Effect of winnowing on armored bed stability

Abstract

Winnowing failure, edge failure and shear failure are indicated as three main factors to cause armored bed failure in typhoon-induced flood in Taiwan. This report investigates the fundamental reasons of winnowing failure by studying the motion of particles in a two-dimensional rectangular cavity under steady flows. Factors affecting the threshold of winnowing are discussed and the flow vorticity related to the cavity’s dimensions is studied. Particle’s property, cavity’s dimensions and flow property are three main variables in influencing the threshold of winnowing. Under the same flow property and cavity dimensions, the value of the critical Shields Parameter T*c for the threshold of winnowing increases with the particles’ size and density. For a certain cavity length, as the ratio of d50/h increases, T*c for the threshold of winnowing decreases. Comparing data in the present study with Sumer et al.’s (2001) result, there was a significant difference. This is mainly because winnowing occurs in different ways due to the different cavity length used. In the present study, winnowing occured under the condition of L/h≥ 5 and particles were directly flushed away by the roll up shear layer. In Sumer et al.’s case or in the present study with L/h≤ 4, a counter-rotating vortex formed near the cavity bottom, which dominated the particle’s entrainment. Thus a very large T*c was required for the entrainment as the impulsive force induced by the counter-rotating vortex is much smaller than that of the shear layer.