Debris flows and fatalities

One question that I am quite often asked is why it is that fatalities in landslides occur more at night than in the day. In some ways this is counter-intuitive as one might expect that people would be safer in their houses than they are when they are out and about.

At least a part of the answer lies I think in the destructive potential of comparatively slow landslides. Most people have probably seen the video of the recent Hong Kong debris flow, which I blogged about earlier in the year. Clearly when a landslide is as rapid and destructive as this one then the chances of escape are quite limited, although running sideways out of the path, rather than away from the slide, would give the best chance.

However, many fatal landslides are much slower than this. The following Youtube video shows a debris flow from Clear Creek County in Colorado.



If you cannot see this above, click here.

This is a fairly typical debris flow. They very often occur in a series of pulses, each of which start with the movement of large boulders that are moving quickly but not exceptionally fast. This is clear in the video above. These boulders continually roll-over each other, bulldozing everything in their path. Behind the boulders comes of a flow of much finer and more fluid material, which often moves quite quickly, as in this case. The flow slowly declines before the next pulse arrives.

In many cases people who are awake and alert are able to move out of the way of one of these flows, not least because they are pretty noisy. However, people who are asleep in a house often do not know that the debris flow is coming until it hits the building. Unfortunately very few buildings can withstand the huge forces that the flow can generate, as the USGS image below shows. Even where people do survive the initial event, they are quite often then hit in the dark by the next pulse.

USGS image of the aftermath of the Vargas debris flows in Venenzuela

Clearly in this case the required mitigation is first to try to prevent these flows from occurring at all - for example by stopping deforestation and by placing catch structures in the tributary valley areas in which the flows start. Second, it is important to identify where flows might occur and then to try to ensure that people are not in the way of them. Finally, in some cases a warning system can be helpful, but this should be seen as a last resort once the above two measures have been implemented as effectively as is reasonably practicable.