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 125

9.3.2 Damage and Recovery Relations Based on Tax Revenue

Otsuchi village in Iwate Prefecture is located just north of Kamaishi city in the
Sanriku ria coast area. The water depth in the middle of Otsuchi Bay is similar to
that of Kamaishi, so that the previous Showa Sanriku tsunami in 1933 and expected
tsunami heights were similar for the two locations. For example, the measured inun-
dation heights from the Showa Sanriku tsunami at Otsuchi Bay and Kamaishi Bay
were 5.4 m and 6.0 m, respectively. However, the offshore tsunami breakwater was
constructed at Kamaishi Bay after that thus the difference of damage can be mainly
regarded offshore breakwater effects. A 6.4 m high onshore breakwater protected
Otsuchi village. An offshore breakwater protected Kamaishi city.
These two locations show large differences as shown in Fig. 9.5 . Two pictures of
Kamaishi port and Otsuchi port were taken at April 2011. The damage of Kamaishi
port is signifi cantly smaller than Otsuchi port, qualitatively. Collapse of small struc-
tures can be seen but larger buildings were relatively undamaged on the Kamaishi
waterfront. On the other hand, the most of structures and onshore breakwaters near
the port in Otsuchi were totally destroyed as shown in Fig. 9.5b . The fatality ratios
of Otsuchi area and Kamaishi area are 8.12 and 2.63 %, respectively. Although the
damage to Kamaishi port was severe, it was not at the level of destruction observed
at Otsuchi, reinforcing the conclusion that the offshore breakwater signifi cantly
reduced the extent of damage in this area.
Based on analysis of numerical calculations, it can be seen that the infl uence of
offshore tsunami breakwaters signifi cantly reduced the tsunami impact on onshore
damage, in comparison with other similar areas such as Otsuchi. Not considering
the cost of construction, effectiveness of tsunami mitigation through the use of
breakwaters was verifi ed for the fi rst time by the experience of the Tohoku
Earthquake tsunami. The degree of damage to the residential area strongly infl uence
on the recovery of the devastated area. Table 9.2 and Fig. 9.6 show the time history
of local tax at Kamaishi city and Otsuchi village based on public release, respec-
tively. Kamaishi city includes the southern part of Otsuchi bay, Ryoishi Bay and

Fig. 9.5 Difference of damage at two different bays (April, 2011); Kamaishi port ( left ; a ) and
Otsuchi port ( right , b )