Environmental Engineering Reference
In-Depth Information
Fig. 13.3
Tsunami runup observed on backshore dune, Kashima Beach, Ibaraki Prefecture (March
12, 2011); watermark measured at 7.0 m T. P., 150 m from the shoreline (UTMS-0174 in the TTSG
database, location marked in Fig.
13.2
)
owing to the presence of seawalls. Figure
13.4
shows a typical example in Hokota,
where a wooden house located behind the seawall was inundated to the level of
6.0 m T.P. but remained unbroken. Seawalls appeared to decrease the volume and
the fl ow intensity of overfl owed tsunami in case the overfl ow depth of tsunami was
of the order of 1-2 m above the top of the seawall.
The tsunami fl ooding reduction by sand dune was also observed on Kujuukuri
Beach in Chiba Prefecture. Figure
13.5
shows a typical example of watermark mea-
sured on the top of an artifi cial sand dune with top height 6.2 m T.P., constructed in
parallel to the shore on the sea side of coastal pine trees. Wood fences around the
pine trees were partially destroyed by the tsunami. The tsunami overfl owed the
4.0 m T.P. seawall on the shore as well as some sections of the artifi cial sand dune.
Since the erosion of natural/artifi cial sand dunes was found insignifi cant, along-
shore sand dunes appeared to be effective in protecting the areas on the landside.
Signifi cant damage was observed however near the river mouth where sand dune
was discontinuous due to the river fl ow. The most signifi cant inundation was
observed near the Kido River mouth (location marked in Fig.
13.2
), where tsunami
overfl ow from the sea was prevented by high sand dunes but tsunami intruded into
the river and destroyed river banks, resulting in fl ooding over a large area. The larg-
est inundation depth around the Kido River mouth was measured at 1.2 m (UTMS-
0155 in the TTSG database).
The tsunami fl ooding through the river was minimized in case a water gate was
installed at the river mouth. Figure
13.6
shows a water gate at the Shimbori River
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