Takumi Okabe

Characteristics of Rip Current Drowning on the Shores of Japan

ABSTRACT Ishikawa, T.; Komine, T.; Aoki, S.I., and Okabe, T., 2014. Characteristics of rip current drowning on the shores of Japan. The characteristics of rip current drowning on the shores of Japan was investigated using the analysis of the lifesavers rescue reports of 2013 and field observations of some beaches. Although the necessary rescue equipment and the water safety information such as signboards were mostly prepared, a large amount of rip current drowning occurred. On most re-occurring rip current drowning beaches, there are permanent rips and fixed rips associated with the characteristic of topography and the coastal structures at most beaches under the relatively high wave condition. Especially, in the case lifesavers cannot determine the swimming areas, swimming areas are generally determined by the local regulations from local governments and shop owners on most beaches. The most important outbreak factor regarding the rip current drowning is the human factor such as the management problems and coastal structures, which was concluded.

Temporal changes in the ebb-tidal delta bathymetry of Imagire-guchi inlet in Japan

ABSTRACT This study focused on the morphologic changes in an ebb-tidal delta of Imagire-guchi inlet, Japan, at several temporal scales and the characteristics. The bathymetric information of the inlet was collected from multiple small fishing vessels. The position and depth of the vessel during operation were acquired with a GNSS antenna and fish finder, respectively, and these spatial data were logged with timestamps. The inlet bathymetry was consequently prepared to evaluate changes in the inlet morphology and delta-front volume. An assessment of the bathymetry monitoring data from 2008 to 2016 showed several reversals of the trends and various rates of accretion or erosion in the terminal lobe. The rate of the terminal progradation ranged from 7 to 30 m/y, and the rate of retrogradation ranged from 3 to 100 m/y over the eight years. The trend of progradation reversed in correspondence to three characteristic typhoons. The morphologic impact appeared in the terminal lobe and topset bed. The inlet channel also responded to typhoons with an expansion of shallows. The volume of delta-front sediment was annually evaluated, and the annual changes were compared with the equilibrium.

Preliminary study on performance of a coupled hydrodynamic and sediment transport model on small domain

Numerical simulation is one of the useful tools to analyze natural phenomena in the earth such as the tsunami disaster. Several numerical models can simulate the tsunami wave from its generation, propagation, and inundation. However, most tsunami models do not include the sediment transport module. The tsunami wave actually induces a lot of sediment during the propagation in the coastal area. In the case of Indian Ocean Tsunami in 2004, massive morphological changes were caused by the tsunami waves around Sumatra coast. In Aceh, some areas eroded by the tsunami wave were living place for a local community. It is indispensable for the resident in the coastal area to estimate the risk of morphological changes due to a tsunami wave. Therefore, a model that can investigate the morphological changes due tsunami wave is necessary. The result of this model can be used to consider a countermeasure for tsunami wave impact in the coastal area, such as land-use management and planning. The COMCOT-SED model had been developed by several researchers. This model combines the hydrodynamic module and the sediment module. The aim of this study is to get general information about performance of the COMCOT-SED model and to modify the model for more accurate results. Firstly, the model was demonstrated in the ideal condition to confirm the model validity. Then, we evaluated the model performance comparing the model results and the laboratory experiment data which was conducted by other researcher. The authors found that the results of water level and bottom profile by the original model in the ideal condition are not suitable. The model modification will give us more suitable results. The modified model will be applied to simulate the tsunami wave and sediment transport in the small area.Numerical simulation is one of the useful tools to analyze natural phenomena in the earth such as the tsunami disaster. Several numerical models can simulate the tsunami wave from its generation, propagation, and inundation. However, most tsunami models do not include the sediment transport module. The tsunami wave actually induces a lot of sediment during the propagation in the coastal area. In the case of Indian Ocean Tsunami in 2004, massive morphological changes were caused by the tsunami waves around Sumatra coast. In Aceh, some areas eroded by the tsunami wave were living place for a local community. It is indispensable for the resident in the coastal area to estimate the risk of morphological changes due to a tsunami wave. Therefore, a model that can investigate the morphological changes due tsunami wave is necessary. The result of this model can be used to consider a countermeasure for tsunami wave impact in the coastal area, such as land-use management and planning. The COMCOT-SED model had been ...

Effect of Regulation of the Inlet Channel on Tides and Water Levels in Lake Hamana, Japan

Hamanako is an economically and culturally important tidal lake on the Pacific coast of Honshu, Japan. Between 1954 and 1973, entrance works were constructed to regulate the channel to the lake. In a previous paper, the tidal records from Maisaka just inside the lake, prior to regulation, were analyzed to explain the responses of the lake to the ocean tide and other water level changes. The present paper compares water level responses with those following regulation of the entrance. The principal differences are that at Maisaka the tidal range increased up to 1965, following the first stages of regulation, but it has been constant since then. In the inner part of the lake, regulation has increased the tidal range but reduced its variability and has reduced the mean water level in the lake. The southern sand shoal divides the lake into two tidal basins, with Maisaka lying in the smaller outer basin. Studies of the inner basin have shown that its tidal range continues to increase. A hydrodynamic model has been used to show that this increase is primarily due to scouring of the inner channel through the southern shoal. The model has also shown that the constancy of the Maisaka tidal amplitude since 1965 is not a static equilibrium, but is a balance between the effects of the outer entrance channel scouring and increasing the Maisaka tide, offset by the inner channel scouring and reducing the Maisaka tide.