Submerged breakwaters are gaining more popularity as a potential coastal protection structure resulting in moderate wave transmission with significant wave energy dissipation. Submerged breakwaters are mainly adopted to prevent erosion and to dissipate the incident wave energy. In addition, the premature wave breaking facilitates the wave surfing activities by proper designing of submerged breakwater. In the present study, the experiments are conducted on submerged breakwaters in a two dimensional wave flume to investigate the influence of stepped and smooth front slope of the submerged breakwater, its height and width in reducing wave energy. A total number of eighteen sets of experiments has been conducted for three different breakwater heights (31cm, 28cm and 26cm) and three different breakwater widths (10cm, 20cm and 30cm) with stepped and smooth front slope of breakwater. The submerged breakwater models are subjected to regular waves of four different wave heights and five different wave periods in a constant water depth of 31cm, to determine wave transmissions characteristics. The influence of relative breakwater width, relative depth of submergence of the breakwater and roughness of breakwater front slope on wave transmission are analyzed and discussed in this paper.DAA

Kerpen, Nils B.

Lokesha

Sannasiraj, S.A.

Schlurmann, Torsten

Sundar, V.

AbstractSubmerged breakwaters are gaining more popularity as a potential coastal protection structure resulting in moderate wave transmission with significant wave energy dissipation. Submerged breakwaters are mainly adopted to prevent erosion and to dissipate the incident wave energy. In addition, the premature wave breaking facilitates the wave surfing activities by proper designing of submerged breakwater. In the present study, the experiments are conducted on submerged breakwaters in a two dimensional wave flume to investigate the influence of stepped and smooth front slope of the submerged breakwater, its height and width in reducing wave energy. A total number of eighteen sets of experiments has been conducted for three different breakwater heights (31cm, 28cm and 26cm) and three different breakwater widths (10cm, 20cm and 30cm) with stepped and smooth front slope of breakwater. The submerged breakwater models are subjected to regular waves of four different wave heights and five different wave periods in a constant water depth of 31cm, to determine wave transmissions characteristics. The influence of relative breakwater width, relative depth of submergence of the breakwater and roughness of breakwater front slope on wave transmission are analyzed and discussed in this paper

Lokesha, 

Kerpen, N.B.

Schlurmann, T.

