18 April 2008

EGU Day 4

Posted by Dave Petley

I spent the morning in the session on the Characterization, monitoring and early warning related to large landslides. I was struck during the presentations by the degree to which the technologies for monitoring landslides have improved over the last decade or so. For example, Casagli and his colleagues gave a very polished presentation on the application of their ground-based radar LISA to the monitoring of the Ruinon landslide near to Bormio in Italy. This system, which costs about 100,000 Euros per annum to buy and operate, can produce a map of deformation with a 1 mm movement resolution at ranges of up to 1.8 km. In this case it is being used to monitor the movement of this somewhat hazardous slope failure to great effect. On the other hand, David Toll (a colleague from Engineering at Durham) presented the results of a new set of high precision tensiometers that are capable of measuring suction forces down to -2 MPa, a significant step forward for tropical soils for example. Thoeny and his collegaues introduced us to the use of dynamic fluid electric conductivity logging, which looks at changes in the electric consuductivity of saline water introduced into a deep boreholed drilled through the landslide. This technique was able to determine the locations at which water is entering and leaving the borehole, which gives a good indication of the location of, for example, the shear surface. This looks like a complex technique, but one that shows a great deal of promise once fully developed. The problem would seem to be the rather cumbersome and specialised processes involved in saturating the hole with saline water. I will watch this development with interest. Glimsdal et al. presented the results of tsunami modelling for the same slide as Thoeny, the Aknes landslide in Norway. The danger is that a large failure would trigger a tsunami that would rapidly inundate local villages. The paper introduced the initial results of modelling a potential tsunami. This is clearly work in progress, but looks interesting.

To me the key issue was highlighted by the paper of Ronchetti and colleagues, who attempted to analyse the data obtained from monitoring of a very large earthflow on Mount Modino near to Modena in Italy. They attempted to use the so-called Saito approach, which I have worked on extensively, to see whether prediction of the failure of the landslide was possible. The paper itself was interesting and there was some sign that movement could be used in this way. However, the data proved to be very noisy, such that identifying trends in real time would be an awesome challenge. This issue that this highlights is that we desperately need to develop better ways to analyse the data that wed are producing. At this meeting it has been clear that analysis is lagging well behind data production. This is surely the next challenge. I have been working with three first class PhD students – Angel Ng, Jon Carey and Chris Massey, all of whom are making really substantial progress in this area for landslide movement at least. It is clear that this is a fertile area for development and that considerable more work is needed.

In the afternoon, I attended the sessions on Rockfalls and large catastrophic landslides. This was a bizarre session in that the quality of the science presented was exceptionally variable. I won’t go into detail regarding the weaker end of the spectrum. At the strong end, I was taken by a couple of presentations in particular. Pichler and his colleagues presented a very comprehensive and detailed analysis of the threat to hydrocarbon pipelines associated with rockfalls. This included dropping large boulders 20 m from a crane onto a gravel pit to see what happened, wich looked great fun at least! The result of the subsequent analysis was a clear demonstration that a 3 m thick layer of gravel placed over the pipeline is enoough to protect it against a 10 tonne boulder falling from 100 m, which would seem to be a rather valuable thing to know! Meanwhile Sausgruber presented a great paper on a massive (1.3 cubic km) ancient kink band landslide in Austria. This slide has in the past moved about 60 m, but the computer model of the slides showed that development of flexural toppling has mobilised shear strength between the beds, which has effectively stabilised the slide. Waldmann and his colleagues reported attempts to model the potential generation of a landslide induced tsunami in Nordfjord in Norway, based upon the development of failure scenarios plus an analysis of two earlier events, one in 1905 (which killed 61 people) and one in 1936, which killed 74 people. Historic photos suggest that the earlier events had a run up height of up to 33 m, illustrating the threat posed. Again, this is work in progress, but it is a very nice piece of research. Finally, Agliardi and his colleagues have untaken the massive task of mapping deep seated gravitation slope deformations (these are massive creeping slope failures that occur in high mountain areas). The study is notable because:
1. It has made use of Google Earth as a primary data source, which shows waht a fantastic resource this has now becaome;
2. The task is epic – it took the individual concerned 2 months to do the mapping alone, which must have been somewhat tiresome;
3. The statistical analysis of the results show very nicely that these types of failure represent a part of the spectrum of mass movement types, and are not a seperate class of landslide in their own right.
Again, there is probably more to do here, but the study is really valuable and needs to be continued.

All-in-all a most interesting day. Once again I was struck by the tremendous range of material presented and by the very high quality of the science in general.