:Cause of deathEven though the slides were rapid debris flows, 39 out of the 43 deaths occurred through asphyxiation, with only four resulting from trauma. Even for those who died outside of buildings, the majority of deaths occurred through asphyxiation.Types of injuriesFor those 31 victims who were injured by the landslides and required hospital treatment, and for whom information on their injuries was available, 16 suffered lacerations, nine contusions/abrasions, four concussions and only two fractures.Gender of victimsAbout 58% of those killed by landslides were female, even though women represent 39% of the general population. The age of victimsThe fatalities were recorded on individuals aged between two months and 84 years, with a median age of 14 years old. The comparison group of survivors had a median age of 29 years, suggesting that landslides preferentially killed the young. Location of victimsVery surprisingly, the study found that "being inside a house or building during the landslide was associated with a higher risk of mortality", although the difference in risk is not statistically significant.DiscussionThis study is really interesting - we desperately need more research of this type! The observation that most of the fatalities occurred as a result of asphyxiation is consistent with avalanche research, but given the greater potential for the materials in landslides to cause trauma, it is a slightly surprising observation. It is also notable that the landslides appear to have preferentially killed the young - probably a reflection of ability to escape an oncoming event. Finally, the observation that structures offered no obvious protection is also interesting, but may reflect both the poor quality of buildings in a less developed country, and the fact that a structure may prevent an individual from seeing / hearing an approaching landslide, and may inhibit escape once it occurs.ReferencesSanchez C, Lee TS, Young S, Batts D, Benjamin J, & Malilay J (2009). Risk factors for mortality during the 2002 landslides in Chuuk, Federated States of Micronesia. Disasters, 33 (4), 705-20 PMID: 19459918... Read more »
Sanchez C, Lee TS, Young S, Batts D, Benjamin J, & Malilay J. (2009) Risk factors for mortality during the 2002 landslides in Chuuk, Federated States of Micronesia. Disasters, 33(4), 705-20. PMID: 19459918
- December 19, 2009
- 08:57 PM
- 847 views
On the perils of Lake Sarez (Usoi) in Tajikistan
by Dr Dave in Dave's Landslide Blog
Science this week has an article (Stone 2009) on the perils associated with Lake Sarez in the Pamirs. Sarez is a huge lake (56 km long and with a volume of 17 billion cubic metres of water) that was formed by a landslide triggered by the 1911 earthquake in Tajikistan (see image below).Google Earth image of Lake Sarez. The landslide dam is to west (left).Google Earth image of the landslide dam at Usoi. The source of the landslide was to the north of the current deposit.The landslide dam (see image above) stands 567 metres tall. To put that in perspective, the image below shows Taipei 101, until recently the world's tallest building. It is 501 metres tall:Taipei 101 (source Wikipedia)Since its creation Lake Sarez has been steadily filling, which has long been a concern. There are an estimated 5.5 million people living downstream of the dam in the Amu Darya river valley, which flows through Tajikistan, Afghanistan, Turkmenistan, and Uzbekistan. There are really three key concerns with this dam:The dam could fail through seepage - a few years ago water started to seep through the landslide deposit, the concern is that this will erode out the core of the landslide;The dam could fail in an earthquake - this is a seismically-active zone, but the threat is considered to be quite low as the dam is considered to be quite stable;The dam could fail as a result of another landslide going into the lake, creating a displacement wave (similar to the Vaiont landslide) that causes the dam to overtop. Of course this is most likely to be triggered by an earthquake landslide.The article points out that the third of these is the most likely, such that the site has a sizable warning system just in case.The article points out that given the number of people downstream the risks are now considered to be too high. The dam itself cannot be stabilised, so there is a need to draw down the level of the lake by at least 50 m. However, there can be little doubt that this falls in the "easier said than done" category.The key component of the article is highlighting that there are a range of views as to the level of danger at this site, both in terms of the possibility of another landslide and of the stability of the dam itself. The article quotes a number of notable landslide experts:Jorg Hanisch is quoted as saying that "the probability is 1 in a million,"of the dam being overtopped by a wave created by a landslide. He also rules out any possibility of the dame being eroded by seepage.Jean Schneider from BOKU in Vienna is quoted as saying that "The risk of even a partial outbreak is exaggerated...the dam will only possibly be overtopped in the far future."On the other hand, Kadam Maskaev (deputy director of the emergency situations committee in Tajikistan) views the seepage in a different way: "The filtration regime of the dam is changing, and that makes me nervous."Kyoji Sassa, the chair of the International Consortium on Landslides, has a different view again. The article claims that he argues that the threat from a further landslide is significant. The suggested optimum mitigation approach is a diversion tunnel that would be used to generate hydroelectric power, with the water also being made available to downstream communities. However, the costs are high ($500 million) and such a project is not without risks. In the sort term it appears that there will be a research campaign that will culminate in a conference in 2011, the 100th anniversary of the dam. That would be an interesting meeting to attend!ReferenceStone, R. (2009). Peril in the Pamirs Science, 326 (5960), 1614-1617 DOI: 10.1126/science.326.5960.1614... Read more »