Elsevier - Publisher Connector 

 Procedia Engineering  116 ( 2015 )  713 – 719 
Available online at www.sciencedirect.com
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer- Review under responsibility of organizing committee , IIT Madras , and International Steering Committee of APAC 2015
doi: 10.1016/j.proeng.2015.08.356 
ScienceDirect
8th International Conference on Asian and Pacific Coasts (APAC 2015)
Department of Ocean Engineering, IIT Madras, India.
Experimental Investigations on Wave Transmission at Submerged
Breakwater with Smooth and Stepped Slopes
Lokeshaa , Kerpen.N.B.b , Sannasiraj.S.A.c , Sundar.V.c , Schlurmann.T. d
aResearch Scholar, Department of Ocean Engineering, Indian Institute of Technology Madras
Chennai 600 036, India
bResearch Scholar, Franzius-Institute for Hydraulic, Estuarine and Coastal Engineering, Leibniz Univ. Hannover 30167, Germany
cProfessor, Department of Ocean Engineering, Indian Institute of Technology Madras
Chennai 600 036, India
dProfessor, Franzius-Institute for Hydraulic, Estuarine and Coastal Engineering, Leibniz Univ. Hannover 30167, Germany
Abstract
Submerged breakwaters are gaining more popularity as a potential coastal protection structure resulting in
moderate wave transmission with significant wave energy dissipation. Submerged breakwaters are mainly adopted
to prevent erosion and to dissipate the incident wave energy. In addition, the premature wave breaking facilitates
the wave surfing activities by proper designing of submerged breakwater. In the present study, the experiments are
conducted on submerged breakwaters in a two dimensional wave flume to investigate the influence of stepped and
smooth front slope of the submerged breakwater, its height and width in reducing wave energy. A total number of
eighteen sets of experiments has been conducted for three different breakwater heights (31cm, 28cm and 26cm)
and three different breakwater widths (10cm, 20cm and 30cm) with stepped and smooth front slope of breakwater.
The submerged breakwater models are subjected to regular waves of four different wave heights and five different
wave periods in a constant water depth of 31cm, to determine wave transmissions characteristics. The influence of
relative breakwater width, relative depth of submergence of the breakwater and roughness of breakwater front
slope on wave transmission are analyzed and discussed in this paper.
© 2014 The Authors. Published by Elsevier B.V.
Peer-review under responsibility of organizing committee of APAC 2015, Department of Ocean Engineering, IIT Madras.
Keywords: Submerged breakwaters; transmission; wave energy dissipation; relative breakwater width; relative depth of submergence of
breakwater.
© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
eer- R iew under responsibil ty of organizing committee , IIT Madras , and International Steering Committee of APAC 2015
714   Lokesha et al. /  Procedia Engineering  116 ( 2015 )  713 – 719 
1. Submerged Breakwaters
Submerged breakwaters are effectively used to maintain tranquility inside the harbor basin, to reduce the
siltation at the harbor entrances and along the approach channel, against coastal erosion, as a shelter for marine
habitats, as an eco-friendly structure. Submerged breakwaters allow the exchange of water masses between
offshore and onshore region reducing the pollution on the beaches. The level of coastal protection provided by the
submerged breakwater depends on the various factors such as width and depth of submergence of breakwater;
distance of breakwater from the shore, hydrodynamic characteristics of the structure, wave climate and angle of
wave attack (Pilarczyk et al., 2003). A comprehensive experimental investigation is conducted on submerged
breakwater to study the performance characteristics (Dattatri et al., 1978). The results on this experimental study
conclude that submerged breakwaters can be effectively used for wave attenuations. Also, the author states that the
crest width and depth of crest submergence have greater influence on the performance of submerged breakwater;
the shape of breakwater has significant effect on the reflection characteristics of the structure. A parametric study
is conducted on the hemi-cylindrical, rectangular and flexible breakwater models to evaluate the transmission and
reflection characteristics (Dimitrios et al., 2003). The experimental results declare that the rectangular model
performs better in dissipating wave energy in comparison with the hemi-cylindrical model for the case of rigid
