Role of Chikugo River`s Old Meandering and Inflowing Tributaries with Cut-off Channels

Chikugo River is a largest river in Kyushu island, Japan with a trunk length of 143 km and a catchment area of 2,860 km². This river has many cut-off channels nowadays but there are many old meandering waterway still existing. All inflowing tributaries into Chikugo River do not connect directly to main stream of present Chikugo River but flow into the old meandering waterway of this river. In the former research, Shimoda cut-off channel and old meandering waterway with inflowing tributaries are analyzed, where the tidal effect of Ariake Sea exists. In this study, the role of old meandering waterway and tributaries with cut-off channels is considered hydraulically, but the present study considers the interval where there is no tidal effect. one-dimensional open channel flow analysis of the interval of Chikugo River, and its tributaries is implemented. The tributaries are Houman River, Akimitsu River, Daigi River, Yasura River, etc. which flow into the old meandering waterway, where there is no tidal effect existing like lower part of Chikugo River system. In order to verify the influence of the inflows of the tributaries on the water level of the old meandering waterway, virtual channel models of tributaries are set up and each case model is compared by their water levels. Komorino cut-off channel was built during early 20th century to shorten the old meandering part of Chikugo River for flood control. Moreover, 2D inundation analysis in this area is conducted for flood control consideration. Cut-off channels can drastically have increased the capacity of flowrate of Chikugo River but direct connecting between the cut-off channel of Chikugo River and its tributaries can increase the water level fluctuation range of the tributaries and ferry of Chikugo River. The old large meanderings are considered to be used for the ship transportation. Moreover, from 2-D inundation analysis, during flood period, the connecting system between inflow tributaries and the old meandering waterway can decrease the overflowing velocity in the case of excessing the designed flood discharge of each tributary.