Volume 2, Issue 3 September 2017

Special Contribution

2011 Great East Japan Earthquake Tsunami and Future
Tsunami Disaster Mitigation
Dr. Masahiko IsobePresident, Kochi University of Technology
E-mail: isobe.masahiko@kochi-tech.ac.jp
The Great East Japan Earthquake Tsunami on March 11, 2011, caused unprecedented damage in northeast Japan. In thefollowing, the characteristics and damage of the tsunami are described. It has been proved that coastal structures canreduce inundation depth and resulting damage even though the structures themselves are partially damaged. A newpolicy that has been adopted for recovery of the damaged area and for preparation for future tsunamis, especially Nankai
Trough earthquake tsunami, is introduced. In the policy, development of resilient coastal structures plays a key role.
Keywords: Great East Japan Earthquake, Nankai Trough, tsunami, two-level tsunami disaster mitigation, resilient coastal structure

1. Tsunami Generated by Great East Japan Earthquake
Japan was attacked by the Great East earthquake on March 11,2011 [1]. In particular, the tsunami generated by theearthquake caused devastating damage in the coastal area ofnortheast Japan. Figure 1 compares the maximum tsunami
run-up heights in each city, town or village among the past
three major tsunamis in the region. Red, blue and grey bars
correspond to the Great East Japan earthquake tsunami, Meiji
Sanriku earthquake tsunami in 1896 and Showa Sanriku
earthquake tsunami in 1933, respectively. The heights of theGreat East Japan earthquake tsunami are far higher than theother two tsunamis. Figure 2 compares the city of Rikuzentakada, one of the most seriously damaged areas, before andafter the tsunami [2], [3]. The city, which had 23,000
populations, was totally inundated and 1,800 people werekilled or missing. The total number of deaths and missing inJapan is 22,118 as of March 1, 2017.
Figure 1 Run-up height of the Great East Japan earthquake tsunami
in comparison with Meiji and Showa Sanriku tsunami

Figure 2 Damage due to tsunami (Rikuzen-takada City, Iwate Prefecture)4

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2. Damage and Effectiveness of Coastal Structures
Coastal structures were constructed as the first line ofcoastal defense. But, many of them were damaged by thetsunami.
Figure 3 shows the process of breaching of coastal dikes.
In each photo, the right side is the bay side, and the left
side the land side. When the tsunami came from the bay,
compressive force act on the sea-side. But, the seaward
surface withstood the pressure. However, when thetsunami overflowed beyond the dike, negative pressure
acts on the landward side due to centrifugal force and flow
separation. The negative pressure peeled off the surface
concrete. Then the sediment was exposed to the flow andgradually flew out. Finally, the seaward concrete lost thesupport from the sediment and broken.

Figure 3 Process of breaching of armored sloping dikes
This lead the dike to total breaching. Actually, experimental
result shows the surface pressure on the seaward concrete
drops significantly as shown in Fig. 4 [4]. This will trigger thebreaching of coastal dikes. Therefore, the design standard
has been modified to increase the thickness of concrete
cover. Many other mechanisms for damage of coastal
structures have been found through investigation after thetsunami.

Figure 4 Possible reason for peeling of seaside concrete

In spite of the serious damage on the coastal structures, theireffectiveness was also confirmed. In Kamaishi bay, offshore
breakwaters were constructed for tsunami protection, butwere seriously damaged by the tsunami. However, since 80%of the projected area of the structure remained, the arrival
time was delayed by 6 minutes and the inundation depth wasreduced by about half.

