Conventional Rubble Mound Breakwaters Research Articles

Numerical Tools For Wave Overtopping At Rubble Mound Breakwaters With Submerged Berms

In the era of climate change, the adaptation of existing coastal structures as a response to potential increasing loads, has become a trending topic in coastal engineering. The combination of sea level rise, storm surge, tides and waves is going to dramatically increase the wave overtopping. In this context, this research is focused on the evaluation of wave overtopping for conventional rubble mound breakwaters, modified by the introduction of a submerged berm. Recent studies illustrated how the introduction of emerged berms on the seaward slope reduces the wave overtopping. The goal then becomes to turn the spotlight on the submerged berms, whose effectiveness has yet to be attested, although it has proven useful in reducing wave loads on the armour layer and seabed pressure under the structure. Therefore, the reliability of two different numerical models in detecting overtopping phenomenon have been assessed. IHFOAM and SWASH have been used (see Figure 1). The first one can solve both Reynolds Averaged Navier Stokes and the Volume Averaged Reynolds Averaged Navier–Stokes equations. The second one solves the non-linear shallow water equations with a non-hydrostatic pressure term, representing a simplified form of the Navier-Stokes equations, with associated limited computational cost. The numerical models have been validated based on experimental tests carried out in a wave flume at the University of L’Aquila.

A NEW SEAWATER LOW-HEAD TURBINE FOR THE OBREC

The OBREC (Overtopping Breakwater for wave Energy Conversion) is based on the overtopping phenomenon [1] where incoming waves run over an overtopping ramp and fall into a reservoir located above the sea level inside a conventional rubble mound breakwater, or into a vertical caisson breakwater. The wave energy is transformed into potential energy, with some contribution of kinetic energy. Then, the flow is driven through a turbine addressing the final transformation into electrical energy. The OBREC prototype is located in the middle of the San Vincenzo breakwater, in Naples harbour (Italy). The key target of the whole pilot project was to demonstrate the high capacity factor of the system (expressed as the ratio of the electrical energy produced to the electrical energy that could have been produced at continuous full power operation), even in a low-energy wave climate. At the study site, in fact, the yearly average wave power was found to be 1.8 kW/m over the last 42 years [2]. However, during last 5 years, a maximum wave height of over 5 m has been registered, confirming the high reliability under severe wave conditions.
 Due to the short monitoring period and to the undersize of the previous power take-off (PTO) systems, no definitive power matrix is available for OBREC [3]. The energy conversion efficiency of OBREC has been deeply investigated over the last years through both physical and numerical model tests. Within the ongoing full-scale monitoring activities in real environments, a new set of power take-off was installed. A propeller-type pico-turbine equipment was in 2021, and its final setting was completed in 2022. Performance and reliability of the entire structure were closely monitored under different “stress tests”, i.e. start & stop cycle in marine environment with a high variable combination of flow rate/hydraulic head.
 This paper presents the design stage for the new equipment and preliminary results from these tests numerical and field tests.
 
 
 Contestabile, P., Crispino, G., Di Lauro, E., Ferrante, V., Gisonni, C., & Vicinanza, D. (2020). Overtopping breakwater for wave Energy Conversion: Review of state of art, recent advancements and what lies ahead. Renewable Energy, 147, 705-718.
 Contestabile, P., Russo, S., Azzellino, A., Cascetta, F., Vicinanza, D. (2022). "Combination of local sea winds/land breezes and nearshore wave energy resource: Case study at MaRELab (Naples, Italy)", Energy Conversion and Management, ISSN 0196-8904, 257, 115356, https://doi.org/10.1016/j.enconman.2022.115356 
 Mariani, A., Crispino, G., Contestabile, P., Cascetta, F., Gisonni, C., Vicinanza, D., & Unich, A. (2021). Optimization of Low Head Axial-Flow Turbines for an Overtopping BReakwater for Energy Conversion: A Case Study. Energies, 14(15), 4618.

