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

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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.