27 August 2008

The influence of the summer monsoon on fatal landslides in Asia

Posted by Dave Petley

I am currently putting together a keynote paper for the forthcoming International Conference on Slopes, which is being held in Malaysia in November. I have been looking at the occurrence of fatal landslides in Asia, using the Durham Landslide Database as the source of data. I have plotted the number of fatal landslides by month for 2002-2007 for three areas – South Asia (i.e. the Indian subcontinent), East Asia (which includes China, Taiwan, Korea and Japan) and SE. Asia (which should be self evident). The graphs are shown below (note the different magnitudes of the y-axis scales by the way).

First, South Asia shows a very strong monsoon signal. Look in particular at the mean trend for the five years. The level of fatal landsliding in the winter months is very low, with most of the events occurring in June to September. It is also clear that the number of landslides increases rapidly at the start of the monsoon as the rain bands push northwards, but then decline slowly as the monsoon disperses. There is also considerable year-on-year variation; in particular the very strong and prolonged monsoon rainfall across parts of India is very clear for 2007.

East Asia (below) also shows a strong monsoon signal, of course slightly modified by the impact of typhoons too. Here though the pattern is quite different – again look at the graph of the mean of the other data. In this case the monsoon signal is far more symmetrical, again reaching a peak in July but rising and declining at about the same rate. Again there is considerable year-one-year variation both in terms of number of events and the variation through the year. The big spike in July 2006 is particularly clear.

SE. Asia present a completely different picture (below). As much of SE. Asia is tropical there is far less monthly variation. Unsurprisingly no monsoon spike is seen. The occurrence of landslides is slightly higher in the latter part of the year as the rainy season affects Indonesia for example. Of particular note are the variations in the distribution between years – this really does reflect the occurrence of extreme tropical cloud bursts.

So the impact of the really large scale climate systems on Asian landslides is absolutely clear. This is particularly interesting in the context of climate change – exactly how the Asian monsoon in particular will change will partially determine the landslide impact. Unfortunately the models are not resolving this issue too well at the moment, but most do suggest an increase in the occurrence of the most intense rainfall events. The implications for landslides of this are stark.