What are debris flows?

DEBRIS flows are a type of landslide that involves rapid, downward mass movement of sediments coarser than sand, often including boulders one meter or more in diameter.

They occur along fairly steep slopes and gullies, and move at 2 to 100 kph.

Debris flows normally occur at the mouth of a mountain drainage, where the river valley meets more gently dipping slopes. In this area, river sediments called alluvium are deposited because of the change in slope where water flow velocity suddenly decreases. Over thousands of years, a landform called an alluvial fan develops at this part of the base of the mountain.

Sediments laid down by river water generally comprise alluvial fans but on occasion are marked by deposits of massive debris that are deposited within minutes. Such massive debris deposits are generated during intense storms or rapid snow melting, which can form torrential rivers of rock or debris flows.

Highly destructive debris flows are known to occur in many areas of the world and in practically every mountainous or hilly region subject to prolonged intense rainfall or snow melting. The volume of debris flows can vary by many orders of magnitude.

The larger events with flow volumes exceeding 10 million to 100 million cubic meters are rare, with many associated with volcanoes that had recent eruptive activity. Relatively small debris flows with volumes in the order of 1,000 to 100,000 cubic meters are the most frequent.

In December 1999, heavy rains spawned floods and landslides in Cordillera de la Costa, Vargas in Venezuela, a densely populated city on an alluvial fan at the mouth of a mountain drainage area, near the base of steeply sided mountains.

Debris flows with a total deposit of 1.9 million cubic meters overwhelmed Cordillera de la Costa, and destroyed 700 apartment buildings and 800 houses. A total of 19,000 people died from the catastrophe.

When debris flows happen in volcanic slopes, they are called lahar. In 2006, Typhoon “Reming” (international code name “Durian”) generated in Albay province lahar that left deposits similar to those that now cover Barangay Andap. Within and on top of the deposit, there lay boulders strewn on the debris field.

Lahar that swept barangays on Mayon’s slopes in 2006 also devastated Cagsawa in Daraga, Albay, the same town with the iconic bell tower, part of the ruins of the Cagsawa church buried by lahar in the early 1800s.