Physical Model Tests on Effects of Wave Action on Graded Sand Sediment Transport of Discharged Mixed Sized Sands from River Mouth
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.
|模型実験||physical model test|
|粒径別漂砂||graded sand sediment transport|