Proceedings of the International Symposium on Engineering Lessons Learned fromthe 2011 Great East Japan Earthquake, March 1-4, 2012, Tokyo, JapanLESSONS AND VIEWS ON HARDWARE
COUNTERMEASURES WITH EARTH BANKSAGAINST TSUNAMI ESTIMATED
IN 2011 GREAT EAST JAPAN EARTHQUAKE

Ken-ichi TOKIDA1 and Ryusuke TANIMOTO2
1

Professor, Department of Civil Engineering, Osaka University,
Osaka, Japan, tokida@civil.eng.osaka-u.ac.jp
2
Undergraduate, Department of Civil Engineering, Osaka University,
Osaka, Japan, rtanimoto@civil.eng.osaka-u.ac.jp
ABSTRACT: Through the field survey after the 2011 Great East Japan Earthquake, 13
damaged and/or non-damaged earth banks and 10 eroded dug pools related tothe tsunami flood stream are investigated to focus on the effects of earthbanks against the tsunami. Based on the investigations, important lessons on
the hardware countermeasures against the tsunami with the earth banks arelearned, and the performance-based design concepts for the reconstruction inthis earthquake and the future tsunami disaster can be proposed.
Key Words: Tsunami, earth bank, dug pool, countermeasures, performance-based design

INTRODUCTION
However it is afraid that the reducing functions of the structures such as sea walls, sandbeaches, forests etc. couldn’t perform against the tsunami enough in the 2011 Off the
Pacific Coast of Tohoku Earthquake, some kinds of the structures can be indicated toresist the tsunami attacks based on the field survey after the earthquake on April 30,
May 1 to 3, July 8 to 10 and September 10 to 12 in 2011 conducted by the authors etc.
(Tokida 2011c, 2011d, 2011e, 2011f). Then the reducing and/or resistant functions of theabove structures should be investigated in detail and estimated more properly for the
future countermeasures against the tsunami.
In this paper, the earth banks are focused on and the 13 damaged and/or non damaged
ones located in Miyagi Prefecture, Chiba Prefecture and Ibaragi Prefecture are found andselected to investigate the resistant functions of the earth banks against the tsunami(Tokida 2011a, 2011g). Furthermore, the 10 dug pools eroded by the tsunami flood stream
temporarily at the back side of the sea walls or earth banks are also investigated to know463

the reducing characteristics against the tsunami to consider the future hardware
countermeasures hinted by the dug pool (Tokida 2011b).
Based on the above investigations, very important lessons on the hardware
countermeasures against the tsunami with use of the earth banks are discussed andobtained, and the performance-based design concepts for the reconstruction in thisearthquake and the future tsunami disaster are proposed from the engineering view
points (Tokida 2011a, 2011f, 2011g).
RESISTANT FUNCTION OF EARTH BANK AGAINST TSUNAMI
Objective Earth Banks
In this paper, the earth banks which aren’t covered with the structures such as blocks and don’t touch
the water area daily are focused on to investigate the resistance characteristics against thetsunami. Then, the river levee and road embankment similar to the above conditions areincluded. The objective earth banks found at the field are shown in Table1 and the theirdetail locations at Sendai Plane in Miyagi Prefecture and Asahi Coast in Chiba
Prefecture are shown in Fig. 1 and Fig. 2, respectively. As shown in Table 1, the research
objects are 2 river levees, 1 road embankment, 2 banks and 4 artificial banks at Sendai
Plane, and as shown in Table 1, the research ones are 3 banks at Asahi Coast.
Furthermore, the one object at Oarai Coast in Ibaragi Prefecture shown in Fig. 3 is a sanddune which is selected because the eroded slope at the sand dune can be referred toconsider the resistance function of the earth banks.
Table 1 Objective earth banks and each condition
No.

1

Content of Earth Structure

Type ofBank

Site of BankHeight (m)Idoura

River
Levee

2

3

Higashiura
Road
Emban
kment

4
Bank
5

Miyagi
Pref.

