Study on recent change of gravel-bed river landscape and river forestation

砾石河床的近期河道特征是河床退化、枯水河道不动、河岸局部冲刷、相对高程的扩大和植被的过度生长。特别是砾石河床河道中河岸树木的出现量大大增加，成为河道流木的来源。首先，在这项研究中，最近的砾石床河的特点和因素，导致河道造林的分析，在河道横截面和林地的变化，在河渡濑，一条支流的砾石段进行了调查。本文以河道造林的代表树种刺槐为研究对象，分析了砾石河床河道的这些特征，这些特征是在交错沙坝影响下，在中等洪水作用下，河床形态发生变化的结果。本文还讨论了反映砾石河床近期河道特征的枯水期河道形成，并通过数值计算考察了形态变化的过程。首先形成交错沙坝的河道，然后降低流量，形成弯曲的枯水河道，造成河岸冲刷和局部冲刷。最后，本文提出了一种描述漂流木运动及其堆积过程的数值模拟方法。该模型由深度平均流分析给出的欧拉流体运动分析和扩展离散元法（DEM）给出的浮木运动的拉格朗日分析组成，可以将浮木运动模拟为刚体。

As recent channel characteristics of gravel-bed river, bed degradation, immobilization of the low-water course, local scour near river bank, extension of relative elevation, and overgrowth of vegetation are mentioned. Especially, the occurrence of riparian trees in the course of gravel-bed rivers has been very much increasing, which produces source of driftwoods in rivers. First in this study, recent characteristics of gravel-bed river and factors that cause the forestation of the river channel were investigated by analysis of the changes in both the channel cross-section and the woodland in the gravel segment of the River Watarase, a tributary of the River Tone. Here we focused on the excessive growth of Robinia pseudo-acacia, the representative species in river forestation.These channel characteristics in gravel-bed river are produced as results of the morphological change under the medium-size floods in the low-water course affected by alternate bars. We also discuss the low-water channel formation where recent river channel characteristics of the gravel-bed river are reflected, and the process of morphological change was examined from numerical calculation. First, a channel with alternate bar was made, afterwards, the flow discharge was lowered and the meandering low-water channel, which brings about the bank erosion and local scour, was formed. These processes were investigated from the results of present numerical calculation.Finally, a numerical simulation method has been developed for describing the behavior of driftwoods motion and their accumulation process. The present model has been composed of Eulerian analysis of fluid motion given by depth-averaged flow analysis and of Lagragian analysis of driftwoods motion by using the extended Distinct Element Method (DEM), which can simulate the motion of driftwood as the rigid body.