5:15 PM - 7:15 PM
[MIS11-P04] Experimental study on the formation of Antidune cross- stratification in a short time due to tsunami intrusion
Keywords:Antidune cross-stratification, Tsunami deposits, Flume experiments
Tsunami deposits are key to estimating the scale of past tsunamis, and their distribution, grain size, and sedimentary structure have been studied. In particular, various studies have been conducted on the deposits in the Sendai Plain by the 2011 Tohoku-oki tsunami (e.g., Goto et al., 2021). Imura et al. (2024) consider stripped samples of tsunami deposits obtained in a large-scale trench survey conducted in 2013 in Iwanuma City, Miyagi Prefecture. The excavation site was about 1.2 km into the plain from the coastline, on the land side of the third beach ridge closest to the sea (Iwanuma City Board of Education, 2017). This tsunami deposit is about 20 cm thick, and a structure in which shallow dish-shaped laminae cut each other can be seen at the upper half. Imura et al. (2024) showed that a similar structure was formed by forming and stacking equilibrium antidunes in a flume experiment. In addition, a tsunami simulation using a fault model showed that the tsunami that entered this area was in the the condition of antidune formation 1-2 minutes after the tsunami intrusion. Therefore, in this study, we conducted an experiment to verify whether antidunes can form and deposit in the very short time by highly suspended supercritical flow.
A small circulating flume (4m long, 40cm high, 8cm wide) at the Department of Information Science, Osaka Institute of Technology was used for the experiment. Considering the topography of the excavation site in Iwanuma City, a sand pile (25cm long, 120cm wide) was placed in the upstream part of the flume to reproduce the condition in which a highly suspended supercritical flow running, as occurs during a tsunami intrusion. Water was directly supplied from the lower tank to the upstream side using a pump. The experiment was conducted under five different conditions with different flume gradients: Experiment 1: 1.8°, Experiment 2: 3.0°, Experiment 3: 1.5°, Experiment 4: 1.0°, and Experiment 5: 1.2°. The flow over the sand pile flows down the downstream slope of the sand pile in a supercritical flow, eroding it. Starting from the weir at the downstream end (approximately 3 cm), antidunes formed in the downstream part of the flume in three of the five conditions (Experiments 1, 4, and 5), leaving a sedimentary structure. This took approximately one minute. In Experiment 1, breaking wave was also observed in association with the formation of antidunes. The sedimentary structure observed in all three conditions was a shallow dish-shaped structure in which laminae cut each other, and laminae gently sloping upstream. The structure in which shallow dish-shaped laminae cut each other was most frequently seen in Experiment 1, where wave breaking occurred.
In this way, it was shown that antidune cross-stratification can be formed and preserved in a short time in unsteady high-veloocity flows containing a large amount of suspended sand.
References
Goto, K., Ishizawa T., Ebina Y., Imamura F., Sato S., Udo K., 2021, Ten years after the 2011 Tohoku-oki earthquake and tsunami: Geological and environmental effects and implications for disaster policy changes. Earth Science Reviews, https://doi.org/10.1016/j.earscirev.2020.103417
Imura, H., Goto, K., Yokokawa, M., and Kawamata, T., 2024, Antidune structure in tsunami deposits. Abstracts of the Annual meeting of the Sedimentological Society of Japan 2024 Kumamoto, p.3, pp.71-72.
Iwanuma City Board of Education, 2017, Report on the investigation of buried cultural properties related to the reconstruction of the Great East Japan Earthquake V. Report on the investigation of cultural properties in Iwanuma City, Miyagi Prefecture, Vol. 18.
A small circulating flume (4m long, 40cm high, 8cm wide) at the Department of Information Science, Osaka Institute of Technology was used for the experiment. Considering the topography of the excavation site in Iwanuma City, a sand pile (25cm long, 120cm wide) was placed in the upstream part of the flume to reproduce the condition in which a highly suspended supercritical flow running, as occurs during a tsunami intrusion. Water was directly supplied from the lower tank to the upstream side using a pump. The experiment was conducted under five different conditions with different flume gradients: Experiment 1: 1.8°, Experiment 2: 3.0°, Experiment 3: 1.5°, Experiment 4: 1.0°, and Experiment 5: 1.2°. The flow over the sand pile flows down the downstream slope of the sand pile in a supercritical flow, eroding it. Starting from the weir at the downstream end (approximately 3 cm), antidunes formed in the downstream part of the flume in three of the five conditions (Experiments 1, 4, and 5), leaving a sedimentary structure. This took approximately one minute. In Experiment 1, breaking wave was also observed in association with the formation of antidunes. The sedimentary structure observed in all three conditions was a shallow dish-shaped structure in which laminae cut each other, and laminae gently sloping upstream. The structure in which shallow dish-shaped laminae cut each other was most frequently seen in Experiment 1, where wave breaking occurred.
In this way, it was shown that antidune cross-stratification can be formed and preserved in a short time in unsteady high-veloocity flows containing a large amount of suspended sand.
References
Goto, K., Ishizawa T., Ebina Y., Imamura F., Sato S., Udo K., 2021, Ten years after the 2011 Tohoku-oki earthquake and tsunami: Geological and environmental effects and implications for disaster policy changes. Earth Science Reviews, https://doi.org/10.1016/j.earscirev.2020.103417
Imura, H., Goto, K., Yokokawa, M., and Kawamata, T., 2024, Antidune structure in tsunami deposits. Abstracts of the Annual meeting of the Sedimentological Society of Japan 2024 Kumamoto, p.3, pp.71-72.
Iwanuma City Board of Education, 2017, Report on the investigation of buried cultural properties related to the reconstruction of the Great East Japan Earthquake V. Report on the investigation of cultural properties in Iwanuma City, Miyagi Prefecture, Vol. 18.