breakwaters. Whereas, the hemi-cylindrical model performs better in reducing wave energy compared to
rectangular shapes for the case of flexible breakwaters. The physical model tests are conducted to evaluate the
performance of culvert pipe blocks against conventional blocks (Jian-Wui Lai et al., 2008). They stated that the
culvert pipe blocks with constrictive sections dissipate more wave energy compared to the conventional blocks.
Also, the authors declare that the wave energy dissipation is almost the same for culvert pipe with constrictive
compared to permeable rubble mound breakwaters. The experimental studies are conducted on stepped slope
floating type breakwater (single, double and triple rows) to investigate the transmission and reflection
characteristics under various wave conditions (Teh.H.M et al., 2013). The experimental results conclude that the
wave attenuation for the triple-row breakwater is 35% more compared to the single row breakwater. Also, the
energy loss coefficient of triple-row breakwater is 20% higher compared to single row-breakwaters. The reflection
has less effect for all the cases such as single, double and triple-row floating breakwater. The experiments are
conducted to evaluate transmission and reflection characteristics of the submerged rectangular stepped breakwater
(El Sai Yasser Moh., 2014). He declares that maximum and minimum wave energy dissipation occurs in case of
three rectangular vertical submerged breakwaters (71.25%) and single rectangular vertical submerged breakwater
(11.26%) respectively. The hydraulic model tests are conducted on two stepped embankments with slope 1:2 and
1:3 to evaluate the mean overtopping rate under various wave conditions (Kerpen et al., 2014). He states that for
the relative free board height, the mean overtopping rate for 1:2 sloped stepped embankments is 2.5 to 5 times
greater than 1:3 sloped stepped embankments. Also, the authors state that
2. Experimental Setup
The laboratory experiments are conducted at Hydraulic laboratory of Franzius-Institute for Hydraulic, Estuarine
and Coastal Engineering, Leibniz University, Hannover, Germany. The submerged breakwater models are tested in
a 700cm long, 30cm wide and 54cm deep unidirectional wave flume equipped with wave a flap type wave
generator and wave absorber on the either ends. A total number of eighteen sets of experiments are conducted for
three different breakwater heights (31cm, 28cm and 26cm) and three different breakwater widths (10cm, 20cm and
30cm) with stepped and smooth front slope of breakwater. The front (smooth, stepped) and rear (smooth) slopes
considered for the submerged breakwater model are 1:2 and 1:1 respectively. The submerged breakwater models
are subjected to regular waves of four different wave heights (3cm, 5cm, 7cm, 9cm) and five different wave
periods (0.55s, 0.65s,0.75s, 0.85s, 0.95s) in a constant water depth of 31cm, to determine wave transmission
characteristics. The photographic view of wave flume and experimental setup is presented in Fig. 1. The
submerged breakwater model is placed at a distance of 231cm from the wave maker. Three wave gauges are used
to measure the surface elevation with a resolution of 0.5mm. For each test run, wave heights are measures at two
locations in front of the submerged breakwater and at one location behind the submerged breakwater. The first and
715 Lokesha et al. /  Procedia Engineering  116 ( 2015 )  713 – 719 
second wave probes are located in front of the submerged breakwater model at a distance of 77cm and 60cm
respectively. The third wave probe is located at a distance of 45cm from the submerged breakwater model. The
submerged breakwater models are made up of wood material with smooth and stepped front slope are presented in
Fig. 2.
Fig. 1. A photographic view of wave flume and experimental setup
Fig. 2. A view of the submerged breakwater model set-up with smooth and stepped front slope
3. Results and Discussion
The hydrodynamic performance of the submerged breakwater is measured in terms of transmission coefficient,
reflection coefficient and energy loss coefficient. The transmission coefficient is the ratio of transmitted wave
height (Ht) to the incident wave height (Hi), for instance a lower transmission coefficient indicates effective wave
attenuation.
3.1. Effect of relative crest width (B/L) on wave transmission coefficient (Kt )
A sufficient crest width is required to trigger the wave breaking resulting in a higher amount of wave energy
