Effects of the Offshore Barrier Against the 2011 Off the Pacifi c Coast of Tohoku Earthquake Tsunami and Lessons Learned - Post-Tsunami Hazard: Reconstruction and Restoration - page 127

9.4 Conclusion

The 2011 Tohoku Earthquake Tsunami was the fi rst case where modern,
well-developed tsunami countermeasures were put to the test for such an extreme
event. One of the most important issues in natural science, engineering, and social
science is to understand the relationships among tsunami forces, local damage, and
the community resilience. For future improvement of tsunami disaster countermea-
sures much can be learned from this catastrophic event.
The damage to coastal structures, ports, houses, buildings, bridges and other
infrastructure was strongly dependent on location and protection methods. Here we
selected one typical area in the ria coastal region of Sanriku for study, Kamaishi,
where an offshore tsunami breakwater was installed as an expensive hardware pro-
tection. The offshore tsunami breakwaters, which were partially destroyed, were
still effective in mitigating the level of destruction in the Kamaishi port area. We
quantitatively discussed the estimated error of the effectiveness of offshore break-
water protection by three different numerical methods, 2D, Q3D and 3D models.
The validation results of maximum inundation height on the landside against survey
results gave 20 % error of impact assessment of offshore breakwater depends on the
numerical method. It indicates that further investigation of numerical modeling on
the landside behavior of tsunami is necessary for understanding tsunami dissipation
onshore. Furthermore, our analyses of numerical results indicate that such structural
protection may have resulted in lower overall inundation heights and could avoid
catastrophic destruction of the city. The local tax revenue indicates more than 30 %
difference of economic impact due to offshore breakwater and its effect continues a
few years after disaster.
From important lessons learned from the 2011 Tohoku tsunami, we have the
opportunity to further vital preparation against tsunamis in future.

Acknowledgements We gratefully acknowledge efforts on the survey results of the 2011 Tohoku
Earthquake Tsunami Joint Survey (TTJS) group. This study is dedicated to all who have been
affected by the earthquake in Japan on March 11, 2011.

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