Stone, R. (2009) Peril in the Pamirs. Science, 326(5960), 1614-1617. DOI: 10.1126/science.326.5960.1614
- November 28, 2009
- 08:43 AM
- 820 views
The link between rainfall intensity and global temperature
by Dr Dave in Dave's Landslide Blog
The aftermath of a landslide in Taiwan caused by very heavy rainfallOne of the most interesting aspects of the global landslide database that we maintain at Durham is the way in which it has highlighted the importance of rainfall intensity in the triggering of fatal landslides. Generally speaking, to kill people a landslide needs to move quickly rapid, and rapid landslides appear to be primarily (but note not always) triggered by intense rainfall events (indeed in the reports the term "cloudburst" often crops up). So, a key component of trying to understand the impacts of human-induced global climate change on landslides is the likely nature of changes in rainfall intensity, rather than that of rainfall total. Put another way, it is possible that the average annual rainfall for an area might decrease but the occurrence of landslides increase if the rainfall arrives in more intense bursts.There is of course a certain intuitive logic in the idea that rainfall intensity might increase with temperature. Warmer air is able to hold more moisture (as anyone who has been in the subtropics in the summer will know only too well!) and of course increased temperatures also drive greater convection, responsible for thunderstorm rainfall. Of course this is a very simplistic way to look at a highly complex system, so it is not enough to rely upon this chain of logical thought. However, until now there have been surprisingly few studies to actually quantify whether there is a relationship between global temperature and precipitation intensity, which has meant that for landslides understanding the likely impact of climate change has been quite difficult.However, an important and rather useful paper examining exactly this issue has sneaked under the radar in the last few months. The paper, by Liu et al (2009) (see reference below), was published in Geophysical Research Letters a couple of months ago. The paper uses data from the Global Precipitation Climatology Project (GPCP). These data can be accessed online here (so no claims that climate scientists don't publish their data, please!) The dataset provides daily rainfall totals for 2.5 x 2.5 degree grid squares across the globe, extending back almost 50 years. Liu et al. (2009) looked at the data from 1979 to 2007, comparing precipitation density with global temperature in this time period.Their results are both unsurprising and surprising. The unsurprising part is that they found that the occurrence of the most intense precipitation events does increase with temperature. The surprising part is the magnitude of the change - they found that a 1 degree Kelvin (Centigrade) increase in global temperature causes a 94% increase in the most intense rainfall events, with a decrease in the moderate to light rainfall events. Indeed the median rainfall increased from 4.3 mm day−1 to 18 mm day−1, which is a surprisingly high shift as well.So why is this important in the context of landslides? Well, I think that there are probably two key implications:1. It has long been speculated that anthropogenic warming will lead to an increase in landslides, but with little real quantitative evidence to confirm or deny this. The demonstration that higher global temperatures does lead to increased precipitation intensity starts to put some meat on the bones of this idea. Furthermore, if it is possible to directly link rainfall intensity to landslide occurrence (and there is some evidence both from my own work and from that of others that this may be possible), then it should be possible to start to examine the likely increase in landslides as warming proceeds.2. The current global climate models assume a much lower increase overall in precipitation intensity with increasing temperature than Liu et al. (2009) suggest. Indeed most of the models assume about a 7% increase per degree Kelvin (Centigrade) warming. For the most intense precipitation events this means that the models predict about a 9% increase, which is an order of magnitude lower Liu et al. (2009) found. This suggests that the rainfall projections that are derived from the models are probably overly-conservative, and possibly very much so, which is a concern. If so, then forecasts of landslide occurrence that are derived from these models are likely to under-estimate the true impact.Of course, this is only one study, and it should also be noted that the most intense rainfall events are usually associated with tropical areas and with those in the path of hurricanes and in particular typhoons. There is a great deal more work to do on this topic, but the initial results provide real cause for concern.ReferenceLiu, S., Fu, C., Shiu, C., Chen, J., & Wu, F. (2009). Temperature dependence of global precipitation extremes Geophysical Research Letters, 36 (17) DOI: 10.1029/2009GL040218... Read more »
Liu, S., Fu, C., Shiu, C., Chen, J., & Wu, F. (2009) Temperature dependence of global precipitation extremes. Geophysical Research Letters, 36(17). DOI: 10.1029/2009GL040218
- November 21, 2009
- 12:19 PM
- 815 views
A very large ancient rockslide in Chile
by Dr Dave in Dave's Landslide Blog
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 kilometresOriginal surface area: 22.7 square kilometresLength (travel distance): 7.6 kmThe 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).ReferenceAntinao, 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... Read more »
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
- November 1, 2009
- 04:53 PM
- 703 views
A very surprising paper - movement of a landslide controlled by atmospheric tides
by Dr Dave in Dave's Landslide Blog