dissipation, which in turn reduces the transmission characteristics of the structure. The transmission coefficient at
smooth sloped and stepped submerged breakwater is plotted against the relative crest width (crest width/wave
length) for crest width B=10cm, 20cm and 30cm are shown in Fig. 3 and 4 respectively. For crest width B=10cm,
20cm and 30cm, from the Fig. 3 and 4, it is observed that transmission coefficient Kt decreases with an increase in
B/L. It is also observed that when crest width is less than the one fourth of the wave length, a lot of oscillations are
seen in the variation of transmission coefficient for all ranges of relative submergence of crest width (Rc/Hi=depth
of Submergence/wave height) considered in the present study. Whereas, for the crest width greater than one fourth
wave length, the transmission coefficient decreases with the increase in B/L for all ranges of relative submergence
of crest width considered. The average reduction in the transmission coefficient for submerged breakwater with
smooth front slope, relative submergence of crest width Rc/Hi=0 and crest width of B= 20cm and 30cm are 8.67%
and 22.75% respectively compared to crest width B=10cm. Whereas, for Rc/Hi= -3.10 ~ -1.40, the average
reduction in the transmission coefficient for the crest width of B= 20cm and B=30cm are 11.16% and 38.15% %
716   Lokesha et al. /  Procedia Engineering  116 ( 2015 )  713 – 719 
respectively compared to crest width B=10cm. Also, for Rc/Hi= -4.64 ~ -2.39, the average reduction in the
transmission coefficient for submerged breakwater with smooth front slope and crest width of B= 20cm and 30cm
are 20.83% and 31.82% respectively compared to crest width B=10cm.
Similarly, the average reduction in the transmission coefficient for submerged breakwater with stepped front
slope, relative submergence of crest width Rc/Hi=0 and crest width of B= 20cm and 30cm are 4.98% and 16.30%
respectively compared to crest width B=10cm. Whereas, for Rc/Hi= -3.10 ~ -1.40, the average reduction in the
transmission coefficient  for the crest width of B= 20cm and B=30cm are 12.7% and 30.21% respectively
compared to crest width B=10cm. Also, for Rc/Hi= -4.64 ~ -2.39,  the average reduction in the transmission
coefficient for submerged breakwater with smooth front slope and crest width of B= 20cm and 30cm are 17.55%
and 28.69% respectively compared to crest width B=10cm.
Fig. 3. Transmission coefficients at smooth slope submerged breakwater with respect to the relative
submergence of crest width for B= 10cm, 20cm and 30cm
Fig. 4. Transmission coefficients at stepped slope submerged breakwater with respect to the relative
submergence of crest width for B= 10cm, 20cm and 30cm
0 0.2 0.4 0.6 0.8
B / L
0
0.2
0.4
0.6
0.8
1
K t
Smooth slope submerged breakwater
RC / Hi = 0 , B=10 cm
RC / Hi = 0 , B=20 cm
RC / Hi = 0 , B=30 cm
RC / Hi = -3.10 ~ -1.40 , B=10 cm
RC / Hi = -3.10 ~ -1.40 , B=20 cm
RC / Hi = -3.10 ~ -1.40 , B=30 cm
RC / Hi = -4.64 ~ -2.39 , B=10 cm
RC / Hi = -4.64 ~ -2.39 , B=20 cm
RC / Hi = -4.64 ~ -2.39 , B=30 cm
0 0.2 0.4 0.6 0.8
B / L
0
0.2
0.4
0.6
0.8
1
K t
Stepped slope submerged breakwater
RC / Hi = 0 , B=10 cm
RC / Hi = 0 , B=20 cm
RC / Hi = 0 , B=30 cm
RC / Hi = -3.10 ~ -1.40 , B=10 cm
RC / Hi = -3.10 ~ -1.40 , B=20 cm
RC / Hi = -3.10 ~ -1.40 , B=30 cm
RC / Hi = -4.64 ~ -2.39 , B=10 cm
RC / Hi = -4.64 ~ -2.39 , B=20 cm
RC / Hi = -4.64 ~ -2.39 , B=30 cm
717 Lokesha et al. /  Procedia Engineering  116 ( 2015 )  713 – 719 
Fig. 5. Transmission coefficients at smooth and stepped slope submerged breakwater for Rc/Hi=0 with respect to
the relative submergence of crest width for B= 10cm, 20cm and 30cm
Fig. 6. Transmission coefficients at smooth and stepped slope submerged breakwater for Rc/Hi = -3.10 ~ -1.40 with
respect to the relative submergence of crest width for B= 10cm, 20cm and 30cm
The transmission coefficient at smooth sloped and stepped submerged breakwater is plotted against the relative
crest width for Rc/Hi = 0, Rc/Hi = -3.10 ~ -1.40 and Rc/Hi = -4.64 ~ -2.39 are shown in Fig. 5, 6 and 7 respectively.
The average reduction in the transmission coefficient for submerged breakwater with stepped front slope, relative
submergence of crest width Rc/Hi=0 and crest width of B= 10cm, 20cm and 30cm are 20%, 16.54% and 16.28%
respectively compared to submerged breakwater with smooth front slope. Whereas, for Rc/Hi= -3.10 ~ -1.40 and
submerged breakwater with stepped slope, the average reduction in the transmission coefficient for the crest width
of B= 10cm, 20cm and 30cm are 17.81%, 17.02% and 6.86% respectively compared to submerged breakwater with
