Rubble Mound Research Articles

SUBMERGED GROYNES FOR BEACH STABILISATION

Typically, rubble mound groynes are constructed by end tipping from trucks for which the roadway level must be above high tide. Some adverse effects of such surface piercing groynes include the generation of rips along their trunks (Fleming 1990; Scott et al., 2016), which can transport sand off the beach and out of the groyne compartment. Further, rubble mound groynes have large footprints that may smother benthic habitat. A submerged groyne may obviate such potential adverse impacts. Submerged groynes are used in England to stabilise shingle beaches (Simm et al., 1996). The groyne extends offshore but underwater, protruding far enough above the seabed to arrest alongshore transport of littoral drift. However, scale modelling (Jensen 1997) showed that groynes remaining below the water surface allow for the expansion of rip currents and, hence, a reduction in their velocity and their capacity to transport littoral drift offshore and beyond the groyne compartment. Further, submerged groynes can comprise sheet piling, which may be timber (as used in UK), fibre-reinforced plastic, steel or concrete, which present a negligible footprint, having a minimal impact on benthic habitat.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/UbmLhdXCVj0

Instability evaluation of rubble mound for breakwaters subjected to tsunami-induced overflow and seepage flow

This paper presents a formula to estimate the stable weight of a rubble mound for breakwaters, considering the tsunami-induced overflow and seepage flow, based on the theoretical force balance among the frictional resistance of the rubble mound, the tractive force of the water flow over the caisson to the rubble mound and the seepage force of the rubble mound through the breakwaters. A series of hydraulic model tests was conducted on the scale of 1/100 for the Kamaishi Harbor Mouth Breakwaters in order to confirm the effectiveness of the proposed formula. Using the test results, the stability of rubble mounds for breakwaters was evaluated from the geotechnical engineering and hydrodynamical points of view.

Subsidence prediction of a rubble mound breakwater at Yantai port: A application of FSSI-CAS 2D

This study is an application of the integrated numerical model FSSI-CAS 2D, to predict the subsidence of a huge rubble mound breakwater in the west part zone at Yantai port. The modified Cambridge clay soil model and poro-elastic model are adopted to describe the quasi-static mechanical behavior of the seabed foundation, which is consists of muddy-clay, clay and silt soil layers. The model parameters of soil used in computation are estimated based on a great number of laboratory tests (over one thousand). A novel point is that the buoyancy applied on the rubble mounds under SWL (Static Water Line), as well as its nonlinear variation dependent on the deformation of seabed foundation are considered in computation. Through analytical and experimental verification, as well as the comparison with observation data monitored in-situ, it is indicated that FSSI-CAS 2D is reliable to numerically predict the subsidence of offshore structures, and has the ability to be applied in practical engineering.

Numerical and experimental modeling of water wave interaction with rubble mound offshore porous breakwaters

In the present study, regular water wave interaction with rubble mound offshore breakwaters (RMOB) is investigated. The porosity of the RMOB is uniform, and the breakwaters are placed on the uniform seabed. Three different types of breakwaters such as (i) RMOB with still water level at the crest, (ii) emerged RMOB, and (iii) submerged RMOB are considered for the present analysis. The associated boundary value problem is handled for the solution using the boundary element method. Further, model tests are conducted, and the experimental results are compared with the numerical solutions. Quantitative analysis is carried out to show the effect of wave height and the height of RMOB’s on the wave reflection, wave transmission, and wave energy dissipation. An emerged RMOB can effectively reduce the wave transmission compared to the submerged RMOB and RMOB with crest level at SWL (still water level). The results indicate that the minimum transmission coefficient (<0.2) can be achieved in the short wave regime. This study also suggests that 90% incident wave energy can be dissipated by emerged RMOB with 46% porosity.

Wave Force Characteristics and Stability of Detached Breakwaters Consisting of Open Cell Caissons Interlocked via Crushed Stones

The maximum external force acting on a long continuous harbor structure can be reduced by controlling the phase difference of forces acting longitudinally. This strategy can be used to increase the structural stability of breakwaters consisting of caissons. Breakwaters have been developed using interlocking caissons to effectively respond to the constant increase in wave height due to climate change. In this study, we investigated the wave force characteristics and stability of a detached breakwater consisting of open cell caissons interlocked via crushed stones. We performed wave basin experiments and compared the results with analytical solutions of linear diffraction waves. The results revealed that the maximum wave force acting on the front of the breakwater decreased as the incident angle increased, reducing by as much as 79% for an incident angle of 30°. Although the variability of the maximum wave force for each caisson is large owing to the influence of the diffracted waves, the maximum wave force acting on the entire detached breakwater was not significantly affected by this variability. The analytical solutions based on linear wave theory agreed with the experimental results, indicating that the findings can be applied to actual designs. The structural stability of the breakwater was enhanced, even for low incident wave angles, compared to that of a single integral structure, as the frictional resistance produced by the sliding structure increased due to the shear resistance between the filled crushed stones and the rubble mound.

