STUDY ON THE DESTRUCTION CRITERION OF RIPARIAN TREES IN A GRAVEL-BED RIVER CAUSED BY FLOOD DISTERBANCE

Recently riparian trees (Robinia pseudo-acacia) 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 this paper, the fall down process and the damage conditions of the riparian trees caused by the 1999, 2001 floods were examined in the segment-1 reach of Watarase River. Especially, from the field surveies after floods, it was clear that the damage, which the floods gave to the trees, could be classified by the scale of the flood external force. Based on the field study and numerical calculation with 2D-flow model, the critical tractive force to be a key index of the scale of flood disturbance was evaluated and results showed that the fall down process of riparian trees is based on the degradation of movable river-bed