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 200

buffer zone against the tsunami attack, which increases the fl ow resistance, is
obviously demonstrated in this fi gure. Further beyond, the ground level keeps
around 2 m and the corresponding inundation depth is only 0.5 m, leading to insig-
nifi cant damage in this area. The most inland run-up location was found at location
820 m from the seawall.
Figure 13.15c demonstrates the cross-shore distribution of ground elevation
and tsunami inundation height at survey lines S2 and S3 in Sekita District. It is
clear that the local seawall height is 6 m at these two locations with a relative
seawall height being 1.2 m. The inland ground elevation is maximum at the sea-
wall and gradually decreases to 2.0 m with an approximate slope of 1/100. The
inundation depth near the seawall is about 1 m, which decreases to 0.4 m, 120 m
landward along the line S2, and to 0.3 m, 50 m landward along the line S3.
Compared with the region north of the Samegawa River, the damage in Sekita
District was minor, which appeared to be due to slightly smaller tsunami height in
the southern region and due to the high and intact seawalls. Relatively high eleva-
tion of land topography as confi rmed in Fig. 13.15c is considered to be another
reason of the minor damage.
The effi ciency of seawalls in decreasing tsunami overfl ow volume is also
observed in Minami-Soma, Fukushima Prefecture, where tsunami height is about
4-6 m larger than that of Nakoso. The inland inundation level in a coastal section
with surviving seawalls was signifi cantly smaller than that in the area where more
than 30 % of the seawalls were completely broken (Sato et al. 2014 ). This sug-
gests that unless they were completely destroyed, the seawalls must have reduced
somewhat the inland inundation level by decreasing the tsunami overfl ow
volume.

13.3 Reconstruction Plan Based on Two

Tsunami Hazard Levels

Based on the lessons learned in the Tohoku Tsunami, the Japanese Government
decided to introduce two-level tsunami hazards to deal with tsunami disaster mitiga-
tion. The fi rst-level tsunami hazard (Level-1 tsunami) is defi ned as a frequent tsu-
nami with the return period of a hundred years. Seawalls are designed for a Level-1
tsunami, which dictates crown elevations along with other design criteria. On the
other hand, the second-level tsunami hazard (Level-2 tsunami) represents the prob-
able maximum tsunami and is used to establish evacuation strategies and plans. The
return period of the Level-2 tsunami is considered at a thousand years.
Figure 13.17 illustrates the reconstruction plan of the Iwama District compiled
by Iwaki City, Fukushima Prefecture, in February 2013 (Iwaki City 2013 ). It is
composed of the construction of a 7.2 m T.P. seawall, alongshore green belt and
relocation of a part of communities to higher ground. The height of the new seawall
was determined by the runup height of the design storm waves, considering the
signifi cant erosion in nearshore bathymetry as well as the co-seismic land subsidence