Underwater structures such as bridge abutments are damaged by a flood and longtime erosion. So far we do not have an effective way to check the safety for this kind of such underwater structures. Therefore, it is considered that there are a lot of potential needs of precise geomorphology and geometry observation. Recently an imaging sonar system C3D, integrated bathymetry and side scan sonar, can provide a high resolution, wide swath and high accuracy in bathymetry, and has been expected to be used for underwater floor mapping in shallow water. However, in the area under a bridge, even the C3D is used, high quality geomorphologic results cannot be obtained because GPS signals will be blocked out and the positioning precision deteriorates tremendously. Though total station (TS) integrated laser ranging and transit has been expected to take the place of GPS in its outage area, it shows difficult to maintain the positional precision as the sampling interval precision is low. Therefore, we developed a new TS with short delay and small dispersion in its data output in cooperation with Topcon, which is synchronized with GPS time. The new positioning system can obtain enough positioning accuracy even in case of GPS outages. With this we constructed a high accuracy geomorphologic measuring system utilizing the C3D imaging sonar which has three times swath width of traditional multi-beam sonar and two thousand measured point data each ping. In order to validate the new system, a survey was conducted under two bridges in the upper and lower reaches of the Tone River respectively. The results showed that the proposed system can be used to measure underwater structures with adequate accuracy.

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The object of this study is to predict the distribution of dioxins deposited on the bottom of Tokyo Bay using a 3-dimensional flow model (a Hybrid Box Model) linked with a diffusion model.
On the basis of the load fluxes from rivers, the distribution of dissolved and particulate (small and large fractions) in seawater and the mass of dioxins accumulated in the bottom of the bay, were calculated by the models. We have divided the bottom sediment layers into 3 layers: the boundary layer (between sea water layer and the particle layer), the bioturbated layer, and the diffusion layer; and constructed an analysis model that treats the coupling of matter between sea water and the bottom sediment layer.
Results were compared to the published observation data in order to examine applicability of the used models.
Estimated dioxins concentration in the sediment showed good agreement with the observed data.
From the analyses, it was estimated that 50.83% of the dioxins inflow into the bay was deposited onto the bottom, and that 48.10% flowed out of the bay. Dioxins sedimentation was greater in the inner and middle parts of the bay, and decreased toward the bay entrance.

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The Izena Cauldron in the Okinawa Trough has two sites of hydrothermal massive sulfide deposits named the Jade Site and the Hakurei Site. The present study conducted observation of ADCP/CTD towings and AUV dives to obtain the over-all distribution of bottom-currents in the caldera. In the ADCP observation, current velocity data and echo intensity data were obtained in multiple-layers between the seabed and the 100-150 m altitude. The analysis shows the characteristics of the bottom-currents; they have an organized structure with almost same directions in the whole region within the caldera, and averaged speed of horizontal velocity of approximately 10 cm s^-1, and the directions change temporally with tidal frequency. There are descending currents with a 0-2 cm s^-1 downward velocity component along the eastern slope of the caldera. The echo intensity data shows that high echo intensity anomalies distribute patchily at depths of approximately 1500 m. They are well-corresponded with turbidity maxima and upward velocity components, and two anomalies have an upward velocity component of more than 2 cm s^-1. It is suggested that these high echo intensity anomalies show the distribution of particulate matter in hydrothermal plumes from the Hakurei Site. These results indicate a circulation pattern of hydrothermal plumes in the Izena Cauldron, as plumes rise to approximately 1500 m depth from the seabed in the Hakurei Site.

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