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Understanding ‘debris flows’

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At Large

Understanding ‘debris flows’

/ 05:07 AM November 01, 2017

Until subsequent weather disturbances, most notably “Yolanda” in 2013, hit the Philippines, the “world’s worst storm” for 2012 was Typhoon “Pablo” (international name: “Bopha”).

Torrential rains hit Mindanao, which until then had but little experience with typhoons and floods, and worse, triggered “enormous debris flow” in the Mayo River watershed, devastating the village of Andap in New Bataan in Compostela Valley province.

A study of this disaster, which led to the loss of over 600 lives in Andap with hundreds more missing, with lead author Dr. Kelvin Rodolfo and a team of meteorologists and geologists, including former Project NOAH head Mahar Lagmay, points out that the “catastrophe” could be blamed not just on the “huge volume and rapidity of the flow itself,” but also on “human factors.”

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Debris flows, write the authors, “although among the world’s most destructive natural phenomena, are remarkably misunderstood.” While it is, technically, a type of landslide, using the generic term “landslide” “makes most people mistakenly think of rock masses detaching from a cliff and accumulating near its base.”

Oftentimes mistakenly called “floods, mudslides, or mudflows” not only by the media but by decision-makers as well, the accumulation of debris flows makes for “unsafe sites to occupy.” Note the authors: “Such lack of understanding may have tragic consequences for communities like Andap in mountainous terrain.”

The authors describe debris flows as “fast-moving slurries of water, rock fragments, soil and mud.” These can be triggered by “sudden downpours, reservoir collapses or landslides dislodged by earthquakes into streams,” but can also be associated with volcanic eruptions.

There is also the “problem” of Mindanao residents’ relative unfamiliarity (until recently) with typhoons and related phenomena. In the case of Andap, “New Bataaan was settled much too recently (in the 1960s) for its founders and inhabitants to be familiar with super typhoons and debris flows.” Worrisome, they say, is that “the rapidly growing Philippine population continues to expand into increasingly disaster-prone areas, and it does so with insufficient hazard evaluation. Unregulated logging deforested the steep slopes, facilitating runoff, erosion and the landslides that fed the debris flow.”
In sum, a combination of faulty site assessment and deficient human decision-making (and negligence and greed) combined to create the disaster that was Pablo.

New Bataan was created in 1968 by an act of Congress from public lands in Compostela Valley. The town’s name was in honor of Luz Banzon-Magsaysay, widow of President Ramon Magsaysay, who had helped lobby for the creation of the town. “The town was laid out thoughtfully,” say the authors, with streets radiating out from a circular central core for government and social functions. But, they add, “the founders were not aware of the natural hazards (the town) faced. Indeed, they add, “debris flows were not widely understood at that time.”

There is a “positive outgrowth” of the Andap disaster though. In reaction, Project NOAH compiled all “alluvial fan areas” in the country, with more than 1,200 alluvial fans identified throughout the country. Better yet, “communities under the threat of debris flows are being educated about them.”

In October 2015, Typhoon “Lando” generated devastating debris flows in Nueva Ecija. “Fortunately, communities living on those alluvial fans had been warned and evacuated. No one was killed.”

In December that same year, Typhoon “Nona” struck Mindoro, also triggering massive debris flows. “Houses and buildings were buried or washed out in several communities, but no one died because of timely warnings and evacuations.”

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The authors warn of increasing numbers of “super typhoons” due to climate change, and more disasters the likes of the Andap debris flow. But with growing awareness, especially among officials charged with disaster preparedness and mitigation, it is hoped that communities will have enough time and warning to get out of harm’s way.

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TAGS: At Large, Bopha, debris flows, Haiayan, New Bataan, Rina Jimenez-David, supertyphoon ‘yolanda’, typhoon Pablo
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