ANALYSIS AND PRELIMINARY DESIGN OF A TANGENT BORED PILE WALL AS AN ALTERNATIVE TO CONVENTIONAL BREAKWATERS

Based on existing construction practices, and the difficulty of sourcing rock armor materials in the Philippines, an alternative to conventional rubble mound breakwaters was considered for a beach resort development in Cebu, Philippines. A tangent bored pile wall, consisting of circular piles placed diametrically along a specified alignment, was analyzed and designed as a breakwater. It relies on pile embedment depth and soil friction for stability, unlike conventional breakwaters which rely on the weight of the armor rocks and the interlocking of individual armor units.

Influence of nonstationarity and dependence of extreme wave parameters on the reliability assessment of coastal structures - A case study

The current general consensus is that because of global climate change, coastal infrastructures may be subject to more extreme loads in the future than in the past. There has been an increased recognition that more attention should be paid to incorporating nonstationarity within the design process of coastal structures. In this regard, the present study examines the reliability of a conventional rubble mound breakwater using a variety of probabilistic techniques, incorporating ten nonstationary models to study the impacts of nonstationarity, as well as six copula functions to explore the dependence effects of wave extremes. The Gumbel copula most accurately represents the dependence structure of wave heights and periods in the study area. When the dependence of wave parameters is not correctly modeled, the failure probability of the studied structure is significantly underestimated. Assuming the Gumbel copula for modeling dependence of wave parameters, the results indicate that the maximum failure probability of the structure is underestimated by about 33% when a stationary extreme value distribution is fitted to wave height extremes, compared to a nonstationary Generalized Pareto Distribution (GPD). The insights gained from this study suggest that considering nonstationarity in extreme coastal events has more significant impacts on the reliability of coastal structures when the dependence structure of the marine variables is accurately modeled.

Long wave absorption by a dual purpose Helmholtz resonance OWC breakwater

In this paper, a novel hybrid breakwater-WEC system consisting of a Helmholtz-resonance oscillating water column (OWC) caisson array and a perforated wall is proposed to attenuate long waves. The principle of Helmholtz-resonance OWC is analogous to the acoustic Helmholtz resonator for noise reduction. A semi-analytical model was developed based on potential flow theory using the matching eigenfunction method to investigate wave interaction with the OWC-breakwater system. The velocity singularity at the tip of the wall is resolved using the Galerkin method to improve the model convergence. The semi-analytical model of the present wave-air-structure interaction problem is verified using Haskind relation and energy conservation rule. Furthermore, we validate the model using numerical and experimental data in terms of wave reflection and wave power absorption. Hydrodynamic characteristics of the proposed hybrid breakwater-WEC system and the influence of key geometrical and wave parameters were investigated. Theoretical results indicate that long waves can be effectively absorbed by the OWC caisson through Helmholtz resonance for both normal and oblique waves. The resonance frequency of OWC varies with the chamber wall draft and chamber width in the along-shore direction. It was found that installing a perforated wall in front of the OWC array could broaden the frequency bandwidth of significant wave attenuation. The along-shore length of each OWC chamber ( 2l ) and the incident wave angle ( θ ) are identified as the key parameters that affect the hydrodynamic performance of the system at high frequency range due to the along-shore sloshing of water column, but have a relatively minor effect on long waves attenuation. The present OWC-breakwater system demonstrates advantage and superior performance over conventional rubble mound breakwaters and breakwaters of multiple perforated walls in attenuating long waves. The findings of this paper provide guidance for a novel design of breakwaters for absorbing long waves. • Novel Helmholtz-resonance OWC breakwater with a perforated wall is proposed to absorb longer waves．. • Semi-analytical model of wave interaction with OWC array with perforated wall is developed and validated．. • Present hybrid OWC-breakwater system is more effective in absorbing long waves than conventional breakwaters. • Significant effect of incident wave angle on wave reflection is observed at higher frequency range but not at low frequency range.

