Proceedings of Coastal Engineering, JSCE, Vol.3, 2012, November

COMPARISON OF CASUALTY AND BUILDINGDAMAGE BETWEEN SANRIKU RIA COAST AND
SENDAI PLAIN COAST BASED ON THE 2011
GREAT EAST JAPAN TSUNAMI
A. Suppasri1, K. Imai2, F. Imamura3 and S. Koshimura4
1 International Research Institute of Disaster Science, Tohoku University, suppasri@irides.tohoku.ac.jp
2 International Research Institute of Disaster Science, Tohoku University, imai@irides.tohoku.ac.jp
3 International Research Institute of Disaster Science, Tohoku University, imamura@irides.tohoku.ac.jp
4 International Research Institute of Disaster Science, Tohoku University, koshimura@irides.tohoku.ac.jp

The 2011 Great East Japan tsunami caused damage to casualty and building in wide range. Different
damage characteristic can be analyzed from the damage data that was resulted by human experience andgeography background. Casualty data and measured tsunami height were used to calculate a fatality ratio
for each area. The fatality area in Sendai plain coast shows that 10% fatality occurred when tsunami
height was 10 m. However, tsunami height in Sanriku area was much higher than 10 m but the fatality
ratio stops at 10%. This might be an effect from high tsunami awareness and fast evacuation of coastal
residence in the Sanriku coast. Building damage ratio against tsunami inundation depth was calculatedusing surveyed data from Ishinomaki city. The data was separated into two locations, ria coast and plain
coast. Result shows that the damage ratio of building in ria coast is much higher than plain coast in thesame inundation depth. This is because of the high hydrodynamic force as a result of high flow velocity in
ria coast. Measured tsunami flow velocity data was summarized and numerical simulation was performed
to support the result and explanation. Results of fatality ratio and building damage ratio will help to assess
future tsunami and support town construction plan in case of different coast type.
Keywords: Great East Japan Tsunami, casualty damage, building damage, Sanriku coast, Sendai coast

1. INTRODUCTION
Tohoku region in the east of Japan had longexperience as they were attacked by many tsunamis
in the past such as in 1611, 1896, 1933 and 1960.
(Suppasri et al., 2012). Only the 1611 tsunami thatcaused devastated damage in both Sanriku ria coastof Iwate prefecture and Sendai plain coast of Miyagi
prefecture. Other tsunamis caused large damage
only in the Sanriku coast. Thus, coastal residencesin Sanriku area have high tsunami awareness andevacuation recognition. Because of its ria(saw-toothed) coastline, tsunami can be easily
amplified to be larger than 10 m. In contrast, Sendai
area is a plain coast. Tsunami attack the area
without the amplification like in the Sanriku coast
but the tsunami can inundate inland as far as few
kilometers. Therefore, communities in Sendai plain
was considered to have comparatively low tsunami
hazard in terms of tsunami height or velocity but thedamage is expected to be in wide area.
The great tsunami in 2011 caused serious damage
in both areas again. However, different casualty andbuilding damage characteristics were found due totheir historical background, experience of tsunami
and geography in the Sanriku ria coast and Sendai
plain coast. Maximum tsunami height measured in
the Sanriku ria coast area was as high as 40. On theother hand, maximum tsunami height in Sendai
plain coast of about 10 m was observed but thetsunami penetrated inland as far as 5 km (Suppasri
et al., 2012).

2. OBJECTIVESThis study used damage data from the 2011
Great East Japan tsunami that caused serious
damage to both Sanriku and Sendai coasts to explain
the different of damage characteristics as mentioned
above. This is to prove that tsunami experience andawareness of residence in Sanriku area that led to
fast evacuation helped to reduce casualty damage.
Also this study will show the different damage
characteristics of building based on their coastal
type using measured inundation depth, flow velocity
and numerical simulation. Results from data

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analysis mentioned above will show effect fromtsunami experience and evacuation recognition to
the reduction of casualty damage and effect fromstrong tsunami power to the vulnerability ofbuilding damage.

northeast Japan along the ria coast. Therefore, 400
years had passed since the great tsunami in 1611
where a tsunami had not caused severe damage inthe plain areas, leading to a similar fatality ratio tothat observed for other historical events in Japan.