Just occasionally a paper appears that makes me sit up with surprise, with a strong sense of "I did not see that one coming!". Just such a paper has appeared in pre-publication form in Nature Geoscience today - namely Schultz et al. (2009). This will undoubtedly represent the most surprising landslide paper of the year, and indeed for a few years in fact. I suspect that this paper will generate a fair amount of media interest in the next few days as well.The subject of the paper is the Slumgullion landslide in SW. Colorado. This slide is a classic landslide site - it is large and moves almost continually. The slide is pretty obvious on Google Earth:All the more so when viewed obliquely:And there are a fair number of websites and online materials that describe it (see here and here for example).This latest study is based on an intense monitoring campaign undertaken on the landslide using extension transducers. There is little doubt that the monitoring has been designed with great care by a team that is well-versed in such techniques. The team have used this movement data to examine the variation in movement through time. There is nothing terribly unusual about this approach except perhaps the level of precision with which the monitoring has been undertaken, but the results are very intriguing. The data suggest that the slide moves almost every day. This style of behaviour is quite unusual - most landslides are very episodic in terms of movement - but certainly not unique. However, examination the movement patterns on a daily basis suggested that the slide moved in distinct eight hour periods, essentially at night. During the day the landslide was to all intents and purposes stationary. This periodic movement of the slide was found to be correlated with the variations in air pressure associated with atmospheric tides - i.e. it appears that the movement of the slide is controlled by air pressure! Atmospheric tides are small variations in atmospheric pressure that result from solar heating of the air. The patterns of atmospheric tides are actually rather complex, but can be examined with a Fourier analysis.The team have compared the Fourier analysis of the movement pattern of the landslide with that of the atmospheric tides - there is little doubt that they are remarkably similar. Thus the movement at least appears to be related to these atmospheric tidal cycles. The authors suggest that these low atmospheric tide conditions induce upward movement of water and air below the shear surface of the landslide, changing the "frictional stress" of the slide, allowing movement to occur. They have modelled the slide to explore this mechanism and present an analysis that, at least on first inspection, appears to be credible and to support their hypothesis.Now, it must be stressed at this point that Slumgullion is not a typical landslide. In particular, the perennial slow landslide movement suggests that the factor of safety is essentially unity. Measurements suggest that the pore pressure on the shear surface varies very little on the landslide, meaning that the slide is potentially highly sensitive to effects such as that reported in Schulz et al. (2009). It is very unlikely that this effect is seen in anything other than a small proportion of slides that have exactly the right combination of stress state, thickness and material properties to allow this to occur, although I am willing to bet that a small number of other slides will be found to display this behaviour. However, as the authors of the paper hint, the most interesting aspect may well be the implications for landslides in areas that encounter conditions with very low atmospheric pressures. The key mechanism to deliver such conditions is the tropical cyclone, especially typhoons in East Asia and hurricanes in the Caribbean. Here, atmospheric pressures can drop very low (Typhoon Tip had a measured sea level air pressure of 870 hPa in 1970), as well as bringing high intensity rainfall of course. Thus, it may well be that the very high incidence of landslides under such conditions is not just the consequence of the rainfall but also the low atmospheric pressure. That is a very interesting observation indeed. Added to the recent observations that tropical cyclones can also trigger slow earthquakes, our understanding of the interactions between the atmosphere and the ground is developing quickly.So, all in all this is a fascinating study for which the authors should be congratulated. It has added an intriguing new dimension to landslides, and will I suspect trigger a new wave of research projects. I should add here that for most landslides this effect is almost certainly very small (no-one living near a landslide should start worrying about atmospheric pressures - it is still rainfall and earthquakes that are the main issue), but for certain slides under certain conditions this may be an important effect.ReferenceSchulz, W.H., Kean, J.W and Wang, G. (2009). Landslide movement in southwest Colorado triggered by atmospheric tides Nature Geoscience : 10.1038/NGEO65... Read more »
Schulz, W.H., Kean, J.W and Wang, G. (2009) Landslide movement in southwest Colorado triggered by atmospheric tides. Nature Geoscience. info:/10.1038/NGEO65
- September 13, 2009
- 01:20 AM
- 881 views
On the loss of life in landslides during the 1949 Khait earthquake
by Dr Dave in Dave's Landslide Blog
Regular readers will know that one of my interests lies in trying to get a better understanding of the loss of life associated with landslides. A key realisation of this work for me has been that earthquake-triggered slides cause a very substantial proportional (probably in fact the majority) of fatalities is mass movement events. Unfortunately our understanding of seismically-driven landslides, and their impacts, remains poor, certainly in comparison with rainfall induced slides. For that reason, work to re-examine past seismically-driven events is very welcome, helping us to get a much better understanding of the range of processes and impacts in these events.One significant but until now slightly elusive such event has been the 1949 Khait earthquake. This was a Mw=7.4 event on 10th July 1949 in the Tien Shan mountains of what is now Tajikistan, but was then the Soviet Union. The timing and location of this event, soon after the war in an area about which the Soviet Union was very secretive, has meant that it has been very difficult to determine any details about the landslides that were triggered