Rubble Mound Breakwaters Research Articles

Damage progression in rubble-mound breakwaters scale model tests, under a climate change storm sequence

Damage progression in rubble-mound breakwaters scale model tests, under a climate change storm sequence

STABILITY EFFECTS OF CUBE ARMOR UNIT PLACEMENT CONFIGURATIONS IN THE BERM OF A BREAKWATER

For the sake of overtopping, stability and economy, rubble mound breakwaters have been built with a berm in the seaward slope. Here, a breakwater model with cube armour units in the lower slope and a horizontal berm, and rock in the upper slope was tested experimentally. The purpose of the study was to investigate the stability of berms for two different configurations of the transition of armor units from the lower slope to the berm. Based on the test results, an effective configuration of cube units at the transition has been obtained.

WAVE TRANSMISSION OVER LOW-CRESTED POROUS BREAKWATERS

Low-crested (LC) rubble mound breakwaters are used for coastal protection. The main advantage of these structures is their mild aesthetic impact on the natural environment. As the waves approach and transmit over these structures, significant hydrodynamic processes occur in their proximal area, such as wave breaking, wave reflection, wave overtopping and transmission (Garcia et al., 2004). Many researchers have studied the hydrodynamics of flow in the vicinity of such structures, as well as the influence of their geometrical characteristics on the flow field. However, in most studies, the structures are either emerged or submerged, while the case in which the crest level of the breakwaters is at the still water level (SWL) has to be further investigated.

ROCK ARMOR DAMAGE IN DEPTH-LIMITED BREAKING WAVE CONDITIONS

The armor layer of a mound breakwaters is usually designed with a formula derived from physical tests in non-breaking wave conditions; however, most rubble mound breakwaters are placed in the wave breaking zone where the highest waves break before reaching the structure. The hydraulic stability formulas developed for rock-armored breakwaters in non-breaking conditions are not completely valid to characterize the hydraulic stability of these structures under depth-limited wave attack. In this study, five series of 2D physical tests were carried out on a bottom slope m=1/50 to analyze the hydraulic stability of double-layer rock armored breakwaters in depth-limited breaking wave conditions. Measurements taken by 12 wave gauges placed along the wave flume were compared with estimations of Hm0, H2% and H1/10 obtained from numerical model SwanOne. The significant wave height, Hm0, estimated or measured at a distance 3hs from the toe of the structure was the best characteristic wave to relate armor damage with stability number. The six-power relationship between dimensionless armor damage and stability number, found in this study, explained more than 94% of the variance in the damage observations. This relationship is valid for conventional non-overtopping double-layer rock-armored breakwaters on bottom slope m=1/50 and depth-limited breaking wave conditions.

HYDRAULIC STABILITY AND OVERTOPPING PERFORMANCE OF A NEW TYPE OF REGULAR PLACED ARMOR UNIT

The XblocPlus is a new type of interlocking single layer armour units that is placed with uniform orientation. This is novel and different from all other single layer, interlocking armouring systems. The hydraulic stability of the XblocPlus breakwater armour unit was tested in 2D and 3D hydraulic model tests. Wave overtopping tests were performed to determine the roughness coefficients of the EurOtop overtopping formula for the XblocPlus. Model tests on a rubble mound breakwater with XblocPlus armour included 2D tests with a 1:30 seabed slope and with 1:2 and 3:4 breakwater slopes and 3D model tests with a flat seabed and with a 3:4 breakwater slope. Wave heights up to 150% of the design wave height were tested in the 2D tests and up to 200% with wave directions 0° to 60° in the 3D tests. No armour unit displacements were observed in 2D tests with 1:2 slope. In the 2D tests with 3:4 slope one armour unit was displaced when the wave height reached 159% of the design wave height. No damage to the XblocPlus armour layer was observed in the 3D tests. A roughness coefficient of 0.45 was deduced from overtopping tests with wave heights of 60% to 100% of the design wave height. The model test results indicate little or no influence of wave steepness on XblocPlus stability and no adverse influence of wave obliquity while the seabed slope in front of the breakwater may have some impact on the XblocPlus armour layer stability.

