●Research Trends and Results

Analysis of damage to coastal dikes by tsunami—Damage analysis to discover the direction of future countermeasures—

SUWA Yoshio, HeadKATO Fuminori, Senior Researcher
WATANABE Kunihiro(Ph.D), Researcher
River Department, Coast Division(Key words) Tsunami, coastal protection facility, disaster

Damaged length rate obtained bytotaling total lenght of all sections

structures more strictly than the envelope curve shownin the bottom of Figure 1.
3. Differences in state of damage according towhether or not the toe of the landward slope iscoveredRegardingtherelationshipofcovering/
non-covering of the toe of landward slopes with thetotal destruction length rate, the results of totaling data
for each section by overflow depth from less than 2m
to 4m etc. reveal that within an overflow depth range
from 2 to 6m, the damaged length rate was lower
when the toe of the landward slope was covered (topof Fig 1). It is necessary to conduct experiments toclarify why there were no differences at and above an
overflow depth of 6m.

Damaged length rate by section

1. Outline
In response to the damage by the Great East Japan
Earthquake, the Central Disaster Management Council
compiled an interim report on 26th June, 2012 andmentioned the principles, “With regard to coastal
protection facilities, it is also essential that progress be
made in the promotion of technical developments for,and installment of structures that will rigorously
withstand tsunamis that are higher than those forwhich they were designed”1). We studied structures toensure the toughness of facilities against tsunami by
dividing the coastline from Aomori Prefecture to
Chiba Prefecture into about 1,400 sections according
to the facility structure, then analyzing the relationshipof the state of damage, the structure specifications(landward slope gradient, toe of landward slope
covered/not covered, etc.), and tsunami heights tohypothesize the facilities’ damage mechanisms. As
one example, directions in preventing scouring bycovering the toes of landward slopes of coastal dikes
with three surface-armoring are showed here.
2. Relationship of tsunami overflow depth withstate of damage
Figure 1 shows the relationship of tsunami overflow
depth with damaged length rate (percentage of total
facility length which is damaged) in 118 sections ofcoastline dikes with three surface–armoring. There
were cases where 100% of the dikes were totally
destroyed at an overflow depth of about 3m, while inother cases, even at overflow depth over 10m, thedamage length rate was 0%, revealing that even under
the same tsunami height, the state of damage differed
greatly according to section (bottom part of Fig. 1).
This could be caused by diverse on-site conditions or
facility structures, and it means that a structure which
has been effective on one coastline might not beeffective on another coastline. It will be necessary tocollect as many cases as possible, and clarify thevariation in the data, to carefully study ways ofhandling the data according purpose of use. To
scientifically discuss differences in the state ofdamage, the average values which also consider
differences in the length of each coastline (top of Fig.
1) are also important, but in order to protect human
lives at all cost, it is necessary to assess safety of

1.0

Values in the figure are total
length of the object facility(km)Blue: no covering on the toeof the landward slope
Red: covering on the toe ofthe landward slope

0.1
0.8
0.6
17
0.8

0.2

2.4

0.0
0.0
1.0
1.0

4.0

1.8

0.4

Completely destroyed（nocovering）
Completely destroyed（covering）

4.0

Half or more destroyed（nocovering）
Half or more destroyed（covering）

1.4
3.0 3.1
1.6 1.0

Solid lines in the figure =completely destroyed,
broken lines = envelope
curve of half or more

0.8
0.6

Completely destroyed（nocovering）

0.4

Completely destroyed（covering）
Half or more destroyed（nocovering）

0.2Half or more destroyed（covering）

0.00246810121416

Overflow depth (m)Figure 1. Differences in State of Damage of Coastal
Dikes with Three Armored Surfaces With/Without
Covering on Toe of the landward Slope[Reference]
1) Report of the Committee for Technical
Investigation on Countermeasures for Earthquakes
and Tsunamis Based on the Lessons Learned fromthe “2011 off the Pacific coast of Tohoku
Earthquake"
http://www.bousai.go.jp/jishin/chubou/higashiniho
n/Report.pdf