smooth slope. Also, for Rc/Hi= -4.64 ~ -2.39 and submerged breakwater with stepped slope,  the average reduction
in the transmission coefficient for crest width of B= 10cm, 20cm and 30cm are 13.7%, 10.54% and 6.61%
respectively compared to submerged breakwater with smooth slope.
0 0.2 0.4 0.6 0.8
B / L
0
0.2
0.4
0.6
0.8
1
K t
RC / Hi = 0
Smooth , B= 10 cm
Smooth , B= 20 cm
Smooth , B= 30 cm
Stepped , B= 10 cm
Stepped , B= 20 cm
Stepped , B= 30 cm
0 0.2 0.4 0.6 0.8
B / L
0
0.2
0.4
0.6
0.8
1
K t
RC / Hi = -3.10 ~ 1.40
Smooth , B= 10 cm
Smooth , B= 20 cm
Smooth , B= 30 cm
Stepped , B= 10 cm
Stepped , B= 20 cm
Stepped , B= 30 cm
718   Lokesha et al. /  Procedia Engineering  116 ( 2015 )  713 – 719 
The total average reduction in the transmission coefficient for submerged breakwater with stepped front slope,
relative submergence of crest width Rc/Hi=0, Rc/Hi= -3.10 ~ -1.40, Rc/Hi= -4.64 ~ -2.39 are 17.61%, 13.9% and
10.29% respectively compared to submerged breakwater with smooth front slope. Analyzing the results of present
study, it is recommended that the values of Rc/Hi should be in the range of -3.10 ~ -1.40 and crest width should be
greater than one fourth of the wave length in order to achieve the wave transmission coefficient less than 50%.
Fig. 7. Transmission coefficients at smooth and stepped slope submerged breakwater for Rc/Hi = -4.64 ~ -2.39 with
respect to the relative submergence of crest width for B= 10cm, 20cm and 30cm
4. Conclusion
The experimental study is carried out to investigate the transmission characteristics of submerged breakwaters
with smooth and stepped front slope. The crest width, breakwater submergence and slope roughness are the
important geometry factors influencing the wave transmission coefficient in evaluating the performance of
submerged breakwater. The result on the present study indicates that submerged breakwaters with stepped front
slope dissipate more energy compared to smooth front slope. The total average reduction in the transmission
coefficient for submerged breakwater with stepped front slope, relative submergence of crest width Rc/Hi=0,
Rc/Hi= -3.10 ~ -1.40, Rc/Hi= -4.64 ~ -2.39 are 17.61%, 13.9% and 10.29% respectively compared to submerged
breakwater with smooth front slope. In order to achieve the wave transmission coefficient less than 50%, it is
recommended that the values of Rc/Hi should be in the range of -3.10 ~ -1.40 and crest width should be greater than
one fourth of the wave length. The results of this investigation can be used in design of submerged breakwater for
medium wave transmission in dissipating wave energy.
Acknowledgements
The study presented in this work has been conducted within the frameworks of the DAAD exchange program
called “New passage to India”. The authors would like to extend their gratitude to the Franzius-Institute for
Hydraulic, Estuarine and Coastal Engineering, Leibniz University, Hannover, Germany for allowing to conducting
experiments.
0 0.2 0.4 0.6 0.8
B / L
0
0.2
0.4
0.6
0.8
1
K t
RC / Hi = -4.64 ~ -2.39
Smooth , B= 10 cm
Smooth , B= 20 cm
Smooth , B= 30 cm
Stepped , B= 10 cm
Stepped , B= 20 cm
Stepped , B= 30 cm
719 Lokesha et al. /  Procedia Engineering  116 ( 2015 )  713 – 719 
References
Dattari, J., Raman, H. and Shankar, N.J. 1979. Performance Characteristics of Submerged Breakwaters, 16 International Conference on Coastal
Engineering, American Society of Civil Engineers,New York, pp. 2152-2171.
Dimitrios G.S., Muhammad R.Hajj., and Demetri P Telionis., 2003. Performance of Hemi-cylindrical and Rectangular Submerged Breakwater.
Ocean Engg. Vol.30, pp813-828.
El Saie Yasser Mohamed, 2014. Effect of Submeged Rectangular Stepped Breakwater for the Defence of the Shoreline. International Journal of
Civil Engg and Technology. Vol.5, pp. 106-118.
Jian-Wu Lai., Ching-Ton Kuo., Tai-Wen Hsu., 2008. Physical Model Test on Wave Transmission to Culvert Pipes Block with Constrictive
Sections. Taiwan-Polish joint Seminar on Coastal Protection. Pp.A15-A24.
Nils B. Kerpen, Torsten Schlurmann., 2014. Stepped Embankments –Accessive Water Fronts and Protection Measures. 7th Chinese-German
Joint Symposium on Hydraulic and Ocean Engg.
Pilarczyk, K.W., 2003. Design of low-crested (submerged) structures: An overview, 6th COPEDEC (Int.Conf. on Coastal and Port Engg. in
Develop.Countries), Sri Lanka.
Teh. H. M., Ismail.H., 2013. Performance Evaluation of Arry of Stepped-Slope Sloating Breakwater. IEE Business Engineering and Industrail
Applications Colloquium (BEIAC). Pp279-283.


