Framlingham College Presentation on the typhoon Morakot disaster in Taiwan

Last night I gave a public lecture on the Typhoon Morakot disaster in Taiwan. The talk may be viewed and downloaded here:

10_02 Framlingham comp

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The causes of the Shiaolin landslide disaster in Taiwan

The Shiaolin landslide disaster in Taiwan, which occurred during typhoon Morakot last August, has been the source of huge controversy. To recap, the landslide, which occurred during an exceptional rainfall event, wiped out Shiaolin village, killing about 500 people. The controversy centred on two key aspects - first, the perceived very slow response of the government to the disaster and second the possibility that tunneling associated with the Zengwen Reservoir project may have been a contributing factor to the slope failure. In response the Executive Yuan of Taiwan commissioned an investigation from the Public Construction Commission, which released its final report yesterday.

The report is of course in Mandarin, but very helpfully there is a powerpoint file available that summarises the findings and provides some illustrations of the key issues. This powerpoint file is available here (warning it is a large file in Powerpoint in pptx format):

http://www.pcc.gov.tw/pccap2/FMGRfrontendDownloadQuoteFile.do?fileCode=F2010020012

The report is available here:

http://www.pcc.gov.tw/pccap2/FMGRfronten/DownloadQuoteFile.do?fileCode=F2010020011

The key finding of the report is in my view correct - this is that the tunnel project was not the cause of this landslide disaster - they factor was the exceptional rainfall experienced in this event. The powerpoint file provides a dramatic illustrations of the magnitude and intensity of this rainfall:


Click on the image for a better view in a new window. The map on the left is the recorded rainfall for the storm, the table on the right is the total rainfall for a number of stations in the worse affected area. Note that the precipitation totals are extreme in every sense of the word - c.2500 mm (2.5 metres of rain) is the equivalent of three years total rainfall for the temperate area in which I live. This is the largest rainfall event ever recorded in Taiwan, and probably the most intense rainfall event worldwide for half a century.

The report shows that disturbance associated with the tunnel is not sufficient to be a factor in the landslide - a conclusion that I support. Instead, they show that the slope underwent a dip slope failure that led to a massive rockslide that destroyed the village. The report suggests that the landslide had a maximum depth of about 86 m and a volume of 2.5 million cubic metres. From what I can tell the slide itself was a wedge failure with a dip-slope defining part of the wedge.

There is only one aspect of the report that continues to cause concern. This is the interpretation of the mechanism of failure. This slide shows a long profile of the landslide site, which shows bedding parallel to the slope right down to the river (section A-A'):

This just doesn't seem to accord with what Chris Massey and I observed on site at the toe of the slope:

This picture is taken from the north end of the toe of the slope looking upstream - note the bedding on the far side of the valley - this is near vertical.

This picture was taken at the site of the old bridge abutment at the toe of the slide (the concrete in the middle of the image is this abutment I think) - again, note the very steeply dipping rocks at this point.

The final point to make is that the Shiaolin landslide was of course not the only failure to occur in the area during Morakot. Mapping of this region has identified 880 landslides covering an area of 2058 hectares (20.88 square kilometres).

There can be no doubt that Morakot was an extraordinary event.

Debris flow damage from Typhoon Morakot in Taiwan

In addition to visiting the Shiaolin landslide in Taiwan at the weekend, we also managed to visit a couple of other places that had suffered damage during typhoon Morakot. Damage from debris flows and river floods occurred extensively throughout the upland areas of southern Taiwan, as these images show:


The government has set aside about US$5 billion for reconstruction. Unfortunately this area is threatened by another typhoon, called Parma:

The geological structure of the Hsiaolin slide

Thanks to reader Chingying Tsou, who has answered my request for information about the geological structure of the Hsiaolin landslide. He has provided a link to the website of the Sino-Geotechnics Research and Development Foundation, which provides the key information. The page is in Chinese, but the diagrams are really helpful, as is the Google translation.



So here is a topographic map of the landslide site, with the major faults and of course the landslide itself marked on:

You will note two cross-section lines on the map, one of which (A-A') runs down the axis of the landslide. This cross-section is reproduced below:


It is clear from this that the landslide is a dipslope failure - i.e. the slide has occurred on beds that are orientated parallel to the slope, and thus facilitate failure. The cross-section indicates that the rocks are a mixture of sandstone and shale. This can often cause problems as the shale is weak, impermeable and prone to weathering, whereas the sandstone is often stronger but allows the accumulation of water (i.e. pore pressure generation). The presence of the fault is an additional factor - it may well be that the movement on the fault has caused the beds to be disrupted and thus weakened. It should also be noted that this cross-section is probably only indicative. It would not surprise me to find that the river has actually eroded out the lower portions of these beds, then filled in the spaces with the terrace deposits upon which the village was built, further weakening the slope.

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.

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.