14 April 2008
Over the next few days I will try to write up some comments on issues that arise at the landslides sessions at the European Geosciences Union (EGU) meeting in Vienna. EGU is a massive meeting (c. 10,000 earth scientists), and the natural hazards section is one of the largest. The landslide field is the biggest component of natural hazards, so there is usually something for everyone. In total there are about 240 talks in the landslides sessions, so I clearly won’t comment on them all! I will instead focus on the matters that arise that interest me in a purely selfish way.
The first session on Monday was on remote sensing techniques and slope failures. Unfortunately I missed it as I was trying to track down my luggage, which once again had been lost by British Airways. Thanks to them once again – there are times when you make me so proud to be British. Oh, and by the way, if Terminal 5 is the answer to Heathrow’s problems then I do have to ask what on earth the question was…
I did make it to the second session, which was on a combination of the role of vegetation on slope stability and on landslides and climate change. All six talks were of a very high standard – clear and concise, and covering a good range of material. I am intrigued by the paucity of papers on landslides and climate change, given that this is the topic of the moment. One paper that did address this issue head on, a presentation by Remaitre and colleagues looked at the impact of climate change on shallow and deep landslides in the French Alps. The results were refreshing as they indicated that projected climate changes will reduce the occurrence of slope movements due to a drop of 1-3 m in the groundwater level. Far too rarely do we hear of positive impacts of climate change, but here they are clear.
The previous paper, but Guthrie and colleagues, looked at landslide occurrence on Vancouver Island in Canada as a result of climate change. Whilst this was an interesting approach, the analysis was on the whole a bit over-simplified I think. However, the paper did very clearly highlight the ways in which logging triggers slope failures, demonstrating that clear cutting increases the occurrence of landslides by an order of magnitude or more in this steep, wet environment. The other presentations sought to try to understand why and how vegetation improves slope stability. All were highly competent, but it is clear that more work is needed given the spatial and temporal complexities involved.
All of which seems to highlight the fact that climate change is important, but land use change is much more so in the context of landslides. We should therefore be deeply alarmed by the current situation, in which concerns about climate change are driving land use changes to allow the production of biofuels. For example, felling of forest for palm oil plantations in tropical areas would seem to be deeply unwise. In addition, we are currently seeing massive increases in food and fuel prices, which is also likely to drive increased rates of forest loss.
The first session in the afternoon focussed on landslides triggered by earthquakes. A common theme emerged in terms of the role of topographic amplification in landslide triggering (this is the way in which earthquake waves interact with slopes to cause higher levels of ground shaking, which in turn triggers slope failures). Lee and colleagues presented an investigation of slope failures triggered by the 1999 Chi-Chi earthquake in Taiwan, showing that position on the slope (i.e. proximity to the crest) is the key issue in terms of triggering failure. Meunier et al presented a rather more conceptual analysis that agreed with this, although the topographic role was less strong than some maintain. The latter seems to be let down by the use of the full length of a landslide feature – presumably including the deposit – when really only the source zone should be used. This inevitably biases the results towards the foot of the slope. The observation that slope failures might be more likely on slopes orientated away from the epicentre is interesting, but is not supported by all earthquake events. My own observations from Kashmir do not support their suggestion that landslides cluster around the epicentre either, so I think rather more work is needed here to make this analysis convincing.
Two other presentations in the session focussed upon landslides triggered by MW=7.9 the 15th August 2007 earthquake in Peru. Wartman and his colleagues focussed primarily on an impressive lateral spread that has in places moved by in the order of 20 m. The surface area of this failure is several square kilometres. Joseph Wartman has pictures of some of the landslides triggered by the earthquake linked from his web site, including a narrated slide show. Hermanns and his colleagues reported on landslides triggered by the same earthquake, but raised a different but very interesting issue. This part of the Andes is littered with large, ancient landslides and rock avalanches, which occur on both the coastal bluffs and in the high mountains. Hermanns noted that this earthquake actually triggered surprisingly few large landslides – and indeed no rock avalanches. What sized event is therefore needed to trigger the landslides that observed – is does this require a different type of event (e.g. a crustal earthquake). There are no answers to this at the moment, but it is a question that needs to be investigated urgently. Dating the ancient landslides, which should be possible for the rock avalanches at least using cosmogenic techniques, would be a good start.
The final session of the day was the Union meeting on forecasting natural hazards. Fausto Guzzetti gave a masterclass on assessing landslide hazard and risk, provoking a lively round of questions. The following speaker, who was talking on a non-landslide topic, impressed me rather less, so I called it a day.
I have the rather unlucky task of starting the first session tomorrow at 8:30 am – I am not expecting a big crowd!