Seawall Performance Along Southern Coast of East Japan Impacted by the 2011 Tohoku Tsunami; A Note for the Reconstruction Process - Post-Tsunami Hazard: Reconstruction and Restoration - page 199

Fig. 13.16 Partially broken seawall and coastal pine trees in Suga District. The photograph was
taken at the seawall on the line 'S1' marked in Fig. 13.8 (April 1, 2011). The watermark in the pine
trees is the most seaward measurement plotted in Fig. 13.15b
Along the line N2 in Ooshima District, the local seawall has a height of 6.2 m
and the nearby ground elevation is 2.6 m. Moving to the landward, there is no sig-
nifi cant change on the ground elevation with a certain decrease by 1 m just after the
seawall. The inundation height is around 2.6 m for all measured points (the inunda-
tion depth is approximately 0.6 m). According to the interview to the local residents,
although the overfl owed tsunami inundated the residential area, the tsunami inten-
sity/velocity was signifi cantly mitigated/reduced because of the protection of the
high seawall. Therefore, the damage to the local houses is fairly mild. Increase of
the water level in this district is assumed to be inch by inch and spatially uniform.
However, the inundated area in Ooshima District is relatively large, which covers
more than 1 km landward of the seawall. This is due to the low land elevation and
due to the additional fl ood water overfl owed from the river bank. The inundation
and the resultant damage in Iwama and Ooshima Districts demonstrates a sharp
contrast owing to the difference in seawall height and inland topography.
Figure 13.15b illustrates the cross-shore distribution of the ground elevation and
tsunami inundation height at the survey line S1 in Suga District, which crosses the
coastal pine trees as displayed in Fig. 13.16 . The local seawall has a height of 5.5 m
whereas the nearby ground elevation is 4.2 m (the relative seawall height is 1.3 m).
The inland ground elevation decreases to 1.8 m at 260 m from the seawall. The
tsunami inundation height also drops signifi cantly in the pine trees, i.e . , from 6.8 m
near the seawall to 3 m at the 230 m location. The function of coastal forest as a