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),

FIELD OBSERVATIONS OF TIDAL CURRENT IN THE FIELD OBSERVATIONS OF TIDAL CURRENT IN THE MOUTH OF ISAHAYA BAY BY MEANS OF MOUTH OF ISAHAYA BAY BY MEANS OF

DBF OCEAN RADAR AND ADCPDBF OCEAN RADAR AND ADCPDURING AUTUMN SEASON IN 2005DURING AUTUMN SEASON IN 2005

Kenta TakenouchiKenta Takenouchi1)1), Akihide Tada, Akihide Tada2)2), Shinsaku Someya, Shinsaku Someya1)1), Shinic, Shinichi Sakaihi Sakai3)3), Michihiro Mizunuma, Michihiro Mizunuma4) 4) , Takehiro Nakamura, Takehiro Nakamura5)5)

1)1) Graduate Student, Graduate School of Science Graduate Student, Graduate School of Science2)2) Professor,Department of Civil Engineering, Nagasaki UniversityProfessor,Department of Civil Engineering, Nagasaki University3)3) Central Research Institute of Electric Power IndustryCentral Research Institute of Electric Power Industry4)4) West Japan Engineering Consultants Inc.West Japan Engineering Consultants Inc.5)5) Professor, Faculty of Environmental StudiesProfessor, Faculty of Environmental Studies

1. Location of Isahaya Bay1. Location of Isahaya Bay

50km

Oita

Fukuoka

Kumamoto

Kagoshima

Saga

Nagasaki

Kyusyu map

Miyazaki

Japanese map

1. Location of Isahaya Bay1. Location of Isahaya Bay

Sea-dyke

Isahaya Bay

Ariake Sound

Takezaki Is. ●N

Isahaya Bay & Ariake Sound map

■■ Big National Project of Isahaya BayBig National Project of Isahaya BayPurpose: 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 Project2. Outline of Big National Project of Isahaya Bay of Isahaya Bay

3. Background of This Study3. 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 2005. The laver has gone down

4. Outline of Field Observation4. 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 Observation4. 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 Observation4. 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 radarConstitution of DBF ocean radar

4. Outline of Field Observation4. Outline of Field Observation

(2) Current observation using DBF ocean radar

Observation area in Isahaya BayObservation 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 Observation4. Outline of Field Observation

(3) Current observation using ADCP

Observation line in Isahaya BayObservation line in Isahaya Bay

5km

Isahaya Bay

E

Takezaki Island

Ariake Sound

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 Discussion5. Results and Discussion

DBF radarStation A (Saigo)

DBF radarStation B (Arao)

Ariake SoundN

Takezaki Island

Oura

5km

50cm/s

Sea-dike

Isahaya BayDBF radar

Station A (Saigo)

DBF radarStation B (Arao)

Ariake SoundN

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 current At the maximum ebb current

5. Results and Discussion5. 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 Discussion5. 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) 4.5 02.259 6.75

Kojiro Takezaki Is.

3.05

5.05

7.05

10.0511.05

2.05

4.05

6.05

8.059.05

12.05

Water depth (m)

1m/s

N N

Distance (km) 4.5 02.259 6.75

Kojiro Takezaki Is.

(3) Vertical structure of tidal currents

At the maximum flood current

At the maximum ebb current

6. Conclusion in This Study6. 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.