10:45 AM - 12:15 PM
[HCG22-P01] Submarine sedimentary environment off the Fuji River revealed by ROV dives.
Keywords:Suruga Bay, turbidity current, TCD, ROV, sedimentary environment
Suruga Bay locate in a plate convergence zone and has steep topographical features, with a maximum water depth of 2500 m at the mouth of the bay. The first-class river Fuji kawa(length of 128 km) flows into the bay from the inner part of the bay, and heavy rainfall during typhoons and the rainy season causes increased water levels and flows, resulting in flooding and inundation damage. It can be inferred that during such inundation events, highly turbid river water with high debris content flows into the sea area and debris is supplied to the sea area. Baba et al., (2021) suggested the possibility of typhoon-induced turbidity currents because submarine seismographs installed on the seabed moved and surfaced rapidly during the passage of Typhoon No. 24 in September 2018. Therefore, sediment gravity flow associated with increased river flow due to heavy rainfall is expected to occur in Suruga Bay.
Tokai University has conducted detailed bathymetry and backscatter intensity data acquisition and surface sediment sampling using the research vessels Bosei Maru (2000 t) and Hokuto/Minamijuji (19 t), with the aim of understanding sediment transport due to meteorological events in Suruga Bay. A self-floating Turbidity Current Detector (TCD) was installed on the seabed at the back of Suruga Bay (water depth: 1312 m) in August 2022 for direct observation of turbidity currents. However, after the passage of Typhoon No. 15 on 23 September, it was found to have moved approximately 2.8 km south (depth of 1421 m) of the installation point, suggesting that it had been affected by the typhoon. As the TCD did not surface itself and could not be recovered, a remotely operated vehicle (ROV) was used to search and simultaneously acquire images of the seabed. In this presentation, we report on the seabed observation situation in the vicinity of the TCD movement point (water depth:1400 m) based on seabed images acquired by ROV.
The ROV submarine survey was conducted on 25 December 2022. The ROV search area is approximately 130 m in the north-south direction and 50 m in the east-west direction, centered on the TCD movement point, with a depth difference of 4 m and a slope of 2 degrees. The topography of the that area is almost flat. The exploration route was on bottom at a depth of 1403 m at 10:00JST, proceed southwards along the trough axis for approximately 100 m, turn around at 10:51JST (water depth:1406 m), proceed northwards and discover TCD (water depth:1401 m) at 11:56JST.
The bottom is characterized by a seabed covered by muddy sediments, with sandy sediments deposited below them. Sediment-covered pebbles were observed at a depth of around 1404 m (10:46 JST). When the ROV moved about 2 m to the west, a crest (with a well-developed southerly slope) of a few centimeters in height was identified. A half-buried tire was observed at a depth of 1402 m (10:11 JST). Dense plant fragments and ropes were seen at a depth of 1404 m, about 30 m to the south (10:21 JST). At a depth of 1403 m, 23 m to the south-west from this point, another dense vegetation fragment with a northward convex erosion feature (current mark) around it was identified. Further southwards, uneven terrain was observed in a north-south direction (distance 34 m). At 10:46 JST (water depth 1404 m), a dense concentration of circular gravels not covered by sediment was observed. About 47 m further north, a tongue-shaped feature (11:13 JST) was revealed, and about 9 m north of that point, a loose uneven terrain was identified (11:14 JST) compared to the uneven terrain at the other sites. From this point, the TCD was discovered (11:56 JST) 9 m to the north. TCD was found in the form of sediments and numerous plant fragments entangled in the sediments. Current marks (with convexity to the north) were identified around dense plant fragments and TCD. Its shape suggests that it was eroded by north-south currents.
From the result, many artefacts and plant fragments transported from land were found on the seabed off the Fuji kawa River in the inner part of Suruga Bay. The erosion topography of the seabed surface due to the southward flow was observed, suggesting that the sediments covering the seabed surface in the inner part of the bay were transported by the strong southward flow. Combined with the TCD migration results, it can be inferred that the typhoon generated flood-induced gravity flows. Flood-induced gravity flows of various sizes and factors are estimated to occur throughout the year in Suruga Bay, and continued research will clarify the mechanisms of sediment transport associated with meteorological events.
Tokai University has conducted detailed bathymetry and backscatter intensity data acquisition and surface sediment sampling using the research vessels Bosei Maru (2000 t) and Hokuto/Minamijuji (19 t), with the aim of understanding sediment transport due to meteorological events in Suruga Bay. A self-floating Turbidity Current Detector (TCD) was installed on the seabed at the back of Suruga Bay (water depth: 1312 m) in August 2022 for direct observation of turbidity currents. However, after the passage of Typhoon No. 15 on 23 September, it was found to have moved approximately 2.8 km south (depth of 1421 m) of the installation point, suggesting that it had been affected by the typhoon. As the TCD did not surface itself and could not be recovered, a remotely operated vehicle (ROV) was used to search and simultaneously acquire images of the seabed. In this presentation, we report on the seabed observation situation in the vicinity of the TCD movement point (water depth:1400 m) based on seabed images acquired by ROV.
The ROV submarine survey was conducted on 25 December 2022. The ROV search area is approximately 130 m in the north-south direction and 50 m in the east-west direction, centered on the TCD movement point, with a depth difference of 4 m and a slope of 2 degrees. The topography of the that area is almost flat. The exploration route was on bottom at a depth of 1403 m at 10:00JST, proceed southwards along the trough axis for approximately 100 m, turn around at 10:51JST (water depth:1406 m), proceed northwards and discover TCD (water depth:1401 m) at 11:56JST.
The bottom is characterized by a seabed covered by muddy sediments, with sandy sediments deposited below them. Sediment-covered pebbles were observed at a depth of around 1404 m (10:46 JST). When the ROV moved about 2 m to the west, a crest (with a well-developed southerly slope) of a few centimeters in height was identified. A half-buried tire was observed at a depth of 1402 m (10:11 JST). Dense plant fragments and ropes were seen at a depth of 1404 m, about 30 m to the south (10:21 JST). At a depth of 1403 m, 23 m to the south-west from this point, another dense vegetation fragment with a northward convex erosion feature (current mark) around it was identified. Further southwards, uneven terrain was observed in a north-south direction (distance 34 m). At 10:46 JST (water depth 1404 m), a dense concentration of circular gravels not covered by sediment was observed. About 47 m further north, a tongue-shaped feature (11:13 JST) was revealed, and about 9 m north of that point, a loose uneven terrain was identified (11:14 JST) compared to the uneven terrain at the other sites. From this point, the TCD was discovered (11:56 JST) 9 m to the north. TCD was found in the form of sediments and numerous plant fragments entangled in the sediments. Current marks (with convexity to the north) were identified around dense plant fragments and TCD. Its shape suggests that it was eroded by north-south currents.
From the result, many artefacts and plant fragments transported from land were found on the seabed off the Fuji kawa River in the inner part of Suruga Bay. The erosion topography of the seabed surface due to the southward flow was observed, suggesting that the sediments covering the seabed surface in the inner part of the bay were transported by the strong southward flow. Combined with the TCD migration results, it can be inferred that the typhoon generated flood-induced gravity flows. Flood-induced gravity flows of various sizes and factors are estimated to occur throughout the year in Suruga Bay, and continued research will clarify the mechanisms of sediment transport associated with meteorological events.