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

指出了砾石河床的近期河道特征，包括河床退化、低水位河道的固定作用、河岸附近的局部冲刷、相对高程的延伸和植被的过度生长。特别是砂砾河床河道中河岸树木的出现大大增加，成为河流浮木的重要来源。本研究首先通过对Tone河支流Watarase河砾石段河道断面和林地变化的分析，探讨了砾石河床的近期特征和河道造林的影响因素。本文重点研究了河流造林的代表树种刺槐（Robinia pseudoacacia）的过度生长。砾石河床的这些河道特征，是在交替沙洲影响下的低潮期中等规模洪水作用下河道形态变化的结果。本文还讨论了反映砾石河床近期河道特征的低水位河道形成，并从数值计算中考察了其形态变化过程。首先形成交替沙坝河道，然后降低流量，形成曲流低潮河道，造成堤岸侵蚀和局部冲刷。从目前的数值计算结果出发，对这些过程进行了研究。最后，建立了一种数值模拟方法来描述浮木的运动特性和堆积过程。该模型由深度平均流分析给出的流体运动欧拉分析和扩展离散元法（DEM）给出的浮木运动Lagragian分析组成，可以模拟作为刚体的浮木运动。

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.