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STUDY ON THE REFLECTION, TRANSMISSION AND WAVE ENERGY DISSIPATION
CHARACTERISTICS OF PERFORATED HOLLOW PILE BREAKWATERS
Thai Van. TRAN1, Hung Duc. PHAM1, Ha Hai. NGUYEN 1 and Tam Thanh. NGUYEN1
ABSTRACT: To protect the mangrove forests along the coastal areas of Mekong Delta, the research group on coastal
and river bank protection of Hydraulic Construction Institute has created perforated hollow pile breakwaters to reduce
wave energy. Perforated hollow pile breakwaters (PHPB) were made by vertical perforated piles, they were installed in
the ground and stabilized by the inner stones. This paper presents the results of study on reflection, wave energy
dissipation and transmission characteristics of PHPB on physical model. The results show that the ratio of d/H varied
from 0.7 to 0.93, the transmission coefficient (Kt) increased from 0.45 to 0.6 and the reflection coefficient (Kr)
decreased from 0.55 to 0.47. At the d/H was 0.75, wave energy dissipation was maximum as 70% and Kt approximated
0.5 with Kr from 0.5 to 0.52. Thus, the authors suggested that the top elevation of structures should be chosen as 1.33
times of design sea water level when applying these structures for coastal protection projects.
Keywords: Perforated Hollow Pile Breakwater, Wave reduction effect, wave energy dissipation, wave transmission.
1 Hydraulic Construction Institute, 03/95 Alley, Chua Boc Street, Dongda District, Hanoi City, VIETNAM
1.INTRODUCTION
The coastal zone of Mekong Delta is affected by the
impacts of global climate change. Intensity and
frequency of storm events and floods are expected
increase. As a consequence, the coastal erosion is more
seriously. The double rows of concrete piles which was
applied from 2011, was quite useful but it was costly and
difficult to move. In 2016, hollow cylinder breakwater
was first used as an efficient way to reduce wave energy.
However, it was limited by shallow zones upper than -
1.00m. In Japan, the double-cylinder caisson breakwater
was constructed at deep water areas exposed to high
waves, to protect Sakai Port (outer permeable and inner
impermeable cylinder, stabilized by filled sand layers
inside).
Based on these knowledge, we proposed a new
structure of coastal protection to apply for Mekong Delta
namely perforated hollow pile breakwaters (PHPB).
PHPB was studied with diameter of 300cm, 13cm thick
and 450cm high. The structure was perforated by
25x60cm holes, 20 holes for sea side and 15 holes for lee
side. Stones were filled inside PHPB to stabilize.
Fig.1 Details of Perforated hollow pile breakwater
Proceedings of the 10 th International Conference on Asian and Pacific Coasts
(APAC 2019) Hanoi, Vietnam, September 25-28, 2019
1021
Some of main characteristics have been investigated
such as reflection, transmission and wave dissipation.
These were affected by the perforations, water depth,
diameter of piles and the high of stone layer…etc. This
study focused on the effect of d/H ratio (the depth of
water and wave height) on reflection and wave
transmission characteristics for proposed structure.
2. EXPERIMENTAL INVESTIGATIONS
2.1. Experiment design
The experiment was conducted on the wave flume of
the Key Laboratory of River and Coastal Engineering –
Vietnam Academy for Water Resources. The wave
flume used for experiment is Flander with 30m long,
1.8m deep and 2.0m wide. The wave generator can
generate regular and random waves in the form of
Jonswap, Jonswap Par, Moskowitz, Moskowitz Par and
Sin. The maximum of wave height can create Hsmax =
0,4m and wave period (Tp) = 0.5s ÷ 5.0s.
To simulate the field conditions of wave height,
period and diameter of perforation by application of
Froude’s law (Hughes, 1993) a geometrically similar
model scale of 1:10 was selected for the present
experimental investigations.
piles breakwater W1W3 W2
X
X
12
X
13
W0W4
x
4
wave maker
perforated hollow
wave absorber
dh
h
s
H
s
0.51m
Fig. 2 Longitudinal section of Wave Flume
The 3 probes W1, W2, W3 are arranged to determine
the reflected wave follow by the theory of Mansard and
Funke (1980), the W4 probe was placed on leeside to
determine the transmitted wave height. The distance
requirements below must be tested to eliminate abnormal
values in the measurement.
