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K.Imahayashi 1 ,A.Tada 2 ,K.Takenouchi 3 , S.Sakai 4 ,M.Mizunuma 5

FIELD OBSERVATION ON THE CHARACTERISTICS OF CURRENTS IN THE CENTER OF ARIAKE SOUND USING DBF OCEAN RADAR. K.Imahayashi 1 ,A.Tada 2 ,K.Takenouchi 3 , S.Sakai 4 ,M.Mizunuma 5. 1) Graduate Student,Graduate school of Science and Technlogy, Nagasaki University

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K.Imahayashi 1 ,A.Tada 2 ,K.Takenouchi 3 , S.Sakai 4 ,M.Mizunuma 5

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  1. FIELD OBSERVATION ON THE CHARACTERISTICS OF CURRENTS IN THE CENTER OF ARIAKE SOUNDUSING DBF OCEAN RADAR K.Imahayashi1,A.Tada2,K.Takenouchi3, S.Sakai4,M.Mizunuma5 1) Graduate Student,Graduate school of Science and Technlogy, Nagasaki University 2) Professor,Faculty of Engineering, Nagasaki University 3) Independent Corporation, Japan Water Agency 4) Central Research Institute of Electric Power Industry 5) West Japan Engineering Consultants, Inc.

  2. Fukuoka Saga Oita Nagasaki Kumamoto Miyazaki Kagoshima Kyusyu map 50km Location of Ariake Sound Japanese map

  3. Sea-dyke Regulation pond Isahaya Bay Location of Isahaya Bay Takezaki Is. ● N Ariake Sound Isahaya Bay Sea-dyke Isahaya Bay & Ariake Sound map

  4. 1. INTRODUCTION In Isahaya Bay, the red tide has been generated in summer frequently. ↓ This observation's purpose is obtain basic knowledge of generation mechanism of the red tide on hydrodynamic viewpoint Field observations on tidal current in the center of Ariake Sound including the mouth of Isahaya Bay have carried out by means of DBF ocean radar. ↓ The basic performance of the DBF ocean radar is evaluated

  5. 2. ADVANTAGES OF DBF OCEAN RADAR ・DBF ocean radar transmits radio waves from land toward the ocean and receives back the radio energy reflected by ocean waves.・By analyzing Bragg scattering, the system provides data on directions, velocities and wave height of surface currents.

  6. 2. ADVANTAGES OF DBF OCEAN RADAR ・Conventional ocean surveys have conducted by hanging a current meter from a ship into the sea or mooring it in the sea in order to directly measure current directions and velocities at multiple observation spots. ・With this method, measurements were not possible under adverse weather conditions. Another drawback to it was errors resulting from areal interpolations of point specific data or different measurement times at individual measurement points. ・In contrast, DBF ocean radar allows remote surveysfrom land, that is to say, measurements are possible any time regardless of weather conditions. In addition, it can monitor ocean currents over a wide area simultaneously in a short period of time.

  7. Ariake Sound U5 U4 U3 T5 U2 Station B (Arao) T4 U1 T3 Shelter of DBF Radar System Transmission antenna T2 T1 Isahaya Bay Eight reception antennas Station A (Saigo) 3. OUTLINE OF FIELD OBSERVATION 3.1Current observation using DBF ocean radar Two DBF ocean radars were set at both Station-A (Saigo, Unzen City) and Station-B (Arao, Arao City). The area colored with gray in Figure is a range of the observation of DBF ocean radar. ↓ In order to realize the characteristic of currents in the center of Ariake Sound

  8. Ariake Sound U5 U4 U3 T5 U2 Station B (Arao) T4 U1 T3 T2 T1 Isahaya Bay Station A (Saigo) 3. OUTLINE OF FIELD OBSERVATION 3.2Current observation using ADCP The current observation using ADCP at T1～T5 points on the 12th of August and at U1～U5 points on the 26th of October. ↓ The basic performance of the DBF ocean radar is evaluated

