Debris flow occurs whenever a huge surface runoff causes the transport of large quantities of solid material (from clay to boulder-sized) existent in rivers beds or in the slopes of the valleys, resulting in a viscous flow of a mixture of poorly sorted sediment in water with a great destructive capacity.

Due to their capability of discharge large quantities of debris with high velocity, debris flows result in high risk natural hazard particularly when they encroach urban areas.

Like water floods, debris flows are usually fluid enough to travel long distances and inundate vast urban areas.