in the earthquake. However, some rather speculative reports have suggested that the impacts were very large - for example this article in Mountain Research and Development reported a huge landslide at a site that it termed Borgulchak Rock. This slide was reported to have travelled 12 km. Some reports, such as the Wikipedia article on this landslide, have suggested a death toll as high as 28,000 people, although to be fair this may well be something of a misinterpretation of the original source.An article in press in the journal Engineering Geology, by Steve Evans and colleagues (Evans et al. 2009 in press) seeks to re-examine the landslides triggered by this earthquake. Unusually for a science paper the article is a ripping-good read. The paper re-evaluates the Khait landslide, and the other large slide that was triggered in the earthquake, providing a rational analysis of the likely impacts of the mass movements.First, they look at the Khait landslide (termed in the paper as a rockslide / loess flow), which is still clearly visible in the landscape, even on the low resolution Google Earth imagery available for this area:For the Khait landslide they conclude that the volume was probably rather lower than earlier estimates have suggested. The other landslide considered is a very large and complex flowslide that swept down the Yasman Valley, covering about 20 km. This slide had multiple source areas on the southern side of the valley:They conclude that this is a rather destructive loess flow slide with a volume of about 245 million cubic metres. Remarkably it travelled over a slope with an angle of just two degrees!Evans et al. (2009) then consider the fatalities caused by the landslides. By looking at contempory reports of the population of the Khait area and census data on settlement size and population density they reject earlier estimates of the loss of life. For the Khait landslide itself they conclude that about 800 fatalities is probably a reasonable estimate - note that this very considerably less than earlier estimates. For the Yasman Valley flowslide they estimate about 4,000 fatalities, and they determine that there were probably a further 2,400 deaths. This gives a total fatality count of about 7,200 - far lower than previous counts, but still substantial of course.In conclusion, this is a very important contribution, filling in another gap in our understanding of previous landslide impacts. Steve and his colleagues have also just published a similar paper (Evans et al. 2009) re-examining the 1960 Huascaran rock avalanche in Peru. This will be the topic of an upcoming post.ReferenceEvans, S., Roberts, N., Ischuk, A., Delaney, K., Morozova, G., & Tutubalina, O. (2009 in press). Landslides triggered by the 1949 Khait Earthquake, Tajikistan, and associated loss of life Engineering Geology DOI: 10.1016/j.enggeo.2009.08.007Evans, S., Bishop, N., Fidel Smoll, L., Valderrama Murillo, P., Delaney, K., & Oliver-Smith, A. (2009). A re-examination of the mechanism and human impact of catastrophic mass flows originating on Nevado Huascarán, Cordillera Blanca, Peru in 1962 and 1970 Engineering Geology, 108 (1-2), 96-118 DOI: 10.1016/j.enggeo.2009.06.020... Read more »
Evans, S., Roberts, N., Ischuk, A., Delaney, K., Morozova, G., & Tutubalina, O. (2009) Landslides triggered by the 1949 Khait Earthquake, Tajikistan, and associated loss of life. Engineering Geology. DOI: 10.1016/j.enggeo.2009.08.007
Evans, S., Bishop, N., Fidel Smoll, L., Valderrama Murillo, P., Delaney, K., & Oliver-Smith, A. (2009) A re-examination of the mechanism and human impact of catastrophic mass flows originating on Nevado Huascarán, Cordillera Blanca, Peru in 1962 and 1970. Engineering Geology, 108(1-2), 96-118. DOI: 10.1016/j.enggeo.2009.06.020
- September 3, 2009
- 03:44 PM
- 963 views
On the dangers of Rhododendrons!
by Dr Dave in Dave's Landslide Blog
Rhododendrons are one of those plants that, when planted well, can create an amazing garden:(from: http://www.kelleriisgaarden.dk/rhododendron-eng.html)However, it might surprise you to hear that they can be a major cause of landslides. As the image below shows, rhododendrons are increasingly grown on the mountain slopes of the Appalachians:(from: http://toursinthesmokymountains.com/SmokyMountainsInDepth.aspx)As well as creating a somewhat beautiful landscape, rhododendrons have been grown in the Appalachians as a result of logging and fire suppression policies. Forest fires have long been perceived as a major hazard, and fire-exposed land is highly prone to landslides (a major fear in California given the fires in this El Nino year). The Appalachians have a long landslide history - in 1969 for example heavy rainfall associated with the passage of the remnants of a hurricane triggered 3700 debris flows, causing 150 fatalities and $116 million of economic losses.In a recently published paper, Tristan Hales (now at Cardiff University) and colleagues (2009) looked at the role of roots in providing strength to the soil in the Appalachians. The results are quite interesting. It is clear that in many Appalachian slopes the key thing that prevents landslides is the strength provided to the soil by the roots of the trees and shrubs. Therefore, anything that causes a reduction in this strength will lead to an increased chance of landslides. Hales et al. (2009) set out to measure the strength provided by different plants and trees in plots on slopes in the Appalachians. They found that different varieties of trees had broadly similar root strengths, but that for the native rhododendron species was markedly lower. Furthermore, the roots tend to be concentrated in the upper layers of the soil (tree roots extend much deeper), the rhododendrons are less effective at removing and transpiring water from the soil than are trees, and the thick bushy vegetation starves the forest floor of light, which prevents tree sapling growth.All of this is of course bad news in terms of landslides. Hales et al. (2009) are keen to stress that this should not be seen as a definitive indication that rhododendrons are responsible for landslide initiation in the Appalachians, but they do note that in the last large landslide event, in 2004, many of the landslides were initiated in thickets of rhododendrons.ReferenceHales, T., Ford, C., Hwang, T., Vose, J., & Band, L. (2009). Topographic and ecologic controls on root reinforcement Journal of Geophysical Research, 114 (F3) DOI: 10.1029/2008JF001168... Read more »