6
7
8

Artifici
al Bank

9

10

11

Bank

Chiba
Pref.

12

13

Sand
Dune

Ibaragi
Pref.

Front 3.9
Back 3.2
Teizan Canal 5.0
Back2.7
Teizann Canal 4.5

Condition
Slope ofof Slope Front Slopelawn

1 : 2.7

lawn

1:3

-

Takenohana

6**

Block・
Grass・
Low TreeYuriage

5.6

bare:soil

1 : 2.7

Yuriage Minami～
East Airport

1.5

grass,
low tree

1:7

Bohken Plaza

14.9

tree,
grass

Condition of Tsunami
OthersFront Slope L10.7m/H3.9m, Crest
Width6.9m、Back Slope
L10.9m/H3.2m, As-Pavement
Levee:L9.7m/H2.1m, Crest Width6.5m,
Not paved
4 lanes, Block ：H2m
Front Slope L15m/H5.6m, Crest Width
24m, L150m, Crest Surface :netted
Front Slope W11m, Back Slope W4m,
Surface Layer 0.6m, Covered Net

Upper 1 : 4
Ship Shape W50ｍ, L400m
Lower 1 : 5

Flooded Depth Overflowed(m)
Depth (m)
7.75*

3.85*

6.8*

4.1*

1.6

None

6.1**

0.5**

6.1**

4.6**

10.55～13.8

None

Mt. Hiyoriyama

6.55

grass

1 : 2.5

Top : 12×20m

8.65

2.1

Iwanuma Coast Green Zone :View Stage

9.8

low treeFront 1:2
Back 1:4

Top : φ10m, Circle

6.8

None

Iwanuma Coast Green Zone :High Bank

9.5

grass

1 : 2.8

Top : 20m×30m
Base :60m×280m

3.9

None

Block Wall : H3.5m, Non Parapet,
Slope Shoulder W3.5m＋Walking Way
W3m,

2.5**

1.0～0**

3.2以上**

1.2以上**

2.5～3.0**

0.5～1.0**

Run UpHeight :Parking 9**,Toilet 13.8**

Non atParking,
Toilet 0.8**

East Ashikawahama

1.5～2.5

grass

1:1

Nishiashiarai

2.0

grass

1:1

Mikawa

2.0

grass

1:1

Oarai Coast

Parking15m**、
Toilet13m**

grass,
toe:bareNote * is estimate based on the tsunami height of 10m.

****

Vertical Sea Wall H2m, Palapet H0.7m,
Cycling Way W3m
Vertical Sea Wall H2m, Palapet H0.9m,
Crest W2.5m, Back Slope H1.5m～2m,
Cycling Way W3m

1 ： 3.3
General Section 1 ： 3.3,
～1 : 10** Reef Section 1 : 10** is estimated.

