バンダル型水制の河床変動に関する研究

本研究は, バンダル型水制周辺の流れがもたらす河床変化に着目した。バンダル型水制はインド亜大陸で古くから用いられており, バングラデシュを流れるジョナム川のような沖積河川では, 河岸浸食の防止, 航路の維持を目的として設置されている。しかし, 透過型・不透過型水制のような従来から用いられている治水構造物の代用として, バンダル型水制が真に有効であるかを検証した研究や現地調査は少ない。そこで, バンダル型水制が河川形態にもたらす影響を明らかにするために, バンダル型水制による複雑な流れや流砂メカニズムについて検討を行った。片側に２基の水制を設置した直線水路を用いて, 動的洗掘状態の移動床実験を行い, 水制周辺の河床位の計測と洗掘河床上の流れ場の計測を行った。数値解析モデルでは非構造メッシュを用いており, 各水制タイプについて流れと河床変動の計算を行った。これらの実験および計算結果を分析し, バンダル型水制が従来の水制と比べて周辺の局所洗掘を低減し, 水制背後への土砂堆積を促すのに有効であることを示した。
The present investigation is focused on river bed variation caused by flow patterns around Bandal-like structure in channels. This type of structure have been used from the past in the Indian Sub-Continent to protect the river bank from erosion and to maintain the navigational conditions in alluvial rivers as the Jamuna river in Bangladesh. Few laboratory studies and field observations using Bandal-like structures was realized to verify the real applicability of them as an alternative structure over conventional river training structures as groins (impermeable and permeable ones). To clarify the effectiveness of Bandals in river morphology, detailed investigation about the complex flow structures and sediment transport mechanism influenced by Bandal-like structures was realized. Laboratory experiments were conducted in a straight flume with two structures positioned on one side of channel to the measurements of the velocity field and bed level near the structures under live-bed scour condition. A morphological model based on unstructured meshes was developed to simulate the flow patterns and bed variation around these structures. The experimental and numerical results of flow distributions and bed level variation around each type of structure were analyzed. The Bandal-like structure shows promising results as the reduction of local scour around them and deposition of sediment at downstream of Bandals in comparison with the conventional structures as groins.