Five units of a tidal flat for the filed study were constructed by dredged sludge in Ago bay, Mie prefecture. Sediment quality and benthos habitat had been monitored for three years after tidal flat construction. The monitoring result revealed that the number of the species and individuals of benthos increased more in the tidal flat units constructed by dredged sludge than in the natural tidal flat. This result shows that the sediment containing the adequate amount of organic matter creates preferable ecosystem as compared with clean sandy sediment and the dredged sludge is useful for the construction material of tidal flats, while it had been treated as waste material. The optimum sediment quality for benthos habitat, 4-6% in Ignition Loss, 3-8 mg/g in COD and 20-30% in mud content, is quantitatively found from the filed study in Ago bay and shows good agreement with the property of the results found from the other sea areas, Sanbanze tidal flat and Seto Inlet. This optimum sediment quality condition enables to determine the content of dredged slug in tidal flat sediment in order to increase benthos biomass, resulting in the verification of a new type of tidal flat construction method using dredged sludge as recycling resource.

View full abstract

Recently, the tidal flats were constructed each place. However, it has been reported that the bottom topography and the sediment size were not stable after the construction due to the wave action. In order to analyze the property of bottom topography and sediment size, it was studied by hydraulic model test and field survey. As a result of model test, it was shown that topography shape and the sediment size changed in conjunction with a wave condition, i.e. erosion / coarse grain at the time of high waves, accretion / fine grain at the time of regular waves. The topography of the constructed tidal flat which was constructed by muddy dredged sediment in Ago Bay also changed with the same property as the result of the hydraulic model test, but the sediment size changed contrary finely.

View full abstract

The influence of the Kuroshio meander/straight on the flow field of the Seto Inland Sea was investigated, based on the inter-decadal variation of the meander/straight of the Kuroshio path, the river discharge, the sea level heights at open boundaries, and the water quality field from 1982 to 1999. The duration was classified into three periods: the meander, the transition, and the no-meander period. First, the seasonal flow amount in the Seto Inland Sea was estimated by the two-dimensional numerical analyses in which the observed density and the sea level pressure distributions were considered. The seasonal variation of throughflow and its direction through the Seto Inland Sea agreed well with the motion of observed low salinity water in the bottom layer. Next, the dynamic mechanism of the influence of the Kuroshio on the sea level along the west-Japan coast was examined, and the numerical analysis method was established. In the analyses, the calculation domain was the south sea of west-Japan including the Kuroshio and the Seto Inland Sea. The Kuroshio was modeled as the southern boundary of the calculation domain by means of the approximation functions of the hydraulic properties into the triangle series along the Kuroshio path. The calculation results were verified with an observation results measured by TOPEX/Poseidon: the mean sea level height distribution and the variation rate of sea level difference between the Kii Channel and the Bungo Channel. The difference of flow amount due to the Kuroshio meander/straight corresponds to thirty to forty percents of the seasonal flow amount, and the flow amount differs about 10¹⁰m³/month between the periods.

View full abstract

While a seed-sowing method and a shoot-planting method have been used as conventional methods of transplanting eelgrass, they require hard underwater works for collecting and planting shoots and seeds, and give damage to natural eelgrass bed during collecting works. In this study, in order to solve these problems of the conventional methods, the new method of transplanting eelgrass was developed and the effect of this method was verified by the filed study carried out for five years in Ago bay, Mie prefecture. The transplant process of the method consists of the following two steps: In the first step, mats are placed on the sea bottom near the natural eelgrass bed, and the seeds of eelgrass fall and germinate on the mats, in the second step, the mats on which eelgrass grew, are moved to the seabed where transplanting is planned, and the new eelgrass bed of the mats expands to the large area of the seabed. The method gives no damage to natural eelgrass bed, because collecting shoots and seeds are not required for transplanting. The method is more efficient than the conventional methods, because underwater works required in the method are only placing and moving the mats and can be significantly reduced to compared with the conventional methods. The result of the field study shows that the eelgrass bed transplanted by the method grew and expanded equivalent to the natural eelgrass bed. Therefore, the new method developed in this study was verified to be useful for eelgrass transplantation.

View full abstract

Dynamics of sand movement were described on the Hamamatsu Coast in Enshuu-nada facing the Pacific Ocean. Topographies of coastal sand dune and nearshore region were reproduced by using digitalized contour lines in geographical maps collected since 1891. Nearshore sand movement was discussed by using bathymetry data. The deformation of coastal topographies around the Magome River mouth was found to be influenced by eastward wind blown sand, westward longshore transport and intermittent sand supply from the Tenryu Rivers. The recent rapid deformation of the Nakatajima Sand Dune was developed mainly by the entrapment of aeolian sand movement owing to the forestation on the adjacent region.

View full abstract

In order to clarify the biochemical and the physical processes of the suspended organic matter derived from phytoplankton detritus and inorganic soil, the sedimentation characteristics are investigated using sediment traps and turbidity sensors in the Hiroshima Bay.
The mineralization rate of the suspended organic matter is estimated using the results of analyses of variations in characteristics of the suspended organic matter, and the incubational experiments on the sediment. It is suggested that the mineralization rate could be roughly determined by the C/N ratio.
Based on the estimations above, a model is presented, which can express the process of the decreasing of the mineralization rate with the sedimentary years increasing. Furthermore, the settling velocity of the suspended organic matter could also be determined using the equations presented in this paper, which focuses on the changes of the density and particle diameter of the suspended organic matter during particle settling, estimated using the model above.

View full abstract

Evaluation of hydraulic resistance on trees considering deformation of foliage and similarity law for deformed foliage under fluid experiments were carried out to establish countermeasures used a costal vegetated area for tsunami disaster reduction. A mechanism about deformation of foliage was evaluated by simplified model based on elastic, applicability of the model and characteristics of hydraulic resistance about trees with deformation of foliage was investigated experimentally. To verify influence of hydraulic resistance on trees deformed foliage on the inundation, another experiment and numerical simulation were carried out, and importance to considering deformation of foliage were shown by comparison experimental and numerical results. Furthermore, numerical simulations about tsunami inundation in a coastal forest of Nihonkai-Chubu Earthquake Tsunami were performed, importance to considering hydraulic resistance on trees considering deformation of foliage was suggested when examining countermeasures used a costal forest for tsunami disaster reduction.

View full abstract