Lessons Learned - Tohoku, Japan Earthquake and Tsunami of 2011 - page 64

Chapter4
Lessons Learned
The Great East Japan tsunami was an event of enormous consequences. The
inundation height exceeded the design levels of many coastal barriers, and the inland
inundation exceeded historic extents for communities along hundreds of kilometers of
coast. Despite the vast damage and enormous loss of life, coastal barriers performed
with various degrees of success. One example of successful performance was from a
15-meter high tsunami wall built to protect the community of Otanabe, near the Fudai
River. The wall was 2.5 meters above the runup elevation and the structure protected
the community from inundation. The only damage experienced by the community
was to the fishing port seaward of the barrier wall.
Asecondtypeofsuccessfulperformancewasthatmanystructures,despite
significantdamagefromthetsunami,wereabletoreducedamagestotheinland
community. The breakwaters at Kamaishi, even though the tsunami moved caissons
off their foundations, reduced the inundation height at the community from the
modeled unprotected height of 13.8 meters to the observed height of about 8 meters
(Takahashi et al. 2011).
A third type of successful performance was that many coastal protection structures
survived the tsunami forces, despite having been overtopped. Examples include the
Arahama shore dike (Figure 19), the Naruse River Entrance Jetties (Figure 20), the
Shizagawa Port Bridge (Figure 22) and much of the armoring at Oya Station (Figure
69). While these structures often failed to protect the inland areas from flooding,
scour, high velocity flows and other effects from the tsunami, they remained stable
and can be part of the future community planning and reconstruction efforts.
Many structures, unfortunately, did not perform well. The enormous range of coastal
destruction demonstrates that the shore protection structures, even those that
remained standing, did not provide effective protection of inland communities. Just as
some coastal barriers succeeded, many others failed. Many structures collapsed or
slid off their foundations. Segments of wall detached from main structures. Armor
units broke into pieces and revetments scattered. Lessons come from the successes
and failures. The following sections detail some of the key lessons learned by the
Coastal Structures Team.
4.1 Contingency Planning
The overarching observation from this event was the importance of considering the
unexpected, the “what ifs?” The tsunami exceeded the design conditions for many
coastal structures. The subsequent area-wide loss of power left most mechanical
barriers wherever they stood at the time of power failure, so barriers intended to keep
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