464

No.3 Takenohana

SENDAI
SendaiNanbu
Highway

NATORI
No.7 Mt. Hiyoriyama

Tohoku Railway

ArahamaNatori River

HirouraSendai
Air Port

IWAKUMA
National
Road
Joban
No.4
Railway

WATARI

No.1 Idoura
No.4 Yuriage

Local Road
No.30

No.4 Idoura

Iioka

●

Yokone●No.11 Nishiashiarai
Yasashigaura
No.10 East Ashikawahama

Shimonagai●

No.7 Clean Center

PACIFIC OCEAN

No.9 Iwanuma Coast Green Zone
High Bank

No.* Black : Earth Bank
shown in Table 1

1 km

Fig. 2 Sites for field survey at Asahi Coast
KAMAISHIOHFUNATO

No.8 Arahama Kita Watari

Torinoumi
Yamamoto Box29

Kujyukuri Beach Line
No.12 Mikawa

No.6 East Airport

No.8 Iwanuma Coast Green Zone
View Stage

Abukuma River

Local Road
No.35

No.3 HigashiuraNo.5 Yuriage MInami

No.5 Yuriage Minami
East Airport

Local Road
No.126

ASAHI

No.2 Arahama

No.6 Bohken Plaza
No.2 Higashiura

SendaiTobu
HighwayNo.1 Arahama Kita
Wakabayashi

No.9 Arahama Minami

Yoshidahama
NationalRoad
No.6

Yamamoto Box10

No.10 Yoshidahama Minami

Ushihashi River

YAMAMOTO

No.* Black : Earth Bank
shown in Table 1

SENDAI
Sendai Plane
SOHMA

×Main ShockNo.* Blue : Dug Poolshown in Table 2

ONAHAMA
Nakahama Junior School

MIYAGI Pref.

PACIFICOCEAN

Oarai Coast
７km

CHOSHI
Asahi Coast

FUKUSHIMA Pref.

Fig.1 Sites for field survey at Sendai Plane

50km

Fig. 3 Three sites for field survey

In Table 1, the structure contents of the earth banks such as the height, condition of slope, slope
and others are shown and compared based on the field survey and the related references. Because theconditions of the tsunami such as the flooded depth and run up height can’t be measured, these factors
at Sendai Plane are estimated based on the height of the earth structures and the ground level measured
in the field, the tsunami height of 10m which is estimated in general, and those at the other sites are
estimated based on the overflow and /or run up conditions.
Typical Earth Banks and Lessons on Resistant Function
Typical earth banks which show the resistant function against the tsunami flood and theeffective lessons obtained are shown as follows．As for the other earth banks, it can be
known by the references of Tokida 2011c, 2011d, 2011e and 2011f.
Site No. 1: Idoura
Photograph 1 is the satellite photo (Google 2011) around the levee newly constructedalong the existing Teizan Canal at Idoura of Wakabayashi Area in Sendai City. The arrowin Photo 1 shows the direction of the leading wave which is same as follows in this paper.
The new levee is about 3.9m high and about 1.5km long and located around the lagoon of
200m width. The purpose of the levee is supposed to reinforce Teizan Canal with theheight of 2m along the lagoon. The coastal zone can be seen to be flooded away.
As shown in Table 1, however the overflow depth of the tsunami is estimated to belower than 4m, the lawn planted on the surface at the front slope of the levee shown in
Photo 2 can be seen not to be eroded however the sand washed by the tsunami is settled alittle. Furthermore, the crest of the levee is paved with the asphalt of 10cm thickness ispartially come off shown in Photo 3, but the performance of the levee can be almost kept.
On the other hand, Photo 4 which shows the conditions at the back slope and the toe by
the tsunami indicates that the erosion at the back side of the levee is more severe thanthose of the front slope and crest. Photo 5 shows the water pool made by the flood stream

465

Estimated Coastal Line Before
Sand
Beach
IdouraReef
TeizanCanal
LagoonPhoto 2 Front slope on May 2

