The viscous debris flow has a very strong eroded ability. It can erode a large amount of loose matters by its constantly erosion or lateral erosion and take them away during its flowing process

which makes it become more dangerous. According to the debris flow constitutive model and soil shear strength theory

take the upper bound theorem used in limit analysis as measure

and under the condition of assuming the erosion surface as arc

the startup mechanism of corroding to the ditch is studied. Then the corresponding calculating formula and judging criteria are suggested. At last

the influence of some factors on the erosion start velocity

such as the gravity

the falling gradient

the strength of the soil and so on

is discussed intensively.