5 July 2021
The Atami mudslides in Japan
Over the weekend there was considerable attention paid to the remarkable videos of the mudflows in Atami, Japan. This one in particular gained traction:-
But there are several others, which are helpfully combined into a compilation here:
I particularly draw your attention to the excerpt that begins at 8 minutes 16 seconds into the second video (the same sequence appears in the first video too).
The main, extremely violent, footage is a particularly nasty example of a channelised flow, common in steep terrain that is subject to extreme rainfall. Long term readers of this blog will remember the classic examples from Lantau in Hong Kong in 2008 and South Korea in 2011 for example, but there are many, many other examples.
The Google Earth image below shows the general location of the Atami disaster. I have placed the marker on what is, I think, the location of the prominent red building in the main video:-
— 苅谷愛彦 Yoshi Kariya (@yoshi_kariya) July 3, 2021
A translation of the text is:
Valley of occurrence. It can be seen that the valley head, which is the forefront of slope failure, extends several times along the volcanic slope, and the past debris flow and landslide deposits that have accumulated at the bottom of the valley are formed in the shape of a terraced rice field. It seems that there was a land on the ridge in the upper part of the basin [Geographical Survey Institute 2017 photo].
This suggests that the Atami mudflows started as a conventional landslide high up in the catchment. This is a typical response to very heavy rainfall. The ground will probably have undergone static liquefaction to form the highly mobile flow. Once channelised, it will have entrained water from the channel and deposits within the channel, especially those left from previous landslides, to create this terrifying mudflow.
The destructive power of these events is much greater than that of a river flood as the density of earth materials is much higher.
There are two interesting elements to this landslide. The first is that the initial failure appears to be quite deep-seated. The second is the presence of the road right at the headscarp. In other cases, most notably the Sarno landslides in Italy, roads have played a key role in the destabilisation of the slope. Given the deep-seated nature of the landslide, that may not be the case here, but it is an interesting juxtaposition.
Japan has high quality landslide management programmes and amongst the best engineering prowess for managing these hazards. However, the combination of the geological setting, the climate and the high population levels means that entirely preventing these events is impossible.
Sadly, but unsurprisingly perhaps, the number of people reported missing at Atami has jumped overnight. Latest reports suggest that there are three known fatalities but as many as 80 people may be missing. Hopefully this will reduce in the next few days as people away from home are traced, but the losses may well be high.