New Levee
Dug Pool

Photo 1 Around new river levee on April 6
Photo 3 Bank crest on May 2
A

B

C
New Levee

Dug Pool
Dug Pool

Photo 4 Back slope and toe on May 2
ForestExist

ForestFlown

ForestExist

Photo 6 Dug pool and forest on April 6

Photo 5 Slight slope failure on May 2

named “dug pool” as follows in this paper and the slope failure, but the failure scale, i.e.
the reduced performance level seems to be small from the engineering view point.
The dug pool shown in Photo 5 is very interesting from the engineering view point
which can be known more clearly in the satellite Photo 6 (Google 2011). It can beindicated that the forests located at the back side of the dug pools through the section A
and section C seem not to be flown away severely and existed after the tsunami, and on
the other hand, the forest at the back side of the levee without the dug pool through thesection B seems to be flown away severely and not to be existed. Considering thedifference of the damage level on the forests at these sections, very important lesson thatthe dug pool is effective to reduce the flow velocity of the tsunami can be obtained.
Site No. 4: Yuriage
At the sand beach located at the east side of the Yuriage Fishing Port, the sand bank can
be found during the field survey on July 9. As shown in Photo 7 (Google 2011), the bank is
about 5.6m high and about 150m long and the crest is about 24m wide and the front slope
at the sea side is about 1: 2.7. Photo 8 (Japan Coast Guard 2011) shows the tsunami just
attacking the bank and the tsunami wave around the bank can be seen to be almost
stopped and the forest at the back side of the bank is flooded slightly. However the bankwasn’t aimed to be constructed as the countermeasures against the tsunami, the sandbank indicates the effectiveness against the tsunami. However the tsunami attacking the466

sand bank goes around the both edges of the sand bank and into the forest, the flooded
depth at the cycling terminal in the forest is about 4.3m to 5.0m which seems to be
reduced by the sand bank. If the length of the sand bank was more than 150m, theeffectiveness of the sand bank against the tsunami could become larger.
The net covered on the sand bank seen in Photo 8 can be verified also before theearthquake in the satellite photograph on August 14 in 2009 (Google 2009), and then the
net seems not to be covered with the surface soils when the tsunami attacked. However
the net is almost flown away at the time of July 9, the crest of the bank seems to be
flooded slightly and the flooded depth is estimated about 0.5m shown in Table 1 because
the net and the small pine tree of 30cm high can be observed at the field survey on July 9.
On the other hand, Photos 9 and 10 show the conditions of the front slope and theback slope, respectively which seems to be eroded slightly at the time of on July 9.
Additionally, Photo 10 shows the back side of the bank where the low bank whose heightis 1.6m and the base width is 12.8m and the surface isn’t eroded because of the reductionof the flood by the front sand bank. Photo 11 shows the view at the right edge of the sandbank shown in Photo 7 and can be known that the trees are fallen down in the direction of
from the left side to the right side of the photograph which means that the tsunami flood
went around the edge of the bank. The flown net can be also seen.
As known from the above discussions, the lesson that the sand bank can reduce thetsunami however the small overflow with about 0.5m deep occurs can be obtained.

Bank

Bank

YURIAGE Fishing PortHIROURA

Low Bank

Photo 8 Just attacking by tsunami

HIROURA

Cycling Terminal

Photo 7 Bank after tsunami on April 6
Photo 9 Front slope of bank on July 9Bank

ForestBank
Low Bank
Back Slope

Photo 11 Trace of flow on July 9

Photo 10 Back slope and background on July 9

Cycling Terminal

FUNDAMENTAL CHARACTERISTICS OF DUG POOL ERODED BY TSUNAMI
Objective Dug Pools
At the back sides of the earth bank shown in Photo 6 and the sea wall shown in Photo 12,
many dug pools appeared in this earthquake and as shown in Photo 6, the reducing

467

Maximumof ErodedＤDepth
最大洗掘深(m) D (m)function against the tsunami of the dug pool can be known. Then the dug pools areinvestigated from the views of the reducing functions against the tsunami. The 10 dug
pools are selected and the characteristics such as the scales of the trenches, tsunami
conditions and structural conditions of sea wall and banks are investigated as follows.
The objective sites selected based on the field survey are summarized in Fig. 1 andTable 2 where the scales of the dug pool such as the width B, depth D and eroded area A,
the overflowed depth H0 at the parapet of sea walls or the crest of banks by the tsunami
and the structure conditions such as the height HB of the back slope and the one HF of thefront slope are estimated. Here, each index of W, B, A, H0, HB and HF is identified as inFig. 4. The overflowed depths at the sites of No.4 and No.5 are estimated based on theoverflowed height at the Yuriage Bank which is same with the earth bank of No.4 inTable 1, and the ones at the other 8 sites are estimated based on the tsunami height of
10m assumed at Sendai Plane in general. Typical relations can be obtained as follows.

