20 April 2009

Updated: European Geosciences Union Day 1

Posted by Dave Petley

Updated to include the afternoon sessions
This week is the annual European Geosciences Union assembly in Vienna. This is the biggest annual landslide meeting – there are >300 landslide related papers this time around – and since I am the scientific secretary for the landslide session I cannot allow the opportunity to comment on what I see to pass. So here are my thoughts on Day 1. My intention is not to comment on everything that I see, but instead on those that I found interesting.

Morning sessions
The first, unfortunately poorly attended, session focused on landslides associated with loess deposits. Dr Meng and colleagues from China presented a very well illustrated overview of landslides on the loess plateau. There were some remarkable statistics – for example, in the 20th Century over 60,000 people were killed by loess landslides in China, whilst in Gansu province alone in the decade between 1875and 1985 there were over 1000 disastrous landslides, killing in total >2000 people! Interestingly, the key factor determining the spatial occurrence of landslides was the neotectonic activity – areas of active uplift have far more slides than those that are subsiding, presumably because of undercutting and oversteepening during incision. The presence of sinkholes was also shown to be rather important.

Mamyrova and her colleagues presented a paper on the investigation of the mechanics of two loess landslides in Kyrgyzstan, a place about which we here far too little from a landslide perspective. She presented some rather nice repeat pass Quickbird imagery to show the evolution of the landslides, demonstrating in particular that one of the slopes showed clear tension crack development before the main failure event. Perhaps most interestingly, the initiation of the main failure event occurred in a wet, but not exceptionally wet, year (1994). A month or so earlier there was a magnitude 3.9 earthquake just 17 km from the landslide. The authors speculated as to whether this might have played a role. This project is clearly in its early stages, but the combination of an under-reported area, a very strong research team and an aspiration to combine FLAC (finite difference) and PFC (discrete particle) modelling makes it one to watch for sure.

Later in the morning, in a far better attended session (I ended up sitting on the floor!), there was a very nice presentation by Dewez and his colleagues frim BRGM in France on the use of terrestrial laser scanning to look at coastal cliff hazards in chalk. In many ways this mirrored the work that my colleagues and I do on cliffs in NE. England and, unsurprisingly, the results were similar. Over about 1 km of cliff, and scanning from eight stations, the survey picked up 8567 individual rockfall events. The most interesting thing for me was the investigation that they had done of the role of large vs small events in erosion on the cliffs, showing that over this time a single large event contributed 85% of the volume change, and that 99.8% of the volume change comes from rockfalls that are >1 cubic metre. This is quite a different result from our observations in layered sedimentary rocks in N. Yorkshire, emphasising the role that geology plays. It should be noted though that the comparatively short duration of this study (ours is >twice as long) may mean that the results are biased by the large events that happened to occur. The authors recognised that there is a need to extend the time-base – I do really hope that this is possible as the development of long datasets is crucial if we are to understand rockfalls properly.

Travelletti and his colleagues, also from France, presented a study of the use of terrestrial laser scanning for the monitoring of a large mudslide – in this case the Super Sauze slide. In my view the application of TLS for monitoring this type of slide is too rarely presented. In this case they have used the system to look at the evolution of the steep source area and to examine the movement of the toe of the flow. Both were pretty convincing, but I was particularly impressed with the work that they had done tracing the movement of boulders to generate displacement fields. In some cases they saw 16 m of movement in a single summer, which is far greater than the c.0.25 m error in the technique at this location. This is looking like a mature technique now that really justifies more extensive use.

The afternoon landslide sessions focused upon geophysical techniques, many of which are a little beyond my area of expertise. I will comment on a couple of presentations though. A general point to note is that, as the chair of the session pointed out, geophysical approaches to landslide analysis have moved from being rather esoteric and obscure to generating quite a lot of interest in a very short period. This is primarily because these techniques are increasingly good at discriminating between different types of wet sediment and debris, which means that they are useful. However, I do note that the presentations that I heard were on the whole about the geophysics, and not about the landslide, so these methods are not really in the mainstream yet.

There were two presentations on the Trieve landslide complex in France, the first by Kneiss and the second by Renalier. The former focused upon the use of seismic noise to map out the subsurface geology of the slope. In particular, they used a technique that they called the H/V method to delineate the soft sediment – bedrock boundary. The story was pretty nice in that the 3D topography that they showed explains the dynamics of the movement, although it does seem to be based on a slightly simplistic view of the mechanics of the slides. Personally I would like to see some verification of the 3d model (perhaps a couple of extra boreholes to see if they can predict where the drill will meet rockhead), but overall it is a pretty nice piece of work. The second was by Florence Renalier, who was a little nervous I suspect, but did a good job (well done if she reads this). The intriguing result here was that they showed that shear wave velocity is inversely correlated with displacement rate for the landslide that they studied (in the same complex as the previous paper). That seems to me to be a pretty fundamental result, but it was not really dwelt upon. In questions the team said that they think that the shear wave velocity is related to microcrack density. I would like to hear a lot more about this to be honest.

The next paper that caught my eye was by Clara Levy and her colleagues, also from Grenoble (are they trying to corner the market in research level landslide geophysics?). This was a study of the seismic precursors to a rockfall that occurred from chalk cliffs in 2002. By luck or good design (or both) the team had a seismic network in place before the 2000 cubic metre failure occurred. It was collecting data at 30 kHz, which must be a mind boggling dataset, but what they captured was 200 seismic events in the 2 hours before the collapse. They attribute this to breaking of the remaining rock bridges (which seems reasonable). They have tried to model this effect, but have chosen to do so with a Mohr-Coulomb failure model, which is perhaps not ideal. Nonetheless, the model does show how the failure propagates through the rock bridges, with those at the toe of the slope being the last to go. The model is I think not really faithful to reality, but it is an interesting piece of work.

Finally, Arnhardt and his colleagues from Aachen decided to break up the French – Italian axis by presenting a paper looking at the use of low cost sensor networks for monitoring landslides. This is highly worthy work, but I seem to have heard numerous similar presentations, all of which come to nothing. The problem is that the low cost sensors in question are usually designed f
or use in nice, controlled, clean, stable environments (like in cars, high tech production lines and hospitals). Landslides are the exact opposite – wet, cold (or hot), dirty and mobile, which means that the sensors that work so well in the lab really don’t function well. This is the same issue that Pen Hadow and his friends are having in their misjudged nightmare in the high latitudes. I guess someone will crack this problem in the end – and the team from Aachen seem sufficiently serious to be the ones to do it – but I do hope that they realise just how hard it will be. I really do wish them luck and will watch their progress with interest.

Overall it has been an excellent day (thanks to all the speakers), with some great science presented. Tomorrow we have two sessions on landslide forecasting and two on landslide risk. I can’t wait!