Environmental Engineering Reference
In-Depth Information
9.2.2
Numerical Conditions
The horizontal mesh size was set as 50 × 50 m for all models. The vertical
discretization of the 3D model were set to 2 m near the free surface and down to 5 m
below the water column, while 10 vertical layers were used in the Q3D model. The
wet-dry condition and offshore radiation boundary condition were applied in the
three models.
The target area for computation is Kamaishi Bay area. Bathymetry for both
Kamaishi Bay (southern part) and Ryoishi Bay (northern part) are provided at a
resolution of 50 m by the Cabinet Offi ce, Government of Japan. Due to the coarse
bathymetry information, land areas are described with a uniform height. The com-
putation was carried out with Dt = 0.1 s time intervals from the time of the earth-
quake until 2 h later. The astronomical tide was not included in all of the 3D, Q3D
and 2D computations. The landside structures and land use are regarded as rough-
ness in terms of Manning's n coeffi cients based on the Cabinet Offi ce dataset. The
time series of measured tsunami by the Kamaishi GPS buoy (Kawai et al.
2011
) was
used as a lateral boundary condition for the offshore side. The local inundation
heights were validated with the measured inundation and run-up heights (TTJS
2011
; Mori et al.
2011
,
2012
).
9.3
Results and Discussion
9.3.1
Numerical Results
A series of numerical computations was performed for Kamaishi Bay and Ryoishi
Bay at Iwate Prefecture together because two bays are surrounded by the two long
peninsula at the south and north borders, although they are separated by the short
peninsula at the middle (see Fig.
9.1
). Kamaishi City in Iwate Prefecture is located
in the Sanriku ria coast area. An offshore breakwater for tsunami protection is
installed at the mouth of Kamaishi Bay. The construction of offshore tsunami break-
water began in 1978. A pair of offshore breakwaters with lengths of 990 and 670 m
was fi nally completed in 2006 in a water depth of 63 m, making it the deepest
caisson breakwater in the world (Tanimoto and Goda
1991
).
It is important to examine the effectiveness of offshore breakwater for 2011 event
and estimate numerical errors dependent on the scheme. We have to note that the
water depth of Kamaishi Bay and Ryoishi Bay are 50 and 100 m, approximately,
therefore direct comparison of the two bays are physically incorrect.
Figure
9.1
shows the maximum water surface elevation for computations with
the offshore breakwater for three different methods. The amplifi cation of the tsu-
nami can be seen for several steep valleys in both bays. The infl uence of the off-
shore tsunami breakwater can be seen clearly around the breakwater in Fig.
9.1
.
Within the bay, the maximum surface elevation is reduced from about 12-15 m to
9-11 m depending on the location. This corresponds to a 20-40 % reduction of
Search WWH ::
Custom Search