Sea WallDug Pool

Photo 12 Dug pool at sea wall on June 9

5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0

D=0.27B
D=0.23B

Partially Destroyed
Completely Destroyed0

Table 2 Conditions of dug poolTunami

B (m)

D (m)

A (m )

1

Arahama Kita
Wakabayashi

19.9

1.9

22

5.7*

2.6

2.3

2

Arahama Kita
Wakabayashi

12.2

2.2

20

6.8*

1.4

1.2

3

Higashiura

13.6

3.4

34

4.1*

2.7

2.1

4

Idoura

16.8

4.6

54

3.85*

3.2

3.9

5

Yuriage Minami

12.5

1.5

19

4.6**

1.5

1.5

6

East Air Port

8.8

2.2

17

4.6**

3.0

1.5

7

Clean Center
Iwanuma Coastal
Green Zone

11.8

2.8

29

2.8*

2.8

5.2

8

Arahama Kita
Watari

13.1

3.4

37

3.05:

2.6

2.0

9

Arahama Minami
Watari

26.3

4.1

88

3.05*

2.7

2.0

10

Yoshidahama
Minami

16.0

3.7

48

5.5*

3.4

1.5

Width

No.

Depth

Area
2

15

20

25

30

Fig.5 Relation between D and B

Banke or Sea Wall

OverflowedDepth
H 0 (m)Site

10

落堀幅 ＢB (m)
Width
(m)

Height of
Back Slope
H B (m)Height of
Front Slope
H F (m)Maximumof ErodedDDepth
最大洗掘深(m) D (m)Dug Pool
Objective5

6

D=1.5HB

Partially Destroyed
Completely Destroyed

5
4
3
2
1

D=1.14HB0012
裏法高 ＨB (m)34Height of Back Slope HB (m)Fig.6 Relation between D and HBNote :* : sumani height is assumed to be 10m.

Tsunami

洗掘断面積
A (m2)A (m2)
Area of
Eroded Section

Note ** : This is estimated based on the overflowed depth at Yuriage Bank.

H0
Crest

Sand
Beach

HF

Section Before Tsunami

HB

B

D

A

Section After Tsunami

100
90
80
70
60
50
40
30
20
10
0

Partially Destroyed
Completely Destroyed

A=15HB

A=12.5HB0

1

2
裏法高 HB (m)3Height of Back Slope HB (m)Fig.7 Relation between A and HB

Fig.4 Identification of dug pool

468

4

The relation between D and B can be drawn in Fig. 5 and the average relation and theupper limit can be estimated, respectively as
D = 0.23 B(1.1)
D = 0.27 B(1.2)
where D : depth of dug pool (m), and B : width of dug pool (m).
The relation between D and HB can be drawn in Fig. 6 and the average relation and theupper limit can be estimated, respectively as
D = 1.14 HB(2.1)
D = 1.5 HB(2.2)
where HB : height of back slope of sea wall or earth bank (m).
The relation between A and HB can be drawn in Fig. 7 and the average relation and theupper limit can be estimated, respectively as
A = 12.5 HB(3.1)
A = 15 HB(3.2)
where A : area of eroded section of sea wall or bank identified in Fig. 4 (m2).
Based on the above mentioned relations, the width or depth or area of the dug pool can be
estimated with use of the height of the back slope at the objective sea wall or earth bank
simply and applied to the design of the sea walls or earth banks in the future.

