Study on Critical Conditions of Generation and The Functions of Medium-Scale Bedforms in Mountain Rivers

从形成的临界条件和对其他河床和水流的作用两个角度，阐明山地河流中尺度河床演化的性质。在研究期内，采用非均质材料进行陡槽试验和分析，得到以下结果：从沙流系统的线性不稳定性分析中发现，河床位移、速度等扰动的最大增长率有两个显著解。一个对应于交替沙洲的生长，另一个对应于河床砾石和沙子的分选波。生长在具有非均质物质的陡峭河床上的交替沙洲的波长往往比生长在冲积河床上的沙洲的波长长3倍。交替的沙洲将河床上的砾石和沙子分选，使水流集中，形成台阶池。在高剪切应力的水力条件下，分选失稳主导河床标高失稳，在河床上生长出平坦的河床，形成间距为1.5倍杆波长的大粒径卵石的交替分段。两种河床形态的输沙量差异不明显。在实验河道中，当肋层叠加在交替的沙洲上时，形成了与天然溪流中的台阶和水池非常相似的河床。这种床型被认为是一种新型的适用于鱼类的床型。在高流量试验中，床上出现了多排木条。小排量下床时，形成辫状河道，形成中等规模的坝肋；其形态特征与天然山地河流十分接近。多排沙坝引起的大规模河床起伏和辫状河道的变化对实验河道末端的输沙量有较大影响。

The aim of the research is to elucidate the properities of medium-scale bedforms evolving on mountain rivers from view points of the critical condition for generation and the functions to other bedforms and flows. Following results were obtained from steep channel experiments with heterogeneous materials and analyses within the research term :1. It was found from the linear instability analysis for the system of sediment and flow, that there are two significant solutions for maximum growth rate of the perturbations of bed displacement, velocity etc. One corresponds to the growth of alternating bars and the other corresponds to sorting waves of bed gravel and sand.2. A wavelength of an alternating bar growing on a steep channel bed with heterogeneous materials has a tendency to become 3 times longer than a wavelength of bar on an alluvial bed. The alternating bars concentrate the flow with sorting the gravel and sand on the bed to form step-pools.3. Under the hydraulic condition of high shearing stress such the sorting instability dominates the bed elevation instability, a flat bed grows on the bed to form alternating fractions of large size pebbles with spacing in 1.5 times of a bar wavelength.4. Sediment discharge to the two kind of bed form is not clearly different each other.5. Very similar bedform to steps and pools in a natural stream evolved in the experimental channel when ribs superposed alternating bars. The bedform was seen applicable to a fish way as a new type.6. In the experiment of high discharge, multiple rows bars rose on the bed. When a small discharge flow was run on this bed, braided channels occurred with medium-scale bars and ribs ; the morphological feature was very close to that of a natural mountain river.7. Large-scale bed undulation caused by the multiple rows bars and change of the braided channels give a great influence to the discharge of sediment at the end of the experimental channel.