Study on the mechanism of the riparian trees expansion in a gravel-bed river and an approach to that management

近年来，沙砾河床的河岸树木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，我们应该了解沙砾河床中河流植被的生长和覆盖范围的扩大。本文对瓦塔拉萨河一段河段刺槐过度生长的原因进行了研究。特别是从树木年轮的调查中，我们可以清楚地看到，一些沙洲刺槐林是在主洪过后形成的。结果表明，洪水引起了树木的世代变化，并通过倒下树木的发芽迅速扩张，使河流树木过度生长。在野外研究和二维流模型数值分析的基础上，对临界牵引力作为洪水扰动尺度的指标进行了评价，结果表明，临界牵引力的大小和河床退化程度决定了河岸树木的破坏尺度。

Recently riparian 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 the gravel-bed river. In this study, the cause of the excessive growth of Robinia pseudo-acacia was examined in the segment-1 reach of Watarase River. Especially, from the investigation of the annual rings of tree, it was clear that some forests of Robinia pseudo-acacia in bars were formed right after of the main flood. We show that the floods cause change of generation of the trees and rapidly expansion by the germination from fallen trees makes the excessive growth of riverine trees. Based on the field study and numerical analysis with 2D- flow model, the critical tractive force to be an index of the scale of flood disturbance is evaluated and results show that the damage scale of the riparian trees depends on the size of the critical tractive force and bed-degradation.