This blog provides a commentary on landslide events occurring worldwide, including the landslides themselves, latest research, and conferences and meetings. The blog is written on a personal basis by Dave Petley, who is the Wilson Professor of Hazard and Risk in the Department of Geography at Durham University in the United Kingdom.

This blog is a personal project that does not seek to represent Durham University.

Monday, 31 August 2009

Eyewitness account of landslides triggered by Typhoon Morakot in Taiwan

Aerial image of Hsiaolin from CNR-IRPI

I have today returned from my holiday, so normal service should be resumed. Interestingly, the number of readers of the blog appears to have increased in my absence. There is a lesson there I think! Anyway, I have a large backlog of things to post, but unfortunately also have a large backlog of other work as well, so it may take some time.

Anyway, to get things moving, there is a very interesting eyewitness account of landslides triggered by Typhoon Morakot on a blog run by Rich Matheson, a resident of S. Taiwan. The account is at the following link:

http://liefintaiwan.wordpress.com/2009/08/18/typhoon-morakot/

It is well worth a read.

Thanks to Richard Foster of http://barking-deer.com/ (a company that runs adventure tours to the fabulous landscape of S. Taiwan) for bringing this to my attention.

Thursday, 27 August 2009

Op Ed in Taipei Times on landslide management in Taiwan

The Taipei Times, which is the premier English language newspaper in Taiwan, has generously run as an Op. Ed. an improved version of my blog post reflecting on landslide management in the aftermath of Typhoon Morakot. This is available here.

Monday, 24 August 2009

A bad summer for landslides in Darjeeling District, India

Regular readers will know that I occasionally highlight the great work being undertaken by a small group of concerned citizens, Save the Hills, to raise awareness of landslides in Darjeeling District in northern India. This work is profiled in a blog, Visions of Hell, run by Praful Rao. As this is an area affected by the SW Monsoon, the summer is the main landslide season in this region. Although the vearly part of the monsoon this year was quite weak, August has seen heavy rain and many landslides. In particular, on both 15th August the area had about 60 mm of rainfall, followed by a further 190 mm on 19th August. The results were all too predictable, with multiple landslides. Strangely though these incidents have been scarecely reported even by the media within India, or in the daily reports of the National Disaster Management Division there.

Praful Rao has written an illustrated report of the latest destruction caused by the landslides, available here.

Tuesday, 18 August 2009

Some reflections on the Typhoon Morakot landslide disaster in Taiwan

For landslide scientists Taiwan has an almost mythical status, effectively being the nearest thing to a landslide laboratory. To my great benefit I have been able to work on landslides in Taiwan since 1991. As well as being a country of great beauty (there are good reasons why the Portuguese named it Ilha Formasa - Beautiful Island - in 1544), the combination of high rates of tectonic uplift, weak rocks, steep slopes, frequent earthquakes and extreme rainfall events renders the landscape highly susceptible to landslides and debris flows. Indeed, Taiwan has almost every type of landslide, although as an aside the number of known ancient rock avalanches remains surprisingly low given the prevailing conditions.


Of course the reason why Taiwan is of interest to landslide scientists is also the reason why it can be a challenging place in which to live. When the World Bank reported in 2005 on its "Disaster Hotspots" study it noted that "Taiwan may be the place on Earth most vulnerable to natural hazards, with 73 percent of its land and population exposed to three or more hazards". The last great disaster there, the Chi-Chi Earthquake, which happened almost exactly a decade ago, was a wake-up call to the hazards that mountain communities face in Taiwan. I think that a great deal has been achieved in Taiwan since this event, but the Typhoon Morakot disaster shows that there is so much more to do.

Inevitably, and frankly understandably, there is now a great deal of concern in Taiwan about the viability of its mountain communities. The issues are complex - many mountain villages are inhabited by indigenous populations that have strong ancestral links to their land. In other areas the communities were settled by retired soldiers who came to Taiwan with Chiang Kai-Shek in 1949. Both sets of mountain dwellers are poor, but they are often passionate in their desire to remain in their rural communities. There are high stakes here, with the possibility of a new wave of enviornmentally-damaging engineering works in the mountain areas.


Unfortunately, in Taiwan much of the development in mountain areas has been undertaken without fully considering the ways in which humans and natural processes interact in this sort of environment. In many cases this is quite understandable - the processes are so dynamic that our understanding of them is poor - but the consequences are tragic. The race of build mountain hotels is perhaps the most obvious case - as the typhoon so clearly showed - but there are many other examples. As roads, hydroelectric schemes, fruit farms, recreation areas and many other developments cause extensive environmental degradation, the landscape is responding with increased rates of erosion, mainly in the form of landslides, debris flows and floods, that puts the population at risk.

