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2 フェロー 中央大学理工学部特任教授，中央大学研究開発機構教授(同上) 3 正会員 国土交通省 中国地方整備局 岡山河川事務所(〒700-0914 岡山県岡山市鹿田町2-4-36) 4 正会員 国土交通省 中国地方整備局 河川部河川計画課長 (〒730-8530 広島県広島市八丁堀6-30広島合同庁舎2号館) 複断面蛇行河川では大洪水時の水位ピーク時に主流が内岸寄りを走り，内岸砂州が洗掘を受けるため内 岸砂州上の樹木群の倒伏・流失が想定される．太田川では，河道内樹木の管理を目的とした樹木調査を行 い，洪水時の水面形の時間変化を観測する準備を行ってきた．平成17年9月には，計画高水流量相当の洪 水が発生し，内岸砂州上の樹木群が倒伏・流失した．本研究では，観測水面形の時間変化を用いた非定常 二次元洪水流・河床変動解析により，洪水中の流れと河床変動，樹木倒伏の関係について検討した．そし て，流体力と河床洗掘の観点から，砂州河床上の樹木倒伏・破壊の主な要因は河床洗掘であることを明ら かにした．最後に，本調査・研究から明らかにされた樹木管理の教訓とその活用を示している． Key Words : vegetation destruction, observed temporal changes in water surface profile, unsteady 2D analysis of flood and bed variation, vegetation management

In the sandy Hii River, a large amount of sediment yield from upper river basin had brought developments of braided channels covered with sand waves. In the braided channels, sediment materials on the river beds are capable to move in normal discharge conditions. In recent years, however, the sediment yield decreases due to constructions of check dams and ground sills in the upper river basin. Thus, the river beds downstream of the ground sill have gradually degraded and the main channel widths have been narrowed with the progressing bed degradation. Firstly, we clarified that the effects of non-equilibrium sediment transports around the ground sill during normal discharge conditions on the bed degradation and the channel width reduction by using annual observed data and numerical simulations for bed variations. In addition, we provided the river improvement techniques for mitigating bed degradation and channel width reduction by improving state of non-equilibrium sediment transports passing through the ground sill.

Takahisa GOTOH, Shoji FUKUOKA and Akira UEDA 1 正会員 博士(工学) 中央大学研究開発機構 機構助教(〒112-8551 東京都文京区春日1-13-27) 2 フェロー 工博 Ph.D. 中央大学研究開発機構教授 (同上) 3 国土交通省 中国地方整備局 太田川河川事務所 所長(〒730-8530 広島市中区八丁堀3番20) Since the elevation of flood plains in the lower Ota River floodway is located in the intertidal zone, tidal flats formed on the flood plains have become important as one of river functions. However, the tidal flats are gradually reducing in size due to bed scouring year by year. So, the cross-sectional shape in the lower floodway has changed gradually and naturally to the ship-bottom shaped channels at present from compound channels in the beginning by flood flows. Therefore, it is necessary to design the riprap for protecting against the erosion of the tidal flats. In this study, we evaluate effects of ripraps on the topographic changes and provide appropriately ripraps design by using numerical simulation for flood flows and bed variations in the lower Ota River floodway. Finally, issues to be considered in the design of a ship-bottom shaped channel which are found in the present study are summarized.