A Consideration Aimed at Improving the Resiliency of Protective Structures Against Tsunami - Post-Tsunami Hazard: Reconstruction and Restoration - page 204

tsunami countermeasures. One level is a tsunami hypothesized to build
comprehensive disaster prevention countermeasures centered on evacuation of
residents. This level is set based on a survey of tsunami deposits formed over long-
term observations of crustal movement, and is a maximum class tsunami which,
although rare, causes devastating damage when it does occur. The other level is a
tsunami hypothesized to build coastal protection facilities such as breakwaters and
seawalls that prevent tsunami from inundating inland regions. This level of tsunami
occurs most frequently and although the class is low, the damage it causes is severe.
To prepare for such times, technological development capable of improving the
resiliency of protective structures under tsunami forces higher than the design loads
must continue for coastal protection to be effective.
In this paper, 'effectiveness' is clarified for resiliency of protective structures
against tsunami forces. First, the damage to breakwaters at Kamaishi port is
summarized. Next, numerical simulations are conducted under deferent states of
breakwaters to clarify the effectiveness of protection. Finally, the decreasing effect
of the disaster against varying tsunami heights is considered.

14.2 State of Damage of Breakwaters at Kamaishi Bay

14.2.1 Outline

The summary of damage of breakwaters in ports was described by Tomita et al.
( 2013 ). Breakwaters at the mouth of Kamaishi Bay consist of three parts as shown
in Fig. 14.1 , the 990 m North Breakwater, the 670 m South Breakwater, and the
300 m submerged breakwater at the mouth of the bay.
TheNorthandSouthpartsofthebreakwatersconsistofadeepregionbreakwa-
ter and a shallow region breakwater. The breakwater at the deep region is formed by
trapezoidal caissons or rectangular caissons. The crest height of all caissons is D.L.
+6.0m(T.P.+5.12m).Figure 14.2 (1) shows the typical cross section of the trap-
ezoidal caissons in the deep region, which have a rubble foundation that extends
fromadepthof−60mto−27m.Intheshallowregion,thecaissonsarerectangular,
as shown in Fig. 14.2 (2). A submerged breakwater is installed at the central open-
ing of the baymouth breakwater to close the baymouth as much as possible; the
heightisfrom10to15mandthedepthatthemouthisD.L.−19m.

14.2.2 Damage Due to the 2011 Tsunami

Figure 14.3 shows caissons which slid or inclined according to the results of a survey
bytheTohokuRegionalDevelopmentBureau.Atthenorthsidebreakwater,7ofthe
22 caissons in the deep region were washed away, 14 were inclined, and 1 was
undamaged.Ofthe22caissonsintheshallowregion11werewashedaway,5were