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.

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.

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.

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.

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.