Messages from Departments and Centers of NILIM

Tsunami-Resistant Design Guideline for Breakwaters
KOIZUMI Tetsuya, Director
Port and Harbour Departmen(Key words) Design tsunami, Technical Standards and Commentaries for Port and Harbour Facilities in Japan,
hydraulic model experiment

1. Introduction
The Great East Japan Earthquake damaged many
breakwaters, primarily because the giant horizontal
force of the tsunami acting on the breakwater and theoverflow of the breakwater by the tsunami scoured thefoundation
moundbehindthebreakwater,
destabilizing the breakwater. If a breakwater is a toughstructure which is not overturned even when it isdeformed by a large-scale tsunami, damage behind thebreakwater will be mitigated, because its toughness
will restrict the flow rate of the tsunami behind thebreakwater, delaying the arrival of the tsunami in theland behind the breakwater. So breakwaters must betough structures, which are as resistant as possible tooverturning, even by a tsunami that is larger than thedesign tsunami.
The Ports and Harbours Bureau, National Institutefor Land and Infrastructure Management, and the Portand Airport Research Institute of the Ministry of Land,Infrastructure, Transport and Tourism havecooperatively conducted a comprehensive study of theresults of surveys of damage to breakwaters and of theresults of a series of hydraulic model experiments
conducted since the earthquake disaster, haveorganized basic concepts of the design of toughbreakwaters, then in September 2013, they partiallyrevised the Ministerial Ordinance for the TechnicalStandards for Port and Harbor Facilities and othertechnical standards, defined design tsunami andprescribed toughness against tsunami for structures
such as breakwaters, seawalls etc., and at the sametime, the Ports and Harbours Bureau has proclaimed
the Tsunami-Resistant Design Guideline For
Breakwaters. Below the major revisions to technology
standards and major contents of this Guideline areintroduced.
2. Basic concepts
Past concepts of tsunami-resistant design of port
and harbour structures have been revised from theirfoundations. Two levels of tsunami are hypothesized
and based on its concept, the aim is, for tsunamis
which occur frequently, to provide disaster protection
using structures to do all possible to protect human
lives and property, and for the largest class of tsunami,
which occur extremely rarely but have catastrophic

damage when they do occur, the aim is to minimize
damage under the goal of, at the very least, protecting
human lives. If a structure has toughness which
prevents its overturning even as it is deformed under atsunami large enough that it exceeds tsunami which
occur frequently, it will restrict the quantity of waterflowing into the area behind the structure, delaying thearrival of the tsunami behind the structure, and thereby
reducing damage. So structures that can provide thistoughness are necessary in cases of a tsunami higher
than the design tsunami.
3. Definition of the design tsunami
The definition of a design tsunami considering
tsunami resistant design is clearly stipulated as atsunami which is unlikely to occur during the design
service life of the facility but would have severe
impacts on the facility if it did occur. The design
tsunami used for performance verification and atsunami with strength greater than the design tsunami
are considered to be at least the tsunami scale which
occurs relatively often with a recurrence period fromseveral decades to one hundred years plus several
decades, and is set appropriately for the degree ofimportance of the facility. The Guideline stipulates
that the design tsunami is set as the tsunami withdesign external force from the highly frequent tsunami
to the maximum class of tsunami according to theimportance of the structures behind the breakwater
based on a regional disaster prevention plan or a basic
plan for coastal preservation.
4. Breakwater performance verification
The overall stability of a breakwater in the casewhere the tsunami and the earthquake motion
preceding the tsunami act on the breakwater first sets
the initial section under actions other than the tsunami
or other waves. Next, the section specifications are set
based on the wave design for the design tsunami.
Finally, for a tsunami with scale greater than that ofthe design tsunami, the section of the tough structure
is set based on an overall judgment made considering
the importance of the facility and cost-benefit
performance. The structure analysis factor whenstability under the design tsunami is verified can be set
with reference to a value in Table 1, becauseaccording to the example of damage shown in Figure
1, damage occurs when the slip safety factor is lower
than approximately 1.2.
Figure 1. Occurrence/non-occurrence of Damage toa Breakwater in the Overflow Depth – Slip Safety
Factor Relationship
Flow caused by tsunami

