4 May 2010

EGU Day 2 – morning landslide sessions

Posted by Dave Petley

For this year’s EGU General Assembly I intend to only blog on talks that really catch my eye.  This morning there was a fabulous session on Landslide Forecasting with series of great talks.  Of these,  Samuele Segoni and colleagues presented a very interesting paper on a hugely ambitious project to develop a regional landslide warning system for Tuscany based on rainfall thresholds.  The project appears to be extraordinarily successful – it appears to work with very few false alarms or missed forecasts.  However, to do this the area had to be split into 25 warning zones, using c.330 rain gauges.  As most such systems do, the approach uses an intensity – duration power law relationship. Thus, to make these systems work requires a huge infrastructure.  Interestingly, now that it is clear that such a system can work from a technical perspective, the emphasis needs to shift to the societal problems of trying to disseminate warnings effectively, and getting people to react appropriately to them.  That is a real challenge.

The second paper that caught my attention was by Peter Lehmann and Dani Or, looking at precursor events in the initiation of landslides using concepts of self-organised criticality.  Their starting point was that in the home country, Switzerland, 6% of landscape is prone to instability. In 2005 a rainfall event triggered over 1000 landslides, causing damage estimated at over $3 billion.  Essentially they seek to explain landslide initiation by considering processes that provide a cascade effect, as in the sand pile models of criticality.  Here they model the landslide as being controlled by fibres and fibre bundles, which are analogues of the loss of strength of the landslide material.  They showed that precursor events can be observed as weakening and breakage of the bundles occurs, replicating observed behaviour.  It was very neat and very interesting, and provided potentially important insights into failure initiation.

Next up was Nejan Huvaj-Sarihan from Turkey, presenting her doctoral work undertaken at the University of Illinois.  This was an experimental investigation of failure time prediction in landslides, using creep-rupture as the basic concept.  She built a simple direct shear machine to investigate the creep-rupture process, and showed two key things:
1.    She observed creep at even very low shear stresses (<20% peak strength).  She used this to infer that all slopes creep, which is correct;
2.    She could initiate creep rupture failure when factor of safety was greater than one, but that the time to failure depended upon how close to FoS = 1 the system is. 
Using this data she then explored whether failure prediction can be undertaken using the range of techniques available, concluding that it can in all three cases.  This is a very neat study, although I am surprised to see creep-rupture in pre-sheared materials.

During questions someone made the point that there is a “geotechnical disease” to ignore the time element.  This was a very provocative statement, but is quite correct.

Oded Katz and his colleagues followed this up with a model based study that sought to look  material disintegration in controlling the geometry and size of landslides.  Again, this was a neat bit of work that came to some key conclusions.  For me the most important one was that the power law roll-over in landslides is indicative of the change in material properties, and that the collapsing of the power law relationships onto each other occurs because there is such a narrow range of residual strengths available in natural systems.  However, the talk also demonstrated beautifully that failure is associated with breakage of inter-particle bonds.  Initiation of movement in the models occurred only when shear surface was fully developed through bond breakage, which links with the previous three presentations.

Those three talks (Lehmann, Huvaj-Sarihan and Katz) together present an extraordinary level of insight into landslide processes that on their own justifies my attendance at the meeting.

In the after coffee session I would like to highlight just one talk, that of Monique Fort and her colleagues on debris flow initiation in the Ghatte Khola watershed of Nepal.  This watershed suffers extraordinary pre-monsoon rainfall events – she quoted a storm in 1974 that had over 300 mm of rainfall in an hour – can this really be right?  Anyways this small ( 7.8 square kilometre) catchment generates debris flows that result from shallow slides that block the tributary valley, then collapse, creating flows.  These in turn enter the main valley, which is then blocked in turn, and another flow occurs down the main channel.  There are two interesting things here – first, how a small failure can initiate a bigger flow that in turn blocks the main valley, generating an even larger one – who would try to forecast hazards when this sort of situation occurs?  Second, she highlighted the ignorance of road builders in Nepal to these processes, resulting in inappropriate designs that then fail in the next storm.  This is a hobby-horse of mine; I could not agree with Monique more.