PERFOMANCE-BASED DESIDN CONCEPT WITH USE OF EARTH BANKS
Performance against Tsunami
In order to consider the software and/or hardware countermeasures against the tsunami
for the future, the fundamental concepts against the tsunami should be establishedfirstly and clearly. In this study, the fundamental design concept is considered not only tosave the human life but also to protect the assets such as houses etc. And this designconcept can be conducted based on the performance-based design which can show theperformance of the objectives protected from the tsunami flood actually. However theview point of the performance-based design against the tsunami is similar to that in caseof other design codes such as highway bridges etc. considering the safety, usability andrecovery, another performance concerning the protected objects such as the houses should
be considered in case of the tsunami flood.
The two fundamental view points on the performances necessary for the hardware
countermeasures against the tsunami flood can be indicated as follows firstly.
View Point 1: The hardware countermeasures should be stable even if the tsunami
doesn’t overflow.
View Point 2: The hardware countermeasures should be stable when the tsunami
overflows, in other words, the damage level is small however thecountermeasure is overflowed.
Additionally the third following performance should be considered to estimate theeffects of the countermeasures against the tsunami flood.
View Point 3: The damage level of the houses flooded by the tsunami should be satisfied
the required performance level of the objectives.
The classification of the performance levels considering the flood levels and thedamage ones of the wooden houses quantitatively is necessary for the performance-based
design and can be proposed as follows.
Class 1: A house isn’t flooded.
Class 2: A house is flooded at the depth under the floor of the first floor. For example, theupper limit of the flooded depth is about 0.5m.

469

Class 3: A house is flooded at the depth of 1m high above the floor of the first floor, andthe house isn’t destroyed severely and flown away. This damage condition isnamed “middle level”. The upper limits of the flooded depth and the water flow
velocity are 1.5m and 4.2m/s, respectively.
Class 4: A house is flooded at the depth over the first floor and the house is destroyed
severely and/or flown away. This damage condition is named “large level”. The
flooded depth and the flow velocity are more than 1.5m and 4.2m/s, respectively.
These classifications are summarized in Table 3. The identification of the middle andlarge levels of the damaged house, the limits of the flooded depth of 1.5m and the waterflow velocity of 4.2m/s are referred the research results by Iizuka & Matsutomi 2000.
Table 3 Performance class and level based on flooded depth and flow velocity
PerformanceClass

Performance Level

Flooded Depth
H (m)Flow Velocityu ((m/s)

001

Not flooded

2

Flooded under the floor of first floor

3

Flooded under the 1m high above thefloor and house isn't destroyed in the
middle level and flown away.

0.5 < H ≦ 1.5

4

Flooded over the first floor and house
is destroyed in the middle and/or large
level and/or flown away.

1.5 < H

0 < H ≦ 0.5
0 < u ≦ 4.2

4.2 < uNote: Identification of the middle and large level of house, flooded depth of 1.5m and water flow
velocity of 4.2m/s are referred from Iizuka&Matsutomi(2000)

The fundamental design steps to apply the performance-based design for the
hardware countermeasures against the tsunami can be proposed as follows.
Step 1: The fundamental concept is that the tsunami should be contained in the water
areas such as the sea, river and sea port etc., in other words, the flood by tsunami
into the back ground isn’t permitted. The containment of the tsunami is
performed with use of the fundamental structures such as detached breakwater,
sea wall, sand dune, river levee etc. considering the collaboration with the banks
whose reduction effects against the tsunami is discussed in this paper.
Step 2: When the containment of the tsunami is difficult or the possibility of the tsunami
overestimated is considered, the overflow of the tsunami is permitted. The
reducing performance of many kinds of the hardware countermeasures such as
the bank, levee, forest, canal, water zone etc. which are located between the sea
coast zone and the objectives such as the houses should be estimated.
Step 3: The flood by the tsunami should be estimated based on the performance level of
the wooden houses or the lightweight steel frame houses considering theestimated indexes of the flooded depth and the flow velocity which are very
related the damage level of the houses. The permitted performance for the houses
should be designed selecting from the Class 1 to Class 3 shown in Table 4 bywhich the houses are not destroyed severely and flown away.
The above proposal is tentative at present and necessary to be checked and improved
to recover the damage in this earthquake and reduce the future damage induced by thetsunami.

