ABSTRACT
Step-pool system is one of the most important and widespread bed structures in mountain rivers and its failure mechanism plays a significant role on the stability and fluvial process of mountain river bed. Combined with the effects of water discharge, downstream scour of the step, surrounding grains, and grain impact, the forces acting on the keystone of a step-pool were schemed and analyzed to advance a physically-based theoretical model for individual step-pool stability. The model illustrates that the channel slope, scour angle, keystone diameter, discharge, impact grain size and interlocking effect are the key factors influencing the stability. With the increase of the scour angle, the critical discharge for keystone destabilization decreases even to a value over 6 times smaller when the scour angle reaches 60 degree. Large floods may break the step when the discharge surpasses the critical value lowered by downstream scour. Grain impact can further decrease the critical discharge for step-pool failure and such effect would be enhanced with increased size, initial velocity and scour angle. Large grains supplied by floods with long return time, debris flows or landslides can even dislodge the keystone directly. The model was applied to step-pool reaches and could evaluate step-pool stability precisely.
