Large fatal landslide in Papua New Guinea

On 25th May there was a large landslide in Papua New Guinea that reportedly killed 19 people. Reports about this event have been rather hard to come by, so I went searching through the Papua New Guinea only newspaper sites. The most dramatic image is this one, from The National:


It is clear that this is very large landslide (see below). The dynamics are fascinating as the source area appears to be very small. The slide has then scoured out what appears to be colluvium deposits from within a channel on the upper part of the hillside, before transitioning into a large -scale, highly energetic flow.

I also came across these images and this report on the pages of the Papua New Guinea Post Courier.

This image shows the track of the landslide

This image reportedly shows the landslide deposit.

The report (shortened here) states that:
"A family is counting its luck after all its members survived a massive landslide that killed 19 of their relatives between
the gorges of Firangka in the Sialum district of Morobe Province. The family of five survived by metres when the landslide swept down the mountainside and engulfed six houses, missing their hamlet in Zongefifi in the early hours of Monday...Locals retrieved three bodies, two male and one female while the other 16 bodies were still buried. The landslide started from the top of the mountain, slid down the gorge for about half a kilometre with debris strewn over
200 metres wide before dropping down the steep terrain.

Mr Gamato [deputy administrator] said the landslide was caused by water underneath the coronus limestone which had built up and caused the ground to become unstable, causing a massive slip. He also warned that the months of May, June and July were a time of continuous rainfall in the province and those travelling by sea, living along river banks and under mountainous terrains should take extra precaution because of the rain in the province in recent weeks."

Landslides from Cyclone Alia in Darjeeling

One of the most admirable landslide websites that I know is the Save the Hills blog, which is the mouthpiece of a community level group trying to raise awareness of the problems of landslides in Darjeeling in NE India. I have featured this site several times before.

As Cyclone Alia has closed in on the area over the last few days See below) they have been warning of the danger of landslides triggered by the rainfall that the cyclone would inevitably bring. The Cyclone passed across the area yesterday - and sure enough reports are now emerging of the landslides that the rainfall has triggered. At the time of writing Sify is reporting 22 killed in landslides, with 10 injured and another six people reported missing. Over 100 landslides have been reported. The Times of India reports that 245 mm of rainfall fell in a 24 hour period.

Meanwhile, in the last few days there are also reports of a large landslide in Sialum District in Morobe province of Papua New Guinea, which is reported to have killed 19 people (although the picture is still rather unclear), landslides in Bhutan with at least one fatality, mudslides in Uttar Pradesh in India that killed eight people, and landslides in Guangdong that have killed four. Finally, there are some very unclear reports about the magnitude of the impact of heavy rainfall in Afghanistan, with reports of about 95 people killed in landslides and floods.

Sadly, none of this is particularly unusual as we are seeing the impacts of the development of the summer monsoon across Asia.

Updated: 25th April: The Kaiapit Landslide, Papua New Guinea, 1988

Update: Mark Drechsler has provided some images of the Kaiapit landslide, which are now integrated into the text below. Thanks to Mark for this - it is much appreciated. All images copyright of Mark Drechsler. In addition, I have placed a copy of the Drechsler et al (1989) paper on the International Landslide Centre website here. It provides a fascinating level of detail about the slide. I recommend this paper over and above the alternative.

Click on the images for a full-sized view - it is well worth it!

Original text: Thanks for Mark Drechsler of Parsons Brinkerhoff Australia and the University of Adelaide for reminding me of the Kaiapit Landslide, which is surely one of the largest landslides of the last two decades (can anyone think of any larger?). Even though its impact was not enormous (74 deaths in three villages) the scale of this landslide is staggering. The landslide was described in a paper by Dreschler et al. (1989) and one by Peart (1991), from which these details are taken.

The slope failed at 10:43 am on 6th September 1988 when approximately 1.8 cubic kilometres of mountian collapsed, leaving a scar 1600 m high. The debris ran out over a distance of 6.5 km, covering an area of over 11 square kilometres. The estimated velocity is up to 180 km/hour. The slide inundated three villages and formed four valley-blockage lakes.

What is particularly interesting about this slide is the lack of an obvious trigger. Peart (1991) reported that the time of initiation was not the rainy season (indeed it was towards the end of the dry season) and that the period in question was not particularly wet. No seismic activity was recorded. The area has not been glaciated, so over-steepening and loss of support cannot be a cause. Clearly the area is affected by high levels of uplift, which may have allowed the formation of over-steepened slopes. These are probably a causal factor, but do not explain the triggering.

Peart (1991) sagely suggested that there may have been a time-related factor in the initiation of the landslide. I am convinced that time-dependent behaviour is important for large-scale landslide triggering, which means that the time of failure might lag significantly behind the trigger event. This is not always the case, but quite often very large landslides do not have an obvious trigger event. Other examples include the Mount Cook failure in New Zealand and the Leyte landslide in the Philippines.

Sadly the Google Earth imagery of the slide site is covered by clouds. Comments are welcome as ever.

Reference
Drechsler, M. Ripper, I., Rooke, E. and Warren, E. 1989. The Kaiapit Landslide, Papua New Guinea. In: Engineering Geology in Tropical Terrains, Universiti Kebaangsan Malaysia.
Peart, M. 1991. The Kaiapit landslide: events and mechanisms. Quarterly Journal of Engineering Geology, 24, 399-411.

Hat-tip: Mark Drechsler from Parsons Brinckerhoff and University of Adelaide.