I should stress that I am not calling for a moratorium on development in the mountains in Taiwan, and nor am I advocating that the mountain populations are relocated. I do believe that development, including road construction and the building of hydroelectric power schemes, is viable. In addition, I also believe that it is possible for people to live, relax and work in the mountains with tolerable levels of risk, although as Shiaolin showed some of the existing communities are built in highly dangerous sites. I do think that some activities in the mountains probably cause too much environmental damage - in the highland areas fruit farming appears to me to be causing extreme levels of degradation for example, and the construction of "hot spring resorts" is often insensitive and poorly planned - but most mountain communities should be helped to change their way of life, not to be forced into relocating.

So here is my suggested manifesto for reducing (but not eliminating) the risk from landslides in Taiwan:
  • Develop and implement a comprehensive national plan for managing slopes, covering design standards, training, land use management, emergency response, etc. This effectively mirrors the very successful slope management programme developed in Hong Kong and, more recently, in Malaysia.
  • Develop a comprehensive research programme to understand the natural processes occurring in the mountain areas of Taiwan. The National Environmental Monitoring Center being proposed by National Taiwan University seems to me to be an essential element of this that could also provide world class science outputs. Operate in the network in a manner similar to GEONET in New Zealand, having both science and public understanding ofd science benefits, Ensure that the outcomes of the research are fed back into the planning and management process.
  • Undertake a properly coordinated and sensitive programme to evaluate the safety of all upland communities. Where risk is found to be high, identify the best mechanism to bring this to a tolerable level, through combination of well-designed engineering works, education programmes, warning systems and, in extremis, relocation to nearby safe sites. This will need to be accompanied by a programme to determine the level of tolerable risk in this environment, and to ensure that there is an understanding that the aim is not to elimate risk, but rather to manage it.
  • Constitute a national disaster management agency to coordinate disaster risk reduction and disaster response.
Templates for all of the above exist in other countries. To achieve this will take considerable political will, and both resource and time. However, management of risk is achievable without forced relocations or catastrophic environmental damage.

A first decent view of the Shiaolin (Hsiao-Lin) Landslide

First, apologies if my posts are a little sporadic (and if I am not replying to your emails). I am currently on leave in the Swiss Alps. However, I will continue to make short posts over the next fortnight, especially in light of the Taiwan landslides.

Thanks again to Tsou Ching-Ying of Kyoto University for bringing to my attention a Youtube video of the Shiaolin site. This is of course the village that was most seriously impacted by Typhoon Morakot. The video appears to have been shot from a UAV. The quality is a little marginal, but you will get a very clear impression of the landslide if you view it below:



If you cannot view it above it should be visible here.

This is not really a mudslide as the media have described it. It is a little hard to characterise from this view, but it looks to be a large-scale, catastrophic rock slope failure that has transitioned into a debris flow. It appears to have had a long runout - I guess it is unsurprising that the loss of life was so high.

Typhoon Morakot mudslides: Before and after photos of Shiaolin (Hsiao-lin) village in Taiwan

Thanks to Tsou Ching-ying from Kyoto University for providing this pair of images, showing the impact of the mudslides on Shiaolin village in Taiwan as a result of Typhoon Morakot.

Before (from an online album here):

After (from an online album here):

Wednesday, 12 August 2009

Taiwan mudslides - a very lucky escape for most


This morning various news agencies are reporting the very good news that four days after Typhoon Morakot 726 survivors have been found from the villages destroyed by the mudslides. This does not mean that the mudslides had no victims - the Taiwan police are saying that it is difficult to know how many people were buried.

Tuesday, 11 August 2009

A strange update on the Nachterstedt landslide

A strange update has appeared in the "The Local" regarding the Nachterstedt landslide. This reports that "Scientists at the Collm Observatory at Leipzig University registered an underground movement in the area measuring 1.0 on the Richter Scale, just six minutes before the first call to the emergency services on July 18. An earthquake can be ruled out, the scientists told Der Spiegel magazine, but they suggested it would be feasible that an old mining tunnel collapsed, causing the landslide. "

For info a magnitude 1.0 earthquake is the equivalent of about 30 kg of TNT. I cannot understand why the scientists believe that this was a tunnel collapsing. To me it is far more likely that the signal was the landslide itself occurring. I must admit that I cannot understand why the collapse of an underground mine working would cause this catastrophic slide anyway.

On another matter, the paper also reports that a further collapse is considered likely:
"Surveyors have been pulled back out of the Auf der Halde community, where the ground has sunk by a further 0.4 millimetres and is expected to also collapse. Preparations are now being made for emergency measures to be taken should the remaining houses also fall into the Concordia Lake, which was created by flooding an old open-pit mine."

