Abstract: Firstly, according to the force analysis of acting on a sand and the view-point of shear stress and suction stress, the author found out the dominant factor in the incipient motion of noncohesive sediment is shear stress. Secondly, the former researchers who followed the theories of noncohesive silt thought that the mechanism for erosion of clay is similar to noncohesive silt. They focused on the shear stress of clay and got many empirical formulas. The calculated results obtained by these formulas have greater differences. Finally, in accordance with the clay erosion process and its characteristics, the author designed an experiment in open-channel. The result of experimentation proved the existence of suction stress. Afterwards, the author got the relationship between suction stress and vertical mean velocity. The maximum suction stress is up to 941 times of shear stress. We draw a conclusion that the suction stress is the extrinsically dominant factor of clay erosion, the tensile strength of clay itself is the inherent factor that resists the destruction of flow.

Key words: clay ,    erosion mechanism  ,   suction stress  ,   shear stress