X12 = L/10; Where L: Wave length;
L/6 < X13 < L/3 and X13 ≠ L/5 and X13 ≠ 3L/10;
X12 ≠ n*Lp/2, with n=1, 2…;
X13 ≠ X12, with n=1, 2…;
Fig.3 Test model of PHPP, scale:1/10
2.2. Method
Random waves in the form of JONSWAP spectrum
(created by wave generator) for the experiments with
heights ranging from 0.1m to 0.14m and the peak wave
period Tp=1.9s as well as the water depths changed in
four different values 0.21m, 0.226m, 0.234m and 0.28m.
Total time for each experiment was 1000.Tp (1000
waves). There were 18 experiments, included 9 options
of wave and 2 options of stone layer height inside the
structure.
1022 T. V. Tran et al.
Table 1 Experiment scenarios
No Scenarios
Wave height Period Water depth Inner stone
Hs (m) Tp(s) d(m) hs (m)
1
PA1
0.14
1.9
0.28
0.05; 0.15
2
PA2
0.12
1.9
0.28
3
PA3
0.10
1.9
0.28
4
PA4
0.12 1.9 0.234
5
PA5
0.10
1.9
0.234
6
PA6
0.12 1.9 0.226
7
PA7
0.10 1.9 0.226
8
PA8
0.12 1.9 0.210
9
PA9
0.10 1.9 0.210
3. EXPERIMENTAL RESULTS AND DISCUSSION
3.1. Experimental results
The results for experiments were summarized below:
Table 2 Reflection and transmission coefficient, in case of hs=0.05m
No
Scenarios
Wave
hei
ght
Water
depth
W1 W2
W3 W4 Hi Hr Kt Kr Kl
Hs
(m)
d/H (m)
(m)
(m)
(m)
(m)
(m)
(m)
(m)
(m)
1 PA1 0.14 0.93
0.154
0.158
0.160
0.096
0.144
0.069
0.67
0.48
0.57
2
PA2
0.12 0.93
0.136
0.137
0.141
0.083
0.127
0.061
0.65
0.48
0.59
3
PA3
0.10 0.93
0.124
0.128
0.133
0.084
0.115
0.053
0.73
0.46
0.51
4
PA4
0.12 0.78
0.141
0.142
0.144
0.072
0.130
0.067
0.55
0.52
0.65
5
PA5
0.10 0.78
0.125
0.124
0.126
0.066
0.114
0.058
0.58
0.51
0.64
6
PA6
0.12 0.75
0.136
0.140
0.141
0.069
0.126
0.064
0.55
0.51
0.66
7
PA7
0.10 0.75
0.120
0.123
0.126
0.069
0.112
0.056
0.62
0.50
0.61
8
PA8
0.12 0.70
0.131
0.133
0.135
0.070
0.119
0.064
0.59
0.54
0.60
9
PA9
0.10 0.70
0.123
0.123
0.124
0.064
0.113
0.059
0.57
0.52
0.64
Table 3 Reflection and transmission coefficient, in case of hs=0.15m
No
Scenarios
Wave
height
Water
depth
W1 W2 W3
W4 Hi Hr Kt Kr Kl
Hs(m)
d/H
(m)
(m)
(m)
(m)
(m)
(m)
(m)
(m)
(m)
1
PA1
0.14
0.93
0.152
0.157
0.160
0.087
0.142
0.069
0.61
0.49
0.62
2
PA2
0.12
0.93
0.135
0.138
0.143
0.076
0.127
0.062
0.60
0.49
0.64
3
PA3
0.10
0.93
0.125
0.126
0.133
0.068
0.117
0.057
0.58
0.49
0.65
4
PA4
0.12
0.78
0.135
0.142
0.144
0.062
0.127
0.067
0.49
0.53
0.69
5
PA5
0.10
0.78
0.120
0.125
0.128
0.056
0.113
0.059
0.50
0.52
0.70
6
PA6
0.12
0.75
0.139
0.144
0.146
0.064
0.129
0.068
0.50
0.53
0.69
7
PA7
0.10
0.75
0.123
0.125
0.128
0.061
0.114
0.059
0.54
0.52
0.67
8
PA8
0.12
0.70
0.139
0.142
0.145
0.063
0.127
0.071
0.49
0.55
0.67
9
PA9
0.10
0.70
0.124
0.124
0.126
0.051
0.113
0.060
0.45
0.53
0.72
1023
Study on the Reflection, Transmission…
Where:
W1, W2, W3, W4: wave height measured at probes (m);
Hr: Reflection wave height (m);
Hi: Incident wave height (m);
Kt: Transmission coefficient;
Kr: Reflection coefficient;
Kl: Loss coefficient;
22
1
3.2. Discussion
When the ratio of d/H increased, the transmission
coefficient increased in all cases where wave heights
were tested. The transmission coefficient increased from
0.55 to 0.73 (in case of the height of the stone layer was
0.05 m), and from 0.45 to 0.61 (in case of the height of
the stone layer was 0.15m) when ratio of d/H increased
from 0.7 to 0.93.
