• Debris flow is a combination of fast moving water and a great volume of sediment and debris that surges down slope with erosive energy and tremendous force.
• A debris flow can be divided into two layers: a fast moving upper layer and a relatively slow moving lower layer.
• Debris flow velocity varies with sediment size, water and clay content and valley-side slope. Velocities of granular flows range from 3 to 10 m/s, and velocities of mud flows range from 2 to 20 m/s.
• The front, or 'head' of a debris-flow surge often contains an abundance of coarse material such as boulders and logs. The following flow contains a higher percentage of sand, silt and clay.
• Gravels are deposited by debris flows which deform underlying sands.
• The debris flow fan is built up because the wide river width and flat valley slope at the end of river which decreases the debris flows velocity. Hence, debris flow usually drops its loads here.

Most debris flow occurrence on the steep slope ranging from 15°–30°, and the mass movements of debris flow is usually deposited on the flat slope ranging from 3°–6°.

The large boulders deposited at the leading edge of the mud flow or debris flow.

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Larger grain floats within a matrix of smaller stones

In the case of Typhoon Toraji (2000), there was a flat, wide land at downstream of Chunkeng Village and Shangan Village in Shuili Township of Nantou where had a great condition to form a debris fan.

• Abundant sources: large amounts of loose debris, including fine (clay, silt and sand) and coarse (gravel, cobbles and boulders) materials.
• Sufficient water: water acts as a lubricant, reducing friction among debris and allowing solid materials to flow.
• Steep slopes: in areas of quite steep slopes where lead debris to transport and flow down with lower friction.

Descriptions

Debris flows are triggered by amount of debris flow sources from upstream area, quite steep slopes and large volumes of water. Near unstable mountain slopes, weathered rock, sand and soil accumulate in valleys or foothills by slide falls, landslides and rock falls. If those debris sources experience heavy rain one day, water will difficult to drainage from the heavy load of rock materials. In the state of high undrained loading water, the top debris sources become heavy and enhance debris flow mobility which often end up in devastating debris flow hazards.

Debris flows may happen when the three main causes (sufficient water, debris sources and steep slopes) exist in the same time. Here are the cause reasons and signs before the debris flow:

The types of debris flows are classified according to forming causes, topography, debris material, debris sources, flow types, triggering factors, movement mechanisms, developmental stages and etc.