[1] Residual currents due to internal wave breaking on a uniform slope were investigated in a two-layer system using laboratory experiments and numerical computations for different layer configurations. Internal wave-induced currents over a slope were measured in an experimental tank using PIV and also reproduced by a hydrodynamic model to quantify the detailed velocity field. The present results reveal that the critical level derived from the KdV theorem is a useful parameter for classifying the dynamics of internal waves breaking over a slope. As the horizontal distance from the critical level point to the internal wave breaking point increases, internal waves break more dynamically over the slope. Consequently, residual currents are enhanced near the breaking point. These findings increase our capacity to understand flux paths of biological and chemical substances in the stratified coastal ocean.Citation: Nakayama, K., T. Shintani, K. Kokubo, T. Kakinuma, Y. Maruya, K. Komai, and T. Okada (2012), Residual currents over a uniform slope due to breaking of internal waves in a two-layer system,

This study aims to reveal the factors most useful for evaluating the influence of wind on entrainment from the lower to upper layer in a two-layer system. Lake Abashiri, which is a typical brackish lake, is chosen as a study area because a distinct two-layer system exists due to salt-wedge intrusion from the ocean. To evaluate entrainment velocity between two layers, a 3D numerical computation is applied, showing good agreement with field observations. Computations suggest that the entrainment velocity estimated using the Richardson number is smaller than that from field observations. Thus, other factors are computed suggesting that use of the Lake number is more effective in estimating entrainment velocity due to internal wave breaking and interfacial fluctuation than use of the Effective Wedderburn number.

Abstract:Production and transportation of suspended sediment was investigated by sampling surface soils in 18 domains over an entire river basin. Particle size distributions alone suggested that all domains were a source of suspended sediment. Chemical composition analysis however showed that the transport rate of suspended sediment was more closely related to the type of surface soil than to land use. A method was developed which enables estimation of the relative suspended sediment transport rates from each domain of a river basin to its downstream end. The model's validity was verified through good agreement with field sampling results.

Abstract:This study aims to evaluate nutrient flux to the coast from the inland areas of Shiretoko in order to understand the key factors controlling the ecological systems of the coast. As an external force, rainfall is considered one of the most significant components controlling nutrient supply to coastal systems in this area. Therefore, to estimate nutrient supply in the future, the bias correction was applied by using Meteorological Research Institute Global Climate Model, which shows good agreement with Automated Meteorological Data Acquisition System data. A synthetic generation technique is used to produce hourly rainfall data, which is necessary for evaluating nutrient supply in Shiretoko. The robustness of the duplicated hourly rainfall intensity was investigated, which reveals that its standard deviation controls nutrient flux when nonlinearity becomes stronger for the evaluation of nutrient supply from a river basin.

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Abstract:Climate change has been revealed to increase the occurrence of natural disasters and to damage ecological systems. In coastal regions, there are many environmentally significant areas from the viewpoint of biodiversity, such as areas registered under the Ramsar Convention. The Kushiro Wetland is the largest wetland and one of the most significant natural systems in Japan, remaining mostly unaffected by human activity. In the wetland, there are many species, which do not tolerate saltwater. When the sea level rises, the communities of freshwater plants and fish may die out due to the extension of salt wedge intrusion along the Kushiro River. Therefore, in this study we make an attempt to understand the impact of sea level rise on the ecological system of the wetland by using a three-dimensional numerical model, which allows us to evaluate the effect of sea level rise on salt wedge intrusion. It is revealed that the National Park area may be affected by salt water when sea level rise reaches about 0.15 m. Also, the maximum sea level rise may induce saltwater intrusion into the Ramsar area. As a result, we proposed an adaptive management strategy for protecting the ecological system of the wetland from salt wedge intrusion.

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