Coastal and harbor structures should be resilient so that the hydraulic performances of the structures will not decreased rapidly at the ultimate state where the incident wave height exceeds the design wave. In this study, the behavior of the caissons of composite breakwaters and the hydraulic performance at the ultimate state are measured through a hydraulic experiment. The influence of the damage to a caisson on the behavior of the adjacent one is also investigated. The authors show that the sliding distance of the caisson for the largest incident wave height in the case where the caissons with the same safety factors are installed in a row is larger than those in the cases where one of the caissons has a smaller safety factor and where one of the caissons is intentionally overturned

Araki, S.

Ogi, H.

Hasanuddin University Repository

 527 
 
 
Proceedings of the 7th International Conference on Asian and Pacific Coasts  
(APAC 2013) Bali, Indonesia, September 24-26, 2013 
 
30cm 
17.0m 
 
 
 
 
 
 
BEHAVIOR AND HYDRAULIC PERFORMANCES OF COMPOSITE BREAKWATER 
UNDER HIGHER WAVE THAN DESIGN WAVE 
 
S. Araki 
1 
and H. Ogi 
2
 
 
 
ABSTRACT: Coastal and harbor structures should be resilient so that the hydraulic performances of the structures will 
not decreased rapidly at the ultimate state where the incident wave height exceeds the design wave. In this study, the 
behavior of the caissons of composite breakwaters and the hydraulic performance at the ultimate state are measured 
through a hydraulic experiment. The influence of the damage to a caisson on the behavior of the adjacent one is also 
investigated. The authors show that the sliding distance of the caisson for the largest incident wave height in the case 
where the caissons with the same safety factors are installed in a row is larger than those in the cases where one of the 
caissons has a smaller safety factor and where one of the caissons is intentionally overturned. 
 
Keywords: caisson, sliding distance, transmission coefficient, composite breakwater, safety factor 
 
 
INTRODUCTIION 
A huge tsunami hit the pacific coasts of the 
northeastern part of Japan on March 11, 2011. Many 
coastal and harbor structures were seriously damaged by 
the tsunami. One of the reasons for the damages to the 
structures is that the incident wave exceeded the design 
wave. Coastal and harbor structures should be resilient 
so that the hydraulic performances of the structures will 
not decreased rapidly at the ultimate state caused by 
tsunami as well as extreme wind waves. 
Bremner et al. (1980) reported that the rubble mound 
breakwater damaged by a cyclone still functioned 
effectively as a submerged breakwater and investigated 
the  breakwaters  which  was  designed  to  be  damaged 
under extreme wave and storm surge conditions. Ahren 
(1989) investigated the change in the hydraulic 
performance of the rubble mound breakwater to keep 
tranquility behind it with the decrease in the crest height. 
Araki et al. (2005) investigated the deformation of the 
submerged breakwater and the change in the wave height 
behind it with deformation through a three-dimensional 
experiment.  However,  the  deformation  of  breakwaters 
and the hydraulic performances of them have not been 
investigated sufficiently. 
The  purpose  of  this  study  is  to  investigate  the 
behavior of the caissons of composite breakwaters and 
the hydraulic performance at the ultimate state where the 
incident wave height exceeds the design wave through a 
hydraulic experiment. They are discussed further by 
investigating the influence of the damage to a caisson on 
the behavior of the adjacent one in the hydraulic 
experiment. 
 
HYDRAULIC EXPERIMENTS 
 
Outline of Hydraulic Experiments 
The hydraulic experiments were conducted in a 25.0 
m long and 0.7 m wide wave flume shown in Fig. 1. The 
composite breakwaters were installed on a fixed flat bed 
where the water depth was 0.30 m. The caissons were 
placed on a 10 cm high mound. The main part of the 
mound consisted of concrete blocks so that the friction 
between the caisson and the mound would be as uniform 
as possible. The rest of the mound, i.e., the seaward and 
landward slope of the mound and the gap between the 
concrete blocks and the side wall of the wave flume, 
consisted of crashed stones whose diameter ranged from 
2 to 4 cm. In this experiment, the state where the caisson 
falls down from the crest of the mound was not taken 
into account. Therefore, the width of the mound on the 
inside of the harbor was larger than that of usual 
composite breakwaters in this experiment. 
 
