A debris avalanche is a large-scale （commonly > 10⁶m³ ）and  rapid （up to150m/s） collapse of a part of a volcano (Ui, 1987; Takarada et al, 1999）.  A  pyroclastic flow and a volcanic mud flow,  phenomena in which pyroclastic materials flow down and are deposited at the foot of a mountain, differ from debris avalanche in some points;

1. A  pyroclastic flow is a hot flow formed mainly of material of magmatic origin, while a debris avalanche is not a hot flow and is generally consists of non-essential and accessory or accidental material.
2. The medium of a volcanic mud flow is water, while a debris avalanche is not saturated with water, although it takes in water when it flows down and may contain up to a few percent of water. Water does not play an essential role as a medium  in the behavior of a debris avalanche.

Debris avalanche deposits generally have the following characteristics (Ui, 1987);

1. Debris avalanche deposits consists of debris-avalanche blocks and the debris- avalanche matrix. The debris-avalanche blocks are large blocks that preserve the structure of the source. The debris-avalanche matrix surrounds the debris-avalanche blocks and is composed of non-graded granular rocks.
2. There is hummocky topography reflecting the preceding characteristic. 
3. Debris avalanche deposits show low apparent friction coefficient.
4. An apparent friction coefficient is smaller for lager debris avalanche deposits (volume effect).