Experimental Investigations on Wave Transmission at Submerged Breakwater with Smooth and Stepped Slopes 

Institutionelles Repositorium der Leibniz Universität Hannover

 Procedia Engineering  116 ( 2015 )  713 – 719 Available online at www.sciencedirect.com1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer- Review under responsibility of organizing committee , IIT Madras , and International Steering Committee of APAC 2015doi: 10.1016/j.proeng.2015.08.356 ScienceDirect8th International Conference on Asian and Pacific Coasts (APAC 2015)Department of Ocean Engineering, IIT Madras, India.Experimental Investigations on Wave Transmission at SubmergedBreakwater with Smooth and Stepped SlopesLokeshaa , Kerpen.N.B.b , Sannasiraj.S.A.c , Sundar.V.c , Schlurmann.T. daResearch Scholar, Department of Ocean Engineering, Indian Institute of Technology MadrasChennai 600 036, IndiabResearch Scholar, Franzius-Institute for Hydraulic, Estuarine and Coastal Engineering, Leibniz Univ. Hannover 30167, GermanycProfessor, Department of Ocean Engineering, Indian Institute of Technology MadrasChennai 600 036, IndiadProfessor, Franzius-Institute for Hydraulic, Estuarine and Coastal Engineering, Leibniz Univ. Hannover 30167, GermanyAbstractSubmerged breakwaters are gaining more popularity as a potential coastal protection structure resulting inmoderate wave transmission with significant wave energy dissipation. Submerged breakwaters are mainly adoptedto prevent erosion and to dissipate the incident wave energy. In addition, the premature wave breaking facilitatesthe wave surfing activities by proper designing of submerged breakwater. In the present study, the experiments areconducted on submerged breakwaters in a two dimensional wave flume to investigate the influence of stepped andsmooth front slope of the submerged breakwater, its height and width in reducing wave energy. A total number ofeighteen sets of experiments has been conducted for three different breakwater heights (31cm, 28cm and 26cm)and three different breakwater widths (10cm, 20cm and 30cm) with stepped and smooth front slope of breakwater.The submerged breakwater models are subjected to regular waves of four different wave heights and five differentwave periods in a constant water depth of 31cm, to determine wave transmissions characteristics. The influence ofrelative breakwater width, relative depth of submergence of the breakwater and roughness of breakwater frontslope on wave transmission are analyzed and discussed in this paper.© 2014 The Authors. Published by Elsevier B.V.Peer-review under responsibility of organizing committee of APAC 2015, Department of Ocean Engineering, IIT Madras.Keywords: Submerged breakwaters; transmission; wave energy dissipation; relative breakwater width; relative depth of submergence ofbreakwater.© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).eer- R iew under responsibil ty of organizing committee , IIT Madras , and International Steering Committee of APAC 2015714   Lokesha et al. /  Procedia Engineering  116 ( 2015 )  713 – 719 1. Submerged BreakwatersSubmerged breakwaters are effectively used to maintain tranquility inside the harbor basin, to reduce thesiltation at the harbor entrances and along the approach channel, against coastal erosion, as a shelter for marinehabitats, as an eco-friendly structure. Submerged breakwaters allow the exchange of water masses betweenoffshore and onshore region reducing the pollution on the beaches. The level of coastal protection provided by thesubmerged breakwater depends on the various factors such as width and depth of submergence of breakwater;distance of breakwater from the shore, hydrodynamic characteristics of the structure, wave climate and angle ofwave attack (Pilarczyk et al., 2003). A comprehensive experimental investigation is conducted on submergedbreakwater to study the performance characteristics (Dattatri et al., 1978). The results on this experimental studyconclude that submerged breakwaters can be effectively used for wave attenuations. Also, the author states that thecrest width and depth of crest submergence have greater influence on the performance of submerged breakwater;the shape of breakwater has significant effect on the reflection characteristics of the structure. A parametric studyis conducted on the hemi-cylindrical, rectangular and flexible breakwater models to evaluate the transmission andreflection characteristics (Dimitrios et al., 2003). The experimental results declare that the rectangular modelperforms better in dissipating wave energy in comparison with the hemi-cylindrical model for the case of rigidbreakwaters. Whereas, the hemi-cylindrical model performs better in reducing wave energy compared torectangular shapes for the case of flexible breakwaters. The physical model tests are conducted to evaluate theperformance of culvert pipe blocks against conventional blocks (Jian-Wui Lai et al., 2008). They stated that theculvert pipe blocks with constrictive sections dissipate more wave energy compared to the conventional blocks.Also, the authors declare that the wave energy dissipation is almost the same for culvert pipe with constrictivecompared to permeable rubble mound breakwaters. The experimental studies are conducted on stepped slopefloating type breakwater (single, double and triple rows) to investigate the transmission and reflectioncharacteristics under various wave conditions (Teh.H.M et al., 2013). The experimental results conclude that thewave attenuation for the triple-row breakwater is 35% more compared to the single row breakwater. Also, theenergy loss coefficient of triple-row breakwater is 20% higher compared to single row-breakwaters. The reflectionhas less effect for all the cases such as single, double and triple-row floating breakwater. The experiments areconducted to evaluate transmission and reflection characteristics of the submerged rectangular stepped breakwater(El Sai Yasser Moh., 2014). He declares that maximum and minimum wave energy dissipation occurs in case ofthree rectangular vertical submerged breakwaters (71.25%) and single rectangular