20cm 
 
 
 
25.0m 
 
Fig. 1   Experimental setup 
 
 
1  
Dept. of Civil Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871 JAPAN 
2  Dept. of Civil Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871 JAPAN 
S. Araki
 
and H. Ogi 
 
528 
 
A set of irregular waves was generated by the piston- 
type wave maker installed at the end of the wave flume. 
The target spectrum was Bretschneider-Mitsuyasu 
spectrum. The significant wave period of the irregular 
wave was T1/3    = 1.4 s and 1.7 s. The significant  
wave height for the irregular wave of T1/3  = 1.4 s 
ranged from 
6.2 to 8.1 cm under the condition that the breakwater 
was not installed. The significant wave height for the 
irregular wave of T1/3  = 1.7 s ranged from 4.8 to 7.3 cm 
under  the  same  condition.  The  distance  between  the 
wave paddle and the caisson was 17.0 m. In each 
experimental run, the duration of the incident irregular 
wave   was   approximately   250   waves.   The   sliding 
distance of the caisson and the water surface elevations 
in front of and behind the caissons were measured. The 
sliding distance of the caisson was measured after the 
incidence of 250 waves. The details of measuring the 
sliding distance is mentioned later. The surface elevation 
was measured by capacitance-type wave gauge and 
recorded at 20 Hz. Fig. 2 shows the set up of the 
composite breakwater in the wave flume. The incident 
wave comes from the right hand side of the photo. The 
left hand side of the photo is the inside of the harbor. 
Caisson 
The caisson was 17.0 cm long in the direction which 
is perpendicular to the wave propagating direction. 
Therefore, four caissons were able to be placed within a 
70 cm width wave flume in a row. The crest width of the 
caisson was 16.0 cm and 26.0 cm, which means that each 
 
 
 
 
  Fig. 2   Caisson and mound in hydraulic experiment 
 
 
26cm 26cm 
 
16cm 
 
 
 
 
 
 
wave wave 
 
 
(a) Case 1 (b) Case 2 
 
 
intentionally 
overturned 
 
26cm 26cm 
 
 
 
 
 
 
 
 
wave wave 
 
 
(c) Case 3 
 
 
(d) Case 4 
Fig. 3  Combinations of caissons 
Behavior and Hydraulic Performances of Composite Breakwater under Higher Wave than Design Wave 
 
529 
 
P
u
lli
n
g
 F
o
rc
e
 [
N
] 
caisson used in the experiment has a different safety 
factor.  The  height  of  the  caisson  was  28.0  cm.  The 
caisson was made with polyvinyl chloride. The inside of 
the caisson was filled with crashed stones and sand. 
Both  safety factors for sliding  and  rotating  of the 
caisson with a 26.0 cm crest width were larger than 1.2 
for the incident wave of H1/3   = 3.0 cm and T1/3   = 1.6 
s. On  the  contrary,  both  safety  factors  for  sliding  
and rotating of the caisson with a 16.0 cm crest width 
were larger than 1.0 and smaller than 1.2 for the same 
incident wave. 
In this experiment, the differences in the behavior of 
the caissons and the tranquility behind the breakwater 
were investigated under the condition that one caisson 
with a shorter crest width, i.e., smaller safety factor, was 
installed in a row of the caissons with a longer crest 
width. The combinations of the caissons are as follows: 
- Case 1 
Four caissons with a 26.0 cm crest width are installed. 
The safety factors for all the caissons are the same. 
- Case 2 
One caisson with a 16.0 cm crest width and three 
caissons with a 26.0 cm crest width are installed. The 
sliding distances of the two kinds of caissons were 
measured  in  order  to  investigate  the  effect  of  the 
inclusion of the caisson with a smaller safety factor. 
- Case 3 
This is the case where the caisson with a 16.0 cm 
crest width was assumed to be overturned toward the 
inside of the harbor. The wave incidence and the 
measurement started from the state where the caisson 
was overturned in order to investigate the effect of the 
intentional overturn of the caisson. 
- Case 4 
This is the case where one of four caissons in a row 
is completely disappeared. Therefore, only three caissons 
with a 26.0 cm crest width are installed. This is one of 
the worst cases for the harbor tranquility. The result 
measured in this case is compared with those in other 
cases. 
Fig. 3 shows the rough sketch of the combinations of 
(iii) The mass of the caisson is changed by putting 
crashed stones into the caisson. 
(iv)  Repeat the steps from (i) to (iii). 
(v)  Find the friction coefficient from the relationship 
between the pulling force and the normal force to 
the concrete. 
 