Design of a rubble mound breakwater under the combined effect of wave heights and water levels, under present and future climate conditions

Design of coastal structures without considering dependence of environmental variables can lead to over- or under estimation of their cross-sectional dimensions, imposing significant economic costs on marine projects. In this paper, the design of a conventional rubble mound breakwater at the Makran coasts on the southern coast of Iran has been carried out using a commonly applied, as well as a joint probability approach. The results of these two design methods have been compared. In this regard, joint probability analysis of water level and significant wave height has been performed. For this purpose, first an appropriate threshold was determined using a systematic and methodological approach, and then using the Copula functions, a joint probability distribution was fitted. In the next step, the critical point of the rubble mound breakwater design was determined, acting as the basis of further investigations. The results indicate that the volume of armor and filter layers of the breakwater are reduced by about 5% and 10%, respectively, which is very economically attractive for engineering projects. Also, suggestions were made for an appropriate selection of 1- and 50-year tidal levels for the common design method to be used in the design codes. Finally, the effects of climate change on the overtopping failure mechanism of the breakwater were discussed, indicating that a higher freeboard and a lower overtopping discharge in the new design approach can reduce vulnerability of the structure against climate change effects in the future.

The role of submerged berms on the momentary liquefaction around conventional rubble mound breakwaters

Berms deployed at the toe of conventional rubble mound breakwaters can be very effective in improving the stability of the armor layer. Indeed, their design is commonly tackled by paying attention to armor elements dimensioning. Past research studies showed how submerged berms can increase the stability of the armor layer if compared to straight sloped conventional breakwaters without a berm. To fill the gap of knowledge related to the interaction between breakwaters with submerged berm, waves and soil, this research aims to evaluate how submerged berms configuration influences the seabed soil response and momentary liquefaction occurrences around and beneath breakwaters foundation, under dynamic wave loading. The effects of submerged berms on the incident waves transformation have been evaluated by means of a phase resolving numerical model for simulating non-hydrostatic, free-surface, rotational flows. The soil response to wave-induced seabed pressures has been evaluated by using an ad-hoc anisotropic poro-elastic soil solver. Once the evaluation of the seabed consolidation state due to the presence of the breakwater has been performed, the dynamic interaction among water waves, soil and structure has been analyzed by using a one-way coupling boundary condition. A parametric study has been carried out by varying the berm configuration (i.e. its height and its length), keeping constant the offshore regular wave condition, the berm and armor layer porosity values, the water depth and the elastic properties of the soil. Results indicate that the presence of submerged berms tends to mitigate the liquefaction probability if compared to straight sloped conventional breakwater without a berm. In addition, it appears that the momentary liquefaction phenomena are more influenced by changing the berm length rather than the berm height.

Prediction of wave transmission over submerged reef of tandem breakwater using PSO-SVM and PSO-ANN techniques

ABSTRACT Protection of the damaged breakwater from the high-intensity wave action has become inevitable. Submerged reef can act as a protective structure in reducing the wave action. Further, placed the reef structures on the sea side of a conventional rubble mound breakwater will reduce the effects of wave action. The conventional breakwater and reef structure combination is a tandem breakwater. Keeping in mind the end goal to decrease the complexities associated in model scaling, time constraints and cost in conducting the experiments, an attempt is made to apply soft computing techniques such as an Artificial Neural Network (ANN) and Support Vector Machine (SVM) to model various problems of real case scenario, where mathematical modelling is also difficult. In the present study, Particle Swarm Optimization (PSO) optimizes various parameters of ANN and SVM model in predicting the wave transmission over a submerged reef of the tandem breakwater. The performance of proposed hybrid models such as PSO-ANN and PSO-SVM is evaluated using statistical indices. The results show that PSO-SVM tool performs better in predicting the wave transmission compared to PSO-ANN.

Effects of submerged berms on the stability of conventional rubble mound breakwaters

Berms deployed at the toe of conventional breakwaters may be needed to reduce bottom settlements and to limit scour in front of the structure due to coastal currents. In the mean time, they may be effective in increasing the stability of the armor layer and also in minimizing the wave overtopping discharge compared to straight sloped conventional breakwaters without a berm. This research aims to provide a new design criterion for the armor layer of conventional breakwaters with submerged berms marked by small thickness compared to water depth. Indeed, past researches focused on the influence of relatively high berms on the stability of the armor layer. The design of the berm itself is not tackled herein. The effects of submerged berms on the incident waves transformation have been evaluated by means of a numerical model, validated by using experimental data. Then, a parametric correction factor of the incident significant wave height at the toe of the structure is provided and included in well established design criteria. The experimental comparison confirms the reliability of the proposed method by highlighting the importance to use design criteria within their validity ranges, in order to avoid an unsafe dimensioning of the armor elements.