vertical submerged breakwater(11.26%) respectively. The hydraulic model tests are conducted on two stepped embankments with slope 1:2 and1:3 to evaluate the mean overtopping rate under various wave conditions (Kerpen et al., 2014). He states that forthe relative free board height, the mean overtopping rate for 1:2 sloped stepped embankments is 2.5 to 5 timesgreater than 1:3 sloped stepped embankments. Also, the authors state that2. Experimental SetupThe laboratory experiments are conducted at Hydraulic laboratory of Franzius-Institute for Hydraulic, Estuarineand Coastal Engineering, Leibniz University, Hannover, Germany. The submerged breakwater models are tested ina 700cm long, 30cm wide and 54cm deep unidirectional wave flume equipped with wave a flap type wavegenerator and wave absorber on the either ends. A total number of eighteen sets of experiments are conducted forthree different breakwater heights (31cm, 28cm and 26cm) and three different breakwater widths (10cm, 20cm and30cm) with stepped and smooth front slope of breakwater. The front (smooth, stepped) and rear (smooth) slopesconsidered for the submerged breakwater model are 1:2 and 1:1 respectively. The submerged breakwater modelsare subjected to regular waves of four different wave heights (3cm, 5cm, 7cm, 9cm) and five different waveperiods (0.55s, 0.65s,0.75s, 0.85s, 0.95s) in a constant water depth of 31cm, to determine wave transmissioncharacteristics. The photographic view of wave flume and experimental setup is presented in Fig. 1. Thesubmerged breakwater model is placed at a distance of 231cm from the wave maker. Three wave gauges are usedto measure the surface elevation with a resolution of 0.5mm. For each test run, wave heights are measures at twolocations in front of the submerged breakwater and at one location behind the submerged breakwater. The first and715 Lokesha et al. /  Procedia Engineering  116 ( 2015 )  713 – 719 second wave probes are located in front of the submerged breakwater model at a distance of 77cm and 60cmrespectively. The third wave probe is located at a distance of 45cm from the submerged breakwater model. Thesubmerged breakwater models are made up of wood material with smooth and stepped front slope are presented inFig. 2.Fig. 1. A photographic view of wave flume and experimental setupFig. 2. A view of the submerged breakwater model set-up with smooth and stepped front slope3. Results and DiscussionThe hydrodynamic performance of the submerged breakwater is measured in terms of transmission coefficient,reflection coefficient and energy loss coefficient. The transmission coefficient is the ratio of transmitted waveheight (Ht) to the incident wave height (Hi), for instance a lower transmission coefficient indicates effective waveattenuation.3.1. Effect of relative crest width (B/L) on wave transmission coefficient (Kt )A sufficient crest width is required to trigger the wave breaking resulting in a higher amount of wave energydissipation, which in turn reduces the transmission characteristics of the structure. The transmission coefficient atsmooth sloped and stepped submerged breakwater is plotted against the relative crest width (crest width/wavelength) for crest width B=10cm, 20cm and 30cm are shown in Fig. 3 and 4 respectively. For crest width B=10cm,20cm and 30cm, from the Fig. 3 and 4, it is observed that transmission coefficient Kt decreases with an increase inB/L. It is also observed that when crest width is less than the one fourth of the wave length, a lot of oscillations areseen in the variation of transmission coefficient for all ranges of relative submergence of crest width (Rc/Hi=depthof Submergence/wave height) considered in the present study. Whereas, for the crest width greater than one fourthwave length, the transmission coefficient decreases with the increase in B/L for all ranges of relative submergenceof crest width considered. The average reduction in the transmission coefficient for submerged breakwater withsmooth front slope, relative submergence of crest width Rc/Hi=0 and crest width of B= 20cm and 30cm are 8.67%and 22.75% respectively compared to crest width B=10cm. Whereas, for Rc/Hi= -3.10 ~ -1.40, the averagereduction in the transmission coefficient for the crest width of B= 20cm and B=30cm are 11.16% and 38.15% %716   Lokesha et al. /  Procedia Engineering  116 ( 2015 )  713 – 719 respectively compared to crest width B=10cm. Also, for Rc/Hi= -4.64 ~ -2.39, the average reduction in thetransmission coefficient for submerged breakwater with smooth front slope and crest width of B= 20cm and 30cmare 20.83% and 31.82% respectively compared to crest width B=10cm.Similarly, the average reduction in the transmission coefficient for submerged breakwater with stepped frontslope, relative submergence of crest width Rc/Hi=0 and crest width of B= 20cm and 30cm are 4.98% and 16.30%respectively compared to crest width B=10cm. Whereas, for Rc/Hi= -3.10 ~ -1.40, the average reduction in thetransmission coefficient  for the crest width of B= 20cm and B=30cm are 12.7% and 30.21% respectivelycompared to crest width B=10cm. Also, for Rc/Hi= -4.64 ~ -2.39,  the average reduction in the transmissioncoefficient for submerged breakwater with smooth front slope and crest width of B= 20cm and 30cm are 17.55%and 28.69% respectively compared to crest width B=10cm.Fig. 3. Transmission coefficients at smooth slope submerged breakwater with respect to the relativesubmergence of crest width for B= 10cm, 20cm and 30cmFig. 4. Transmission coefficients at stepped slope submerged breakwater with respect to the relativesubmergence of crest width for B= 10cm, 20cm and 30cm0 0.2 0.4 0.6 0.8B / L00.20.40.60.81K tSmooth slope submerged breakwaterRC / Hi = 0 , B=10 cmRC / Hi = 0 , B=20 cmRC / Hi = 0 , B=30 cmRC / Hi = -3.10 ~ -1.40 , B=10 cmRC / Hi = -3.10 ~ -1.40 , B=20 cmRC / Hi = -3.10 ~ -1.40 , B=30 cmRC / Hi = -4.64 ~ -2.39 , B=10 cmRC / Hi = -4.64 ~ -2.39 , B=20 cmRC / Hi = -4.64 ~ -2.39 , B=30 cm0 0.2 0.4 0.6 0.8B / L00.20.40.60.81K tStepped slope submerged breakwaterRC / Hi = 0 , B=10 cmRC / Hi = 0 , B=20 cmRC / Hi = 0 , B=30 cmRC / Hi = -3.10 ~ -1.40 , B=10 cmRC / Hi = -3.10 ~ -1.40 , B=20 cmRC / Hi = -3.10 ~ -1.40 , B=30 cmRC / Hi = -4.64 ~ -2.39 , B=10 cmRC / Hi = -4.64 ~ -2.39 , B=20 cmRC / Hi = -4.64 ~ -2.39 , B=30 cm717 Lokesha et al. /  Procedia Engineering  116 ( 2015 )  713 – 719 Fig. 5. Transmission coefficients at smooth and stepped slope submerged breakwater for Rc/Hi=0 with respect tothe relative submergence of crest width for B= 10cm, 20cm and 30cmFig. 