水制周辺の河床変化および砂の分級現象

Abstract

水制周辺の洗掘現象において, 水制高と河床の粒度分布が局所洗掘に与える影響, そして, 水制がもたらす河床表層の粒度分布変化と水制高の関係性について明らかにするために, 越流型および非越流型水制に対していくつかの混合砂河床の移動床実験を実施し, 考察を行った。実験結果より, 越流型水制の場合も非越流型と同様に, 河床粒度の幾何標準偏差が最大洗掘深評価に重要なパラメータと成り得ることが示され, そして非越流型水制は越流型に比べて水制頭部の下流域で顕著な洗掘傾向が見られることが明らかとなった。また, 越流状態の異なる流れ場の計測により, 水制が高いほど水制先端近傍の下降流が大きくなり, その影響で水制が高いほど水制頭部から下流に伸びる河床表層の粗粒化域の横断幅が広くなることが明らかとなった。

This paper presented an experimental study on the impact of both spur dyke height and grain size distribution on the bed topography and bed surface composition around an impermeable spur dyke. The importance of governing parameter, i.e. the relative spur dyke height, which is inverse number of overtopping ratio, was emphasized through quantitative evidences. It was found in the current experiments that the spatial progress pattern of scour-deposition in the non-submerged and nearly non-submerged condition was different compared with full-submerged condition. Furthermore, in the submerged experiments using non-uniform sediment bed, it was found that the maximum scour depth decreased with the increased of the standard deviation as well as in non-submerged spur dyke experiments. The flow measurement around the spur dyke revealed the difference of three-dimensional flow structure under the different spur dyke submergences, and the relationship to the variation of the surface bed composition. It was also found that with the increase of the spur dyke height, the longitudinal coarse sediment region was expanded to transverse direction in the downstream region of spur dyke head.