Hales, T., Ford, C., Hwang, T., Vose, J., & Band, L. (2009) Topographic and ecologic controls on root reinforcement. Journal of Geophysical Research, 114(F3). DOI: 10.1029/2008JF001168
- March 6, 2009
- 05:27 PM
- 1,119 views
The role of landslides in global warming
by Dr Dave in Dave's Landslide Blog
A rather extraordinary paper has just been published in Geophysical Research Letters about landslides triggered by the Wenchuan (Sichuan) earthquake. Why is it extraordinary - well, let me quote from the abstract. The paper suggests that the landslides caused destruction of vegetation such that "the cumulative CO2 release to the atmosphere over the coming decades is comparable to that caused by hurricane Katrina 2005 (~105 Tg) and equivalent to ~2% of current annual carbon emissions from global fossil fuel combustion."Wow! In case you are struggling to decode the above, this suggests that the landslides triggered by the earthquake caused a massive loss of vegetation that will now decay. In decaying it will release CO2, which will add to the effects of global warming. This is a pretty interesting result - and it has already been picked up by the mainstream media.So, how do the authors reach these remarkable conclusions, and are they valid? Well, I am afraid that I have some serious doubts about this study, which seem to be based on some misunderstandings of earthquake-induced landslides. Lets base the analysis on Fig 2 of the paper, reproduced below, in which the authors highlight one of the landslides that blocked the valley on the river upstream of Beichuan:So why do I object so strongly to the paper? Well first, the use of terminology is inexcusably weak. For example, the authors describe the landslides thus (referring to Fig 2a): " (a) Living carbon scars left by mudslides, which indicates the geographical locations of the landslides for this region." NO - these are not mudslides - these are clearly shallow rockslides - a very different beast. Of Figure 2b they say "A quake surface wave triggered basal sliding that initiated the movement through liquefying the top ~2 m slab." NO. Failure was not due to liquefaction, and even the most cursory view of the image shows that more than 2 m of material was displaced. Finally, they say of Fig. 2d "An aerial photo taken on May 26, 2008, showing the landslide mud that formed the Tangjiashan quake lake". No again - this is most definitely not mud (see image below) - it is bouldery / fragmented debris (if it was mud then the problems would have been far less serious). They say that there model suggests that "The material reaches a maximum speed of 5 m s−1 but only briefly because the resistance stress is strong for the still coherent sliding material. " Again, this is poppycock. 5 m/sec is 18 km/hour - there is no way that this failure was as slow as that - look at how the debris fragmented and at how it spread across the valley (see image of the landslide deposit being excavated for the drainage channel below):This is not a deposit that was emplaced at 5 m/sec, and nor is it mud. Pretty poor stuff, frankly. Note finally that figs 2b and 2d are supposed to represent the same area. However, in 2d the debris is clearly in the valley floor, with the source being the slopes above. In 2d the debris is above the 1155 m contour line. There is no debris between 1100 m and 1155 m - so the deposit areas are completely different.So now lets turn to the modelling. The paper is ridiculously short of proper detail of what they have actually done - I cannot understand how the editors/referees let this through. It states that they have used an "advanced modeling tool—a scalable and extensible geo-fluid model—that explicitly accounts for soil mechanics, vegetation transpiration and root mechanical reinforcement, and relevant hydrological processes. The model considers non-local dynamic balance of the three dimensional topography, soil thickness profile, basal conditions, and vegetation coverage ... in determining the prognostic fields of the driving and resistive forces, and describes the flow fields and the dynamic evolution of thickness profiles of the medium considered, be it granular or plastic."Hmmm! Not sure what this means really. However, they do state that "we need to use the finest possible digital elevation model (DEM) and soil profile data". However, they have actually used the SRTM data-set, which has a spatial resolution of 30 metres at best, and possibly 90 metres (!). I cannot believe that this is anything like good enough. Where velocity exceeded 1m/s in their model they assume that vegetation is destroyed. They have used this to determine the total amount of vegetation lost, and then calculated the contribution of the CO2 to the atmosphere.There are several problems with this. First, the landslide model appears to be erroneous, as described above. Second, they seem to omit to include the uptake of CO2 by vegetation as it re-establishes on the slide scars, which will in the long term balance that emitted. Finally, note that they say 2% of CO2 emitted by burning fossil fuels, not 2% of all anthropogenic sources. This makes the contribution sound larger than it actually is. Indeed, 2% of annual anthropogenic emissions spread over a substantial period (it doesn't say how long) indicates a comparatively minor annual total.In my view the basis of the paper is iffy, although it would have helped if the methodology had been properly outlined. Unfortunately, the work is already being picked up the climate change denier community. Read this and weep. The logic used by Paul Fuhr in this opinion piece makes no sense at all to me, but the fact that he can use this paper in this way is deeply unfortunate, providing yet more ammunition for the pseudo-science community of climate change deniers.ReferenceDiandong Ren, Jiahu Wang, Rong Fu, David J. Karoly, Yang Hong, Lance M. Leslie, Congbin Fu, Gang Huang (2009). Mudslide-caused ecosystem degradation following Wenchuan earthquake 2008 Geophysical Research Letters, 36 (5) DOI: 10.1029/2008GL036702... Read more »