Landslides from the Fiordland earthquake, New Zealand

GNS Science in New Zealand have released details of the mapped landslide distribution from the Fiordland earthquake on 15th July. As I noted a day later, the number of landslides appears to be surprisingly low:



The earthquake produced only 187 landslides, most of which were shallow and comparatively small. The area affected by landslides was 5600 square kilometres. Comparison with the graph below (see my original posting) suggests that this is at the bottom end of the expected number for a Magnitude 7.8 earthquake.


According to Graham Hancox the reasons for this lower than expected level of landslides are thought to be:

1. The ground motions were smaller than would normally be expected for a magnitude 7.8 earthquake;

2. The dominant fault rupture motion was away from land;

3. Lower than average rainfall occurred in the two months prior to the 2009 quake.

The map suggests that there is a very strong relationship between the ground motion and the occurrence of landslides, with most of the mass movements being concentrated in the vicinity of the epicentre. This is consistent with, for example, the Chi-Chi earthquake in Taiwan.

Monday, 10 August 2009

Another catastrophic landslide - this time in China

As soon as I had posted the round-up of catastrophic landslides below another one appeared on the newswires, this time in Mainland China. Xinhua is reporting that Typhoon Marakot has triggered a landslide in Pengxi Township, Taishun County in Zhejiang Province, burying half a dozen apartment blocks:

"A massive landslide occurred in an eastern Chinese town Monday night, causing six or seven apartment buildings to collapse and burying an unknown number of residents, local authorities said Tuesday. The accident took place at about 10:30 p.m. in Pengxi Township, Taishun County of Wenzhou City in Zhejiang Province, when the unexpected landslide soon destroyed the 4-story apartment buildings at the foot of a mountain, rescuers said. Rescuers have pulled six people alive from the debris, but one of them is in critical condition. It is not immediately known how many people were buried, rescuers said, adding that it is very difficult for them to carry out search operation due to a huge amount of mud and rock. The landslide was triggered by continuous torrential rain brought by Typhoon Morakot, which has left six people dead and three others missing on the Chinese mainland after a powerful landing in east China Sunday. "

A dreadful weekend of landslides

The last few days have been a dreadful period for landslides. I really cannot cover all that has happened in enough detail, so all I can do is to outline the main events:

1. Rotorua rockfall, New Zealand
Actually the period started with a very lucky escape in New Zealand on Thursday when a rockfall on a slope under maintenance crushed a car with two passengers. Both were trapped but were extracted from the car with minimal injuries. Given the size of the blocks and the state of the car this is pretty remarkable image from here:



2. Shunhe township, Hanyuan county, Sichuan province, China
The large valley-blocking landslide in Hanyuan County also occurred on Thursday. This slide, which is reported here, is believed to have killed 31 people. The partial blockage of the river continues to cause problems:

3. Mount Pinatubo, Philippines
On Friday the Philippines was hit with very intense rainfall that appears to be from the outer edges of Typhoon Marakot, which hit Taiwan and then China. There were two disastrous landslides - in the first a lahar hit a tour group and their guides on the flanks of Mount Pinatubo, killing two locals and three tourists.

4. Kias, Baguio, Philippines
In the second incident, also on Friday, a landslide hit a group of miners at Kias, near to Baguio in the Philippines. A group who went to their rescue were then hit by a second landslide, killing 14 people in total.

5. Pithoragarh district, Uttarakhand
On Saturday a large landslide occurred in northern India (see image below from here). The true impact of this is a little unclear, but the current estimate of loss of life appears to be 43 people. Thanks to David Hopkins for the heads-up on this one, and to Sekhar for the photo).

6. Typhoon Marakot, Taiwan
The true impact of Typhoon Marakot is far from clear, with reports of debris flows that might have killed hundreds. It is important to stress that these are unconfirmed, but with 2.4 m of rainfall in a weekend who knows? A slightly clearer report suggests that Taoyuan Village in the county of Kaohsiung was hit by a debris flow that killed 16. We will wait for the morning to see whether the reports of much higher loss of life are correct.

Breaking news? Hundreds missing in Taiwan landslides after Typhoon Morakot?

The Taiwan Government's news agency is reporting that hundreds of people may be missing after mudslides triggered by Typhoon Morakot:

"An estimated 500 to 600 people remain unaccounted for Monday after mudslides triggered by Typhoon Morakot hit an isolated mountain village in southern Taiwan's Kaohsiung County, according to rescued villagers."

