Characteristics of beach erosion due to the 2011 Tohoku Earthquake and Tsunami and its recovery across the entire inundated area

The 2011 Tohoku earthquake and tsunami caused the disappearance of beaches over a wide area because of inundation accompanying the land subsidence caused by the earthquake and because of tsunami erosion. This study aims to investigate the general characteristics of the 2011 earthquake and tsunami and identify the factors that caused beach damage. The study focuses on 74 beaches from Aomori Prefecture to Chiba Prefecture, including those where the damage was insignificant. Satellite and aerial image analyses demonstrated that the main factor in beach disappearance was not inundation following land subsidence caused by the earthquake but erosion because of sediment transport by the tsunami. There were many cases where a significant increase in the amount of beach retreat occurred when the tsunami overflowed a seawall, leading to the collapse of the seawall. It was also quantitatively confirmed that beach erosion could have been significantly reduced by the survival of the seawall. Three years after the 2011 tsunami and earthquake, the recovery rates differed between beaches on the Sanriku coast and those in other areas. The rate was 25% for many beaches on the Sanriku coast of the Rias terrain and 75% throughout the south of the Sendai plain.