Diandong Ren, Jiahu Wang, Rong Fu, David J. Karoly, Yang Hong, Lance M. Leslie, Congbin Fu, & Gang Huang. (2009) Mudslide-caused ecosystem degradation following Wenchuan earthquake 2008. Geophysical Research Letters, 36(5). DOI: 10.1029/2008GL036702
- February 25, 2009
- 03:52 AM
- 1,098 views
A new mechanism for landslide initiation
by Dr Dave in Dave's Landslide Blog
It is not often that one reads a paper and finds a new and exotic landslide mechanism being suggested. I was somewhat surprised yesterday to find that in a paper just published in Geomorphology, Steve Evans and his co-authors have done just that. Although it requires further research, the mechanism is intriguing and undoubtedly has some very interesting implications for glacial hazard management as well.The origin of the theory is the extraordinary Kolka Glacier landslide of 22nd September 2002. There is quite a nice basic description of this event at here. In a nutshell, this was a catastrophic collapse of the Kolka Glacier in the Genaldon Valley, which is located in the Caucasus Mountains in the Republic of North Ossetia, part of the Russian Federation. A catastrophic debris avalanche swept down the valley (Fig. 1) at velocities of up to 65 m/sec. The flow travelled a total of about 19 km, transitioning to a debris flow when it hit a narrowing of the valley. In all, about 125 people were killed.Figure 1: This pair of ASTER images, from NASA, taken before and after the collapse, shows the vast extent of the disaster. Debris and ice filled the Genaldon Valley from the Kolka Glacier Cirque to the Gates of Karmadon—a distance of about 18 kilometers . (Images by Robert Simmon)The controversy about this event comes in trying to determine what initiated this massive landslide. Two theories have been postulated:The simple one is that the flow started because a large rockfall detached from the mountain behind and fell onto the glacier. This instantaneously loaded the ice, inducing massive pore pressure increases through the mechanism known as undrained loading. In consequence the resistance to movement rapidly decreased, and the flow started. This is the model proposed by Huggel et al. (2005).The alternative, rather minority view, is that the event was a conventional glacier surge that rapidly developed into a full failure (see Kotlyakov et al. 2004). Whilst glacial surges do occur, the rates are usually much lower than was observed here.Figure 2: NASA image, hosted by Wikimedia, showing the source area of the glacial surgeThis paper suggests a third model. Most importantly, they propose that there is no evidence that the flow was initiated by a large rockfall event. The authors have looked at satellite imagery immediately before the event, which suggests that there were smaller scale failures occurring almost continually in the summer months of 2002. This is supported by observations by mountaineers at the site. Perhaps most importantly, they have obtained a Landsat ETM+ image that was collected on 20th September 2002, 8.5 hours before the failure event. Comparison with post event imagery suggests no major differences (Fig. 3), suggesting that a large failure did not occur.Fig. 3: This is Fig. 4 in Evans et al. 2009, captioned: Fig. 4. (A). Landsat ETM+ satellite image obtained 20 September 2002, 11:31 am (local time); Kolka Glacier (1) is covered by new snow, with a very fresh and large (0.17 km2) debris trail (2). Also note exposed bed (3) of the former hanging glacier that entirely collapsed between 19 August and 20 September 2002, a pronounced shadow (4) indicating a 50-m-high margin of a northward glacier surface rise, and another shadow (5) of a high ice cliff where Kolka Glacier has already started to deform 8.5 h before the catastrophic detachment at about 20:05 h local time. (B) QuickBird image taken on 25 September 2002 (©2007 Google™, 2008 DigitalGlobe). Note that there is very little difference in morphology of the mountain slope above the Kolka glacier (arrow), compared to the image of 20 September 2002 in (A).However, it is clear that the glacier itself shows signs of extensive deformation in the pre-failure image. Most notably, they observe that a 50 m high ice cliff had develped on the glacier (at point 4 on Fig. 3A), suggesting that extensive movement was occurring. Evans et al. (2009) therefore suggest that the Kolka Glacier started to deform in response to loading from ice and debris. This disrupted the internal drainage of the glacier, triggering the development of excess water pressures at the base of the ice body, which in turn triggered a catastrophic decrease in effective stress and thus an almost complete loss of frictional resistance at the base of the glacier. As a result the glacier detached from its bed (85-175 m below the surface). As the upper part of the ice slide it loaded the lower portion of the glacier, which also began to move through undrained loading.The theory is supported by seismic data, which do not show a signal that could be interpreted as the impact of a large rockfall (these are usually picked up by seismic stations).AnalysisSo how likely is this? Well, technically it is undoubtedly feasible. However, two things should be noted. First, the authors have not provided any numerical analysis to support the hypothesis that very high basal fluid pressures can be generated in this way. It would be interesting to try to model this to see if it can happen in reality, although this modelling is far from easy. Second, the analysis is based upon post-event reconstructions and the use of pre-event satellite imagery whose resolution is really not good enough. It is still possible that a rockfall occurred that can't be resolved on the imagery - vertical images with comparatively low resolutions are not good at observing vertical rock faces. It could be that a hybrid model is appropriate - i.e. that the massive glacial deformations observed created the