旭ダム排砂バイパストンネルによる石礫の排出機構および下流河道の瀬淵の回復;旭ダム排砂バイパストンネルによる石礫の排出機構および下流河道の瀬淵の回復;Study on Flushing Mechanism of DAM Reservoir Sedimentation and Recovery of Riffle-Pool in Downstream Reach by a Flushing Bypass Tunnel
In the Asahi River, riffles and pools had been scaled down in the downstream reach of Asahi Dam due to sediment deposition in Asahi Dam reservoir. To improve this environmental problem, a flushing sediment tunnel was constructed so as to bypass Asahi Dam reservoir. In this study, first, the authors demonstrated the recovery of riffles and pools and the grain size distribution in the downstream reach by using the data of cross-sectional surveying, air photographs and field investigation. Next, authors’ two-dimensional model was applied to a series of flood events in the Asahi River. As a result, the flushing mechanism of Asahi Dam reservoir sedimentation, the recovery process of riffles and pools and the change in grain size distribution in the downstream reach were clarified by the model.
Tools for Sediment Management in Rivers
Sediment management, including supply and continual redistribution of sediments for channel maintenance and channel forming, is critical for effective environmental water programs. Many tools have been developed to more effectively manage sediment in rivers for ecosystem maintenance and improvement. This chapter reviews some of these tools, including sediment mobilization theory, environmental water requirements for geomorphic maintenance, and complementary options. Sediment mobilization theory addresses entrainment, transport, and deposition of sediment particles. Environmental flows assessment approaches that support geomorphic processes include threshold-based approaches, field/laboratory methods, and holistic methods. Complementary options for sediment management include catchment-scale conceptual models and budgets for sediment, basin land management (e.g., reforestation, better agriculture practices, etc.), sediment management through dams, and sediment augmentation below dams. These complementary options often have cobenefits with other management objectives. The chapter emphasizes that sediment management is generally complex, such that continual research is needed, ranging from improved sediment mobilization models to large-scale river experiments.
CHARACTERISTICS OF RIVER CHANNEL PROCESSES WITH BANK EROSION AND DEVELOPMENT OF THEIR NUMERICAL MODELS
本研究は, 河岸侵食によって生じる2次元的な流路変動を対象としており, 流路平面形状の変化すなわち河岸侵食量の空間的分布とその時間変化特性と, それに影響を与える要因を明らかにすることを目的としている. まず, 実河川の流路変動現象を再現可能なモデルを提示し, バングラデシュ国・メグナ川に適用を行い, 実河川での平面流況と侵食位置との関係について考察を行った. 次に, 侵食性河岸を有する蛇行および直線流路において実験・数値解析を行った. 蛇行流路においては, 初期平面形状を同一とし水理条件を変化させて行った複数ケースの結果より, 水理量分布が流路変動過程に及ぼす影響について検討した. また, 直線流路においては, 交互砂州の発達・移動特性と河岸侵食との関連性こついて考察した.