FIELD OBSERVATIONS OF TIDAL CURRENT IN THE MOUTH OF ISAHAYA BAY BY MEANS OF DBF OCEAN RADAR AND ADCP DURING AUTUMN SEASON IN 2005 Kenta Takenouchi 1), Akihide Tada 2), Shinsaku Someya 1), Shinichi Sakai 3), Michihiro Mizunuma 4), Takehiro Nakamura 5) 1) Graduate Student, Graduate School of Science 2) Professor,Department of Civil Engineering, Nagasaki University 3) Central Research Institute of Electric Power Industry 4) West Japan Engineering Consultants Inc. 5) Professor, Faculty of Environmental Studies

1. Location of Isahaya Bay 50km Oita Fukuoka Kumamoto Kagoshima Saga Nagasaki Kyusyu map Miyazaki Japanese map

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

■ Big National Project of Isahaya Bay  Purpose: Disaster prevention and land reclamation  Start of Constructing : 1989  Main construction : ① Sea-dyke :about 7 km in length ② Reclamation by drainage: ・ Land development ・・・ about 1,800 ha ・ Regulation pond ・・・ about 1,700 ha Isahaya Bay Regulation pond Honmyo River Sea-dyke 2. Outline of Big National Project of Isahaya Bay

3. Background of This Study ■ In Isahaya Bay,a big project that consists of the construction of sea-dyke and reclamation by drainage reaches the last lap. ■ These caused severe environmental problems such as a change current, eutrophication, oxygen depression and so on. ■ In this study, therefore, field observations on tidal current in the middle of Ariake Sound including the mouth of Isahaya Bay are carried out by means of both DBF ocean radar and ADCP. during autumn in The laver has gone down

4. Outline of Field Observation ■ Three kind of observation method  The current observation using ADCP  The current observation using DBF ocean radar  The vertical distributions of various water quality indexes

4. Outline of Field Observation ・ 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. How ocean radar works as observation instrument (1) ADVANTAGES OF DBF OCEAN RADAR

4. Outline of Field Observation (1) 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 surveys from 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.

Transmission antenna eight reception antennas Shelter of DBF Radar System DBF ocean radar Constitution of DBF ocean radar Constitution of DBF ocean radar

4. Outline of Field Observation (2) Current observation using DBF ocean radar Observation area in Isahaya Bay 5km Isahaya Bay DBF ocean radar Station A (Saigo) Takezaki Island DBF ocean radar Station B (Arao) Oura N  Observational Term → Central Research Institute of Electric Power Industry has developed  DBF ocean radar ： Digital Beam Forming Marine Rader Sea-dyke → September 20 to October 5, 2005 Two DBF ocean radars were set Station-A (Saigo, Unzen City) Station-B (Arao, Arao City) Ariake Sound

4. Outline of Field Observation (3) Current observation using ADCP Observation line in Isahaya Bay 5km Isahaya Bay E Takezaki Island Ariake Sound E’E’ Oura N  Observational Term → In order to reveal a dimensional structure of tidal currents during spring tide in Isahaya Bay, the current observation using ADCP.  ADCP ： Acoustic Doppler Current Profiler Sea-dyke → September 20, 2005 The E-E’ Line:about 9km in length Current measurements were collected 9 times during one tidal cycle.

5. Results and Discussion DBF radar Station A (Saigo) DBF radar Station B (Arao) Ariake Sound N Takezaki Island Oura 5km 50cm/s Sea-dike Isahaya Bay DBF radar Station A (Saigo) DBF radar Station B (Arao) Ariake Sound N Takezaki Island Oura 5km 50cm/s Sea-dike Isahaya Bay (1) Horizontal distribution on velocity vectors of tidal current in surface layer ( DBF ocean radar ) It is found that the maximum flood current branches off to both Isahaya Bay and the head of Ariake Sound significantly. At the maximum flood currentAt the maximum ebb current

5. Results and Discussion (2) Horizontal distribution on velocity vectors of tidal current in surface layer ( ADCP ) At the maximum flood current 1m/s At the maximum ebb current It is obvious that the maximum velocity in flood current and ebb current occur near an offshore where is about 2km out from Kojiro, respectively. These tidal currents are similar to the observation results not only in October 2001 but also in August 2002 (Nakamura et al. 2002, 2003). 5km Isahaya Bay Takezaki Island Kojiro Sea-dike 5km Isahaya Bay Takezaki Island Kojiro Sea-dike

5. Results and Discussion The outflow into Ariake Sound occurred in the northern part of the E-E’ Line. These tidal currents are similar to the observation results not only in October 2001 but also in August 2002 (Nakamura et al. 2002, 2003). 0.5m/s Distance (km) Kojiro Takezaki Is Water depth (m) 1m/s NN Distance (km) Kojiro Takezaki Is. (3) Vertical structure of tidal currents At the maximum flood current At the maximum ebb current

6. Conclusion in This Study ■ There is outflow into Ariake Sound from Isahaya Bay near Takezaki Island in the best part of one tidal cycle. ■ Tidal current along the north coastline of the Shimabara Peninsula is dominant in Isahaya Bay. ■ DBF ocean radar is one of the powerful remote sensing tools to measure surface currents from lands widely and continuously without being affected by weather conditions.

Isahaya Bay DBF ocean radar Station A (Saigo) Takezaki Island Ariake Sound DBF ocean radar Station B (Arao) Oura Change of tide level at Oura N DBF ocean radar analysis result (2005/9/20) Horizontal distribution on velocity vectors of tidal current in surface layer Sea-dyke Scale on current velocity magnitude