conditions that allowed a much smaller rockfall to initiate failure.If the authors are right, and they may well be, then this will change the way that we view hazards associated with surging glaciers. Given the changing dynamics of glaciers associated with anthropogenic climate change (global warming) (see here for example), this will become increasingly important.Main ReferenceStephen G. Evans, Olga V. Tutubalina, Valery N. Drobyshev, Sergey S. Chernomorets, Scott McDougall, Dmitry A. Petrakov, Oldrich Hungr (2009). Catastrophic detachment and high-velocity long-runout flow of Kolka Glacier, Caucasus Mountains, Russia in 2002 Geomorphology, 105 (3-4), 314-321 DOI: 10.1016/j.geomorph.2008.10.008Other referencesC. Huggel, S. Zgraggen-Oswald, W. Haeberli, A. Kaab, A. Polkvoi, I. Galushkin and S.G. Evans, 2005. The 2002 rock/ice avalanche at Kolka/Karmadon, Russian Caucasus: assessment of extraordinary avalanche formation and mobility, and application of QuickBird satellite imagery, Natural Hazards and Earth System Sciences 5 (2005), pp. 173–187. This paper is avilable online for free here.V.M. Kotlyakov, O.V. Rototaeva and G.A. Nosenko, 2004. The September 2002 Kolka Glacier catastrophe in North Ossetia, Russian Federation: evidence and analysis, Mountain Research and Development 24, pp. 78–83.... Read more »
Stephen G. Evans, Olga V. Tutubalina, Valery N. Drobyshev, Sergey S. Chernomorets, Scott McDougall, Dmitry A. Petrakov, & Oldrich Hungr. (2009) Catastrophic detachment and high-velocity long-runout flow of Kolka Glacier, Caucasus Mountains, Russia in 2002. Geomorphology, 105(3-4), 314-321. DOI: 10.1016/j.geomorph.2008.10.008
Stephen G. Evans, Olga V. Tutubalina, Valery N. Drobyshev, Sergey S. Chernomorets, Scott McDougall, Dmitry A. Petrakov, & Oldrich Hungr. (2009) Catastrophic detachment and high-velocity long-runout flow of Kolka Glacier, Caucasus Mountains, Russia in 2002. Geomorphology, 105(3-4), 314-321. DOI: 10.1016/j.geomorph.2008.10.008
Stephen G. Evans, Olga V. Tutubalina, Valery N. Drobyshev, Sergey S. Chernomorets, Scott McDougall, Dmitry A. Petrakov, & Oldrich Hungr. (2009) Catastrophic detachment and high-velocity long-runout flow of Kolka Glacier, Caucasus Mountains, Russia in 2002. Geomorphology, 105(3-4), 314-321. DOI: 10.1016/j.geomorph.2008.10.008
- February 9, 2009
- 04:48 AM
- 1,096 views
Australian wildfires and risks of increased erosion rates
by Dr Dave in Dave's Landslide Blog
The extraordinary wildfires in Australia are dominating the headlines in the UK, half a world away. Wildfires are quite common events, but the number of fatalities that this particular episode has caused is really quite unusual. Below in Figure 1 I have plotted the recorded worldwide recorded number of deaths from wildfires for the period since 1980, using data from the CRED EM-DAT database . I have added the 131 reported deaths from this event so far as an extra column, although note that reports suggest that this total may rise substantially:Figure 1: Global numbers of reported fatalities from widlfires, based upon the CRED EM-DAT database. The 2009 value is the reported number of deaths from the Austrlian wildfires. The average annual global total number of deaths is 59.5 fatalities per annum. Care is needed in the interpretation of the above as CRED only record events that kill ten or more people, thus these values consistently underestimate the true toll, but nonetheless the unusual impact of these events is clear.In the context of this blog it is also interesting to think through the likely long term impact of these fires in terms of erosion and landslides. A recent paper by Smith and Dragovitch (2008) looked at the long term consequences of wildfires in SE. Australia. These two researchers have published extensively on sediment production and erosion in Australia, so have a strong pedigree.The study focused on a fire that occurred in January 2003 during a drought in the Snowy Mountains near to Thredbo (Fig. 2), a sub-alpine environment. The study used erosion pins to monitor surface level change on both burnt and unburnt hillslopes over a period of 795 days after the fire.Figure 2: Google Earth perspective image of the area around Thredbo, the location of the study reported by Smith and Dragovich (2008) The study showed that after the fire the areas that had been burnt suffered a net loss of soil representing an average of 3.8 mm of material, with the most intense erosion occurring on the lower slopes (Fig. 3). On the other hand, the unburnt areas saw a net accumulation of soil of an average of 2.6 mm, again with the greatest accumulation at the lower slopes.Figure 3: Mean net soil loss and gain for burnt and unburnt areas as reported by Smith and Dragovich (2008).Thus, the burnt areas clearly suffered a net loss of material in the aftermath of the fires. The study showed that this loss of soil declined with time after the fire, with a slight increase again during snow melt, presumably as vegetation re-established. However, these values are perhaps surprisingly low compared with those recorded in other environments, especially in N. America, given the steep slope angles seen in Figure 2. Interestingly, Shakesby et al. (2007), who studied post-fire erosion in Eucalyptus forests in SE. Australia, came to similar conclusions, stating that "except under extreme post-fire rainfall conditions, present-day wildfires affecting south-east Australia seem to be less potent in geomorphological terms than might be expected given the severity and frequency of the wildfires". They attribute this to the rapid rate of plant growth in the aftermath of fires plus the resistance of the soil to erosion.The conclusion is therefore that although the fires have devastated vast areas, and made thousands homeless, there should not be a serious increase in erosion in the burnt areas. This will help greatly in the post-fire recovery of the burnt areas.References:H SMITH, D DRAGOVICH (2008). Post-fire hillslope erosion response in a sub-alpine environment, south-eastern Australia CATENA, 73 (3), 274-285 DOI: 10.1016/j.catena.2007.11.003R SHAKESBY, P WALLBRINK, S DOERR, P ENGLISH, C CHAFER, G HUMPHREYS, W BLAKE, K TOMKINS (2007). Distinctiveness of wildfire effects on soil erosion in south-east Australian eucalypt forests assessed in a global context Forest Ecology and Management, 238 (1-3), 347-364 DOI: 10.1016/j.foreco.2006.10.029... Read more »
