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水中堆積物斜面が崩壊し流動化すると、重力の作用のもとで遠距離にわたって流走することがある。このような水中堆積物重力流の基本的な流動機構と再堆積過程を調べるために、一連の二次元水槽実験を行っている。高速CCD カメラによる流況画像情報にPIV (particle image velocimetry) 法を適用し、堆積物重力流の下方から次第に流動化土が骨組構造を回復していく過程、すなわち進行性凝固過程を明瞭にとらえている。さらに、提案解析法に基づいて水槽実験における堆積物重力流の流況の再現を試みている。また、河川堤防の破堤にともなう三次元重力流問題をとりあげ、提案解析法の適用を試みている。, The flow-out potential of a body of fluidized sediment following abrupt collapse is discussed through numerical and physical modelling. The emphasis of the analysis procedure is placed on considering the multi-phased nature of sediment gravity flows. Specifically, it combines Navier-Stokes/continuity equations for a liquefied soil domain with a consolidation equation for a solidifying soil domain. Evolutions of the flow and solidification surfaces are traced as part of solution by using a volume-of-fluid (VOF) technique. The physical modelling is concerned with two-dimensional flume tests, in which a column of fluidized sandy sediment was subjected to abrupt collapse and then allowed to flow out into ambient water. The observed and calculated results show that the sediment flow manifests itself as a decelerating gravity flow due to interactions between the flowing liquefied soil and the underlying soil with grain-supported framework being reestablished. Furthermore, a three-dimensional analysis is worked out for a class of levee breaching problems, with the aim of clarifying spread characteristics of flood water through the breached section.

沿岸域の未固結堆積土斜面において海底地すべりが発生すると、崩土は長距離にわたって沖合に流出することがある。実際、沿岸域と深海域を結ぶ重要な堆積物運搬作用として混濁流が知られているが、その物理機構には不明の点が多い。本研究は、堆積物の水中重力流れの発達、減速、停止の一連の過程を整合的に記述し得る解析コードの開発を行なったものである。液状化堆積物を高密度粘性流体と理想化し、自由表面を有する3次元重力流の解析コードを開発した。さらに、その予測能力を堆積物ダム決壊問題に照らして検討した。, A three-dimensional analysis procedure for describing the behaviour of liquefied soil with afree surface has been developed in the present study. The liquefied soil is modelled as a heavy, incompressible viscous fluid. A set of three-dimensional Navier-Stokes equations and thecontinuity equation are numerically solved under moving boundary conditions. For the purpose ofidentifying the moving interface between the ambient fluid and the liquefied soil, thevolume-of-fluid technique (Hirt and Nicholas, 1981) is adopted. The predictive capability of theproposed analysis procedure is discussed with reference to a class of dam-break problems in whicha column of water in air collapses under its own weight. The predicted performance comparesfavourably with results of two-dimensional experiments of Martin and Moyce (1952), encouragingfurther test of the three-dimensional analysis code developed.

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.