Short note: Debris flow

A debris flow is a rapid flow of a water-saturated mixture of soil, rock fragments, boulders and organic matter down a steep channel, under gravity. It contains 50-80% solids by volume and behaves like wet concrete.

• Causes: intense, prolonged rainfall; abundant loose debris from old landslides; steep gullies; sudden release of landslide dams or glacial lakes; earthquakes.
• Features: three parts: initiation zone (source), transport channel with levees, and a deposition fan at the foot. Boulders concentrate at the front.
• Speed: from 1 m/s up to over 10 m/s, so it is destructive.
• Occurrence: frequent in the Siwalik and Lesser Himalaya during the monsoon.
• Control: check dams, debris basins, channel lining, deflection walls, planting, early warning, and keeping settlement away from fans and gully mouths.