A Study on the Origin of Step-Pool Formation in A Mountain Stream Considering the Water Surface Waves.

在实验室砂砾床水槽上进行了肋形和阶梯形水池的形成试验，并对其成因进行了理论分析。结果表明：(1)在超临界水流条件下，河床上形成肋状结构，具有二维横向形态，输沙活跃，顶端最大尺寸的石块滞留；(2)在肋状结构的触发下，水面产生三维斜波；(3)当水面波与肋状结构发生共振时，水面波以较大幅度出现，并迅速将床面波变形为三维起伏。在短时间内，水波和床波都不复存在。只有在共振状态发生时停止供水，床上才会出现阶梯和水池。(5)阶梯和水池系统的局部出现是…的原因更多的是由于水面波和床面波之间的拍打现象，这种拍打现象是由不同但接近的波长产生的。(6)航道内存在多种模式的阶跃和水池系统；用非静力压力分布的平面二维动量方程对活动边界进行了线性稳定性分析，结果表明：(1)给出最大增长速率的理论阶梯和水池波长与实验阶梯和水池波长吻合得很好。(2)阶梯和水池成长的不稳定波数范围很小。(3)通过分析可知，阶梯水池是由对角水面波的共振效应形成的三维反沙丘。(4)对于肋骨，理论上很难推导出与实验波数域对应的不稳定波数域。要将其应用于人工肋骨和梯级池的设计，还需对理论进行改进。较少

Experiments of ribs and step & pools formation using a laboratory flume with sand-gravel bed, and theoretical analysis for the origins were conducted. They resulted in the followings :(1) In the supercritical flow condition, ribs were formed on the bed, which had a 2-dimensional transversal shape, with active sediment transport and with stagnation of the maximum size stones on the crests.(2) With the trigger of rib formation, 3-dimensional diagonal waves were generated on the water surface.(3) When the resonant condition between water surface wave and rib arose, the water surface waves stood with large amplitudes and deformed the bed waves into 3-dimensional undulations very rapidly.(4) However, the resonant state was not continued so long time ; both the water waves and bed waves ceased to exist with short duration. Only when the water supply was stopped during the occurrence of resonant state, step & pools were found on the bed.(5) Partial occurrence of step & pool system were caused … More by the beat phenomenon between water surface waves and bed waves, which was yielded with different but close wave lengths of them.(6) Multi modal step & pool systems could be found in the channel ; they arose with corresponding to the transverse mode of diagonal water surface waves.Linear stability analysis for the movable boundary, using the plane 2-dimensional momentum equations with non-hydrostatic pressure distribution, revealed the followings :(1) The theoretical step & pool wave length to give the maximum growth rate agrees with the experimental step & pool wave lengths well.(2) Extent of unstable wave number with which step & pools grow up is very narrow, and hence it causes the rapid failure of the step & pools.(3) It can be understood from the analysis that step & pools are 3-dimensional anti-dunes which are formed by the resonant effect of diagonal water surface waves.(4) Concerning with ribs, the unstable wave number domain corresponding to the experimental one is rather difficult to be derived theoretically. Improvement of the theory is needed to apply it to the design of artificial ribs and step & pools. Less