6. Transmission coefficients at smooth and stepped slope submerged breakwater for Rc/Hi = -3.10 ~ -1.40 withrespect to the relative submergence of crest width for B= 10cm, 20cm and 30cmThe transmission coefficient at smooth sloped and stepped submerged breakwater is plotted against the relativecrest width for Rc/Hi = 0, Rc/Hi = -3.10 ~ -1.40 and Rc/Hi = -4.64 ~ -2.39 are shown in Fig. 5, 6 and 7 respectively.The average reduction in the transmission coefficient for submerged breakwater with stepped front slope, relativesubmergence of crest width Rc/Hi=0 and crest width of B= 10cm, 20cm and 30cm are 20%, 16.54% and 16.28%respectively compared to submerged breakwater with smooth front slope. Whereas, for Rc/Hi= -3.10 ~ -1.40 andsubmerged breakwater with stepped slope, the average reduction in the transmission coefficient for the crest widthof B= 10cm, 20cm and 30cm are 17.81%, 17.02% and 6.86% respectively compared to submerged breakwater withsmooth slope. Also, for Rc/Hi= -4.64 ~ -2.39 and submerged breakwater with stepped slope,  the average reductionin the transmission coefficient for crest width of B= 10cm, 20cm and 30cm are 13.7%, 10.54% and 6.61%respectively compared to submerged breakwater with smooth slope.0 0.2 0.4 0.6 0.8B / L00.20.40.60.81K tRC / Hi = 0Smooth , B= 10 cmSmooth , B= 20 cmSmooth , B= 30 cmStepped , B= 10 cmStepped , B= 20 cmStepped , B= 30 cm0 0.2 0.4 0.6 0.8B / L00.20.40.60.81K tRC / Hi = -3.10 ~ 1.40Smooth , B= 10 cmSmooth , B= 20 cmSmooth , B= 30 cmStepped , B= 10 cmStepped , B= 20 cmStepped , B= 30 cm718   Lokesha et al. /  Procedia Engineering  116 ( 2015 )  713 – 719 The total average reduction in the transmission coefficient for submerged breakwater with stepped front slope,relative submergence of crest width Rc/Hi=0, Rc/Hi= -3.10 ~ -1.40, Rc/Hi= -4.64 ~ -2.39 are 17.61%, 13.9% and10.29% respectively compared to submerged breakwater with smooth front slope. Analyzing the results of presentstudy, it is recommended that the values of Rc/Hi should be in the range of -3.10 ~ -1.40 and crest width should begreater than one fourth of the wave length in order to achieve the wave transmission coefficient less than 50%.Fig. 7. Transmission coefficients at smooth and stepped slope submerged breakwater for Rc/Hi = -4.64 ~ -2.39 withrespect to the relative submergence of crest width for B= 10cm, 20cm and 30cm4. ConclusionThe experimental study is carried out to investigate the transmission characteristics of submerged breakwaterswith smooth and stepped front slope. The crest width, breakwater submergence and slope roughness are theimportant geometry factors influencing the wave transmission coefficient in evaluating the performance ofsubmerged breakwater. The result on the present study indicates that submerged breakwaters with stepped frontslope dissipate more energy compared to smooth front slope. The total average reduction in the transmissioncoefficient for submerged breakwater with stepped front slope, relative submergence of crest width Rc/Hi=0,Rc/Hi= -3.10 ~ -1.40, Rc/Hi= -4.64 ~ -2.39 are 17.61%, 13.9% and 10.29% respectively compared to submergedbreakwater with smooth front slope. In order to achieve the wave transmission coefficient less than 50%, it isrecommended that the values of Rc/Hi should be in the range of -3.10 ~ -1.40 and crest width should be greater thanone fourth of the wave length. The results of this investigation can be used in design of submerged breakwater formedium wave transmission in dissipating wave energy.AcknowledgementsThe study presented in this work has been conducted within the frameworks of the DAAD exchange programcalled “New passage to India”. The authors would like to extend their gratitude to the Franzius-Institute forHydraulic, Estuarine and Coastal Engineering, Leibniz University, Hannover, Germany for allowing to conductingexperiments.0 0.2 0.4 0.6 0.8B / L00.20.40.60.81K tRC / Hi = -4.64 ~ -2.39Smooth , B= 10 cmSmooth , B= 20 cmSmooth , B= 30 cmStepped , B= 10 cmStepped , B= 20 cmStepped , B= 30 cm719 Lokesha et al. /  Procedia Engineering  116 ( 2015 )  713 – 719 ReferencesDattari, J., Raman, H. and Shankar, N.J. 1979. Performance Characteristics of Submerged Breakwaters, 16 International Conference on CoastalEngineering, American Society of Civil Engineers,New York, pp. 2152-2171.Dimitrios G.S., Muhammad R.Hajj., and Demetri P Telionis., 2003. Performance of Hemi-cylindrical and Rectangular Submerged Breakwater.Ocean Engg. Vol.30, pp813-828.El Saie Yasser Mohamed, 2014. Effect of Submeged Rectangular Stepped Breakwater for the Defence of the Shoreline. International Journal ofCivil Engg and Technology. Vol.5, pp. 106-118.Jian-Wu Lai., Ching-Ton Kuo., Tai-Wen Hsu., 2008. Physical Model Test on Wave Transmission to Culvert Pipes Block with ConstrictiveSections. Taiwan-Polish joint Seminar on Coastal Protection. Pp.A15-A24.Nils B. Kerpen, Torsten Schlurmann., 2014. Stepped Embankments –Accessive Water Fronts and Protection Measures. 7th Chinese-GermanJoint Symposium on Hydraulic and Ocean Engg.Pilarczyk, K.W., 2003. Design of low-crested (submerged) structures: An overview, 6th COPEDEC (Int.Conf. on Coastal and Port Engg. inDevelop.Countries), Sri Lanka.Teh. H. M., Ismail.H., 2013. Performance Evaluation of Arry of Stepped-Slope Sloating Breakwater. IEE Business Engineering and IndustrailApplications Colloquium (BEIAC). Pp279-283.

Experimental investigations on wave transmission at submerged breakwater with smooth and stepped slopes

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Experimental investigations on wave transmission at submerged breakwater with smooth and stepped slopes

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