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

近年来，砾石河床上的沙棘树在沙洲和洪泛平原上的覆盖面积急剧增加和扩大。这种情况带来了环境方面的河流管理问题，以及河流的防洪安全。为了弄清这些问题，我们应该了解砾石河床中河流植被的生长和覆盖范围的扩大。本研究在Watarase河上游河段进行了野外调查，该河段平均河床坡度约为1/150，具有一段河段的特征。调查结果表明，刺槐林在沙质表层的沙坝和洪泛平原上扩展。该层由0.5mm ~ 1.0mm的细砂沉积而成，其资源可能来源于主河道的河床物质。它是刺槐生长和树木扩大面积形成的重要因子。研究结果还提出了一种砾石河床中逆行树扩张形成的情景。此外，还验证了考虑植被阻力效应的二维数值流模型的适用性，该模型适用于Watarase河上游河段，该河段具有大量逆行树木的1段河段特征。将计算结果与洪水后的观测结果进行比较，根据植被密度预测水位的增加，并预测了含有过多黄树的砾石河床的形态过程。

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.