The impact of the 2017 Mocoa debris flows

The 2017 Mocoa debris flows in Colombia, which resulted in the deaths of 409 people, were the worst landslide disaster in that country for some years. I wrote about the tragedy at the time, and have also highlighted an article published earlier this year investigating the causes and effects of this rainfall induced disaster.  A new paper has been published in the journal Landslides (Prada-Sarmiento et al. 2019) that explores this event in detail.  This is a good piece of work that complements the previous study, most notably by mapping in detail the impacts of the debris flows. There are two aspects that are worth highlighting in particular.

First, Prada-Sarmiento et al. (2019) note that significant debris flows have occurred in this basin on 11 known occasions in the 70 years prior to the 2017 Mocoa debris flows, and indeed a subsequent event occurred on 12th August 2018.  This of course emphasises once again that this was a foreseeable event.  The 1960 debris flow event in particular generated deposits that are spatial similar to those from the 2017 events, and many of the houses that were destroyed were built within this deposition zone.

Second, the study includes some really interesting analysis of the location of the damage from the 2017 Mocoa debris flows:-

The damage to structures caused by the 2017 Mocoa debris flows. The shading indicates the 2006 hazard map that formed the basis for land use planning. Map from Prada-Sarmiento et al. (2019).

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As Prada-Sarmiento et al. (2019) observe:-

“In terms of damage, we can see that the hazard zoning strategy defined prior the event correctly identified several areas in which flooding and mudflows took place. However, it must be highlighted that this strategy does not explicitly consider large debris flows, which were responsible for most of the casualties and damage caused by the 2017 event. This analysis clearly reveals a need to incorporate good-quality information about multiple hazards in the municipal land used plans and the design of consequent mitigation strategies.”

This is a lesson that applies in many other places.  The town is now protected by an early warning system, which reportedly performed well in the 2018 event, and will hopefully prevent a repeat of the 2017 Mocoa debris flows.

Reference

Prada-Sarmiento, L.F., Cabrera, M.A., Camacho, R. et al. 2019. The Mocoa Event on March 31 (2017): analysis of a series of mass movements in a tropical environment of the Andean-Amazonian Piedmont.  Landslides. https://doi.org/10.1007/s10346-019-01263-y.