Study on the Excessive Growth of Reverine Trees and Its Effects on Morphological Processes in Gravel-Bed River

近年来，砾石河床河道中的树木在沙洲和河漫滩上的覆盖面积大大增加和扩大。这种情况给河流管理带来了环境方面的问题，也给河流防洪安全带来了问题。为了弄清这一问题，需要了解卵石河床中的植被是如何生长和扩大覆盖范围的，本研究在渡濑河上游河段进行了实地调查，该河段平均河床比降约为1/150，具有1段河段的特征。调查结果表明，刺槐格罗夫斯林扩展到沙洲和河漫滩上，表层为细沙。该层由细砂（0.5mm-1.0mm）沉积而成，其资源可能来自主航道的底质。它是刺槐生长和林木扩展面积形成的重要因子。研究结果还提出了砾石河床中逆向树木扩张的形成情景。此外，二维数值水流模型，包括植被拖曳效应的渡濑河的上游河段，具有段1的特点，与过多的reverine树的适用性进行了验证。计算结果与洪水后的观测结果进行了比较，并预测水位的增加取决于植被密度和预测的形态过程中的砾石床河与过多的reverine树木。

Recently reverine trees in the course of gravel bed-river have extremely increased and enlarged their covered-area over bars and flood plain. Such a situation brings about problems of river management for the environmental aspects as well as the safety of rivers against flood. In order to clarify such problems, we should get the knowledge about how the instream vegetation grow up and expand their covered zones in gravel-bed river.In this study, the field surveys were carried out in the upstream reach of the river Watarase which has the averaged bed slope of about 1/150 and has the characteristics of the segment-1 reach. The surveys suggest that the groves of Robinia pseudo-acacia expanded over bars and flood plains with the surface layer of the fine sand. This layer is formed by the deposition of fine sand(0.5mm-1.0mm) and its resources might originate from bed materials in the main channel. It is significant factor for the growth of Robinia pseudo-acacia and the formation of trees expanded area. The results also propose a scenario of the formation of the reverine trees expansion in gravel-bed river. Furthermore, the applicability of a 2D-numerical flow model including vegetation drag effect is verified for the upstream reach of the river Watarase which has the characteristics of the segment-1 reach with the excessive reverine trees. Calculated results are compared with the results obtained by the observation after flood and predict an increase in water-level depending on the vegetation density and predict morphological processes in gravel-bed river with the excessive reverine trees.