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

REBUILDING COMPOSITE BREAKWATERSFOLLOWING THE 2011 TOHOKU TSUNAMI:LESSONS LEARNT AND DOES IT MAKE SENSETO REINFORCE?
Miguel Esteban1, Rafael Aranguiz Muñoz,2, Izumi Morikubo3, Takahito Mikami4,Nguyen Danh Thao5, Koichiro Ohira6 and Tomoya Shibayama7
1Dept. of Civil and Environmental Engineering, Waseda University, esteban.fagan@gmail.com
2Dept. of Civil Engineering, Universidad Catolica de la Ssma Concepcion, raranguiz@ucsc.cl
3 Nihon Unisys Ltd., izu_morikubo_413@yahoo.co.jp
4Dept. of Civil and Environmental Engineering, Waseda University, t.mikami@asagi.waseda.jp
5Dept. of Civil Engineering, Ho Chi Minh City University of Technology, ndthao@gmail.com
6 Chubu Electric Power Company, chonchan.factory@gmail.com
7Dept. of Civil and Environmental Engineering, Waseda University, shibayama@waseda.jp

Currently there are no formulas to design armour units against tsunami attack. The present work
described laboratory experiments and field work that attempts to clarify the failure mechanism of thesetypes of structures, and compares them with well established formulas such as that of Van der Meer orHudson. It thus appears that the Hudson formula can be used to provide an indication of how large
armour units would have to be to survive a tsunami attack with relatively little damage.
Key Words : breakwaters, Tohoku, tsunami, armour units, laboratory experiments

1. INTRODUCTION
The reliability of the different available tsunami
counter-measures is being re-assessed following theMarch 2011 Tohoku tsunami. In this area a numberof composite breakwaters and sea dykes wereprotected by armour units on their seaside face.
Although many of these structures were designed
primarily against storm waves, they neverthelessresisted comparatively well the forces exerted onthem by the tsunami. The most important question
at this point, however, is whether any such
armoured structures should be given preference
when designing tsunami counter-measures, andwhether these counter-measures should be attempted
at all. To date, a fair amount of research has been
carried out on vertical structures and dykes.
Tanimoto et al. (1984) performed large-scale
experiments on a vertical breakwater by using a sine
wave and developed a formula for the calculation of
the wave pressure. Ikeno et al (2001) conducted
model experiments on bore type tsunamis andmodified Tanimoto’s formula by introducing anextra coefficient for wave breaking. Subsequently
Ikeno et al (2003) improved the formula to include
larger pressures around the still water level, where
the largest wave pressure was observed to occur.
Mizutani and Imamura (2002) also conducted modelexperiments on a bore overflowing a dike on a level
bed and proposed a set of formulae to calculate themaximum wave pressure behind a dike. Esteban etal. (2009) calculated the deformation of the rubble
mound foundation against different types of solitary
waves, allowing for the determination of the caisson
tilt.
However, to the authors’ knowledge no researchhas been carried out on the behaviour or armour
units (in rubble mound or composite breakwaters)
against tsunami attack. This paper will explore thisfrom the dual point of view of breakwater design
methodology and disaster risk management.

2. FIELD SURVEY
The authors of the present chapter conducted
field surveys as a part of the larger 2011 TohokuEarthquake Tsunami Joint Survey Group in Iwate,- 81 -Miyagi, Fukushima, Ibaraki and Chiba prefectures(Mori et al. 2012, Mikami et al. 2012). During these
inspections detailed surveys of the failure of armour
layer at various breakwater locations along theaffected coastline were made. Two of these werecomposite breakwaters, those of Hikado and Ooya
ports (see Fig. 1), located close to Kesennuma City.
These two breakwaters are situated fairly close toeach other and are facing the open sea, meaning thatthe tsunami waves would have attacked themdirectly. Three different measurements of wave
heights were taken in this area, 15.7m (by theauthors themselves) and 16.55 and 15m (by othermembers of the Tohoku Earthquake Tsunami Joint
Survey Group). In the present analysis the authors
have thus adopted their own value of 15.7m for thetsunami height at the breakwater, as it sits roughly
half way between the other two values measured by
the other teams.
Three different types of armour units were present
at the breakwaters surveyed. Ooya port is made of
Sea-Lock, and Hikado port has both X-block andHollow Pyramid units along the breakwater(X-blocks in the body of the breakwater and heavier
Hollow Pyramids at the head). The X-blocks andSea-Blocks armour completely failed, and thearmour units were scattered over a wide area in frontof the breakwater, with only the top of some of theunits still showing above the water surface. The
exact failure mechanism is unclear, and whether theunits were displaced by the incoming or theoutgoing wave is not so easily established. In anycase, the breakwater was overtopped with the wholeof the area being completely underwater at one point
during the tsunami attack (which would have also
generated large underwater currents in the area).
A summary of these units can be found in Table 1(along other typical armour units shown forcomparison). The table also shows the Hudson
Damage Coefficients (KD) that were used for each ofthese armour units in the analysis.
Table 1. Summary of armour units surveyed