3. DATA AND METHODS(1) Casualty damage
Number of casualty and population for eachlocation (Onagawa town, Ishinomaki city, Natoricity, Iwanuma city and Watari district) was obtained
from a report from each city and town. Maximum
tsunami heights for each area were obtained by the2011 tsunami joint survey group (2011). These data
were used to calculate the fatality ratio against
maximum tsunami height.

1933 Showa

2011 Heisei-Ria

2011 Heisei-Plain

100

Fatality ratio (%)10(2) Building damage
Building damage and tsunami inundation depth
data provided by Ishinomaki city which is separated
into two categorizes plain coast and ria coast wereused to plot the vulnerability curves for damagedbuilding. The building damage was classified into
five levels, washed away, collapsed, major damage,moderate damage and partial damage for every 0.5
m interval of the measured maximum tsunami
inundation depth. Therefore, building damage ratio
for each damage level (among damaged buildings)
can be plotted against the maximum inundation
depth.

1

0.1

(a)

0.01
1

10
Maximum tsunami height (m)100Ishinomaki Plain

Ishinomaki Ria

Onagawa Ria

Iwanuma Plain

Natori Plain

Watari Plain

10
Fatality ratio (%)

4. RESULTS(1) Fatality ratio
Fig. 1(a) depicts a comparison of fatality ratio
between historical Sanriku tsunamis in 1896 and1933, and the 2011 tsunami in ria and plain coasts.
Fig. 1(b) shows a comparison of the fatality ratio forthe 2011 tsunami for each local area in ria and plain
coast in some areas of Miyagi prefecture. Bothfigures show very clear results that although the
maximum tsunami height in ria coast is larger than
10 m, the fatality ratio is limited to about 10%. On
the other hand, just 3 m height of tsunami could
cause 10% of fatality in Sendai plain area.
The 1896 Meiji-Sanriku tsunami led to thehighest fatality ratio because of the weak groundshaking characteristic of a tsunami earthquake that
occurred at night. Because the 1933 Showa-Sanriku
tsunami occurred only 37 years later, people inmany areas still had strong tsunami awareness.
People in the Sanriku area were again awakened by
the 1960 Chile tsunami. Therefore, the fatality ratio
from the 2011 Great East Japan tsunami, occurring
41 years later, was greatly reduced. However, thetsunamis in 1896, 1933 and 1960 affected mainly

1896 Meiji

1

(b)
0.1
0

5

10

15

20

Inundation height (m)Fig.1 Tsunami fatality ratio in Tohoku region from (a)Historical tsunamis in Sanriku and (b) the 2011 tsunami(2) Building damage ratio
Building damage and tsunami inundation depth
data provided by Ishinomaki city which is separated
into two categorizes plain coast and ria coast wereused to plot the vulnerability curves. The building
damage was classified into five levels, washed away,
collapsed, major damage, moderate damage and- 77 -

partial damage for every 0.5 m interval of themeasured maximum tsunami inundation depth.
Therefore, building damage ratio for each damagelevel (among damaged houses) can be plottedagainst the maximum inundation depth. Curves to fit
the calculated damage ratio were constructed using theregression analysis and normal or lognormal
distribution function as explained by Suppasri et al.
(2011). Tsunami vulnerability curves for plain andria coasts in Ishinomaki city are shown in Fig. 2(a)and (b) respectively. It can be seen that, for example,
damage ratio for washed away at 2 m inundation
depth is less than 0.05 in plain coast while about 0.4
in ria coast. Damage ratio in plain coast increase to
0.5 at 5 m inundation depth while the damage ratio
is 0.8 for ria coast. Table 1 show a summary ofparametersforconstructing
thetsunamivulnerability curves
1.0
0.9

Damage ratio

0.8
0.7
0.6
0.5
0.4

(a)

0.3

Washed away

0.2

Collapsed or greater

0.1

Major damage or greater
Moderate or greater

0.0
0

1

2

3 4 5 6 7 8
Inundation depth (m)

