A Consideration Aimed at Improving the Resiliency of Protective Structures Against Tsunami - Post-Tsunami Hazard: Reconstruction and Restoration - page 208

Table 14.1 Calculation conditions about breakwaters

Openingratioatthemouthof
bay with still water level (%) Case Statusofbreakwaters

1 Bottom topography (no breakwaters) 100.0

2 Breakwaters of complete shape 6.5

3 Perfectly closed mouth of the bay 0.0

4 Breakwaters with all damage due to the 2011

tsunamis

36.0

5 Breakwaters with damage to submerged breakwaters 11.9

6 Breakwaters with damage in the shallow region and

the south part

18.0

7 Breakwaters with damage to submerged breakwaters

and to breakwaters in the shallow region and south
part

23.3

away.So,inCase6,thebreakwatersintheshallowregionandsouthsidearegone.
All damage except for the north side breakwater in the deep region is included in
Case7.ThesummaryofthestateofbreakwatersisshowninTable 14.1 , where the
number in the right column is the opening rate under each condition when an opening
rate without breakwaters is assumed to be 100 %. The cross section is shown in
Fig. 14.7 . As the tsunami wave source, data of the Central Disaster Prevention
Council (2011) were used.

14.3.2 Results of Decreasing Rate

Figure 14.8 shows that the inundation height in each case. Figure 14.9 shows that
the relation between the opening ratio and the tsunami decreasing rate, which is
calculated by the following equation:

1 − HH

i

/

1

Where, H 1 is the inundated tsunami depth calculated with case 1, which means
no breakwaters, and Hi is that of each case. The black dot shows the relation
between the decreasing rate calculated using the watermark in the East Japan
Great Earthquake tsunami and the opening rate calculated using the remain
breakwaters.
This result revealed that the effect of breakwaters would be almost zero if the
opening ratio were 40 %. But the decreasing rate in the case of the 2011 tsunami is
around50%,whichisalmostthesameresultasthemiddlepointofcases5and6.
This indicates that the submerged breakwaters were washed away before the
maximum tsunami arrived, so the protective effect was almost zero by the submerged