  9. Ariake Sound ● ● U5 V2 V1 U4 U3 T5 U2 Station B (Arao) T4 U1 T3 T2 T1 Isahaya Bay Station A (Saigo) 3. OUTLINE OF FIELD OBSERVATION 3.3Lagrangian observation using drifting buoy The Lagrangian observation using a drifting buoy on the 25th of October in 2007. Two buoys (Buoy-1, Buoy-2) were set at the point V1 and another (Buoy-3) at the point V2 as shown in Figure2. ↓ • In order to realize the characteristic of currents in the center of Ariake Sound • The basic performance of the DBF ocean radar is evaluated

  10. DBF(deg) DBF(deg) DBF(cm/s) ADCP（cm/s） ADCP（deg） ADCP（deg） DBF(cm/s) ADCP（cm/s） 4. RESULTS AND DISCUSSION 4.1 Precision of DBF ocean radar compared with ADCP Figures show the scattering plot of the surface velocity by the DBF ocean radar and by the ADCP classified into current speed and its direction. ↓ R(the correlation coefficient) 0.866～0.968 ↓ DBF ocean radar can grasp the surface current with the same precision as common current meters.

  11. observation time (cm) 300 200 100 0 -100 -200 -300 18:00 24:00 6:00 12:00 0:00 4. RESULTS AND DISCUSSION 4.2 Precision of DBF ocean radar compared with drifting buoy The drifting buoys were threw in at just about high tide. Buoy-1 and Buoy-2 went south about 10km, afterward the tide was flooding, they went north. Similarly, Buoy-3 went south about 12.7km, and moved north. Buoy-1,2 Buoy-3

  12. Upperfigure is time series of the zonal and meridional surface current components interpreted by the movement of the drifting buoy, one of the three buoys, and by the DBF ocean radar measurement with a time interval of 15 minutes. E-W component (m/s) N-S component (m/s) DBF ocean radar Drifting buoy DBF ocean radar DBF ocean radar Drifting buoy Drifting buoy ↓ The tendency of the current variability derived from the DBF ocean radar agrees well with that by the drifting buoy N-S direction distance(m) 2000 4000 8000 Lower figure shows the comparison of a pursuit result of trajectories between the drifting buoy and the imaginary particle moved by the velocity of DBF ocean radar every 15 minutes. -2000 2000 6000 10000 -2000 -4000 ↓ -6000 Both trajectories show the same moving pattern qualitatively -8000 -10000 E-W direction distance(m)

  13. (dig) Wind direction Wind speed Rainfall 4. RESULTS AND DISCUSSION 4.3 Daily average current vector detected by DBF ocean radar The wind speed on the observation day is less than 1.0m/s, and amount of the rainfall for ten days in the past is less than 1.0mm. 0.25(m/s) ↓ Typical average current pattern for one day when the tidal motion is the predominant factor in Ariake Sound. Daily average current vectors detected by the DBF ocean radar on the 20th of September, 2005.

  14. (dig) Wind speed Rainfall Wind direction 4. RESULTS AND DISCUSSION 4.3 Daily average current vector detected by DBF ocean radar It has not been raining for six days about the observation day. And south wind blew all day long and the averaged wind speed was about 3.4m/s on the observation day 0.25(m/s) ↓ Typical wind-driven current pattern. Daily average current vectors detected by the DBF ocean radar on the 12th of August, 2007.

  15. 5. CONCLUSION Field observations on tidal current in the center of Ariake Sound including the mouth of Isahaya Bay have carried out by means of DBF ocean radar. • It is obvious that the DBF ocean radar can grasp the surface current with the same precision as common current meters,namely ADCP. • The tendency of the current variability derived from the DBF ocean radar agrees well with that by the drifting buoy. • According to a daily average current vector, it realized that the weather conditions and tidal fluctuation had a great influence on the characteristics of surface currents in the center of Ariake Sound ↓ The basic performance of the DBF ocean radarwas proven it was accurate.

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