9

10

1.0
0.9

Damage ratio

0.8
0.7
0.6

Table 1 Summary of parameters for constructing the tsunami
vulnerability curves

X forvulnerability
function P(x)D4
Sendai
D3
plain
D2
coast
D1
D4
Sanriku
D3
ria
D2
coast
D1

μ or μ'σ or σ'

R2

4.982
2.009
-0.2178-1.00280.84650.2498-0.5135-0.8598

1.625
0.515
0.39050.64020.90870.62780.51850.4575

0.89
0.91
0.98
0.96
0.95
0.88
0.93
0.99

(D4=Washed away, D3=Collapsed or greater, D2=Major
damage or greater and D1=Moderate damage or greater)

5. DISCUSSIONS(1) Hydrodynamic characteristics of tsunami
estimated by survivor videos
To verify the different damage characteristic at
the same inundation depth, flow velocity should be
investigated. Foytong et al. (2012) estimatedtsunami flow velocity using moving object shown insurvivor videos in Kamaishi city, Ofunato city,
Kesennuma city, Iwaki city and Oarai city.
Koshimura and Hayashi (2012) performed the same
analysis of survivor video in Onagawa town andalso aerial videos in Sendai city and Natori city. In
addition, Fritz et al. (2012) measured flow velocity
from survivor videos at Kesennuma using LiDAR.
All of their results are summarized in Table 2. It isclear from the results that tsunami flow velocity in
Sanriku ria coast (4-11 m/s) is higher than in plainarea in Sendai and Fukushima (1-3 m/s in average).
Although, there are some areas in Sendai city thatthe flow velocity as high as 6-8 m/s. This is because
they were located near the Natori river and will notuse in the comparison. Tsunami flow along rivermight faster than inundation flow inland. Based on
the tsunami flow velocity results, hydrodynamic
force of tsunami can be estimated using the dragforce from a formula here.
FD = 0.5ρv2CdA

0.5
0.4

(b)

0.3

Washed away

0.2

Collapsed or greater
Major damage or greater

0.1

Morderate damage or greater

0.0
0

1

2

3 4 5 6 7 8
Inundation depth (m)910Fig.2 Vulnerability curves developed from damaged buildingsin Ishinomaki city in (a) Plain coasts and (b) Ria coasts(1)

Where, Cd = drag coefficient, ρ = water density(1,000 kg/m3), V = flow velocity (m/s) and A = force
projected area (m2). Inundation depth of tsunami canbe roughly estimated using tsunami inundation
height data measured by the Tohoku Tsunami Joint
survey Team (2011) and subtract by land elevation
data provide by the cabinet office. The results ofcalculated hydrodynamic force per unit width arealso shown in Table 2. In general, hydrodynamic
force in Sanriku ria coast (70-180 kN/m) is higher
than in plain area (2-52 kN/m). It can be confirmed- 78 -

that at 3.0-3.5 m inundation depth, hydrodynamic
force estimated in Kesennuma city (182 kN/m) is
greatly higher than in Sendai city and Natori city
(16-52 kN/m).
Table 2 Summary of tsunami flow velocity in each area

Location

Velocity

Average

(m/s)
4.53

(m/s)

Depth
(m)

(kN/m)