Design Optimization of a Porous Box-Type Breakwater Subjected to Regular Waves

Breakwaters are used to suppress the energy of waves for providing shelter to coastal and offshore facilities. Very often, the conventional rubble mound breakwaters result in high construction cost and several environmental problems, such as water contamination and wave amplification in front of the structures due to severe wave reflection. One way to alleviate the above-mentioned problem is to appropriately increase the porosity of the breakwaters. This paper aims at developing the optimum design of a porous box-type breakwater comprising multiple scrapped pipelines via physical modelling approach. Herein, the best geometrical design of the breakwater under the governing factors of porosity, width and internal tube length is proposed. A series of experiments have been conducted under the influence of regular wave environment through the analysis of wave transmission, reflection and energy loss. Several geometrical design criteria were derived to maximize the hydraulic performance of the breakwater, when adopted at sites. The proposed breakwater is a reasonably good wave attenuator and anti-reflection structure as well as an effective energy dissipator.

Computational Intelligence on Hydrodynamic Performance Characteristics of Emerged Perforated Quarter Circle Breakwater

Protecting the lagoon area from the wave attack is one of the primary challenges in coastal engineering. Due to the scarcity of rubble and to achieve economy, new types of breakwaters are being used in place of conventional rubble mound breakwaters. Emerged Perforated Quarter Circle Breakwater (EPQCB) is an artificial concrete breakwater consisting of a curved perforated face fronting the waves, a vertical wall on back and a base slab resting on a low rubble mound base. The perforated curved front face is having advantages like energy dissipation and good stability with less material as it is hollow inside. Computational Intelligence (CI) can be adopted for the evaluation of performance characteristics like reflection, dissipation, run-up and rundown which are complex, time consuming and expensive to perform in laboratory. The paper presents the work carried out to predict the reflection coefficient (Kr) for input parameters, wave period (T) beyond the data range used for training and of wave height (H) along with the data on input parameters of water depth (d), spacing-perforation ratio (S/D) and radius (R) of the EPQCB. The data on various parameters are taken in two categories for training and testing of ANN as mentioned below in order to understand the effect of using non-dimensional data in place of parametric values: 1) Input in the form of parametric data (H, T, d, R, S, D), and 2) Input in the form of non-dimensional values (H/gT2, d/gT2, S/D, R/H). Better correlation was found when individual dimensional parametric data was used instead of non-dimensional group values in both the methods of prediction. Similarly, the correlation between the beyond the data range prediction and actual values was found to be good in both methods of prediction.

ICELANDIC-TYPE BERM BREAKWATER FOR THE HAMBANTOTA ARTIFICIAL ISLAND REVETMENT, APPLICATION OF GEOMETRICAL DESIGN RULES

The paper discusses the development of a berm breakwater revetment design for protection of an artificial island on the southern shore of Sri Lanka. The island will primarily be constructed with excavated materials from the Phase 2 Hambantota Port construction activities, which include considerable volumes of slightly weathered rock. A value Engineering Study considered three different design options for the protective revetment: two rock options, conventional rubble mound and berm breakwaters, and an option using Chinapode concrete armour units. There were considerable cost-savings in choosing an Icelandic-type berm breakwater. The geometrical design rules of Icelandic-type berm breakwaters, developed by Van der Meer and Sigurdarson (2014), were used to develop different cross-sections based on different armourstone classes. Based on a quarry yield prediction and hydraulic model studies, the design was optimized using the available rock as efficiently as possible. Refinement of the design during the hydraulic model testing resulted in increased stability of the berm breakwater. Due to lack of experience in the production of large armourstone, the yield of the 5-10 tonnes class was rather low during the first months of construction. A number of measures were recommended resulting in more than tripling the yield.