The pulling force was measured twice for each mass 
of the caisson (It was measured four times for each mass 
of the caisson in several cases). The measurement was 
conducted in the air. However, the face of the concrete 
block was wet by sprinkling water on it. Fig. 4 shows the 
measurement of the friction coefficient. 
Fig.  5  shows  the  relationship  between  the  pulling 
force at the moment when the caisson starts to slide and 
the normal force to the concrete (the product of the mass 
of the caisson multiplied by gravitational acceleration). 
The data are not scattered so much because the concrete 
block was used as a mound in this experiment. The 
gradient of the solid line indicates the friction coefficient. 
Therefore, the friction coefficient was 0.55. 
 
 
 
 
 
Fig. 4   Measurement of friction coefficient 
 
 
 
 
60 
y = 0.554 x 
2
 
the caissons. 
 
Friction Coefficient 
The friction coefficient between the caisson and the 
concrete block which was the main part of the mound 
was measured. The procedure for measuring the friction 
coefficient is as follows: 
40 R 
 
 
20 
 
 
0 
= 0.9808 
 0 20 40 60 80 100 
(i)   Pull a caisson on the concrete block horizontally 
until the caisson starts to slide. 
(ii)   The pulling force is measured by spring balance at 
the moment when the caisson starts to slide. 
Mass * g [N] 
Fig. 5  Relationship between pulling force at the moment 
when caisson starts to slide and weight of caisson 
S. Araki
 
and H. Ogi 
 
530 
 
Measurement of Sliding Distance 
The cross-shore distance between the positions of the 
caisson   before   and   after   the   wave   incidence   was 
measured as the sliding distance. The sliding distances of 
the caissons next to the side walls of the flume were not 
measured because the friction between the caisson and 
the side wall may have the influence on the sliding 
distance.  Therefore,  the  sliding  distances  of  the  two 
center caissons in the row was measured. The details of 
the measurement in Cases 1-4 are as follows: 
- Case 1 
Both two center caissons have the crest width of a 
26.0  cm.  A  larger  sliding  distance  of  the  two  is 
determined to be the sliding distance in a case. 
- Case 2 
In this case, one of the two center caissons has the 
crest width of a 16.0 cm. The caisson with a 16.0 cm 
crest width slides more than that with a 26.0 cm crest 
width. However, the purpose of installing the caisson 
with  a  16.0  cm  crest  width  is  to  prevent  adjacent 
caissons with a 26.0 cm crest width from damaging 
seriously. Therefore, the sliding distance of the caisson 
with a 26.0 cm crest width is determined to be the sliding 
distance in a case. 
- Case 3 
The sliding distance of the caisson with a 26.0 cm 
crest width is determined to be the sliding distance in a 
case as in Case 2. 
- Case 4 
The sliding distance of the center caisson is 
determined to be the sliding distance in a case. 
 
In some cases, the caisson rotated around a vertical 
axis. In those cases, the distance between the offshore 
sides of the initial and the rotated positions was 
determined to be the sliding distance in the case. 
 
 
 
 
Fig. 6 Behavior of caissons in Case 1 at the incidence 
of approximately 250 waves of H1/3   = 6.8 cm and T1/3  = 
1.7 s, taken from the offshore side 
EXPERIMENTAL RESULTS 
Behavior of Caisson 
Case 1 
The behavior of the caisson in Case 1 depended on 
the test conditions. As an example, Fig. 6 shows the 
positions of the caissons at the incidence of 
approximately 250 waves of H1/3  = 6.8 cm and T1/3  = 
1.7 s.  The  photo  was  taken  from  the  offshore  side.  
The sliding   distances   of   the   two   center   caissons   
were measured. 
 
Case 2 
Under the incident wave whose significant wave 
height was smaller than 5.4 cm, the caisson with a 16.0 
cm crest width slid and the caisson with a 26.0 cm crest 
width did not slide. Under the incident wave whose 
significant wave height was larger than 5.4 cm, both 
caissons slid. As an example, Fig. 7 shows the positions 
of the caissons at the incidence of approximately 250 
waves of H1/3   = 6.8 cm and T1/3   = 1.7 s. The photo 
was taken from the inside of the harbor. The sliding 
distances of the two center caissons were measured. 
 
