Numerical investigation of severe dune erosion induced by a tsunami: A case study of the 2011 Tohoku-oki tsunami on Osuka coast, Hachinohe, Japan

Tsunami waves cause severe erosion and deposition in coastal areas. Although tsunami deposits are well surveyed because they reflect tsunami history, tsunami erosion is not examined as intensively. Nevertheless, tsunami erosion can cause remarkable topographic changes and can destroy artificial structures. Understanding tsunami erosion characteristics is important to mitigate tsunami erosion damage. Moreover, understanding tsunami erosion processes of modern tsunamis is necessary to reproduce phenomena caused by paleotsunamis, while erosion traces formed by tsunami waves are useful as paleotsunami evidence. This case study examined a coastal area of Tohoku, Japan affected by the 2011 Tohoku-oki tsunami. A numerical model reproduced the tsunami flow and sediment transportation to elucidate tsunami erosional landform formation processes. Findings indicated that computed positions of the tsunami erosional landforms and the volumes of eroded and deposited sediments are consistent with field observations. Based on the model results, we assessed the timing of erosion occurrence, the flow characteristics causing erosion, and factors affecting the locations at which erosion occurred. The results demonstrate that return flows, rather than run-up flows, generated most of the tsunami erosion. During the return flow, a supercritical flow locally but severely eroded the beach. Tsunami erosion location was pre-tsunami topography-dependent. The return flow was concentrated in lower places, causing localized, severe erosion. Our findings represent an important achievement for elucidating paleotsunamis that caused severe erosion and for preventing future tsunami disasters. • Topographic coastal erosion by the 2011 Tohoku-oki tsunami was modeled numerically. • Findings show return flow, not run-up flow, as the main cause of erosion. • During return flow, supercritical flow contributed to severe foreshore erosion. • Erosion locations depend on pre-tsunami microtopography and return flow concentration.