We examine a wave-current interaction, in particular current effects on waves (CEW), in a nearshore current system developed on a single-barred, rip channel topography. An Eulerian phase-averaged shallow water equation with a vortex-force formalism tightly coupled with WKB refraction equations is exploited to conduct a comprehensive numerical experiment. If CEW is taken into consideration, wave ray bending occurs to induce the prominent reduction of offshore extent of rip currents with near-normal offshore wave incidence. At deeper incident angles, longshore currents dominate over rip currents, resulting in eddying flows associated with shear instability around the bar crest. A diagnostic momentum budget analysis indicates that in the rip-dominant cases CEW alters the momentum balance significantly through modulation of the wavenumber field, leading to the enhancement of the onshore breaker acceleration and the pressure gradient force both in the alongshore and cross-shore directions. For the unstable longshore currents, CEW acts to diminish turbulence kinetic energy in the offshore region around the bar. Since the cross-shore turbulent intensity is more attenuated than the alongshore component, offshore eruption of the surfzone eddies is reduced by CEW.
A particle drift model was developed for precise prediction of spilled oil transport at sea surface. The developed model considers the major factors which affect the floating oil movement. For the mechanical oil spreading, Fay's model, which was widely used but described in Eulerian manner, is modified to meet the particle expression in the new model. The new model also includes effect of turbulent diffusion, tidal current, ocean current and wind-driven current. To verify the model, hindcast simulations were applied to the actual oil spill incidents. They include Hebei Spirit oil spill incident which occurred offshore from the Taean coast in South Korea in 2007, and also Diamond Grace incident which occurred in Tokyo bay in 1997. In the hindcast study, the horizontal turbulent diffusivity was discussed. It was found that the horizontal turbulent diffusivity model introduced in the model can be applicable to the observation results and provides better agreement to the observation than the large eddy simulation. A controversial point on how to model the wind-driven drift effect in the simulation was also discussed. A numerical method, in which the wind-driven current is numerically solved, was applied to the oil spill simulation. It was found that the vertical mesh as fine as 1m at the sea surface is not still enough for resolving the precise surface current velocity required for the oil spill simulation. To the contrary, the wind coefficient method, which is used in our simulation model, was found to have good reproduction of actual oil spills and more suitable for the practical real-time simulation.
After the tsunami disaster induced by the Great East Japan Earthquake, there is a need to develop structures permitting the effectiveness of coastal dike on disaster mitigation, even when a tsunami overflows. The authors conducted hydraulic model experiments to reveal hydraulic conditions and scour process near coastal dike during tsunami overflow, and to study more resilient structures with scour protection at the landward slope toe of coastal dike and improvement of the landward slope armor. The results of the experiment showed that tsunami overflow caused negative pressure at the top of the landward slope and large velocity pressure rise at the landward slope toe, and that depth and width of scour landward of coastal dike increased with overflow depth. We found that it made coastal dike more resilient to tsunami overflow to install the landward slope armor made by interlocked blocks with notches at their top edges and bottom edges, and the foundation work and soil improvement at the landward slope toe.
The 2011 Tohoku-oki tsunami caused sea-level rise at about 20 m height along the coast of Fukushima resulting in devastation on significant power plants including Fukushima No. 1 nuclear power plant. This paper investigated the process of tsunami inundation on Haramachi thermal plant located on the north Fukushima coast in order to ascertain the cause of the larger tsunami height in Fukushima coast than that in the neighboring coasts from field investigations and 2-D numerical simulations. The numerical simulation and inverse wave-ray analysis revealed two significant characteristics of Tohoku-oki tsunami. Firstly, the wave source of Tohoku-oki tsunami covers areas which have the same travel time to Fukushima coast so that tsunamis from wide source areas overlapped at Fukushima coast. Secondly, the tsunami wave concentrated at the coast due to the refraction on the sea-bed topography offshore Fukushima. Additionally, the overlapping of the incident tsunami and the reflected tsunami from the coast caused local sea-level rise near the plant, resulting in reinforcement of the inundated flow. This paper also examined cost-effective measure against future tsunamis. Raising the ground level at where superposition of tsunami occurs can be an effective measure against future regular tsunamis on Haramachi plant. The complement and radical measures, however, is needed for rare giant tsunamis such as Tohoku-oki tsunami.
A Bayesian statistical model was employed to infer the relationship between the number of species of benthic marine animals and the characteristics of sediment in Japanese ports and estuaries. After examining the correlations between sediment characteristics, water content, which was significantly correlated with concentrations of total organic carbon (TOC) and total organic nitrogen (TON), and concentrations of F, Hg, Cd, and As were chosen as explanatory variables. In addition, regions, ports, and estuaries were treated as categorical variables based on the hypothesis that they affected the diversity of benthic marine animals. In the analysis the number of species in four groups of benthic marine animals (molluscs, annelids, arthropods, and other animals) was considered. The number of species in all groups was negatively correlated with water content, and in sediments with a low water content, the number of species of annelids was significantly lower than in other sediments. These results suggest that water content (or TOC or TON) affects the number of species of benthic marine animals. Furthermore, the number of annelids was negatively correlated with the concentration of F.
Meteorologically generated large secondary oscillation along coasts of Kyushu has been explained into two stages; long ocean waves induced by air pressure disturbance are amplified by air-sea interactive resonance when both moving speeds nearly coincide, and the arrived long waves are amplified by the geometric resonance with the eigen frequency of the bay or harbor. The area of west Kyushu coasts facing the East China Sea has suffered the seiche disasters locally called “Abiki”. Recently, the same type disasters have been reported from the countries along the Mediterranean, Adriatic and North Sea, etc. The term “Meteotsunami” has become widely recognized and the importance of the subject is increasingly appreciated. We have studied the meteotsunami events at Urauchi Bay in Kamikoshiki Islands, Kagoshima by seasonal field observations in 2010 and 2012. For obtained meteotsunami events, we have numerically analyzed the atmospheric fields over the East China Sea and southeastern China mountains, as well as the oceanic fields from the offshore to the coastal areas. The present study aims to correlate the findings obtained by our field observations and numerical simulations, and to clarify a series of mechanism from the generation of the air pressure disturbance, and its development, resonance with ocean long waves, and amplification in the bay of the arriving long waves.