Mocoa: the cause of the tragic debris flow in Colombia

The tragic debris flow at Mocoa in Colombia at the weekend has garnered considerable media attention internationally.  As of today, the total number of people who are known to have died is 273, many of them children with varying reports of the number missing.  Recovery operations continue on the site, and Colombia has mobilised large-scale resources to provide assistance.

There are some aerial images of the track of the debris flow online that allow the catchment to be identified.  This image, via Fox6, in particular clearly identifies the section of town devastated by the debris flow:

Aerial image of the aftermath of the Mocoa debris flow in Colombia, via Fox6

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This is a Google Earth image of the same area of the town.  Unfortunately the image is from 2001 – it is clear that there has been a great deal of development since, with many of the houses that have been inundated post-dating the image.

Google Earth image of the area affected by the Mocoa debris flow in Colombia

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It appears that many of these newer houses may have been built on the flood plain of the river.  However, more interesting in the imagery of the catchment upstream of the area affected – i.e. of the source of the debris flow itself.  This is a Google Earth perspective view:

Google Earth image of the catchment that generated the Mocoa debris flow. The image is from 2001.

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The catchment is not large – the river length is about 10 km but it is steep.  Perhaps the most interesting aspect is the obvious existing shallow landslides high in the catchment:

Shallow landslides high in the catchment at Mocoa, based on the 2001 Google Earth image.

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So a good working hypothesis for this event is that one or more shallow landslides slipped into the channel to become a channelised debris flow, which then entrained large volumes of debris from the channel to create the monster that struck Mocoa.  An alternative, but less likely cause might be that a landslide blocked the channel, and then collapsed to release the debris.  A good quality satellite image will reveal the truth in due course.