STABILITY ANALYSIS OF RUBBLE-MOUND BREAKWATERS UNDER TSUNAMI FIRST IMPACT AND OVERFLOW

Tsunamis are relatively infrequent but very destructive phenomena that can cause devastating consequences on coastal areas. In view of recent tragic episodes, the scientific community is strengthening their efforts to develop strategies to mitigate the risk of tsunami consequences, specially focused on the potentially affected areas. One of the strategies in this direction is the design and construction of more efficient coastal structures. In this way, in the frame of the EU FP7 project ASTARTE physical experiments on rubble-mound breakwaters (RMB) under tsunami wave attack have been carried out in the IH Cantabria facilities in Santander, Spain. These experiments focused on gaining a better understanding of tsunami impacts on this kind of structures. Improving the knowledge about their stability and hydrodynamics will contribute to a better design of coastal protection marine structures.

PHYSICAL MODEL TESTS OF WAVE OVERTOPPING AND FORCES ON BREAKWATER CROWN WALLS

This paper describes new physical model tests aiming at measuring both wave overtopping and wave induced forces on rubble mound breakwater crown walls. The physical model and the equipment used for the measurements are described in detail. For the completed tests, a detailed analysis is reported, by evaluating the properties of the incoming waves at the toe of the breakwater and some statistical parameters to describe the wave induced forces and pressures on the crown wall. Careful analysis is also carried out to evaluate how the distribution of the pressures changes with time. It is found that the upper part of the wall is subjected to the first large quasi-impulsive action of the wave; the lower part of the wall is afterwards flooded and a quasi-hydrostatic pressure develops along the height of the wave wall. As far as the pressures on the base of the crown wall are concerned, they develop after a quite large time lag after the maximum of the horizontal force. First attempts to correlate the maximum horizontal force with some explanatory variables such as the ratio of the crest freeboard and of the significant wave height of the incoming waves indicate a promising correlation, also in agreement with the existing literature on the topic. The overtopping rate are also measured and compared with empirical formulas. The correlation between the wave induced forces and the average overtopping discharge on the breakwater is also investigated.

NUMERICAL STUDY OF WAVE INTERACTION WITH A SUBMERGED POROUS BREAKWATER IN COMBINATION WITH A FLOATING BREAKWATER

Conventional emerged rubble mound breakwaters are commonly built to protect ports and marinas from direct wave action. With increased high-valued developments in the coastal region, engineers have to design innovative coastal protection structures that can provide adequate harbor tranquility with minimum visual impact. One such solution is the combination of a submerged breakwater with a floating breakwater. In this paper, the open-source CFD model REEF3D (Bihs et al. 2016) is used to simulate wave interaction with a submerged porous breakwater with a floating breakwater on the lee side. The wave interaction with the submerged porous breakwater is validated by comparison with experimental data from Hieu and Tanimoto (2006). The validated model is then used to simulate the wave interaction with a combination of the submerged breakwater and the floating breakwater. The transmission coefficient across the combination is calculated to be about 12.5%, demonstrating the possibility to provide harbor tranquility with minimum visual impact.

EXPERIMENTS ON THE EFFECT OF FREEBOARD ON THE STABILITY OF A BREAKWATER CROWN WALL

A crown wall is a gravity based structure on top of a rubble mound breakwater which gains stability due to its own weight and friction between the base and contact surface of the rubble mound. An experimental research is carried out in which the global stability of a crown wall is investigated depending on, amongst other things, freeboard. Until now, these structures tend to be designed in a too conservative way, when the vertical distance between base slab and water level (base freeboard) increases. It was concluded that current design methods overpredict vertical loads for increasing freeboard. By implementing a reduction factor for the upward pressure acting on the base slab of the crown wall, a better prediction can be made for the transition region between stability and failure.