470

Example of Use of Earth Bank
The typical structures or factors related to reduce the tsunami which are clarified in thefield survey at the Sendai Plane are the detached breakwater, natural reef, sand beach,lagoon, sea wall, forest, canal, river levee, manmade bank and road embankment and soon (Tokida 2011a, 2011b, 2011c, 2011d ).
When the reducing function of each factor as mentioned above can be estimatedquantitatively, the performance-based design concept shown in Fig. 8 can be proposed ingeneral. As shown in Fig. 8, the input conditions of the tsunami at the coastal line such as
the run up height and the flow velocity are reduced to the output conditions such as therequired flooded depth and flow velocity through each factor gradually. The factors
considered in the above design can be selected and 3 examples are shown in Fig. 8. The
selected factors in the Example 1, Example 2 and Example 3 are the various factors, thesea wall & earth bank and the sea wall & earth bank & canal, respectively. In other words,
the combination of various reducing factors against the tsunami may be the one kind of
the multiple defenses.
Furthermore, considering the reducing functions by the dug pool and the forest
learned from the lessons shown in this paper, the combination of the sea wall, earth bank,trees planted on the earth bank and manmade canal can be proposed under permitting
InlandPermitted
Height & Velocity

Sea
Inputted
Height & Velocity

Ex.1 : Various Factors
Ex3. : Sea Wall & Bank & Canal

Ex.2 : Sea Wall & Bank

High
Bank

ForestSea WallBank

Detached
Breakwater

Sand Dune
Canal Manmade
Road
Water Space Dune
Natural Reef
ForestBank Canal
Sand Beach
River
Water Space
Levee
LagoonFig. 8 Performance-based design concept by combination of each factor

Fig. 9 Design concept against tsunami with sea wall, earth bank and manmade canal471

the overflow by the tsunami as shown in Fig. 9 where the manmade canal is constructed in advanceinstead of the dug pool. Additionally, the design factors necessary to be researched in the future to
apply the design concepts to the sea wall, earth bank and canal are also shown in Fig. 9.
In order to apply the design concepts shown in Fig. 8, the reducing function of the selectedfactors are necessary to be investigated and estimated quantitatively from now on. As for the dug poolor the manmade pool against the tsunami flood, the detail field survey on the conditions of the dug
pool at Idoura was conducted by the authors (Tanimoto 2011).
Flooded Depth and Its Reduction
In order to estimate and/or check the effectiveness of the hard countermeasures such as earth structures
etc. quantitatively, the flooded depth at the back ground by the tsunami is necessary to be clarified. In
this study, the flooded depth and the distance from the coastal line at the Sendai Plane are tried to begathered through the field survey and the related references by other researchers. The results areshown in Table 4 where 14 sites by the authors and 1 site by the Institute of Port and Airport (2011)
and 3 sites by Shibayama etc. (2011) are compared.
Table 4 Flooded depth and distance from coastal line

No.

City

Observed Site

Distance from
Coastal LineX (m)Flooded
Depth
H (m)

223
720
350
650
2400
600
440
1120

4.4
5.05
10.55
3.5
1.6
8.65
8.3
2.98

350

6.7

300
900

6.8
3.7

800

3.9

270
600
810
2450
3150
400

5.0
4.6
3.9
1.5
0.8
9.1

Arahama （Sea Port&Air Port Ins. ）Arahama Junior School （Shibayama etc.)
Sendai
Bohken Plaza、High Bank
Higashiura、House
Takenohana at Sendai Tobu Highway
Yuriage, Hiyoriyama
Natori
Yuriage Fishing Port （Shibayama etc.）
Sendai Air Port (Shibayama etc.)
Iwamuma Coastal Green Zone, Baseball
GroundIwamuma Coastal Green Zone, View Stage
IwanumaFujisone, House
Iwamuma Coastal Green Zone,
Office House
Torinoumi Hotel
Fisshing House
WatariWest Side of Fishing Port, HouseBox 29 at Sendai Tobu Highway
Box 10 at Sendai Tobu Highway
Yamamoto
Nakahama Junior School

1.
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18

Flooded Water Depth H (m)1412●：Tokida et al.