Case 3 
Under the incident wave whose significant wave 
height was smaller than 5.9 cm, the caisson with a 26.0 
cm  crest  width  slid  a  little.  Under  the  same  incident 
wave, the net sliding distance of the intentionally 
overturned caisson with a 16.0 cm crest width was very 
short although it slid offshore and onshore according to 
the  wave  crest  and  trough.  Under  the  incident  wave 
whose significant wave height was larger than 5.9 cm, 
the sliding distance of the caisson with a 26.0 cm crest 
width increased. As an example, Fig. 8 shows the 
positions of the caissons at the incidence of approximately 
 
 
 
Fig. 7 Behavior of caissons in Case 2 at the incidence 
of approximately 250 waves of H1/3   = 6.8 cm and T1/3  = 
1.7 s, taken from the inside of the harbor 
Behavior and Hydraulic Performances of Composite Breakwater under Higher Wave than Design Wave 
 
531 
 
 
 
 
 
x
s
 /
 B
L
 
x
s
 /
 B
L
 
1.5 
 
 
 
1.0 
 
 
 
0.5 
 
Case 1 
Case 2 
Case 3 
Case 4 
 
 
 
 
Fig. 8 Behavior of caissons in Case 3 at the incidence 
of approximately 250 waves of H1/3   = 6.8 cm and T1/3  = 
1.7 s, taken from the offshore side 
0.0 
2.0 
 
2.2 
 
2.4 
H1/3 / Hd 
 
2.6 
 
2.8 
 
 
 
1.5 
 
 
 
1.0 
 
 
 
0.5 
(a) T1/3  = 1.4 s 
 
 
 
Case 1 
Case 2 
Case 3 
Case 4 
 
 
 
 
Fig. 9 Behavior of caissons in Case 4 at the incidence 
of approximately 250 waves of H1/3   = 6.8 cm and T1/3  = 
1.7 s, taken from the offshore side 
0.0  
1.6 
 
1.8 
 
2.0 
H1/3 / Hd 
 
2.2 
 
2.4 
(b) T1/3  = 1.7 s 
 
250 waves of H1/3   = 6.8 cm and T1/3   = 1.7 s. The 
photo was taken from the offshore side. The sliding 
distance of the second caisson from the left on the  
photo was the sliding distance in this case. The second 
caisson from the right is an intentionally overturned 
submerged one. 
 
Case 4 
In  many  incident  wave  conditions  in  Case  4,  the 
center caisson with a 26.0 cm crest width rotated around 
a vertical axis. As an example, Fig. 9 shows the positions 
of the caissons at the incidence of approximately 250 
waves of H1/3   = 6.8 cm and T1/3   = 1.7 s. The photo 
was taken from the offshore side. 
 
Sliding Distance 
Fig. 10 shows the sliding distance of the target 
caisson in each case. The vertical axis shows the sliding 
distance of the target caisson mentioned before xS 
normalized by the crest width of the caisson BL   (= 26.0 
Fig. 10 Sliding distance of caisson 
 
 
 
cm). The horizontal axis shows the incident significant 
wave height H1/3   normalized by the design wave height 
Hd   for which both safety factors for sliding and rotating 
of the caisson with a 26.0 cm crest width are larger than 
1.2 (Hd  = 3.0 cm). 
In Case 2 where the caisson with a 16.0 cm crest 
width is installed, the sliding distance of the center 
caisson with a 26.0 cm crest width is the largest in the 
range of the smaller significant wave height in the figure. 
However, the sliding distance for the largest incident 
significant wave height in Case 2 is smaller than that in 
Case 1 where the breakwater consists of only the caisson 
with a 26.0 cm crest width. In Case 3 where the caisson 
with a 16.0 cm crest width is intentionally overturned, 
the magnitude of the incident significant wave height at 
which the sliding distance increases rapidly is the similar 
to that in Case 1. However, the sliding distance of the 
center caisson with a 26.0 cm crest width for the largest 
S. Araki
 
and H. Ogi 
 
532 
 
H
t 
/ 
H
1
/3
 
H
t 
/ 
H
1
/3
 
 
1.0 
 
 
0.8 
 
 
0.6 
 
 
0.4 
 
 
0.2 
 
 
0.0 
2.0 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
2.2 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
2.4 
H1/3 / Hd 
 