ACCUMULATED DAMAGE EVOLUTION AND INVESTMENT COSTS OF BREAKWATERS

Since the pioneering work by Borgman, (for vertical walls), and Paape, (for rubble mound breakwaters) in the 50´s of the last century, the design of new breakwaters and rehabilitation of existing breakwaters can be based on lifecycle analysis targeting the optimum investment costs, including the construction costs, maintenance, repairs and demolition, costs, and downtime costs. The estimation of these costs is based on the performance criteria of each failure mode for the structural and hydraulic responses related to the design limit stages SLS and ULS, including both a physical measure and a probability of occurrence, ROM 0.0 and 1.0 (2000 and 2007) and PIANC 2017. To evaluate these costs including their uncertainty it is necessary to apply, (1) a spatial and temporal damage evolution model of each failure mode, (2) to define the interdependence between modes including the decision criteria when start to repair, and (3) to compute the costs depending on the real sea conditions, the strategy of construction and the available construction power. The main objective of this work is to present a methodology to estimate the probability distribution function of the investment costs in the useful life of the structure.

OVERTOPPING AND STABILITY OF A RUBBLE MOUND BREAKWATER WITH CORELOC ARMOUR UNDER OBLIQUE CYCLONIC WAVES

This note presents the physical model tests carried out on the new breakwater in the fishing harbour of Mirbat (Sultanate of Oman), designed for a cyclonic wave Hs = 6.0 m and for a 200 year return period wind wave Hs = 3.2 m. Based on this experience, the behaviour of a rubble mound breakwater subject to wind waves that are much smaller than the design cyclonic wave height is discussed. We shall define the wave climate in this case as “cyclonic”. The initial interlocking (during construction) that does not “grow” during the lifetime as usual and the expected cyclonic wave is likely to produce larger damage than predicted by the design formulae, that on the contrary rely on such interlocking. Under oblique waves, the importance of unit interlocking is even greater due to the sub-horizontal loads and therefore the case of cyclonic wave climates requires a larger safety margin.

NUMERICAL SIMULATION OF INTERACTIONS BETWEEN WATER WAVES AND A MOORED-FLOATING BREAKWATER

Floating breakwaters offer a cost-efficient alternative to common emerged rubble mound breakwaters in deep water coastal areas or harbors with poor seabed conditions. Often, simple box type structures with a suitable mooring system are considered. The motion of the moored box and the resulting tension forces in the cables have to be known during the design process. The accurate determination of these properties is therefore of high significance to produce a safe and economical design. In this paper, a numerical model for determining the influence of the mooring system on the floating body dynamics is developed and incorporated into the opensource CFD solver REEF3D.

NUMERICAL MODELING OF WAVE INTERACTION WITH A NON-CONVENTIONAL BREAKWATER FOR WAVE ENERGY CONVERSION

The hybrid non-conventional breakwaters are innovative coastal structures, which have as a primary function the coastal and harbours protection, but with the important benefit of electricity production, due to their combination with Wave Energy Converters. The most recent example of a non-conventional breakwater is called OBREC, standing for Overtopping Breakwater for Energy Conversion (Vicinanza et al., 2014). The device consists of a traditional rubble mound breakwater, in which the seaward armour layer in the upper part is replaced with a frontal sloping ramp and a reservoir. The structure is designed to capture and gather the water that overtops the crest ramp. The potential energy of the water stored in the reservoir is converted into kinetic energy and then into electrical energy by flowing through low head hydraulic turbines coupled with generators, exploiting the different water levels between the reservoir and the sea level.

