碰撞作用下单个阶梯-深潭稳定性模型

阶梯-深潭系统是山区河流广泛分布的控制性河床结构，泥沙输移过程中大颗粒碰撞阶梯关键石块，使其发生位移，强烈影响阶梯-深潭的稳定性。以单个阶梯-深潭的关键石块为研究对象，重点考虑碰撞对阶梯-深潭的影响，量化来沙中大颗粒碰撞作用并改进稳定性理论模型，利用新模型分析阶梯-深潭的临界条件和破坏机制。来沙颗粒对关键石块的碰撞作用受自身粒径、运动速度和阶梯下游冲刷程度影响且皆为正相关关系。颗粒撞击减小阶梯失稳临界流量，且参与碰撞的石块粒径越大，减小作用越明显。当η> 0.55时（η=D₁/D，D₁为碰撞石块粒径，D为关键石块粒径），临界流量下降幅度达到50%以上，表明来沙中卵石漂石对阶梯-深潭稳定性发挥主要影响。山区河流发生低频率洪水或滑坡泥石流，向下游河道输运大粒径石块并与阶梯碰撞，显著增大转动合力矩并降低失稳临界流量，使得单个阶梯-深潭更易达到临界条件发生破坏。

A physical-based model of individual step-pool stability with grain collision

Step-pool is a controlling river bed form which frequently develops in mountain streams. Sediment transport significantly affects the stability of step-pool system by dislodging the keystone with large grain collision. The keystone of an individual step-pool was selected as the object for collision force analysis and the grain impact was quantified to add into the theoretical model for individual step-pool stability, which was then utilized for the analysis of critical conditions and the failure mechanism. The grain impact is positively correlated with grain size, grain initial velocity and the downstream scour of the step. The critical discharge for step-pool failure largely decreases under collision and larger grain size leads to the smaller threshold. The critical discharge would experience a decrease of over 50% if η > 0.55 (η=D₁/D, where D₁ is the diameter of the impacting grain; D is the diameter of the keystone), indicating that the large grains in bedload have a significant influence on the stability of an individual step-pool. When a flood event with long return time, landslide or debris flow occurs, the collision between the large grains and the step would enlarge the driving torque and reduce the critical discharge, resulting in much easier access to the failure of an individual step pool. This model could effectively evaluate the step-pool stability precisely when applied to step-pool reaches.