Beach profile change at Hasaki, Japan, caused by 5-m-high tsunami due to the 2011 off the Pacific coast of Tohoku Earthquake

• Tsunami-caused beach profile change was mostly induced by first two waves.
• Ratios of profile change to wave energy were the same for tsunami and wind wave.
• Mechanisms for profile change were different for tsunami and wind wave.

The beach profile change caused by the 5-m-high tsunami striking the sandy Hasaki coast of Japan in 2011 was investigated using numerical simulation results and a detailed field data set including the beach profiles measured just 6 h before and three days after the tsunami attack. The magnitude of the tsunami-induced profile change had roughly the same relation to the wave energy flux as did the profile changes caused by wind-generated waves at this location. However, the mechanisms underlying the morphological changes were different. The beach profile changes due to wind waves were induced by smaller bottom stresses with longer duration, whereas the profile change due to the tsunami was induced by larger bottom stresses with shorter duration. Simulation results showed that the tsunami-induced beach profile change, including the offshore bar erosion, was largely caused by suspended sediments transported by the uprush flows of the first two tsunami waves. Compared with the amount of beach profile change at Banda Aceh due to the 2004 Indian Ocean tsunami, that at Hasaki due to the 2011 tsunami was small. The main reason for this is likely the difference in tsunami height: 5 m at Hasaki and 10 m at Banda Aceh. The second cause may be the topography of the land.

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