Dangerous debris flows

A little over a month ago, heavy and sustained rainfall in Mocoa, Colombia triggered deadly landslides and debris flows, claiming more than 300 livesPost-event analysis by scientists from GFZ Potsdam show that more than 600 landslides occurred within the watersheds next to town. The mixture of soil, rocks and debris from the landslides mixed with water flowing in the streams to form dangerous slurries called “debris flows.”

Sadly, as with many disasters caused by natural hazards, this event was foretold, but went largely ignored.

What is striking about this event is that this phenomenon is actually fairly common here in the Philippines. The most recent significant events were in Compostela Valley (2012), Nueva Ecija (2015), and Oriental Mindoro (2015). More than 500 people were killed in Compostela Valley; fortunately no one died from the debris flows in Nueva Ecija and Oriental Mindoro.

We can see below the similarities between the texture of the deposits from these events. Notice the large boulders “floating” on top of the deposits, a characteristic of a debris-flow deposit.

debris_flows
Top left: April 2017 in Mocoa, Putumayo, Colombia (via Independent); top right: December 2015 in Baco, Oriental Mindoro, Philippines (via GMA News Online); bottom left: October 2015 in San Vicente, Nueva Ecija, Philippines; bottom right: December 2012 in New Bataan, Compostela Valley, Philippines.

Many events like these go unreported because a lot of people still describe this as either flashfloods or mudflows, which is an inaccurate description. It is neither a flood, which is mainly water, nor composed of mud. In this case, semantics matter because debris flows behave differently from flowing water, hence different strategies have to be employed to mitigate its effects.

Now that the rainy season has officially started here, events like these are bound to happen. The only question now is when and where.

 

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