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

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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.