Approximate
WeightKD

Sea-Block
X-Block
Hollow
Pyramid

3.2 tons
5.76 tons

108

28.8 tons

10

Rock
Tetrapods
TribarModifiedCubeN/A
N/A
N/A

4810

N/A

7.5UnitFig.1 Damaged Sea-Block armour at Ooya Port

3. LABORATORY EXPERIMENTS
Laboratory experiments were carried out usingsolitary waves generated by a wave paddle in awave flume at Waseda University in Japan as shownin Figs. 2 and 3 (dimension 14m long, 0.6m high,0.41m wide). On one side of the tank a caisson
breakwater was placed, which was protected by anarmour layer made up of stones of different sizes (atotal of 3 different stone sizes were used, withmedian weights W of 27.5, 32.5 and 37.5g) to test
the sensitivity of armour weight on breakwater
damage. Two different breakwater configurations
were also tested (with a seaward angle θ of 30 and45°). Each of the breakwater configurations was
also tested for three different water depths, h = 17.5,
20 and 22.5cm. The number of armour units
extracted after one solitary wave was counted withthe aid of a high-speed photographic camera, andeach of the experimental conditions was repeated 15times to ensure accurate results.
The wave profile was measured using two wave
gauges, one located approximately in the middle ofthe tank and the other one just before thebreakwater, in order to measure the incident wave
height. The solitary waves that were used were thus
measured to have a half-period T/2=3.8sec, which
would thus correspond to a T=76sec. The
experiments were carried out in a 1/100 scale, which
would represent a T=76sec wave in the real world.
The waves generated were 8.4cm in height, thus
corresponding to 8.4m in the real world. The wave
profile as recorded by the measuring software isshown in Fig. 4.

- 82 -Number of Stones Moving

76527.5(45°)
432.5(30°)

332.5g(45°)

237.5(30°)

137.5g(45°)

017.5cm

20cm

22.5cmWater depth (d)Fig.2 Experimental layout of the breakwater

Fig.3 Average number of stones moved in each experimental
condition

4. RESULTS

5. ANALYSIS

Fig. 4 shows the average number of armour units(N) extracted from the breakwater for each of thebreakwater layouts, armour unit weighs and depthsof water. Generally damage to the 45° structure wasfar greater than to the 30°, as expected. No patternof damage could be clearly observed from the depthof water in front of the structure, and indeed thewave profile did not appear to significantly change
between them (and which always looked like asolitary wave). This is different from the results ofEsteban et al. (2009) which found that different
types of waves could be generated for different
depths (bore-type, breaking and solitary types
waves), though in this case the depth of water did
not change substantially between each experimentalcondition.
According to these, an armour damage parameter
S similar to that used in Van der Meer (1987) wasobtained for each of the breakwater configurations.

The values of S obtained from the experiments
were compared with the theoretical values thatwould have been obtained using Van der Meer’sequation, using only one wave (N=1). The resultsfor each of the experimental conditions are shown inFigs. 4-8
In order to compare how well the results of thepresent experiments compared with the field data,
the actual weight of armour (R) that would berequired according to the Hudson formula over theactual weight of the armour at the two breakwaters
in the field and the experimental results was plottedin Fig. 9. This figure shows the results for each of
the types of armour layers and how those that havelower values of R failed completely (represented by
S values of 15) whereas those units with higher
values of R did not fail catastrophically. However, it
is important to understand also that the field results
represent breakwaters that were overtopped whereas
those in the laboratory were not, and hence it is notclear that both results are comparable.

Fig.3 Experimental set-up- 83 -

Experiment
0.3

Calculated

0.25

0.15

SS

0.2

0.1
0.05
017.5cm

0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
020cm
22.5cmdepth of water (d)ExperimentCalculated
17.5cm20cm
22.5cmdepth of water (d)Fig.4 Experimental and calculated values for θ=45° and
W=27.5g

Fig.7 Experimental and calculated values for θ=30° and
W=37.5g

0.14

0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00

0.12
0.1
SS

0.080.06
0.04
ExperimentCalculated

0.02
017.5cm

20cm
depth of water (d)SExperiment17.5cm

22.5cm

Fig.5 Experimental and calculated values for θ=30° and
W=32.5g

0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
0.02
0

Calculated

20cm
depth of water (d)22.5cm

Fig.8 Experimental and calculated values for θ=45° and
W=37.5g

ExperimentCalculated
17.5cm20cm*
depth of water (d)22.5cm

Fig.6 Experimental and calculated values for θ=45° and
W=32.5g

Fig.9 Plot of the actual over required weight of armour and thedamage to each breakwater section- 84 -

