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

研究的目的是从生成的临界条件和对其他河床和水流的作用角度，阐明山区河流中尺度河床演变的特性。通过非均质陡槽试验和研究期间的分析，得到了以下结果：1.从泥沙和水流系统的线性不稳定性分析中发现，床面位移、流速等扰动的最大增长率有两个重要的解，一个对应于交替沙洲的增长，另一个对应于河床砾石和沙子的分选波。在具有非均质物质的陡峭河床上生长的交替沙坝的波长有比冲积层上沙坝的波长长3倍的趋势。交替的沙洲通过对床面上的砾石和沙子进行分选，使水流集中，形成阶梯状水池。在以分选不稳定性为主的高剪应力水力条件下，床面上发育一条平坦的床面，形成间隔1.5倍于条形波长的大粒径卵石的交替碎块。来沙量对两种床型的影响并不明显。在实验河道中，当肋骨叠加交替的沙洲时，演变出与自然溪流中的台阶和水池非常相似的床形。这种床型被认为是一种适用于鱼道的新类型。在大流量实验中，床面上升起了多排横杆。在该河床上运行小流量时，出现了中等尺度的沙坝和肋状辫状河道，其地貌特征非常接近天然山地河流。多排沙洲引起的大范围河床起伏和分流河道的变化对试验航道末端输沙有较大影响。

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.