Takumi Yoshii

Practicality of VHF-band ocean radar to surface current observation in open sea area

The fundamental characteristics of the high-resolution ocean radar system with the VHF band under severe winter conditions in the open sea area were evaluated through the field observations in Wakasa Bay. As the results, over 60 percentage of the detection ratio of the horizontal currents by the radar was confirmed in the extent of 15km as to the maximum observation range of 25km. The comparison between the current data of the radar and that of the current meter in one-month observation at the close point to Monjyu Power Station showed good correlation with a correlation coefficient over 0.7 and the standard error of 5 to 10cm/s. It was also indicated that the currents from surface to 2.5m below the surface were mainly driven by the wind fluctuation component of the 25.3 hours with the phase of 30 minutes. The ocean surface radar could detect the horizontal current patterns in the irregular current field under the sever weather and oceanic condition, which might largely contribute to comprehend unknown ocean characteristics in an open sea.

INVESTIGATION OF EFFECTS OF SILTATION SUPPLY FROM A SEA CLIFF ON SAND DEPOSITION INSIDE A HARBOR BY FIELD OBSERVATION AND NUMERICAL SIMULATION

In this study, firstly, the field observation was executed on wave, current and concentration of silty sand in the sea area around the harbor of the Haramachi-thermal power plant site of Tohoku Electric Power Company and in front of the Nitta River and the sea cliff, which are surmised as silt sources. The actual situation of erosion of the sea cliff due to waves is investigated by using the aerial laser beam survey. The physical mechanism of migration of silty sand into the harbor and the supply mount of silty sand are estimated from the field observation results. The facilities of the Tohoku Electric Power Company have stood on the land behind the above sea cliff. So, prevention of the sea cliff from more erosion should be focus for the above facilities. Secondly, the beach deformation numerical model was modified in order to consider supply of siltation from the sea cliff. Finally, using the modified numerical model, effects of siltation supply from the sea cliff on accretion amount and seabed change around the harbor are investigated during a year.

Consideration of impact factors on data acquisition for ocean radar with VHF band

An ocean radar with a VHF band is a useful tool for surveying coastal currents in a wide area. We have conducted a field observation of currents in Ariake Bay, the large inner sea located in the western part of Japan. Although the number of valid data acquired (hereafter, data acquisition) varies temporally and spatially, the mechanism of such variation is not yet well understood. Our aim in this study is to investigate the impact factor affecting data acquisition through field observations of current, salinity and wind in Ariake Bay. As a result, the data acquisition and the maximum measurement distance of the radar decreased owing to the decreases in salinity and wind speed. After receiving much rainfall, the data acquisition decreased rapidly in accordance with the decrease in surface salinity influenced by run-off from adjacent rivers and a flood control dike. Also, when wind speed became less than 2 m/s, the intensity of the signals received was not sufficient for the detection of current velocity. In conclusion, salinity and wind speed are important factors for steady-current observation using ocean radars.

Long-term Real Time Monitoring System by DBF Ocean Radar and its Availability and Effectiveness

DBF ocean radar can detect surface currents widely with high resolution in all weather conditions. We developed 24-hours real-time oceanographic monitoring system, which based on automatic figuring system for surface currents and wireless communication system with high-speed packet communications (up to 384 bps). Figures of surface currents can be delivered every 15 minutes with time lag of about 45 minutes through the Internet. The Dragon System is feasible for emergency observation after sudden ocean accident such as oil spills because of high portability. Field observations showed a measurable rating is more than 90% within 15 km along the receiving beam lines. Amount of measuring points in one operation periodically change over time. Its fluctuations were related with tidal variability.