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 187

Fig. 13.2 Topography and bathymetry of the survey area ( a ), location of measurements ( b ) and
measured watermark heights ( c ) in Chiba and Ibaraki Prefectures. The coast is partly protected by
piecewise seawalls with height 4-5 m T.P.

13.2.1.1 Tsunami Protection by Seawalls and Sand Dunes

Some coastal stretches in the study area were intermittently protected by piecewise
height calculation of design storm waves because the height of the historical tsu-
nami was smaller than the runup of storm waves. Since the height of the 2011
Tohoku Tsunami was mostly larger than the height of seawalls, tsunami overfl owed
the seawalls. However, no large-scale inundation was developed except in the area
near river mouth because the top of the backshore dune was higher than the tsunami.
Figure 13.3 shows a typical example of tsunami runup line traced on the seaside
slope of backshore dune. Distribution of debris and clear boundary between dry and
wet sand surface presents tsunami runup line observed on March 12, 2011, one day
after the tsunami. It is noted that the measurement at this location is registered as
UTMS-0174 in the TTSG database. All the photographs taken in the survey are
archived and available on the Internet in Tsunami Joint Survey Group Photo Archive
2013 . In the study area in Ibaraki Prefecture, large-scale inundation was developed
only around the harbour area at Oarai.
Signifi cant damage concentrated near harbours was also observed in Chiba
Prefecture, in which Choshi and Iioka were among severely damaged communities.
In other areas, tsunami damage was limited to the narrow coastal zone close to the
shore. Even in the narrow affected area, the damage appeared to be minimized