3 July 2020
The Dawn of Hope peat slide: understanding the source area
My tweet yesterday linking to the remarkable video on Youtube by John Flynn that shows the full extent of the Dawn of Hope peat slide attracted a great deal of attention. One of the aspects of landslides that I enjoy the most is that, 30 years into my professional career, I still find slides that cause surprise. This is one, without doubt.
The video provides a source to toe record of the landslide, but of course it is the source area that has generated the most interest. I can only speculate on what is going on here – others will investigate it properly, and I will post their findings when published if possible. On Twitter some suggested that this might be a lateral spread that transitioned into a flow slide. I would speculate that this is unlikely. This image shows the lower part of the source area, and the transition zone into the channelised flow:-
I would hypothesise that the slide initiated here as a bog burst and then retrogressed to create the extraordinary landform seen in the upper reaches of the source area:-
These are extensional landforms, which have generated because there was space vacated by the initial bog burst. The line of trees, mostly intact and upright, following the drainage line, is remarkable. It shows that the failure has propagated up the channel, and that in the channel itself mobility is quite high. The peat adjacent to the channel has failed, but with much lower mobility.
To understand landslides in peat there are a few important aspects to consider:-
- Peat is a predominantly organic material, with some clay and silt and a very large amount of pore space. This means that it has an unusually low density compared with other geomaterials. A typical density for peat is about 400 kg per cubic metre – less than half of that of water – so rafts of peat can literally float.
- The peat itself has low compressive strength, but is quite strong in tension. This is because the body of the peat consists of organic fibres. This may be the reason that the blocks have remained intact.
- Failure in peat slides very often occurs at the boundary between the peat and the underlying substrate. I think there is some evidence that this is the case here.
So, my hypothesis would be that the extreme rainfall, perhaps aided by piping in the peat, led to very high pore water levels in lower part of the topographic depression shown in the first image. This failed to generate a bog burst, and a large amount of degraded organic material vacated the depression and entered the channel. It clearly entrained considerable amount of material from within the channel, and of course the structure of the peat was rapidly lost in the turbulent flow, creating a slurry with high mobility.
Back in the source area, the now vacated depression allowed failure of the adjacent blocks. Failure propagated upwards through the depression, aided by the high pore water pressures. But of course these materials are highly permeable, so failure also induced rapid drainage, especially away from the channels. Thus, the pore water pressures at the base of the blocks quite rapidly declined, leaving the peat rafts stranded.
The video also shows the areas of inundation (there is more than one) downstream:-
The level of damage here is undoubtedly high.
I welcome comments about this landslide, and my interpretation of it. I’m aware that the comment function is not working correctly at the moment, so I will try to add the key comments to the main text below when I get a chance.
Quickslide 1: The Sate Mu jade mine landslide
Loss of life from the Sate Mu Jade Mine landslide is now at least 166 people, making it the worst landslide of 2020 to date. Heavy rainfall is hampering the rescue, so the number remaining missing is unclear. A much better version of the video of the failure has now been tweeted. Tragically, the video appears to show a very large number of people on the slope at the time of failure:
— ABC News (@ABC) July 2, 2020
Quickslide 2: NASA imagery of the Achoma landslide
NASA has now posted before and after satellite imagery of the Achoma landslide in Peru. It is useful, but is less impressive than the Planet Labs image that I posted a few days ago.