Study on the destruction of riparian trees in a gravel-bed river caused by flood disturbance

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Recently, bed degradation has been generated in many gravel-bed rivers. As the result, flooding areas exist only around the low-water channel and on the other hand, non-submerged areas or the regions affected by the weak flood disturbance occur in the river channel. Furthermore, 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 causes the degeneration of the dynamic river system and that of inherent habitat environment in gravel-bed rivers, which brings about problems of river management for the environmental aspects as well as the safety of rivers against flood. In this paper, the magnitude of recent flood disturbance, the fall down process and the destruction mechanism of the riparian trees caused under the degeneration of river channel characteristics were examined through the field investigation in the segment-I reach of Watarase River. Based on the field study and numerical calculation with 2D-flow model, the destruction of the plant was classified into next four types : a)Yield of the tree due to the hydrodynamic force by the increase in the project area of trees to the flow because of trapping of flowing things, b)the disturbance of physical ground in stands, c)the disappearance of trees due to the erosion of the river bed/bank. As an index as such situation occurs, the maximum size of movable gravels and the local bed-degradation to be a key index of the scale of flood disturbance and results showed that the destruction mechanism of the riparian trees is caused by the disturbance of movable bed.