Overflow depth (m)Overflow
depthTsunami

Erect part(caisson)

Infiltration flow

Scouringfailure

Riprap mounddamageno damage (no scouring on back)no damage (scouring on back)Overflow of breakwater
startsDamage

Slip safety factor
0.01.02.0

3.0

4.0

5.0

6.0

7.0

8.0

・If slip safety factor is lower than 1.2, damage occurs.
・It is important to prevent prior scouring failure caused by overflow

Table 1. Reference Values ofStructural Analysis Factor
Item verified

Structural analysis factor

Slip of erect part

1.2

Overturning of erect part

1.2

Bearing capacity of foundation

1.0

scale exceeding that of the design tsunami occurred at alocation where the said breakwater is installedaccording to the structure, severe impact on the stabilityof the structure of the said facility of damage from theaction caused by the action of the tsunami etc., can bedelayed as long as possible.
It is stipulated that a breakwater, which is a facility
prepared for accidents, have a structure devised toensure the maximum possible stability so that evenwhen it is subject to the action of a tsunami which scale
greater than that of the design tsunami for that location,
it displays its disaster mitigating effects and ensurescalmness in the port immediately after the disaster.
The Guideline stipulates that as a result of taking
supplementary measures according to the importance ofthe facility and cost-effectiveness etc. at weak points
revealed by full studies of the form of damage and weak
points in the structure of breakwaters according to thescale of the tsunami while using hydraulic modelexperiments, the breakwater must be a structure with
toughness which prevents it from overturning while
deforming, and to the greatest possible degree,
maintains its overall stability under a tsunami of a scale
exceeding that of the design tsunami.
Figure 3. Breakwater scouring countermeasures[Revision of shape of crown]

The tsunami wave force calculation method used isthe appropriate wave force formula obtained by theprocedure shown in Figure 2, considering whether or
not a tsunami simulation is done, and whether or not
an undular bore or overflow occurs.
Figure 2. Tsunami Wave Force Calculation
Procedure for Breakwaters
Perform tsunami
simulation

Does an undular boreform?
※(Tsunami height is approx.. 60%
or more of water depth, andseabed gradient is approx.. 1/100or less)

No

Does overflow
occur?

Yes

Yes[Correction TanimotoFormula]

Calculation formulabased on hydrostaticpressure difference

NoTanimoto formula

5. Toughness of a structure against tsunami
Among breakwaters, seawalls, etc., highly importantfacilities which, if damaged, would have a severe
impact on human life, property, and public economic
activities, are required to have toughness which will
maintain stipulated stability under tsunami with actionof a scale exceeding that of the design tsunami.
Required performance is stipulated so that in afacility which, if damaged, might have a severe impact
on human life, property, and public economic activities,
even in the case where a tsunami with strength of a

[Covering blocks installed]

No revision
Breakwater
ScouringFoundation moundFoundation mound[Scouring prevention mat placed][Thickening works]

6. Future research
The partial revisions to technology standards andthe Tsunami-Resistant Design Guideline for
Breakwaters, were prepared based on comprehensive
study of survey and research etc. done immediately
after the earthquake, but not all of the many challenges
have necessarily been clarified. In the future, as new
knowledge is obtained, it must be reflected in theGuideline, and we must promote further research andtechnology
development
inthefieldoftsunami-resistant design.

Photo 4. Breakwater Model Experiment[Sources]
1) Tsunami-Resistant Design Guideline forBreakwaters, September 2013
Ministry of Land, Infrastructure, Transport andTourism, Ports and Harbours Bureau
http://www.mlit.go.jp/common/001012142.pdf

2) T. Arikawa, M. Sato, K. Shimosako, T. Tomita, G.
Yeom, T. Niwa (2013): Damage mechanisms andeffectiveness of thickening work of compound banks
during tsunami overflow, Document of the Port andAirport Research Institute, No. 1269