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In Japan it has been difficult to construct new dams because of environmental and financial restrictions. So it is important to maintain existing dams suitably. Especially reservoir sedimentation is one of the most important problems for securing long term functions of dams. It is, therefore, necessary to plan and carry out efficient and economically feasible asset management for existing dam reservoirs. In this study, we focused on long-term effectiveness of reservoir sedimentation management on maintaining flood mitigation functions by several dams in the same river basin. We compared several scenarios based on sedimentation management options and reducing flood risks on the target river sections in the Oyodo River Basin, Miyazaki Prefecture. As the result, we could obtain prioritized reservoir sedimentation management strategy covering all river basin.

In Japan it has been difficult to construct new dams because of environmental and financial restrictions. So it is important to maintain existing dams suitably. Especially reservoir sedimentation is one of the most important problems for securing long term functions of dams. It is, therefore, necessary to plan and carry out efficient and economically feasible asset management for existing dam reservoirs. In this study, we studied efficient asset management measure for several dams in the same river basin. We analyzed the several risks regarding flood control and water utilization based on sedimentation progress, and proposed how to prioritize several dams for sediment management. As the result, we showed the advantage of asset management measure to deal with dams in a group.

流域と沿岸域を繋ぐ流砂系のとらえ方について、最近の進歩を概観している。分布型流出モデルの実流域への適用と貯水池堆砂実績による検討例に基いて豪雨イベントによる土砂流出過程の重要性を指摘し、ついで、漂砂収支解析法の現況にふれ、沖合境界への堆積物流束、特にストーム条件下での堆積物流束の実態把握の重要性を指摘している。さらに、流体―堆積物系のダイナミクスに焦点を当て、土砂流出と河川生態系の関わりを示す現地観測、粘着性／非粘着性堆積物の混合体に対する流送モデルの提案、伝統的水制工による土砂制御機能の再評価、ならびに液状化堆積物の水中重力流れ機構の解明について論述している。, This paper starts with an overview of recent developments pertaining to sediment routing systems that may connect fluvial, estuarine and coastal environments. It discusses sediment runoff analyses on a basin scale and methodologies for developing a budget of littoral sediment. It then focuses the discussion on recent findings obtained through collaborative research in the framework of a Center-of-Excellence program. The topics covered include the following: field observations of offshore-going sediment fluxes in the surf zone; reservoir sedimentation; an interrelationship between the riverbed morphology and habitat of a class of fish as influenced by sediment runoff; modeling of the transport of mixtures of non-cohesive and cohesive sediments with emphasis on wash load; reappraisal of traditional flow-control structures for river stabilization; and the dynamics of sediment gravity flows. Collectively, the coverage emphasizes the importance of addressing highly nonlinear fluid-sediment interactions.

バイパストンネルによる貯水池流入土砂の排除法を比較的大規模な流域に適用するには, 貯水池の上流端付近に副ダムを設置して, 洪水時に流入する土砂を一旦この上流域に貯留し, 流入流量が減少してきた時点で排出する方法が考えられる。この時, 副ダムに設けたゲートから上流向きに貯水池内の水を逆流させれば, 堆砂の再侵食を促進できると考えられる。本研究ではこの「貯水池逆流システム」の機能を明らかにするために, 副ダム上流域における堆砂の進行過程, 並びに堆砂の再侵食過程を再現し得る2次元河床変動モデルを提案し, トンネルの容量やゲートからの放流の有無といった条件を変化させて数値シミュレーションを行っている。, We propose a new flushing system that is a combination of a by-pass tunnel and a sub-dam in order to control reservoir sedimentation. The sub-dam is a small dam constructed at a little downstream of the upstream end of reservoir and equipped with a gate and an introductory channel.During a flood, sediment is deposited upstream of the sub-dam forming a delta. After the flood, water stored upstream of the sub-dam is drained away through the by-pass tunnel. Then, the delta is eroded by the flow introduced from the gate of the sub-dam and the river flow. The eroded sediment is transported into the by-pass tunnel. The deposition process and erosion process are investigated by two-dimensional numerical simulation.The simulated rainfall fields will be used to evaluate the uncertainty of real-time river discharge predictions on a Monte Calro simulation framework.The investigaton of statistical characteristics of real-time rainfall prediction error by a translation model shows that a relative prediction error is modeled as lognormal random field.Then a method to generate rainfall fields with the characteristics of the random field is developed based on a matrix factrization technique of a covariance matrix decomposition into its squre root matrix approximately by using the Chebyshev polynomials.