H SMITH, & D DRAGOVICH. (2008) Post-fire hillslope erosion response in a sub-alpine environment, south-eastern Australia. CATENA, 73(3), 274-285. DOI: 10.1016/j.catena.2007.11.003
H SMITH, & D DRAGOVICH. (2008) Post-fire hillslope erosion response in a sub-alpine environment, south-eastern Australia. CATENA, 73(3), 274-285. DOI: 10.1016/j.catena.2007.11.003
H SMITH, & D DRAGOVICH. (2008) Post-fire hillslope erosion response in a sub-alpine environment, south-eastern Australia. CATENA, 73(3), 274-285. DOI: 10.1016/j.catena.2007.11.003
R SHAKESBY, P WALLBRINK, S DOERR, P ENGLISH, C CHAFER, G HUMPHREYS, W BLAKE, & K TOMKINS. (2007) Distinctiveness of wildfire effects on soil erosion in south-east Australian eucalypt forests assessed in a global context. Forest Ecology and Management, 238(1-3), 347-364. DOI: 10.1016/j.foreco.2006.10.029
- January 11, 2009
- 04:11 PM
- 1,187 views
Future British seasonal precipitation extremes - implications for landslides
by Dr Dave in Dave's Landslide Blog
One of the great questions of the age is of course the ways in which climate change will affect the weather patterns that we are likely to see in the future. In the case of landslides the key issue is the ways in which precipitation patterns will alter, especially the most intensive rainfall events that are responsible for many of the most damaging landslides. One of the most significant steps forward over the last few years has been the ability of global climate models to handle these extreme events, meaning that at last we are starting to develop some capability.This week an important paper has been published by Fowler and Ekstrom (2009), which seeks to look at the likely changes to very intense rainfall events in the UK. Helen Fowler, is based just up the road from me at Newcastle (the city with the chronically under-performing football team), and her co-author have used modelling ensembles to examine how UK precipitation regimes are likely to change in the time period 2070 to 2100 under the SRES A2 emissions scenario, which is currently effectively our best estimate as to how carbon dioxide emissions will change with time (Fig. 1).Fig 1: SRES Emissions Scenarios. A2, as used in this study, is shown in Fig. (b). Source: http://www.grida.no/publications/other/ipcc_sr/?src=/Climate/ipcc/emission/014.htmEnsemble modelling looks at the results of a series of different climate models to examine the range of outputs. Each model operates in a slightly different way, meaning that there will always be a range of results. Therefore, papers presenting ensemble model outcomes always present a range. One of the key issues of interest is whether there is some consistency between them. In this study. Of course the results of such modelling runs are highly complex - in this paper the authors have looked at the 1 day and 10 day precipitation events with a current return period of 25 years. The 1 day event can be thought of as the impact of an intense storm; the 10 day probably simulates a series of low pressure systems tracking across the country, as has happened several times in the last few of years. In landslide terms the 1 day storms might trigger the catastrophic debris flow and sallow failure events, whilst the 10 day events might trigger deeper seated and large slope failures.First the model is run for the a control period (1961-1990) to check that they can realistically simulate observed conditions. They can. The models are then run to look at what would happen in the period between 2070 and 2100, and the results are then pooled using a fairly interesting approach. Well, the first thing to say is that the Global Climate Models (GCMs) do predict a much warmer climate - global mean temperatures are predicted to be 3.1 to 3.56 degrees warmer than at present. Interestingly though the occurrence of these intense rainfall events also greatly increases for three of the four seasons:Winter: Increases in occurrence of extreme precipitation of 5 to 30%Spring: Increases in occurrence of extreme precipitation of 10 to 25%Summer: Very varied results, with some models suggesting decreases and other increases. More work is neededAutumn (Fall): Increases in occurrence of extreme precipitation of 5 to 25%A few of the models do predict larger (and smaller) increases - look at the paper for the full detail. Overall, the authors conclude that "Nevertheless, importantly for policy makers, the multi-model ensembles of change project increases in extreme precipitation for most UK regions in winter, spring and autumn. This change is physically consistent with warmer air in the future climate being able to hold more moisture. The use of multi-day extremes and return periods also showed that short-duration extreme precipitation is projected to increase more than longer-duration extreme precipitation, where the latter is associated with narrower uncertainty ranges."The implications for landslides are stark. Increases on this level of the occurrence of extreme precipitation events will inevitably increase the occurrence of slope failures. Therefore, we should expect to see an increase in the occurrence of slope failures. Unfortunately, as landslides are triggered by just a small proportion of our existing rainstorm events, increases in this range are likely to have a disproportionate impact.Of course the next thing to do will be to build the outputs of these models into slope stability models. This will be a fascinating exercise.Reference:H. J. Fowler, M. Ekström (2009). Multi-model ensemble estimates of climate change impacts on UK seasonal precipitation extremes International Journal of Climatology DOI: 10.1002/joc.1827... Read more »
H. J. Fowler, & M. Ekström. (2009) Multi-model ensemble estimates of climate change impacts on UK seasonal precipitation extremes. International Journal of Climatology. DOI: 10.1002/joc.1827
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