On the dangers of Rhododendrons!

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.

Reference
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

New images of the Shiaolin landslide in Taiwan

The largest landslide triggered by typhoon Morakot occurred at Shiaolin in Kiaosiung County. It is estimated that 450 people died in this landslide. Until now I have been struggling to find a decent image of the slide itself, apart from the poor quality aerial photography and drone video that I have highlighted previously. However, I have today come across the following three images on the Taiwan News website:

Click on the images for a better view in a new window.

The failure is a little difficult to interpret. It is possible that it is a very large-scale translational slide, with a large about of erosion into the scarp to form the channel structure. Alternatively, there could be an element of a wedge failure here, but if so it is surprisingly deep. The slide looks to have been very high energy, and thus very destructive, in the lower part of the track.

Updated: A large landslide triggered by the latest Indonesian earthquake

A large (Mw = 7.0) earthquake struck offshore Java in Indonesia today at 07:55 UTC (2:55 am local time). USGS has a location map (below) and earthquake parameters, a shake map, etc here.

AP is reporting that a large landslide was triggered in Rawa Hideung village, Cianjur district in Java:

"Antara reported that 12 families, or about 30 people, were trapped in houses buried by the landslide in Rawa Hideung village, Cianjur district. Six bodies had been recovered and the fate of many others was unknown, a resident, Agus Sobandi, was quoted as saying."

Java appears as one of the most serious hotspots on the global landslide dataset that I collate, so the occurrence of slides from this earthquake is not surprising. There may well have been many more landslides.

Update: The Indonesian National Disaster Agency BNPB has a report on its web page about the earthquake here. This is in Indonesian, but Google can translate this to English. The key element from a landslide perspective is (unedited translation): Cianjur: "Interim report, there were 10 people died in landslides buried, with details of the 9th district residents and 1 Cibinong Sukanegara citizens. A total of 11 houses in the district Jembrong severely damaged and landslides buried Cikado district offices to the ground."

A video of a river bank failure

The BBC News website has a short video showing the effects of heavy rainfall in Southern China in mid August. Most notable is a pretty remarkable piece of footage of buildings sliding down a river bank and into the torrents below. You can view it here.

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.

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.

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.