 
 
Case 1 
Case 2 
Case 3 
Case 4 
 
 
 
 
 
 
 
 
 
 
2.6 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
2.8 
 horizontal axis shows the ratio of the incident significant 
wave height H1/3  to he design wave height Hd  (= 3.0 cm). 
The transmission coefficient in Case 4 where one of 
four caissons in a row is completely disappeared is the 
largest for all the incident significant wave height. The 
transmission  coefficient  in Case1 where four caissons 
with a 26.0 cm crest width are installed is the smallest. 
The transmission coefficients in Cases 2 and 3 are 
between the coefficients in Cases 1 and 4. The 
transmission coefficient in Case 2 fluctuates according to 
the behavior of the caissons. The variation of the 
transmission coefficient in Case 3 is small. In Case 3, the 
overturned caisson resulted in a breakwater like a 
submerged breakwater. The transmission coefficient in 
Case 3 is not so large because the incident wave broke 
(a) T1/3  = 1.4 s on the overturned caisson. 
 
1.0 
 
 
0.8 
 
 
0.6 
 
 
0.4 
 
 
0.2 
 
 
0.0 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
1.6 
 
Case 1 
Case 2 
Case 3 
Case 4 
 
 
 
 
 
 
 
 
 
 
1.8 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
2.0 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
2.2 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
2.4 
 
SUMMARY 
In this study, the behavior and the hydraulic 
performances of the caisson breakwater was investigated 
under the condition that the incident wave height 
exceeded the design wave. The influence of the damage 
to a caisson on the behavior of the adjacent one was also 
investigated. The sliding distance of the caisson for the 
largest incident wave height in the case where the 
caissons with the same safety factors were installed in a 
row (Case 1) was larger than those in the cases where 
one of the caissons had a smaller safety factor (Case 2) 
and where one of the caissons was intentionally 
overturned  (Case  3).  However,  the  conditions  in  this 
study were very limited. In order to use the caisson with 
a smaller safety factor for reducing the sliding distance, a 
more careful and detailed investigation should be done. 
 
incident significant wave height in Case 3 is smaller than 
that in Case 1. 
The characteristics of the sliding distance mentioned 
above are clear for the conditions of T1/3  = 1.7 s shown in 
Fig. 10(b). Although the characteristics are not so clear 
for the condition of T1/3  = 1.4 s shown in Fig. 10(a), the 
sliding distances for the largest incident significant wave 
height in Cases 2 and 3 is smaller than those in Cases 1 
and 4, which is the same as the condition of T1/3  = 1.7 s. 
 
Transmitted Wave Height 
One of the important hydraulic functions of 
breakwaters is to keep tranquility inside harbors under 
the high wave conditions. Fig. 11 shows the transmission 
coefficient in each case. The vertical axis shows the 
transmission coefficient which is the ratio of the incident 
to transmitted significant wave heights Ht    / H1/3 the 
 
 
 
 
 
 
REFERENCES 
Ahren, J. P. (1989). Stability of reef breakwater. Journal 
of Waterway, Port, Coastal, and Ocean Engineering, 
ASCE, Vol. 115, No. 2: 221-234 
Araki, S., Niijima, H., Fumoto, H., Miyoshi, H. and 
Deguchi, I.    (2005). Change in transmission 
coefficient  with deformation   of  submerged 
breakwater. Proc. of the 15th International Offshore 
and Polar Engineering Conference, ISOPE, Seoul: 
Vol. III, 606-611. 
Bremner, W., Foster, D. N., Miller, C. A., and Wallace, 
B. C. (1980). The design concept of dual breakwaters 
and its application to Townsville, Australia. Proc. of 
International Conference on Coastal Engineering, 
ASCE, Sydney: 1898-1908. 
H1/3/Hd 
(b) T1/3 = 1.7 s 
 
Fig. 11 Transmission coefficient 
 


Behaviour and hydraulic performances of composite breakwter under higher wave than design wave

http://repository.unhas.ac.id/bitstream/handle/123456789/7585/84.OW%2077.pdf?sequence=1

Behaviour and hydraulic performances of composite breakwter under higher wave than design wave

Abstract

Similar works

Full text

Available Versions

Hasanuddin University Repository