4:00 PM - 4:15 PM
[MIS11-09] Sedimentation effects of the 2011 Tohoku-Oki Tsunami on coastal lakes: A case study of Agawa-numa, Shichigahama, Miyagi.
Keywords:Tsunami deposit, Numerical simulation
Tsunami deposit surveys are conducted not only in coastal lowlands, but also in coastal lakes and ponds. Lateral continuity and preservation potential of tsunami deposits in coastal lakes and ponds are considered to be high due to their quiet sedimentary environment and large depositional space. Meanwhile, characteristic features have been reported, such as large-scale erosion by tsunami inflow and deposition of muddy sediments. These features are interpreted to reflect the temporal changes in flow conditions. A simple conceptual model of sediment transport in coastal lakes proposed by Bondevik et al. (1997) is widely accepted to explain tsunami-induced sedimentary process in coastal lakes.
Since it is difficult to observe the water flow in a lake at the time of a tsunami, some flume experiments have been conducted to replicate tsunamis surging into lakes. However, the relationship between topography or bottom sediment of individual lakes and the structures of tsunami sediments is poorly understood. In this study, we excavated Agawa-numa, Shichigahama, Miyagi Prefecture, northeastern Japan to investigate the relationship between lake bathymetry and tsunami deposit characteristics. Agawa-numa is a freshwater lake and located inside the inundation area of the 2011 Tohoku-oki tsunami. In this study, we cored the lake shore and bottom twice and collected 31 sediment cores from 23 sites. We further investigate the formation process of the 2011 Tohoku-oki tsunami deposit by means of numerical simulation.
The collected cores showed sand and silt layers deposited by the Tohoku-oki tsunami over a wide area of the lake bottom, and the sediment structure and distribution were found to be similar to those reported in previous studies. Tsunami sediment transport simulations were performed using bathymetric data acquired during the field survey. The simulations were calibrated to reproduce the waveforms and trace heights well. Based on the simulation results, it was found that the characteristics of the actual sediments could be partially explained. For example, the calculated erosion was large in the seaward area of the lake, and the sandy sediments in the central part of the lake were considerably thinner than in the around areas. The latter may be related to the effect of hydraulic jump due to the tsunami inflow to the lake suggested by previous flume experiments (Yamaguchi and Sekiguchi, 2015). On the inland side, contrastingly, the actual depositional structure was not well explained. For example, large-scale erosion deduced by the core observations was not reproduced in the calculations. This may be due to physical processes not considered in the numerical model or uncertainties in the input parameters. In the future, it will be necessary to recover core samples at higher densities and perform the simulation under different boundary conditions to clarify the factors controlling the sediment transport process.
References
Bondevik, S., Svendsen, J. I. and Mangerud, J., 1997, Tsunami sedimentary facies deposited by the Storegga tsunami in shallow marine basins and coastal lakes, western Norway. Sedimentology, 44, 1115–1131.
Yamaguchi, N. and Sekiguchi, T., 2015, Effects of tsunami magnitude and terrestrial topography on sedimentary processes and distribution of tsunami deposits in flume experiments. Sediment. Geol., 328, 115–121.
Since it is difficult to observe the water flow in a lake at the time of a tsunami, some flume experiments have been conducted to replicate tsunamis surging into lakes. However, the relationship between topography or bottom sediment of individual lakes and the structures of tsunami sediments is poorly understood. In this study, we excavated Agawa-numa, Shichigahama, Miyagi Prefecture, northeastern Japan to investigate the relationship between lake bathymetry and tsunami deposit characteristics. Agawa-numa is a freshwater lake and located inside the inundation area of the 2011 Tohoku-oki tsunami. In this study, we cored the lake shore and bottom twice and collected 31 sediment cores from 23 sites. We further investigate the formation process of the 2011 Tohoku-oki tsunami deposit by means of numerical simulation.
The collected cores showed sand and silt layers deposited by the Tohoku-oki tsunami over a wide area of the lake bottom, and the sediment structure and distribution were found to be similar to those reported in previous studies. Tsunami sediment transport simulations were performed using bathymetric data acquired during the field survey. The simulations were calibrated to reproduce the waveforms and trace heights well. Based on the simulation results, it was found that the characteristics of the actual sediments could be partially explained. For example, the calculated erosion was large in the seaward area of the lake, and the sandy sediments in the central part of the lake were considerably thinner than in the around areas. The latter may be related to the effect of hydraulic jump due to the tsunami inflow to the lake suggested by previous flume experiments (Yamaguchi and Sekiguchi, 2015). On the inland side, contrastingly, the actual depositional structure was not well explained. For example, large-scale erosion deduced by the core observations was not reproduced in the calculations. This may be due to physical processes not considered in the numerical model or uncertainties in the input parameters. In the future, it will be necessary to recover core samples at higher densities and perform the simulation under different boundary conditions to clarify the factors controlling the sediment transport process.
References
Bondevik, S., Svendsen, J. I. and Mangerud, J., 1997, Tsunami sedimentary facies deposited by the Storegga tsunami in shallow marine basins and coastal lakes, western Norway. Sedimentology, 44, 1115–1131.
Yamaguchi, N. and Sekiguchi, T., 2015, Effects of tsunami magnitude and terrestrial topography on sedimentary processes and distribution of tsunami deposits in flume experiments. Sediment. Geol., 328, 115–121.