Modeling field observations of the 2011 Great East Japan tsunami: Efficacy of artificial and natural structures on tsunami mitigation

Abstract Damage to embankments and forests along the coast of the northeast Japan by the 2011 Great East Japan tsunami was observed at twenty-five locations covering a length of about 340 km. A modified two-dimensional depth-integrated shallow water model was applied to analyze the tsunami mitigation by the artificial and natural structures at two coastal sites: Misawa, a site with vegetated dune, and Hachinohe, a site with seawall and coastal vegetation. In addition to configuration of the real sites, a few hypothetical cases were tested with alternative arrangements of these structures to identify their contribution towards tsunami mitigation. The numerical results revealed that the vegetated dune in Misawa was the primary reason for the mitigation of the tsunami and that the shape of access roads (straight or crooked) through the vegetation to the coast had a great effect on the variation of flow velocity increment compared to in the absence of vegetation. An asymmetry in the damage to the vegetation was observed at Hachinohe, where the vegetation behind the seawall was undamaged and the vegetation that was not shielded from the seawall was damaged. The simulation results showed that diversion of the tsunami flow by the seawall supported creation of a higher moment (4–2 kNm) of the vegetation compared to the places behind the seawall (less than 0.2 kNm). The actual flow velocity and Froude number of the wave front at the Sendai Plain were calculated from video footage. The average flow velocity 1 km inland was 6.2 m/s, and the corresponding Froude number was ca. 1.14–1.4. The values are comparable with the maximum Froude number at the time the tsunami hit the shore at Misawa and Hachinohe, which was determined by the modeling to be ca. 1.5.

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