EXPERIMENTAL STUDY OF THE FLOW INDUCED BY WAVES IN THE VICINITY OF A DETACHED LOW-CRESTED (ZERO FREEBOARD) BREAKWATER

The combined action of environmental forcing (waves, coastal currents, sediment transport, e.t.c.), the continuously decreasing supply of coastal areas with sediment from rivers, as well as the intense anthropogenic activity, results in the appearance of severe erosion problems in coastal areas and constantly decreasing beach width. A frequently used coastal protection measure is the construction of detached breakwaters parallel to the coastline. Detached breakwaters have a direct effect on the incoming waves, which contributes to the control of coastal sediment transport, hence the morphodynamics of the coastal bed. There are many examples of such structures, the majority of which are emerged breakwaters. Recently, interest has been directed towards the construction of low-crested (LC) and submerged breakwaters due to the reduced construction cost and a more effective harmonization with the natural environment. These structures are characterized by wave overtopping and breaking over their crest in addition to all other coastal processes that are involved with emerged breakwaters. For the proper design of such structures, one critical aspect is the behavior of the induced flow in their vicinity due to their presence. To this purpose, several studies have been carried out in recent years. In particular, Petti et al. (1994) studied experimentally the large scale vortices developed by waves breaking above a submerged breakwater. Mory and Hamm (1997) performed measurements of wave height, surface elevation and wave generated currents around a detached breakwater for incoming regular and irregular waves. Kramer et al. (2005) performed a series of experimental measurements in order to study the waves - LC structure interaction, in terms of flow velocity and turbulence developing around such structures within the European Project DE.LO.S. Garcia et al. (2004), Losada et al. (2005), Johnson et al. (2005) e.t.c. used the database created within the DE.LO.S. project to develop and validate numerical codes for the simulation of wave-induced flow around LC breakwaters. The aim of the present study was the experimental study of the flow developed by waves in the vicinity of an LC rubble mound breakwater with crest level at the water line (zero freeboard). The geometrical scale of the physical model was 1/30. The breakwater was placed on a beach of constant slope 1/15, which is typical of steep beaches in Greece.

Stability analysis of a non-conventional breakwater for wave energy conversion

This paper presents the stability analysis of a non-conventional breakwater cross-section integrating an overtopping wave energy converter, named OBREC. The device consists of a traditional rubble-mound breakwater in which the upper part of the armour layer is replaced by a smooth ramp and a reservoir.The analysis of the structure is carried out by combining model scale experiments and numerical simulations based on the Volume-Averaged Reynolds Averaged Navier-Stokes (VARANS) equations.The numerical analysis is used to complete and extend the results of the physical model test campaign, providing a deeper understanding of the pressure distribution and resultant force behaviour in locations where laboratory measurements were difficult to obtain or not available. Results show that the maximum vertical and horizontal total forces on the device are not simultaneous. At the time instant of the maximum total horizontal force, the vertical force is zero or directed downward, due to the significant positive contribution of the force acting on the sloping ramp. Additional numerical simulations show the influence of the submerged ramp length on the forces acting on the structure using the global and local stability analysis. The presence of the shaft contributes positively to the global stability against the sliding failure mode by reducing the uplift force exerted on the horizontal base. Moreover, the stability analysis shows that the critical conditions for the global failure modes occur at different time instants.

Wave response to a submerged wedge-shaped rubble mound breakwater

Wave response to a submerged wedge-shaped rubble mound breakwater

Least Squares Support Vector Mechanics to Predict the Stability Number of Rubble-Mound Breakwaters

In coastal engineering, empirical formulas grounded on experimental works regarding the stability of breakwaters have been developed. In recent years, soft computing tools such as artificial neural networks and fuzzy models have started to be employed to diminish the time and cost spent in these mentioned experimental works. To predict the stability number of rubble-mound breakwaters, the least squares version of support vector machines (LSSVM) method is used because it can be assessed as an alternative one to diverse soft computing techniques. The LSSVM models have been operated through the selected seven parameters, which are determined by Mallows’ Cp approach, that are, namely, breakwater permeability, damage level, wave number, slope angle, water depth, significant wave heights in front of the structure, and peak wave period. The performances of the LSSVM models have shown superior accuracy (correlation coefficients (CC) of 0.997) than that of artificial neural networks (ANN), fuzzy logic (FL), and genetic programming (GP), that are all implemented in the related literature. As a result, it is thought that this study will provide a practical way for readers to estimate the stability number of rubble-mound breakwaters with more accuracy.

