Abstract: Seepage erosion has been a major cause of riverbank failure in the geology conditions of sandy and clay soils forming double and multilayer structures. Riverbank collapse caused by seepage erosion is an interaction of water and soil with a complex failure mechanism. A correct understanding of the influence factors is of great significance to engineering management. This paper focuses on analysis and comparison of two major failure modes, i.e. shearing failure and overturning failure, using the theory of seepage failure and wedge stability method and considering the process of actual bank failure. Results show that the erosion development and stability of embankment depend on four key factors: water level variation, embankment height, sand permeability, and the tilt angle of sand layer. Water filling into the cracks on the bank backside greatly enhances bank instability. Due to the progressive feature of erosion development, the bank collapse caused by an abrupt water level drawdown is always delayed.