2:15 PM - 2:30 PM
[HCG22-03] Seafloor migration of pop-up type Turbidity Current Ditector (TCD) observed off the mouth of the Fuji River in northern Suruga Bay, Japan.
Keywords:Turbidity Current, Fuji River, Typhoon origin, Suruga Bay
In October 2018, crustal structure survey was conducted in the submarine fan off the mouth of the Fuji River in northern Suruga Bay under the "Focused Research and Observation in the Fuji River Estuary Fault Zone" project (MEXT, and Univ. Tokyo, 2020). In this exploration, 18 pop-up type Ocean Bottom Seismographs (OBS) were developed. After installation, seven of the 18 OBSs experienced anomalies, such as moving southward from their input position and drifting to the nearby coast. The cause of this was suggested by Baba et al. (2021) to be the generation of turbidity currents by Typhoon No. 24 in 2018.
In this study, in order to investigate and observe turbidity currents caused by typhoons in Suruga Bay, a pop-up type Turbidity Current Detector (TCD) was developed and installed on the 1312 m depth seafloor in the submarine fan off the mouth of the Fuji River during August to November 2021 (the 1st experiment) and 2022 (the 2nd experiment), when many typhoons occur in the area. The TCD is equipped with cameras, lights, and an IMU (Inertial Measurement Unit) inside a glass sphere, which is used in the OBS, to record images of turbidity currents and the progress of movement of the TCD when it is swept away.
In September 2022, typhoons No. 14 and 15 approached Suruga Bay in succession. Typhoon No. 15 in particular had a major impact on the northern part of Suruga Bay, causing a major water outage in Shimizu, Shizuoka. After the typhoon passed, we waited for the sea conditions in Suruga Bay to calm down and attempted acoustic communication to confirm the location of the TCD at sea, where it was originally installed. Subsequently, an attempt was made to surface and retrieve the TCD by sending a detachment command signal, but the TCD did not surface and could not be retrieved, although it responded normally. On December 25, 2022, a search of the seafloor was conducted using a Remotely Operated Vehicle (ROV), and the TCD was found in a state in which it was entangled with a lot of mud and plants that were thought to be terrestrial in origin, and was not able to surface by itself. The TCD was recovered by ROV manipulator. After recovery, we checked the internal data, but the data analysis was not possible because the internal equipment was heavily damaged.
In this presentation, we report on the time sequence from the installation to the recovery of the TCD, including the details of mud adhesion, anchor deformation, and the mechanical energy that is considered to be applied to move the TCD after the TCD has been recovered. In relation to this presentation, Submarine sedimentary environment off the Fuji River revealed by ROV dives (poster presentation: Nakamura et al, Session ID, H-CG22, 5/22PM), and details of the TCD observation system (poster presentation: Nakao et al, Session ID, M-IS07, 5/23PM) will also be presented.
Acknowledgements
We would like to very thanks the crew and ROV operators of the ROV mother ship "Kaiyo-Maru No.1 (1390 tons: Kaiyo Engineering Co., Ltd.)" and Mr. Takeshi Kito, president of Kaiyo Engineering Co., Ltd.
References
Research and Development Bureau, Ministry of Education, Culture, Sports, Science and Technology and Earthquake Research Institute, The University of Tokyo, 2020, Report on the Focused Investigation and Observation at the Fuji River Estuary Fault Zone, 2017-2020, in Japanese.
Baba, H., N. Nakao, T. Nishimiya, M. Shinohara, S. Abe and K. Tsuruga, 2021, Turbidity Current in the Northern Suruga Bay Recorded by Ocean Bottom Seismographs after Passing Typhoon No. 24 in 2018, Zishin, 2, 73, 197-207. in Japanese.
In this study, in order to investigate and observe turbidity currents caused by typhoons in Suruga Bay, a pop-up type Turbidity Current Detector (TCD) was developed and installed on the 1312 m depth seafloor in the submarine fan off the mouth of the Fuji River during August to November 2021 (the 1st experiment) and 2022 (the 2nd experiment), when many typhoons occur in the area. The TCD is equipped with cameras, lights, and an IMU (Inertial Measurement Unit) inside a glass sphere, which is used in the OBS, to record images of turbidity currents and the progress of movement of the TCD when it is swept away.
In September 2022, typhoons No. 14 and 15 approached Suruga Bay in succession. Typhoon No. 15 in particular had a major impact on the northern part of Suruga Bay, causing a major water outage in Shimizu, Shizuoka. After the typhoon passed, we waited for the sea conditions in Suruga Bay to calm down and attempted acoustic communication to confirm the location of the TCD at sea, where it was originally installed. Subsequently, an attempt was made to surface and retrieve the TCD by sending a detachment command signal, but the TCD did not surface and could not be retrieved, although it responded normally. On December 25, 2022, a search of the seafloor was conducted using a Remotely Operated Vehicle (ROV), and the TCD was found in a state in which it was entangled with a lot of mud and plants that were thought to be terrestrial in origin, and was not able to surface by itself. The TCD was recovered by ROV manipulator. After recovery, we checked the internal data, but the data analysis was not possible because the internal equipment was heavily damaged.
In this presentation, we report on the time sequence from the installation to the recovery of the TCD, including the details of mud adhesion, anchor deformation, and the mechanical energy that is considered to be applied to move the TCD after the TCD has been recovered. In relation to this presentation, Submarine sedimentary environment off the Fuji River revealed by ROV dives (poster presentation: Nakamura et al, Session ID, H-CG22, 5/22PM), and details of the TCD observation system (poster presentation: Nakao et al, Session ID, M-IS07, 5/23PM) will also be presented.
Acknowledgements
We would like to very thanks the crew and ROV operators of the ROV mother ship "Kaiyo-Maru No.1 (1390 tons: Kaiyo Engineering Co., Ltd.)" and Mr. Takeshi Kito, president of Kaiyo Engineering Co., Ltd.
References
Research and Development Bureau, Ministry of Education, Culture, Sports, Science and Technology and Earthquake Research Institute, The University of Tokyo, 2020, Report on the Focused Investigation and Observation at the Fuji River Estuary Fault Zone, 2017-2020, in Japanese.
Baba, H., N. Nakao, T. Nishimiya, M. Shinohara, S. Abe and K. Tsuruga, 2021, Turbidity Current in the Northern Suruga Bay Recorded by Ocean Bottom Seismographs after Passing Typhoon No. 24 in 2018, Zishin, 2, 73, 197-207. in Japanese.