Thanks to a former student for bringing the following video to my attention. Hopefully it will play directly when you click on the box below – if not then take a look at:

This channelised debris flow occurred on Lantau Island, near to Hong Kong airport, on 7th June when the area was affected by an exceptional (“black”) rainfall event. In a single hour Hong Kong observatory recorded 140 mm of rain, the highest intensity on record there. During the day, which saw over 300 mm of rain across most of the area, about 400 landslides were triggered on Lantau alone, including one that killed two people.

Helpfully, the Hong Kong Observatory make rainfall isohyet charts available online. I have annotated below the map for 7th June and the approximate location of the landslide. Note that the landslide occurred in an area in which rainfall totals were at their highest (>400 mm).

Hong Kong Observatory isohyet map of the rainfall distribution for 7th June 2008. The approximate location of the landslide in the Youtube video is indicated.

A couple of words about the landslide. This is technically a channelised debris flow. It appears to have started as a comparatively small shallow slip in weathered materials high on the slope. The mobile material has then picked up (technically this is termed “entrained”) debris and water in the channel. Once such flows start, and assuming that they have a steepish slope to travel down, they build momentum and volume to create a highly turbulent and destructive pulse of material that moves very rapidly. Slides of this type typically occur in pulses, as can be seen in the video, which is of course particularly nasty for people in the way. Needless to say these types of flow are very dangerous, particularly if they overflow the channel banks. Fortunately, in this case the flow appears to have remained mostly in the channel itself.

Hong Kong has a long history of these types of landslides, and is adept at managing them. To enhance this the Hong Kong government has just started a large project to identify natural terrain areas prone to these failures and to mitigate areas of high risk.