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Iida, T.; Kure, S.; Udo, K.; Mano, A., and Tanaka, H., 2016. Prediction of the 2011 Tohoku Tsunami scouring near structures. In: Vila-Concejo, A., Bruce, E., Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 872-876. Coconut Creek (Florida), ISSN 0749-0208. The 2011 Tohoku Tsunami left innumerable sites with large scale scouring near failed coastal structures. This was a major cause of failure, and the reconstruction of these structures in a robust way requires determination of the mechanisms and prediction of the scouring. Two mechanisms are known for tsunami scouring: erosion by the bottom shear stress and liquefaction by the pore pressure, however, these mechanisms have not been compared yet in their effects. This study aims to quantify these mechanisms and predict the scouring by the 2011 Tohoku tsunami. For this approach, we developed a numerical model by the quasi-3D finite volume method by meshing horizontally in water and vertically in ground. After validating our model, we have applied it to the breach point of a coastal levee hit by the tsunami. We can conclude that erosion by the bottom shear stress has the major role, however, the fluidisation still has innegligible effect in 28% to the maximum erosion depth. [ABSTRACT FROM AUTHOR]

The 2011 tsunami induced sandy coast breaching in Akaiko area, Miyagi Prefecture, Japan. It also caused severe erosion of sand barrier-lagoon system adjacent to a river mouth on Sandai Coast in which Akaiko is located. These damages resulted in forming concave shoreline in these areas after the tsunami. The morphological recovery of this area is presented through analysis of aerial photography. In addition a new analytical solution of one-line model, which describes the evolution of shoreline of concave landform bounded by rigid boundaries, has been introduced. The recovery time of morphology is evaluated from the analytical solution of one-line model. Theoretical results indicate that coastal structures at both ends have major impact on the shoreline evolution. Moreover, the recovery time of the concave portion is highly depended on the total length of sandy coast on both sides. Good agreement between measured shoreline positions and simulated results is also obtained. [ABSTRACT FROM AUTHOR]

The 2011 Great East Japan earthquake and tsunami caused significant damage along the coastal region in Miyagi Prefecture. In the Natori River of Miyagi Prefecture, the recovery process of the estuarine morphology has been observed since the tsunami arrived. In this study, detailed analyses of collected aerial photographs and beach topography in front of the river mouth were conducted to evaluate whether the river mouth has reached a new equilibrium. The shoreline analysis suggests that the river mouth has obtained an equilibrium state after 2014 in which the sediment volume can be preserved within the littoral system. In addition, the convergence process to the stable topography was detected by the first spatial and temporal eigenfunctions obtained from the empirical orthogonal function (EOF) analysis of the beach profiles. From the results, it can be concluded that the river mouth has obtained a new equilibrium of morphology after the tsunami. As the recurrence of the tsunami can be expected in the next several centuries, the findings of this study can be useful for long-term coastal and riverine management against future disasters in this river mouth, and other coastal regions that are prone to large-scale disasters in the near future. [ABSTRACT FROM AUTHOR]

The Nobiru Coast is situated on the southwest of the Ishinomaki Bay. The 2011 Great East Japan Tsunami severely devastated the Nobiru Coast and the adjacent Naruse River mouth. In this study, an investigation was conducted based on the available historic maps and images combined with in situ surveys that revealed the century-to-decade morphology change and sediment budget alteration in the Nobiru Coast. During the past two centuries, the longshore transport on the northeast coast and sediment supply from the Naruse River were the principal sediment supply onto the Nobiru Coast and the estimated annual net sediment input into the coast was 87,000 m3/y. Until several decades ago, the construction of the Ishinomaki Port and the erosion preventing constructions (breakwaters, headlands) along the Ohmagari Coast on the northeast areas caused a dramatic reduction of longshore transport to the Nobiru Coast. Hence, the net sediment input fell to 46,000 m3/y. After the tsunami, the sediment input was further reduced to 29,000 m3/y and this loss was closely related to the intruded sediment into the Naruse River. The outcomes of this study are highly valuable for the government authorities to manage the long-term coastal and riverine morphological changes after the 2011 tsunami. [ABSTRACT FROM AUTHOR]

On 11 March 2011, the Great East Japan Earthquake generated huge tsunami waves. Then, tsunami propagation occurred in rivers, resulting in further expansion of the flooded area. Full data sets of tsunami characteristics such as tsunami inland and river propagation distances as well as the river geometries from all rivers within the Tohoku District were compiled and analyzed. It was found that tsunami propagation distance in rivers was about 1.2 to 4.5 times that of the inland area. There was a good correlation between propagation distance in rivers and the river bed slope. Furthermore, new empirical formulae for calculating the damping coefficient of tsunami wave height and tsunami intrusion length were successfully derived based on the current comprehensive data sets covering a wide range of river geometry and bottom slope. Moreover, tsunami-induced flow discharge was evaluated by using measured water level variation. [ABSTRACT FROM AUTHOR]