報告書番号

V06026

タイトル（和文）

河口流出土砂の周辺海域への粒径別移動に及ぼす波の影響に関する実験

タイトル（英文）

Physical Model Tests on Effects of Wave Action on Graded Sand Sediment Transport of Discharged Mixed Sized Sands from River Mouth

概要 （図表や脚注は「報告書全文」に掲載しております）

河口流出土砂の周辺海域への粒径別移動を調べる実験を実施した。平面水槽の1/50勾配固定床上に河川模型を設置し砂試料を敷き詰めた。細砂,粗砂,レキ（粒径0.08mm,0.5mm,5mm）用い、均一粒径の場合、粗砂、レキ2種混合砂（重量比1:1）、細砂、粗砂、レキ3種混合砂（重量比4:3:3）の場合の河口流出土砂実験を行った。右斜め入射の規則波を作用させた場合としない場合を比較した。粗砂・レキ2種混合砂の場合、レキによる遮蔽効果のため、粗砂は一様な粗砂の場合よりも移動しにくく、逆にレキは一様なレキの場合よりも移動しやすくなった結果，河床は細粒化した。細砂・粗砂・レキ3種混合砂の場合、浮遊砂として細砂が多く流出するため、河床は粗粒化した。波が作用する場合、発達した速い沿岸流が水深中層に見られ、河川流は大きく左に曲げられ、河川流が蛇行して河口前面に循環流が形成された。細砂は浮遊砂となってこの沿岸流に乗り河口左側の広い範囲に運ばれるとともに、循環流により河口前面を移動した。粗砂やレキは掃流状態で水底近くを移動するため，沿岸流によってさらわれず，循環流や砕波による戻り流れによって河口前面の限られた範囲を移動した。

概要 （英文）

Physical model tests have been performed in order to elucidate graded sand sediment of out flow sand from river-mouth into surrounding sea area. River model was installed on the solid bottom with slope of 1/50 and sand was covered on the river floor. Fine sand, coarse sand and gravel of grain sizes of 0.08mm, 0.5mm and 5mm were used for experiments. Model tests have been carried out in the cases of uniform sized sand, 2-mixed sized sand of coarse sand and gravel with weight ratio of 1 to 1, and 3-mixed sand of fine and coarse sands and gravel with weight ratio of 4 to 3 to 3. Effects of waves and nearshore currents on graded sand sediment into surrounding sea area were investigated through comparison between the cases with and without obliquely incident waves from right direction.
In the case of 2-mixed sized sand of coarse sand and gravel, movement of coarse sand on the riverbed was restrained than that of uniform sized coarse sand by sheltering effects of gravel. Conversely, gravel on the riverbed moved easier than that of uniform sized gravel. So, riverbed materials became finer and the mean grain size of riverbed surface became smaller.
In the case of 3-mixed sized sand of fine and coarse sands and gravel, much more fine sand is flowed out as suspended sand sediment transport than coarse sand and gravel from river-mouth. So, riverbed materials become coarser and the mean grain size of riverbed surface becomes larger.
Faster longshore current occurred in the middle layer of water depth down river-mouth by obliquely incident waves from right direction. So, discharged flow from river-mouth was significantly turned into left direction (down river-mouth) by waves. So, river flow meandering, circulation flow occurred in front of river-mouth.
Fine sand, which moved as suspended sand sediment transport, was carried into the seabed area in front of river-mouth by circulation flow occurring in the surf zone. Fine sand was also carried into the larger seabed area by longshore current down river-mouth.
Coarse sand and gravel, which moved as bed load sand transport, were carried into the limited seabed area in front of river-mouth by circulation flow alone and undertow due to wave breaking.

報告者

キーワード