Study on performance of Savonius rotor type wave energy converter used in conjunction conventional rubble mound breakwater

In the present study the performance characteristics of a wave energy converter used in conjunction with conventional rubble mound breakwater is investigated using physical model studies. Savonius rotor type converter is used in the present study. The rotor is placed in front of the breakwater towards seaward side can cause substantial wave attenuation and thereby reduce the impact on the breakwaters apart from generating electricity. Regular waves of wide ranging heights and periods are considered in the present study. Tests are carried out for different spacing between the breakwater and the wave energy converter (X/d=10 to 40) and for two depth cases viz. shaft of the rotor at SWL (z=0) and rotor fully submerged case (z=−55mm). The dead weight loading (shaft power) capacity for the rotor is also optimised in the present study. From the experimental study, it is observed that at a distance of X/d=22.5 to 30 and for submerged case (z=−55mm) the rotor is found to be most efficient. It results in a wave height attenuation of 15–33%.

WAVE OVERTOPPING AT BERM BREAKWATERS IN LINE WITH EUROTOP

The paper presents the development of a new overtopping formula for berm breakwaters. Overtopping data from hydraulic model tests of berm breakwaters have been gathered and reanalysed in line with the procedure in the EurOtop Manual. The data shows a clear dependency on wave period or wave steepness, which is in contrast to the main conclusion of the CLASH project and the EurOtop Manual for conventional rubble mound breakwaters. The formula is roughly validated on prototype performance.

Review of the research on emerged and submerged semicircular breakwaters

Scientists and researchers are currently focusing considerable attention on the hydrodynamic characteristics of semicircular breakwaters worldwide, particularly in Japan and the People’s Republic of China. A semicircular breakwater is a composite breakwater made of prestressed concrete and has several merits over a conventional rubble mound breakwater, e.g. it is technically advanced, economically viable and aesthetically pleasing. At low water levels, it serves as a rubble mound breakwater and, at high water levels, it acts as a composite breakwater in the emerged condition. It reduces the considerable magnitude of dynamic forces if constructed in front of a vertical wall (the main structure) under the submerged condition and also reduces coastal erosion along the shoreline. Many research studies are being carried out all over the world to evaluate the performance of a semicircular breakwater in the laboratory and in the field. The authors of this review paper have also conducted a large number of experiments to study the hydrodynamic characteristics of a semicircular breakwater and to optimize the parameters such as the rubble mound height, the water depth and the size of perforations. The experiments were conducted for both the emerged condition and the submerged condition subjected to regular and random waves with various wave heights and periods to find the suitability of a semicircular breakwater in the field, and the optimum parameters were recommended in their previous publications. Hence, a sincere attempt has been made in this review paper to present the ongoing research on semicircular breakwaters, which has been carried out all over the world to facilitate the studies of researchers and to provide related information to people working in the area of semicircular breakwaters.

Wave induced loading of the reshaping rubble mound breakwaters

Economic berm breakwater design requires knowledge of the wave induced loading of the breakwater. In order to investigate the wave load of the reshaping berm breakwaters, model tests have been carried out to measure the wave force on a berm breakwater stone before and after reshaping. Wave force measurements were carried out with the force “measuring stone” placed at three elevations: one elevation above the still water line (SWL) and two elevations below SWL. The measurements show that the maximum forces occur at the location above the still water level. The maximum forces at all of the three levels occur at wave run-up and are directed into the breakwater, thus stabilizing the stone. The maximum forces during wave run-down are directed outwards from the breakwater slope, as expected. The comparison between the measured forces before and after the reshaping around the still water level show that the wave force before reshaping is higher than the force after reshaping. This partly explains why the berm breakwater requires lower weight of the armor units than conventional rubble mound breakwaters. An additional explanation is that during the reshaping the slope of the berm breakwater becomes flatter and the destabilization gravity force component down the slope becomes smaller.

On berm breakwaters: Recession, crown wall wave forces, reliability

The paper reviews different formulae, Lykke Andersen (2006), Lykke Andersen and Burcharth (2010), Tørum (2007) and Moghim et al. (2011), for recession of berm breakwaters and compares calculated recession with previous and new recession test data. There are differences of the results by different formulae, but by and large all the formulae give reasonable results on the recession for shallow water berm breakwaters. It seems that the Lykke Andersen (2006) formula works best for the deep water cases although the Tørum (2007) formula seems to work as good for the homogenous berm breakwater case. The Moghim et al. (2011) formula was not included in the comparison for the deep water cases because the deep water cases are outside the validity of the formula. Results of tests on different alternative berm breakwater designs, which may be considered as practical/economic designs, have also been included, but without any comparison with any formulae simply because the formulae do not cover the specific designs. Wave forces from model tests on crown walls on berm breakwaters are presented and compared with an existing formula for wave forces on a crown wall on conventional rubble mound breakwaters, Pedersen (1995). It appears that the wave forces on a crown wall on a berm breakwater are smaller than the wave forces on a crown wall on a conventional rubble mound breakwater. Finally a probability of failure analysis of a site specific berm breakwater, the Sirevåg berm breakwater, Norway, is presented, using Monte Carlo simulations. The probability of failure analysis shows that the Sirevåg berm breakwater is extremely strong.