This story has been expanded by Taiwan News:

"An estimated 500 to 600 people remain unaccounted for Monay after mudslides triggered by Typhoon Morakot hit an isolated mountain village in southern Taiwan's Kaohsiung County, according to rescued villagers. "Only 44 residents of Siaolin managed to avoid the mudslides, and the remaining 500 to 600 residents are unaccounted for, " said a family member of residents believed to be trapped in the village and possible buried alive. The weeping family member urged the government to not waste even a second in launching an operation to rescue the trapped villagers."

I will post again when further details are available. Hopefully the villagers found a safe place to shelter.

Friday, 7 August 2009

Update - emergency evacuations below the Dadu River landslide in China

The Chinese language website www.scol.com.cn has some images of the area upstream of the landslide dam at Dadu here. This also links to what appears to be a Chinese language bulletin board here in which people are posting images of the situation. If this is as it seems then this area has serious problems. although reports do seem to indicate that the river is not completely blocked.

For example, this is a bridge across the Dadu River under normal conditions:

And this is it today:


More details here as well, with some images of the low flow downstream. It appears that the authorities are organising a large-scale evacuation.

Valley blocking landslide in China

The web site China.org.cn is reporting a major valley blocking landslide on the Dadu River in Sichuan:

"A massive landslide in southwest China's Sichuan Province has killed at least one person and injured 19 others, local officials said Friday morning. The landslide, which occurred at around 11:30 p.m. Thursday in Shunhe Township of Hanyuan County, also blocked a major local river, leaving six local residents stranded, said officials from the county government. The local government is still verifying the casualties. Minor landslides still took place from time to time at the site Friday morning.

The landslide has formed a 250m-long barrier blocking the local Dadu River. By 3:30 a.m. Friday, the water had overrun the obstruction, posing a danger to the lower reaches of the river. The runoff at the power station's dam is 240 cubic meters per second, compared with a flow of 2,760 cubic meters per second Thursday. The provincial government has ordered an evacuation of the people living at the lower reaches of the river to avoid further casualties. A provincial-level working team is also rushing to the scene to direct rescue work."

The Dadu River is famous for the disastrous valley-blocking landslides after the 1786 earthquake, which are believed to have killed 100,000 people when they failed. This area was also affected by the shaking from the Wenchuan Earthquake.

The report is somewhat contradictory about whether the river is still blocked, but at this time of year, and considering the amount of rain that the area has received this monsoon, the flow rate into a lake will be high.

Wednesday, 5 August 2009

30 years ago - the Abbotsford landslide, New Zealand

As well as being something of a landmark birthday for my wife Kirstie, Saturday will also be the 30th anniversary of one of the best known and most interesting landslides in New Zealand - the Abbotsford landslide of 8th August 1978. This was a large (5 million cubic metre) slide that caused the loss of 69 houses, with an overall cost of about NZ $10 million. Fortunately, no lives were lost. The landslide was recently described in a paper by Graham Hancox (Hancox 2007) of GNS Science, from which I have gained most of the information here.

The landslide occurred in a suburb of the South Island of Dunedin. The remarkable picture below, from the Hancox paper, shows the state of the site three days before failure:


On the left side of the image (the east) lies a quarry from which about 300,000 cubic metres of sand had been removed. Running across the centre right of the image is a large array of cracks that were opening up as the landslide moved. These cracks clearly extend into the suburb, although they become less easy to discern in this area. In fact cracking was first noted in some of the houses in 1972 or even before. Through time the cracks grew until in 1978 they defined the rear scarp of the landslide as seen above. The cracks caused several water main breakages in late 1978 and early 1979.

The slide was extensively monitored in the period before final failure. The data suggests that the rate of movement was steadily increasing through time, with rates as high as 10-15 cm/day being noted in the week before failure!

Final failure began at about 9 pm on 8th August. It lasted about 30 mins, during which time a large block moved forward by about 50 m, leaving a graben structure behind that was about 16 m deep. This is very clear in the image below. The slide was a deep-seated translational block slide that covered an area of about 18 hectares. It was about 800 m, 400 m long and up to 40 m deep. The average movement was about  × 400 m, up to 40 m thick). According to Hancox (2007) it slid down a 7°-dip slope at an average speed of about 1.7 m/min. The rate of movement was sufficiently slow that no-one was killed, although many people needed to be rescued and, of course, lost their homes.

(source: Teara)

(Source: http://www.kvc.school.nz/Kaikoraistream/Intro_Folder/Geology.htm)

So why did the slope fail? Well, the first key factor is that the site was susceptible to failure under natural conditions. The materials were dipping in the same direction as the slope and were weak and susceptible to sliding. There were ancient landslide deposits on the site that point to previous instabilities, well before humans could have played a major role. Second, the removal of the sand from the quarry removed support from the slope, making it far more likely to fail. In fact, Hancox suggests that the slope only needed groundwater to increase by 0.5 m for movement to start.

