Young River Landslide, New Zealand

Interesting news from New Zealand, where for the last few months there has been some concern about a landslide dam on South Island. The landslide itself occurred in a fairly remote area of the Southern Alps at 4:40 am on 29th August 2007. The landslide, which is well-described in a GNS Science poster here (there are some great images and some good data on that poster), blocked the river valley to a depth of about 100 metres. This was a big debris slide, with a volume of about 11 million cubic metres and a runout distance of about 1.8 km. Interestingly it does not appear to have created an air blast as large slides of this type often do. This may mean that the mass moved as a series of events closely spaced in time, rather than one big, instantaneous failure.

Over time a lake built up behind it, finally overtopping on 5th October 2007. Understandably, there was concern about the possibility of an outburst flood. In this case there are few human assets at risk downstream, but this is a popular hiking area, potentially putting people on the trails at risk. As a result, the trails were closed by the Department of Conservation whilst the landslide was monitored in real time by GNS Science and Otago Regional Council. Unfortunately, such a closure was not good for the local community as it reduced the number of tourists.

The images below show the landslide from the downslope side. I have annotated it below to show the key features. Note that the deposit has banked up on the right side (from this view). The natural spillway that has formed is just on the lee of this banked up deposit. This deposit is coarse-grained (bouldery), which may well be the reason why the channel has not eroded downwards to release the water.

The image below is from the landslide dam itself looking across the lake. Note the size of the boulders - these are part of the landslide deposit. This does show that if the landslide dam is stable then the resultant lake can in some circumstances become an asset.

This week it was reported here that a decision has now been taken to reopen the trails downstream of the landslide from 1st November (i.e. just before the start of the summer). Unsurprisingly, the local people are rather pleased: "Makarora Residents Association deputy chairman Devon Miller said the closed valley had affected some of the tourist operators in the township, so the decision to re-open was a welcome one."It's a good positive announcement and the community is happy," he said." (Otago Daily Times).

The decision to reopen the trails but to maintain some restrictions in heavy rain, using a new warning system, appears to be sensible. So often landslide management is about balancing risks - i.e. what is the risk of a collapse of the dam affecting someone on a trail compared to the risk to the local communities associated with the loss of tourism, etc. In this case the stability of the dam suggests that this risk has now dropped to close to the residual level. Note that this does not mean that there is no risk - there are hazards associated with spending time in remote mountains. The risk from the landslide dam is no greater, and may be substantially less, than those other risks.

All-in-all the approach taken by the parties involved in New Zealand has been exemplary, in terms of picking up the event in the first place, in terms of the ways that they have monitored it and in terms of the decision-making process to minimise risk.

The relationship between global fatal landslides and rainfall in July

I thought it might be interesting to compare the occurrence of fatal landslides in July 2007 in relation to the distribution of rainfall in that month. Let's start off by looking at the July 2007 fatal landslide map (you will probably need to click on the map to get a decent view of it).

As a comparison, below are the statistics for July 2007 and July 2008, followed by the fatal landslides map for July 2008.



The number of fatal landslides is about the same, but the number of fatalities is very different. At first glance these two maps are also quite similar, but if you look carefully there are some key differences:
1. They are similar in that both have a clear cluster in Central China, although the location is subtly different;
2. In 2008 there were a number of landslides in Central America, whereas in 2007 there were none;
3. They both have a clear cluster along the Himalayas, but close examination shows that there are key differences between the two. In 2007 there were far more fatal landslides in Asia in 2007 than there were in July 2008. The southern edge of the Himalayas is particularly interesting - in 2007 there was a big cluster of landslides in northern Pakistan. This is not the case in 2008. In Nepal the 2007 distribution was concentrated in the west, whereas the 2008 focus was in the central part of the country. On the other hand, in 2007 there was a number of landslides in Bangladesh, but this was not the case in 2008, when only one occurred. Finally, in July 2007 there was a cluster of landslides in southern India, but only one landslide in this month is 2008.

So what drives the differences between these two years? The most obvious explanation is that of rainfall patterns between the two years. It is possible to look at this issue because TRMM provides monthly maps of rainfall anomaly - i.e. the difference between the recorded monthly rainfall and the monthly total averaged over several preceding years. Unfortunately, this map is not available for 2008 yet, but 2007 is online. Given that most of the landslides in July 2007 occurred in Asia, let's take a look there. First the landslide map:

Now the TRMM rainfall anomaly data. The units are mm / day difference from the long term July mean.

It should be fairly clear that there is a strong correlation between the distribution of fatal landslides and the rainfall anomaly in July 2007. In particular, the clusters along the southern edge of the Himalayan arc are focused almost exactly where the rainfall was unusually high. The apparent gap in central Nepal corresponds to an area of lower than average rainfall. Similarly the cluster in south-west India is also related to a high rainfall anomaly.

Finally, below is a NASA image of population density for Asia. Dark red colours represent high population density, orange is low. It is clear that in Asia the population is very unevenly distributed. Note the high population densities in places that were particularly badly affected by landslides in July 2007. Thus, it is clear that the distribution of rainfall probably determines the distribution of landslides, and the density of population then determines the fatality total. Of course this is a bit of an over-simplification, not least because landslides only appear on the database if they kill someone, which means that the population density plays a role here too, but I am sure that you get the general idea.

Finally, it should be noted that Dalia Bach, Yang Hong and the TRMM team have also been looking at this issue and have completed an analysis for landslides in general. See this web page for details:
http://trmm.gsfc.nasa.gov/publications_dir/potential_landslide.html

Landslide sizes (numbers killed) around the world

One of the interesting aspects of our landslide fatality database is that it is possible to produce maps of the size of the impact of landslides (which in this case means the numbers of people killed) in each event around the world. I tried to do that for my EGU presentation, but my colleague Nick Rosser has produced a rather more attractive version (Fig. 1) showing the 2007 dataset.

Fig 1: Map of the size of the impact of landslides, as measured by the number of fatalities in each event, for 2007. Click on the map to open a larger version.

The variation in size is quite interesting. Note that in Europe, N. America and Japan the recorded landslides tend to be comparatively small, almost always resulting in less than five fatalities. South Asia and Central America on the other hand have a scatter of landslide sizes from small events through to the very large. China also has a high frequency of large landslides, but appears to have comparatively few small ones. Africa has surprisingly few recorded fatal landslides, but those that do occur tend to be quite large.

So what to make of this? Well, first and foremost it shows the different levels of mitigation of landslides around the world. In the more developed world (i.e. Europe and N. America), large landslides tend to be very well mitigated (i.e. engineering measures have been used to stabilise them). The few fatal landslides that do occur tend to be small and in fairly remote locations, resulting in low fatality counts. On the other hand, in South and South-East Asia, mitigation is much less likely and there is a high density of people in affected areas, meaning that fatality counts are much higher. There might also be an effect of the both the tectonic setting (i.e. the high mountains of S. Asia in particular might mean that the fatal landslides are larger) and the climate (i.e. torrential monsoon rainfall might tend to trigger very energetic landslides, which are more likely to kill, perhaps). Finally, the data also perhaps show that the current database still misses smaller landslide events in China and in particular in Africa. We are trying to improve this situation.

As ever your comments are really welcome.