H= 13－0.009X▲ ：Port and Airport Institute■：Waseda University

10
8

H = 13e-0.001x
64

H= 5.5－0.0015X
2
0
0

500

1000

1500

2000

2500

3000

3500

Distance from Coastal Line X (m)Fig. 10 Relation between flooded depth and distance from coastal line at Sendai Plane

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The relations between the flooded depth and the distance from the coastal line can be drawn as inFig. 10 where the relation is shown as
H=13 ×EXP (-0.001×X)(4)
where H : flooded depth (m), and X : distance from the coastal line (m).
As shown in Fig.10, because the flooded depth at the sites close to and/or far from the coast line
is afraid to be under estimated by Eq. (4), two lines identified by Eq. (5) are drawn shown as in Fig.10
to improve the relation between H and X and to estimate the large flooded depth from the engineeringview point to estimate the flooded depth safely.
0m ≦ X ≦ 1,000m
H= 13 – 0.009×X(5.1)
1,000m ≦ X ≦ 3,500m
H=5.5 – 0.0015×X(5.2)
As for the above two lines identified by Eq. (5), the flooded depth at the coastal line is estimatedto be 13m similar to that by Eq. (4). According to Eq. (5), the example of the flooded depth at X
=1,000m and 2,000m can be estimated to be 4.0m and 2.5m, respectively.

CONCLUSIONS
The resistance characteristics of earth banks and dug pools which were found in the fieldsurvey conducted after the 2011 Great East Japan Earthquake are investigated based on
the field survey and other references. The following lessons on the hardware
countermeasures with the earth banks for the future recovery and reconstruction against
the tsunami can be obtained and proposed.
1. The 13 earth banks at Sendai Plane, Asahi Coast and Oarai Coast which areconsidered to be effective to reduce the tsunami flood can be found through the fieldsurvey.
2. The earth banks which are covered with many kinds of materials such as soils, asphalt,
lawn, grass and low or high tree are eroded only at the surface and difficult to be
destroyed severely although the height of the earth bank is larger than about 4m. On
the other hand, the earth banks lower than about 1.5m are possible to be flown awaywhen the overflowed depth of the tsunami is larger than 3m to 4m.
3. When the tsunami flows over the earth banks, the dug pools are easy to be made at theback side of them. Because the tsunami flood can be reduced by the dug pool, the canalor the water area can be estimated to be effective against the tsunami.
4. The 10 dug pools are selected and investigated and the relations between the width,
depth, section area and the height of the back slope are investigated and the simple
equations to estimate their relations are proposed.
5. The ground surface at the background of the earth bank seems to be eroded when the
ground surface isn’t solid and/or covered with plants and the sediment can be observed
when the dug pool is a little large, for example, with the eroded depth of about 4m.
6. For the hardware countermeasures, the performance-based design concepts arenecessary and effective to clarify the required performance of the objectives whichshould be protected.
7. The earth banks are necessary to perform the seismic stability firstly and then very
effective to contain the run up of the tsunami combining with the existing sea wall orsatisfy the required performance of the objectives against the overflow of the tsunami.
8. The flooded depth of the tsunami can be known to be reduced according to the distancefrom the coastal line based on the traces by the tsunami at the houses etc. through thefield survey, and the relation between the flooded depth and the distance from thecoastal line at the Sendai Plane is proposed with the simple equations.

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The future research subjects are necessary to be investigated through theexperiments and the numerical analysis applying the simulated tsunami in detail topropose the hardware countermeasures against the tsunami with use of the earth banks.

ACKNOWLEDGMENTS
The authors would like to express sincere gratitude to Mr. W.Z. CHEN of Soma Construction Office at
Joban Highway of Nippo Co. Ltd., Assistant Professor K. Koizumi and Mr. T. Akita of Graduate
School of Engineering of Osaka University who joined the field surveys.

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