The slide was probably triggered by increased water levels in the slope. This is likely to have come from two sources. First, the few years before the landslide were wetter than had been the previous 20 years or so. Second, Hancox (2007) suggests that there was a leaking water main that may have been allowing 5 million litres to enter the slope each year.

New Zealand is of course a landslide prone environment, but there can be little doubt that many lessons have been learnt from this failure. By modern standards it seems amazing that the houses were still inhabited when cracks as large as those shown in the first photograph were developing, and movement rates of 10 cm per day were being recorded. The authorities in New Zealand are clearly using the anniversary of the landslide to remind people of the need to remain vigilant.

Reference
Hancox, G.T. 2008. The 1979 Abbotsford Landslide, Dunedin, New Zealand: a retrospective look at its nature and causes. Landslides 5: 177-188. Journal page.

Tuesday, 4 August 2009

Fatal landslides in July 2009 - map and report

Below is the fatal landslide map for July 2009. Each yellow dot represents a single landslide that killed a person. The background is the etopo1 DEM, with the darker colours representing higher terrain. As usual clicking on the image should provide a high resolution view. The image is a GIF so you should be able to download it, but please acknowledge the source.

You will see that in July the focus has very much switched to Asia, with the effect of the (weak) monsoon along the length of the Himalayas being fairly clear. Note also the clear evidence of fatal landslides in Central China, reflecting the heavy rainfall there, and in Japan. There is very little going on in the Caribbean and in SE Asia at the moment - this is not unexpected for the time of year.

The statistics are as follows:
Number of fatal landslides: 65
Number of fatalities: 366

The average for July for 2003-2008 was 468 fatalities, meaning that 2009 was substantially below average. The cumulative number of fatalities for 2009 is 1328 to the end of July, substantially lower than the long term average cumulative total of 1574 (excluding earthquakes of course). Indeed, only one year (2008) has had a lower cumulative total at this point.

Comments and corrections are welcomed. Thanks to all who have helped to put this together.

Sunday, 2 August 2009

The low level of NH hurricanes and typhoons in summer 2009

One of the reasons that the northern hemisphere summer is essentially the global landslide season is that landfalling tropical cyclones (typhoons and hurricanes) represent a rather efficient way of triggering slope failures. This is especially the case in the Caribbean, Taiwan, Japan, SE and S China, the Philippines and Vietnam. Typhoon rain is astonishing to experience for the first time - peak intensities of 100 mm per hour are not unusual in the largest events. The hourly rainfall data below is for the passage across Korea of Typhoon Rusa in 2002, taken from Lee and Choi (2007):

Note that at Gangneung the peak hourly precipitation was 100.5 mm (4 inches) and the peak 24 hour rainfall was 870.5 mm (35 inches). It is unsurprising that such events cause landslides on a large-scale.

The northern hemisphere tropical cyclone season is primarily associated with warm sea surface temperatures, and hence runs primarily through the summer and early autumn months. Interestingly, and for reasons that are far from clear, the energy associated with northern hemisphere tropical cyclones has been reducing for a number of years. Ryan Maue at Florida State University runs a superb web page that tracks tropical cyclone occurrence globally. He has the following graph of global and northern hemisphere tropical cyclone energy (note smoothed using 24 month running sums):

Levels of tropical cyclone activity are now approaching a 50 year low. However, even by recent standards the level of Northern Hemisphere tropical cyclone activity to date this season has been exceptionally low. Ryan also provides the following graph of Northern Hemisphere tropical cyclone energy (ACE) for the first three months of each season since 1979:

It is not for me to speculate on why tropical cyclone activity should be at such a low level (Ryan is much better qualified to do so), but it is clear that so far the Northern Hemisphere tropical cyclone occurrence has been a damp squib. The occurrence of landslides reflects this (I will post my monthly update in the next few days). This is of course good news in terms of landslides and floods, but it is bad news for farmers who rely on rainfall for irrigation, the hydro-electric industry in these areas, and many others whose livelihood and.or welfare depends upon water derived from tropical cyclones.

It will be interesting to see whether level of activity dramatically increase later in the season.

Meanwhile, the S. Asian monsoon remains very weak, with the Indian Meteorological Department noting that total precipitation in the monsoon season is 19% below the long term average. Again, the occurrence of fatal landslides that I have recorded is mirroring this pattern. Meanwhile, much of China is suffering from unusually intense monsoon rains. For example, Shanghai has just suffered its heaviest rainfall for 70 years.