Debris flow: materials of pellet and water or mud in the rock stratum depart from each other in the course of flowing or separating. This departing process reduces not only the small material proportion in the rock stratum, but also the thickness of it. In Taiwan, debris flow takes place most in hillsides or mountain valleys when the torrential rain comes. Main characters of debris flows are its rapid flowing speed, high consistency of silt, strong eroding strength, and heavy chocking power. The thickness volume of the solid substance in the debris flow probably lies between 27% and 75%, and it weights about 1.45 to range of 2.24 tons / stere. The characteristic of Debris flows’ motion differ from that of landslides; it also differs from the sand-transporting phenomenon of river courses. On the contrary, it lies between these two different phenomenons.

   Debris Flows Most Happen in Areas as Following:
    ※Edge of the Slope: Because the rock receives the long-term weathering and erosion, the rock especially in the abrupt slope and the valley of the small stream become so fragile and soft that it may easily lead to debris flows. For after absorbing a large number of the rainwater, the foot of slope cannot have enough strength to prevent the gliding of rocks from happening.
    ※Landslide Area: Occasionally, after the landslide, there will be a large amount of mound piled up in the valley. If the mound forms a large claybank, which blocks waters’ draining, it will straightforwardly cause the collapse of the claybank when the stream goes up and crosses the mound. Moreover, it will also bring about debris flows.
    ※Discarded Soil from the Engineering Project: Unnecessary soil from projects, which include roads’ opening, houses’ building, and mining, will become the material source of debris flows, if it’s threw aside improperly.