4.12

8.14

69

2.41

8.46

25

4.63

6.34

68

Force

4.18
Kamaishi

2.88
4.17
4.85

Ofunato

2.75
2.06
5.38
3.65

Kesennuma 1

4.88
4.52
4.74
2.63

2.63

10.84

37

Kesennuma 2

11.00

11.00

3

182

Onagawa

6.30

6.30

6

100

7.99

3.44

110

5.88

3.03

52

3.02

3.45

16

2.80

1

4

9.30
9.26
Sendai 1

8.85
7.68
6.75
6.09
6.04
5.69

Sendai 2

5.69
5.54
5.63
6.70

Natori 1

3.46
2.58
3.43

Natori 2

2.87
0.98
3.91

Iwaki

1.42

1.42

1.7

2

Oarai

2.55

2.55

0.88

3

(2) Hydrodynamic characteristics of tsunami
estimated by numerical simulation
Tsunami numerical simulation was performed toprovide a general overview of tsunami
characteristics such as inundation depth and flow
velocity in the whole region. Tsunami source model
proposed by Imamura et al. (2011) was applied in
the simulation as their model results were verified
with the survey results from the Tohoku Tsunami
Joint Survey Team (2011). Initial tsunami on the sea
surface was assumed to be equal to the sea bottom
deformation calculated by equations proposed by
Masinha and Smylie (1971). Sets of bathymetry andtopography data provided by the Central Disaster
Mitigation Council, Cabinet Office, Government ofJapan were used in the calculation. The datacomprises of three nested regions in Cartesian
coordinates with the grid size of 1,350 m, 450 m and
150 m respectively. Tsunami simulation was
performed followed the methods explain in the
IUGG/IOC TIME project (1997). Linear long wave
theory with neglecting the effects from bottom
friction was applied in region 1 (1,350 m mesh)while non-linear long wave theory with considering
the effects from bottom friction (assuming
Manning’s roughness coefficients of 0.025 m-1/3s)
was applied in region 2 (450 m) and 3 (150 m).
Tsunami simulation time was set as three hours
from the earthquake with the temporal grid size of
2.7 s, 0.9 s and 0.3 sec respectively. In general,
average of the maximum flow velocity obtained
after three hours in the Sanriku ria coast (10-20 m/s)
is higher than in Sendai plain coast (5-10 m/s).
These results agree with the results estimated bysurvivor videos. By this reason, damage in the ria
coast is higher as they experienced higher velocity
and can be confirmed by the distribution ofhydrodynamic force (Fig. 3).
From the simulated results in the plain coast, themaximum hydrodynamic force along shoreline isfound to be larger than 150 kN/m and less than 20
kN/m inland respectively. According to criterion for
degree of damage to building based on historicaltsunami damage data in Japan that concrete andtimber building will be destroyed if the fore ishigher than 155-281 and 9.7-17.6 kN/m (Matsutomi
and Harada, 2010) the force of 150 kN/m candestroy all types of building material along the coast.
The force was reduced to less than 20 kN/m forlocations where buildings were located at more than
2 km from the coast. Therefore, most of buildings
were observed as small or partially damage. In
contrast, Sanriku coast had comparatively smaller
inundation distance but higher force of larger than
500 kN/m in most areas. This caused devastateddamage to all building types in the Sanriku area.

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Results from this study show some examples ofdifferent damage characteristic based on historicalbackground and topography for each tsunami prone
area which are important issues for a future tsunami
damage assessment and city planning for disastermanagement.
ACKNOWLEDGMENT: The building damage
data belong to the damage field survey of the 2011
Tohoku tsunami conducted by the Ministry of Land,
Infrastructure and Transport provided by Ishinomaki
city office. We express our deep appreciation to the
Tokio Marine and Nichido Fire Insureance Co., Ltd.,
Willis Research Network, the Ministry of Education,
Culture, Sports, Science and Technology andIndustrial Technology Research Grant Program in2008 and other related organization for the financial
and other support.
REFERENCES

Fig.3 Simulated hydrodynamic force and estimated flow
velocity in 150*150 m2 grid

6. CONCLUSIONS
Fatality ratios show that experience andawareness of past tsunamis help to reduce casualty
in the Sanriku ria coast as the great tsunami
occurred almost in every people’s generation. On
the other hand, people in Sendai plain coast waslack of experience of the great tsunami for 400 years
and lose their tsunami awareness.
Comparison of tsunami vulnerability curves
show that damage ratio in ria coast increase rapidly
when inundation depth is higher than 1-2 m. Thiswas because not only the higher tsunami height but
also stronger current velocity. Therefore, even thesame inundation depth but due to the stronger
current velocity that cause greater hydrodynamic
force attacking to each building.
Different hydrodynamic characteristics oftsunami in ria coast and plain coast were discussed.
Flow velocity estimated by survivor videos andnumerical simulation agreed that this value is higher
in the ria coast. This leads to the higherhydrodynamic force that attacked the buildings.

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