In Sendai Plain, the tsunami overflew and damage coastal dikes, but the inundation depth and area behind the dikes are similar
to the result of computer simulation on assuming all dikes are sound rather than that without dikes. This means the overflow
rate can be reduced significantly if sufficient projected area of structures is maintained.
3. New Policy for Tsunami Disaster Reduction
In recovering and reconstructing the damaged area, the first step is to establish a policy to construct the coastal structures that are
the first line of the tsunami defense. Before the Great East Japan earthquake tsunami, the maximum recorded hazard level had
been generally used as the design external force on the coastal structures. However, this is neither economical nor resilient to
prepare for extremely rare and extraordinary hazard. In addition, high coastal structures disconnect the continuity of the land andsea, which results in serious adverse impact on the daily human life as well as the coastal ecology. A series of committees organizedby Japanese Government established a policy of two-level tsunami disaster mitigation. For tsunamis of the maximum level, whichis termed as level 2 tsunami and has a frequency of once in the order of one thousand years, we save all human lives by all means,mainly by evacuation. The Great East Japan earthquake tsunami is classified at this level.
For a lower tsunami with relatively high frequency, we design and construct coastal structures to protect land from inundation toprotect assets as well as human lives. This is called level 1 tsunami and has the frequency of once in about one hundred years, i.e.,
once in a lifetime. This means that structures may experience tsunamis over the design level.
Therefore, the design standard requires coastal structures be resilient to avoid total corruption even by tsunamis of over-designlevel. In the area damaged by Great East Japan earthquake tsunami, reconstruction of coastal structures has almost completed
according to the policy.
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Figure 5 summarizes this tsunami disaster mitigation
policy. Even when a tsunami over the design levelattacks, coastal structures can defend until the highest
part of the tsunami arrives. During this period, people
are to evacuate to the safest places among high groundand tsunami evacuation buildings, towers, and shelters.
In addition, even if the tsunami overflows, the coastal
structures can decrease flow rate and inundation depth,
and as a result reduce damage.

4. Preparation for Future Nankai TroughEarthquake Tsunami
The policy is now being applied not only to the damaged
areas but also to all other areas in Japan. Kochi
prefecture is being threatened by a giant tsunami
generated by the Nankai Trough earthquake. TheFigure 5 Tsunami disaster mitigation policy
highest run-up height is simulated as high as 34m.
However, in central and eastern coast, the crown heights of the coastal dikes and seawalls are enough to protect from level 1
tsunamis because they have already been constructed for protection against storm surges which are very severe due to frequentattack of strong typhoons. But, to meet the two-level tsunami mitigation policy, they must be improved to avoid subsidence due
to liquefaction and to increase resilience. Reinforcement by double sheet piles is suitable for this purpose. Figure 6 shows thesketch of improvement of the dike [5].
When a tsunami of the maximum level attacks,improved structures can save enough time forevacuation. During the time, people can climb
up high lands or tall buildings. In the area where
they are not available, tsunami evacuation
towers as shown in Figure 7 have been
constructed. Thus, the two-level tsunami
disaster mitigation policy is being implemented
in Kochi.

5. Conclusion
The first priority in tsunami disaster mitigation isto save all human lives. In parallel to this,economic damage should be minimized. In thisrespect, the two-level tsunami disastermitigation policy is effective and flexible toimplement. If regional economy does not allow
to construct high quality coastal structures, level1 tsunami can be modified to be lower so thatthe policy becomes feasible to implement.
Finally, development of resilient coastalstructures is a key to establish successful disastermitigation system.

Figure 6 Reinforcement of coastal dikes by piles

Figure 7 Tsunami evacuation tower6

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REFERENCES[1]

Sato, S. ed., “Special Anniversary Issue on the 2011 Tohoku Earthquake Tsunami,” Coastal Engineering Journal, Vol. 54, No.
1 (12 papers), 2012.

[2]

Joint Survey Group for Great East Japan earthquake tsunami, http://www.coastal.jp/ttjt/, April 30, 2011.

[3]

Watanabe, H., “List of Disastrous Tsunamis in Japan,”, Univ. Tokyo Press, 238p. 1998. (in Japanese)[4]

Kotake, Y. and M. Isobe, “Experimental study on pressure distribution along landward slope of coastal dike due to tsunami
overflow,” J. JSCE, Ser. B2, Vol. 68, pp. 891-895, 2012. (in Japanese)[5]by courtesy of Seacoast Office, Water and Disaster Management Bureau, Ministry of Land, Infrastructure, Transport andTourism.

◆A brief CV of Dr. Masahiko Isobe
Masahiko Isobe obtained doctoral degree in 1981 in the field of coastal engineering at the Universityof Tokyo. He worked at Yokohama National University from 1981 to 1987 as Assistant Professor andAssociate Professor. He also worked at the University of Tokyo from 1987 to 2013 as AssociateProfessor, Professor and Vice President. Then he moved to Kochi University of Technology as VicePresident, and has been serving as President since 2015. Based on his research career, he took aleadership role at national level in recovering from the damage due to the Great East Japan EarthquakeTsunami.

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