20 May 2015
The timing of the landslide season in Nepal
Posted by Dave Petley
Timing the landslide season in Nepal
In a previous post I highlighted the threat posed by landslides in the SW monsoon in Nepal. A key question now is the timing – i.e. when will the rainy season (i.e. the landslide season) start? The IMD has a tracking map for the advance of the monsoon – at the time of writing the monsoon front is lying to the SW of India in the Andaman Sea, and the advance looks to be about normal for the time of year:
Based upon the timings on this map the rains might be expected to reach the eastern part of Nepal on about 5th June, and to have covered the entire county by about 20th June. These are average figures of course, so there may be considerable variability. But of course the arrival of the monsoon front is not necessarily the start of the landslide season. Landslides can be triggered by pre-monsoon rainfall (especially convective events), and the heavy monsoon rainfall may not be associated with the arrival of the monsoon front. So I have been looking at my Nepal landslide database. In the graph below I have plotted by year for the period from 2004 to 2014 inclusive the cumulative number of fatal landslides in Nepal:-
Today (20th May) is day number 140, so it is clear that the main landslide season has yet to start. In most years the number of fatal landslides starts to increase at around about day 160 (about 8th June), with the main focus starting from about day 180 (28th June). So the main threat will start in about three weeks from now, and will intensify towards the end of next month. The landslide season typically ends at about day 280, i.e. somewhere around early October. Note that there is a great deal of variation between years both in terms of timing and the number of events.
I have highlighted two curves – 2006 and 2009. These were both years that represented the start of El Nino events – a medium-sized event in 2009-10 and a weak event in 2006-7. Interestingly both years saw a later than average start to the landslide season. We are currently in weak El Nino conditions. I am unsure as to whether this is significant.