Fig.4 Relation between Kt and d/H
When the height of the inner stone was 0.05 m, the
maximum transmission coefficient (Kt) was 0.73 with
the ratio of d/H as 0.93 and the experimental wave
height of 0.1 m. In case of d/H as 0.75 and the wave
height was 0.12 m, the transmission coefficient was
lowest of 0.54.
Similarity, when the height of the inner stone
increased to 0.15 m, the maximum transmission
coefficient (Kt) was 0.6 with the ratio of d/H as 0.93 and
the experimental wave height of 0.14 m. In case of d/H
as 0.70 and the wave height was 0.10 m, the
transmission coefficient was lowest of 0.45.
The reflection wave decreased in all tested cases
when the water depth increased. The reflection
coefficient (Kr) decreased from 0.54 to 0.46 (in case of
the height of the stone layer was 0.05 m), and from 0.55
to 0.49 (in case of the height of the stone layer was
0.15m) when ratio of d/H increased from 0.7 to 0.93.
Fig.5 Relation between Kr and d/H
1024 T. V. Tran et al.
It was clear that, for all the tested cases, the loss
coefficient (Kl) ranging from 0.50 to 0.72. At the ratio of
d/H as 0.75, the measurement of Kl were from 0.65 to
0.77 for all conducted wave heights and stone layers
Fig. 6 Relation between Kl and d/H
4. CONCLUSIONS
The experimental work for piles breakwaters was
presented. The following conclusions can be drawn:
When the ratio of d/H increased from 0.7 to 0.93, the
transmission coefficient increased from 0.45 to 0.73. The
minimum of Kt was 0.45 in case of the height of the
stone layer was 0.15 m and the ratio of d/H =0.7.
In contrast, the reflection coefficient (Kr) decreased
from 0.54 to 0.46 when ratio of d/H increased from 0.7
to 0.93. At the ratio of d/H =0.93 and the height of the
stone layer was 0.05m, the reflection coefficient was
minimum as 0.46.
When the ratio of d/H increased from 0.7 to 0.75, the
loss coefficients were from 0.65 to 0.77 while the
reflection coefficient ranging from 0.5 to 0.52 and the
transmission coefficient was approximately to 0.5. Based
on these results, the authors suggested that the top
elevation of structures should be chosen as 1.33 times of
design sea water level when applying these structures for
coastal protection projects.
In addition, the measurement results also showed that
when increasing the height of the inner stone layer from
0.05m to 0.15m, the reflection coefficient increased
slightly (from 3.7% to 4.2%) but the transmission
coefficient decreased significantly (from 20% to 24%).
Therefore, increasing the height of the inner stone does
not affect significantly to the reflection characteristics,
but it influences dramatically to the wave transmission
characteristics of PHPB. This means that when
increasing the stone layer height, the dissipated wave
energy also increased.
REFERENCES
Mansard, E. P. D., and Funke, E. R. (1980). The
Measurement of Incident and Reflected Spectra
Using a Least Square Method. Proc. 17th Coastal
Eng. Conf., Sydney, Australia, vol. 1, pp. 154-172.
K. Sankarbabu, S.A. Sannasiraj, V. Sundar (2008).
Hydrodynamic performance of a dual cylindrical
caisson breakwater. Coastal Engineering 55 (2008)
431446.
Norzana Mohd Anuar and Faridah Jaffar Sidek (2010).
Wave characteristics around perforated piles in a two
rows arrangement. Malaysian Journal of Civil
Engineering 24(1):48-66 (2010).
Sonia y. El.serafy, Yasser E. Mostafa, Yasser M. EL
Saie and Christena F. Gad (2016). Reducing Wave
Energy by Using of Perforated Piles Breakwaters.
IOSR Journal of Mechanical and Civil Engineering
(IOSR-JMCE) e-ISSN: 2278-1684,p-ISSN: 2320-
334X, Volume 13, Issue 6 Ver. VII (Nov. - Dec.
2016), PP 01-09.
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Study on the Reflection, Transmission…