6. IMPLICATIONSFORDISASTER
MANAGEMENT AND CONCLUSIONS
Generally structures in Japan are designed bybuilding them higher than the expected tsunami
wave height, though following the 2011 Tohoku
tsunami there is a general perception that structures
must be designed to fail in a non-catastrophic way,even if their design criteria is exceeded. These weredescribed by Kato (2012) as “tenacious structures”,
representing a structure that would slowly fail overthe course of the event, while retaining somefunctionality, as opposed to a “resilient” structure,
which would indicate a structure that would suffer
limited damage even if its design load was greatlyexceeded. Such a difference could be seen in thefailure of the breakwaters at Kamaishi (which could
be regarded as a “tenacious structure”, as it suffered
great damage but somehow survived the event) andthat at Ofunato (which was completely wiped out).
The present work concerned itself mainly on
breakwaters, though it is also relevant to coastal
dykes, as in Japan these are often protected byarmour units on their seaside, either placed directlyin front of the dyke or a few dozen metres in front ofit (in the beach or just immediately in front of thebeach).
Currently, there is research going on in Japan onwhether protection structures (such as dykes) should
be reinforced in the future. Recommendations arethus being made on how to improve the resilience(“tenacity”) of dykes, by improving their armouring
or placing scour protection measures at theirbackside or at the toe of the structure (especially at
the toe at the landside part of the structure).
Despite this, no formula has been developed in
the past to design the armour of rubble moundbreakwaters against tsunami attack. The question ofwhether to construct such structures to defend
against a tsunami is controversial. Currently it isbelieved that such structures can protect property inthe event of a level 1 event, though will have little
effect for the case of a level 2 event. The erection ofvertical barriers and dykes, however, can clearlygive extra time for residents to escape, as seen by
footage of videos during the disaster. Althoughthese barriers can be ultimately destroyed by a level2 event, it is unclear how much extra protection thearmour units would provide, and whether it would
be preferable to just create a wider concrete barrier.
Much is still not understood about the failure ofprotective measures in the event of a tsunami, andtheir ability to delay the arrival of the flooding watermust be carefully balanced against the extra cost of
the armour units.
In this respect, much research is still needed toascertain the failure mechanism of armour units, andwhether the placement of these armour units will
increase the forces acting on the caissons behind

them, especially if the armour units fail (Esteban atal. (2012). Also, it would be important to establish
the failure mechanism of the breakwater against theoutgoing wave, as this can generate strong currents
that could displace the armour. As a tsunami event
is made of more than one wave, it is important alsothat the structure can offer some protection against
the second or third waves (that can sometimes bestronger than the first)
REFERENCES
Esteban, M., Nguyen Danh Thao, Takagi, H. and Shibayama, T.
(2009) Pressure Exerted by a Solitary Wave on the Rubble
Mound Foundation of an Armoured Caisson Breakwater,19th (2009) International Offshore and Polar Engineering
Conference, Osaka.
Esteban, M., Takagi, H. and Shibayama, T.,(2012) “ModifiedGoda Formula to Simulate Sliding of Composite Caisson
Breakwater”, Coastal Engineering Journal, (under review)
Hudson, R.Y. 1959. Laboratory Investigation of Rubble-Mound
Breakwaters, J. Waterways, Harbors Div., 85, ASCE,
pp93-121.
Ikeno, M., Mori, N. and Tanaka, H.. (2001). ExperimentalStudy on Tsunami force and Impulsive Force by a Drifter
under Breaking Bore like Tsunamis, Proceedings of Coastal
Engineering, JSCE, Vol. 48, pp. 846-850.
Ikeno, M. and Tanaka, H. (2003). Experimental Study on
Impulse Force of Drift Body and Tsunami Running up toLand”, Proceedings of Coastal Engineering, JSCE, Vol. 50,
pp. 721-725.
Kato, F. (2012) Improvements in Coastal Dike Design Based on
the lessons learnt from the Great East Japan Earthquake
Tsunami. Advances in Coastal Disasters Risk Management,
Lessons from the March 2011 Tsunami and preparedness tothe climate change impact Seminar, Sendai, Japan, 7th-8thJune 2012.
Mikami, T., Shibayama, T., Esteban, M. and Matsumaru, R.,(2012) Field Survey of the 2011 Tohoku Earthquake andTsunami in Miyagi and Fukushima Prefectures, Coastal
Engineering Journal (CEJ), Vol. 54, No. 1
Mizutani, S. and Imamura, F.(2000). Hydraulic Experimental
Study on Wave Force of a Bore Acting on a Structure,
Proceedings of Coastal Engineering, JSCE, Vol. 47, pp.
946-950.
Mori, N., Takahashi T. and The 2011 Tohoku Earthquake
Tsunami Joint Survey Group (2012) Nationwide survey ofthe 2011 Tohoku earthquake tsunami, Coastal Engineering
Journal, Vol.54, Issue 1, pp.1-27.
Tanimoto, L., Tsuruya, K., and Nakano, S. (1984) Tsunami
Force of Nihonkai-Chubu Earthquake in 1983 and Cause ofRevetment Damage, Proceeding of the 31st Japanese
Conference on Coastal Engineering, JSCE.
Van der Meer, J. W. 1987. Stability of Breakwater Armour
Layers. Coastal Engineering, Vol 11, p. 219-239.

- 85 -