What are debris flows?

Last time, I mentioned that what hit Mocoa, Colombia were not floods or mudflows, but debris flows. But what exactly is a “debris flow?”

This term isn’t very well-known, at least in popular usage. Indeed, numerous reports describe the Mocoa tragedy as caused by either a mudflow, mudslide, or a landslide (which is technically correct, just a more general term). So far, I’ve only seen one news outlet mention the term.

A debris flow is a kind of landslide, only unlike the popular imagery of a mass of soil, rocks and other debris sliding down from a slope, a debris flow behaves more fluid-like, with a slurry texture similar to that of wet cement. This property allows it to travel much farther even on low angle slopes. Here’s a Youtube video of a debris from Illgraben, Switzerland:

A “lahar” is a type of debris flow. The main distinction is that lahars are made up of primarily volcanic materials, from sands to huge boulders. Since lahars, and debris flows in general, are made up of a wide range of materials, it is incorrect to refer to those as “mudflows” since this term implies that it is made up of mainly mud-sized particles.

The fact that we use unique terms to describe different phenomena isn’t a trivial matter, especially when it comes to natural hazards. Each hazard behaves differently, which means that we have to come up with different ways to deal with it. We wouldn’t call floods as landslides because we know they’re different. Same with debris flows, it behaves differently from water, so we have to find appropriate mitigation measures. Otherwise, money will go to waste, which unfortunately is what has happened many times in the past.

About the photo: Dr. Kelvin Rodolfo walks through the deposits from the debris flow that devastated a village in New Bataan, Compostela Valley, Philippines. To his right is a memorial for the lives lost from disaster. You can read our paper about that disaster here.

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