THE EFFECT OF BRANCHED CHANNEL MAINTENANCE ON RIVER CHANNEL EVOLUTION DURING FLOODS

The river channel of Satsunai River is currently covered with forest, and we have been aiming to regenerate its gravel bar through induction of disturbances. In order to do so, we have been excavating to maintain the branched channel by connecting it to the old watercourse left in the forest. It was under such circumstance that the largest flood in history occurred in August, 2016. The changes in the river channel during the flood has proven that maintenance of the branched channel by excavating to connect it to the old watercourse is effective in promoting regeneration of the gravel bar during floods. Furthermore, by calculating the changes in the riverbed, we demonstrated that maintaining branched channel to the center of the river channel could promote riverbed change in the branched channel, which can be expected to suppress erosion towards the embankments during the flood. Additionally, in terms of coping methods to new challenges such as early regeneration of vegetation due to decline in the frequency of disturbances caused by growth of the relative height difference of water surfaces, and the increased risk of river bank failure due to increased erosion of the embankments, we also demonstrated that digging branched channel to the gravel bar in the riffle area can be expected to renew disturbances in the gravel bar, and reduce erosion of the embankments.