Core made of geotextile sand containers for rubble mound breakwaters and seawalls: Effect on armour stability and hydraulic performance

A systematic armour stability and the hydraulic performance, including wave reflection, wave transmission, experimental study in the twin-wave flumes of Leichtweiss-Institute (LWI) is performed on a geocore breakwater and a conventional rubble mound breakwater in order to comparatively determine the wave run-up and wave overtopping. The geocore breakwater consists of a core made of sand-filled geotextile containers (GSC) covered by an armour made of rock. The geocore is more than an order of magnitude less permeable than the quarry run core of a conventional breakwater. As expected, the core permeability substantially affects the armour stability on the seaside slope, the wave transmission and the wave overtopping performance. Surprisingly, however, wave reflection and hydraulic stability of the rear slope are less affected. Formulae for the armour stability and hydraulic performance of the geocore breakwater are proposed, including wave reflection, transmission, run-up and overtopping.

Effect of perforations and rubble mound height on wave transformation characteristics of surface piercing semicircular breakwaters

The present study investigated how the perforations, water depth and rubble mound height on fully perforated semicircular breakwater (SBW) affects non-breaking wave transformations. SBW model with surface piercing condition for three different perforation ratios with 7 percentage, 11 percentage and 17 percentage were considered to study the variation of reflection, transmission, run-up characteristics and dimensionless horizontal and vertical forces as a function of relative water depth and the results are compared with an impermeable SBW and seaside perforated SBW models. From the results it is understood that, SBW with perforation ratio 17 percentage in the case of seaside perforated case shows reverse trend in hydrodynamic characteristics and for fully perforated SBW, it transmits large amount of wave energy on the seaside, which affects the tranquillity condition in the harbour. In addition, transmission characteristics of SBW models and conventional rubble mound breakwater model are compared to understand the effect of composite breakwater action and also the reflection characteristics of SBW models are compared with field data of Miyazaki Port after Sasajima et al. (1994). The results reveal that the SBW model with perforation ratio of 11 percentage in seaside and fully perforated type gives an optimum performance in terms of energy dissipation and transmission.

Numerical analysis of wave loads for coastal structure stability

Abstract The numerical model COBRAS-UC [Losada, I.J., Lara, J.L., Guanche,R., Gonzalez-Ondina, J.M. (2008). Numerical analysis of wave overtopping of rubble mound breakwaters. Coastal Engineering, Vol 55 (1), 47–62.] is used to carry out a two-dimensional analysis of wave induced loads on coastal structures. The model calculates pressure, forces and moments for two different cross-sections corresponding to a low-mound and a conventional rubble-mound breakwater with a crown-wall under regular and irregular incident wave conditions. Predicted results are compared with experimental information provided in Losada et al. [Losada, I.J., Lara, J.L., Guanche,R., Gonzalez-Ondina, J.M. (2008). Numerical analysis of wave overtopping of rubble mound breakwaters. Coastal Engineering, Vol 55 (1), 47–62.] and Lara et al. [Lara, J.L., Losada, I.J., Guanche, R. (2008). “Wave interaction with low mound breakwaters using a RANS model”. Ocean engineering (35), pp 1388–1400; doi:10.1016/j.oceaneng.2008.05.006.] on a 1:20 scale. Good agreement is found, and the differences between both typologies are explained in detail. Additionally, numerical results are also compared with several semi-empirical formulae recommended for design at both the 1:20 model scale and two prototype cross-sections. Results suggest that COBRAS-UC is able to provide realistic stability information that can be used to complete the approach based on currently existing methods and tools.