Environmental Engineering Reference
In-Depth Information
the southern part of East Japan, that is, Fukushima, Ibaraki and Chiba Prefectures.
In these areas, many measurements were conducted by survey teams organized by
researchers in the University of Tokyo. The survey teams observed clear contrasts in
damage behind collapsed seawalls and survived seawalls, which appeared to provide
valuable hints for tenacious seawall structure that enhances durability against
tsunami overfl ow. This article describes tsunami surveys in the areas south to the
Fukushima Prefecture focusing on the limitation and effectiveness of seawalls.
13.2
The Survey
13.2.1
Kujuukuri and Kashima Beaches
Field surveys in Chiba and Ibaraki Prefectures were conducted on March 12, 15 and
18-19, 2011 (Shimozono et al. 2011 ). Tsunami watermark heights were measured
by a VRS-mode Real-Time Kinematic Global Positioning System (RTK-GPS) in
Ibaraki Prefecture and by an auto-level in Chiba Prefecture. The RTK-GPS presents
the local elevation data with an accuracy of a few centimeters in terms of Tokyo Peil
standard (T.P., T.P. zero corresponds to the mean water level at the Tokyo Bay,
which is the standard datum level for land topography in Japan).
Hereafter, all the measured elevation data are referred to the T.P. standard. The
measurements by the auto-level were based on local sea level, which were converted
to the standard datum T.P., by using tide charts at nearby tide gauges. The watermark
heights measured at the landward boundary of the fl ood area are specifi ed as 'run-up
heights' while those inside the fl ood area are called as 'inundation heights.'
Figure 13.2 shows topography in the study area, location of measurements and
measured height of tsunami watermarks. The topography in Chiba is characterized
by a 40 km stretch concave beach, Kujuukuri Beach, isolated by headlands at both
ends and backed on the land side by a fl at coastal plain with many small rivers. On
the other hand, the topography in Ibaraki is characterized by a relatively narrow
sandy beach like Kashima Beach, which is backed by high hills with 20-30 m
elevation with fewer rivers fl owing to the sea. Sand dunes are formed on both beaches
which develop backshore dune in parallel to the shoreline with height 7-9 m T.P.
The backshore dune is discontinuous at the mouth of a number of small rivers on
the Kujuukuri Beach while the dune develops continuously on the Kashima Beach
because of fewer rivers. Measured tsunami heights shown in Fig. 13.2 varied in a range
from 4 to 8 m T. P. with gradually decreasing trend from north to south. The tsunami
height is large at Oarai and Choshi, located at the end of concave sandy beach.
The large tsunami heights near Oarai and Choshi are due to marine bathymetry
characterized by contour lines convex to the sea that converge tsunami rays due to
refraction. In contrasts, tsunami height becomes small near the central part of the con-
cave beach owing to energy divergence due to concave bathymetry. Such convergence/
divergence of tsunami energy due to convex/concave bathymetry is also observed on
the Sendai Coast (Sato et al. 2014 ) and on the Fukushima Coast (Sato et al. 2013 ).
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