Optimized Reliability Based Upgrading of Rubble Mound Breakwaters in a Changing Climate

The present work aims at presenting an approach on implementing appropriate mitigation measures for the upgrade of rubble mound breakwaters protecting harbors and/or marinas against increasing future marine hazards and related escalating exposure to downtime risks. This approach is based on the reliability analysis of the studied structure coupled with economic optimization techniques. It includes the construction of probability distribution functions for all the stochastic variables of the marine climate (waves, storm surges, and sea level rise) for present and future conditions, the suggestion of different mitigation options for upgrading, the construction of a fault tree providing a logical succession of all events that lead to port downtime for each alternative mitigation option, and conclusively, the testing of a large number of possible alternative geometries for each option. A single solution is selected from the total sample of acceptable geometries for each upgrading concept that satisfy a probabilistic constraint in order to minimize the total costs of protection. The upgrading options considered in the present work include the construction or enhancement of a crown wall on the breakwater crest, the addition of the third layer of rocks above the primary armor layer of the breakwater (combined with crest elements), the attachment of a berm on the primary armor layer, and the construction of a detached low-crested structure in front of the breakwater. The proposed methodology is applied to an indicative rubble mound breakwater with an existing superstructure. The construction of a berm on the existing primary armor layer of the studied breakwater (port of Deauville, France), seems to be advantageous in terms of optimized total costs compared to other mitigation options.

Dynamic response and liquefaction analysis of seabed-rubble mound breakwater system under waves

Dynamic response and liquefaction potential of granular seabed around a rubble mound breakwater is investigated under waves. Dynamic response of the system is assumed to be governed by coupled Biot equations of poroelasticity. Mathematical formulations in terms of governing equations of partially-dynamic and quasi-static cases are considered. Dynamic response of seabed soil is analyzed using classical finite elements and a number of parametric studies are performed to draw a complete picture of the response of the system. Vertical displacement, shear stress and pore water pressure distributions are considered as the most representative variables defining the variations of response of the whole system and which are thus evaluated along some key cross sections around the breakwater. The results indicate that inertial terms associated with solid skeleton have almost no effect on the dynamic response and liquefaction potential. It was found that large amount of wave-induced shear stresses are taken by a relatively rigid core layer contributing to the breakwater stability where also the pore pressures dissipate significantly towards the seabed interface. Standing wave-induced instability of the system is also investigated in terms of instantaneous liquefaction based on the effective mean stress criterion. There is liquefaction observed at the seaward surface of the rubble whose depth increases significantly with decreasing saturation. While the core overall provides more stability, the adverse effect of this is more liquefaction in the outer layer.

Investigation into the use of macro synthetic fibre reinforced concrete for breakwater armour units

A common form of coastal protection in New South Wales, Australia, is the rubble mound breakwater. This consists of quarried rock or unreinforced concrete armour units. However, increased impact forces on these structures due to greater storm surge levels that are predicted by global warming models, suggest that they will require strengthening. Traditionally steel bar reinforcement is used to strengthen concrete; however, as steel is particularly susceptible to corrosion in aggressive coastal environments, it is not a robust solution. This paper reports on the potential of using macro-synthetic fibre reinforcement in reinforcing of breakwater armour units, in lieu of traditional steel reinforcing bars. To this end, a programme to ascertain the tensile capacity of 0.6 percent and 1.0 percent by volume of concrete reinforced with macro-synthetic fibres was implemented. The behaviour of this synthetic fibre reinforced concrete in Dolos type breakwater armour units was then examined using the finite element analysis software ATENA. The results of the numerical analysis indicate that the addition of synthetic fibres at these volume percentages into concrete armour units improves the post peak response under impact loading by a factor of two to three, compared with unreinforced units. Furthermore, the addition of fibres significantly reduced the maximum crack width for the same number of impact loadings and allows the units to withstand larger crack widths before failure.