Experimental Study of Rubble Mound Behavior on the Very Soft Soil Seabed

The construction of rubble mound structures such as a breakwater or access road on a very soft seabed is essential to the development of maritime transportation. This process can result in significant settlement that causes the rock particles to sink into the seabed, leading to loss of material. In the current study, several case studies on breakwaters facing large settlements, which resulted in the use of excessive amounts of rubble, were surveyed. A series of physical models were developed to study the deformation and volume loss of rubble mound breakwaters on very soft clayey soil during construction. It was found that most deformation occurred early in construction. The primary mechanisms influencing settlement were rock penetration into the soft bed, flow of clay material into the porous body of the rubble mound, and lateral deformation of the soil beneath the breakwater, which is associated with soft bed heave at the breakwater toe. During testing of very soft beds, the ratio of material loss to the design weight of the rubble mound increased up to 27%, and the proportion of penetration to total material loss was as high as 56%. The results of the experiments and case studies were used to develop an empirical relation with which to predict settlement.

Separation as a solution for deformation control and material loss reduction in rubble mound structures constructed on very soft clayey bed

In many cases, the construction of rubble mound structures such as breakwaters over soft beds is inevitable in coastal areas. The common problems arising from these structures include sinking of the rubble into the seabed, shear failure, general seabed instability, severe settlement, and soft material flow into the rock pores. The present study investigated the use of sand mattress and geotextile as separation layers through physical-model testing. The results of the model without a separation layer showed that the individual penetration of aggregates into the soft bed and flow of soft clay into the rubble mound pores is the effective mechanisms in wasting rock materials. The results are suggestive of the 25% share of penetration in total material loss. Further, mud flow accounted for a 49% share compared to foundation settlement. It was shown that placing a separation layer under the rubble mound can help significantly reduce material loss and offers control over deformations. Using a sand mattress or a geotextile layer enabled a considerable reduction in material loss—26 and 39%, respectively—by controlling penetration and mud flow. The better performance of geotextile over the sand mattress can be attributed to its capacity for withstanding tension and its reinforcing role, helping reduce lateral deformations.

Composite breakwater of a submerged horizontal flexible porous membrane with a lower rubble mound

An analytical solution of a new composite breakwater consisting of a moored submerged flexible porous membrane and a submerged ruble mound below the membrane is presented. The semi-analytical solution is obtained based on the velocity potential decompositions where the consideration of a dispersion relation with complex wave numbers is not required. The convergence of the series solution is demonstrated by computing the values of the reflection, the transmission, the dissipation coefficients and the vertical force. In addition, the present results are compared against published numerical results and experimental data. In order to analyze the efficiency of the newly developed composite breakwater, the present results are compared with those of a single horizontal submerged moored porous membrane and a single rubble mound breakwater. Furthermore, the effects of design parameters on the performance of the composite breakwater are also analyzed in detail.

Effect of basal reinforcement on sinking and deformation of rubble mounds on a soft seabed

ABSTRACT The construction of rubble-mound breakwaters on soft sea beds is problematic, but is necessary in many coastal areas. The issues faced include the penetration of the rock particles into the very soft clayey bed, which results in the loss some of the material, major deformation during and after construction, and instability. The basal reinforcement method employs both separation and reinforcement and is an effective solution. Most previous research has focused on the role of reinforcement, but less attention has been paid to the combination of reinforcement and separation. In the present study, physical model tests were used to quantitatively compare the action mechanisms of the basal reinforcement method and the gravity replacement method. The experimental results showed that the use of a geotextile for basal reinforcement reduced horizontal deformation up to 42%, but had no significant effect on settlement. The combined reinforcement and separation mechanisms also reduced material loss up to 44%.

Wave-induced uplift pressure on berm revetment with Seabee slope

ABSTRACT The berm revetment with a Seabee slope has been widely used in coastal defense structures. It is necessary to understand the distribution of wave-induced uplift pressure as it may cause serious damage to the cover layer of the revetment. Therefore, a series of physical experiments are carried out to quantify the uplift pressure on the revetment under various wave conditions. It is found that the freeboard of the berm is a key parameter which categorizes the laboratory results into two groups. Under negative freeboard, the distribution of uplift pressure on the upper slope is similar to that on the lower slope. Additionally, the maximum uplift pressure on both upper and lower slopes is strongly related to the surf-similarity parameter ξ p. Under positive freeboard, the uplift pressure on the lower slope is significantly larger than that of the upper slope. The maximum uplift pressure on the lower slope is similar to that of the conventional rubble mound revetment. At last, Empirical equations are proposed to estimate the maximum relative uplift pressure under positive and negative freeboards.

Stability of berm type breakwater with cube blocks in the lower slope and berm

In this study, the stability of cube armour units in the lower slope and berm of rubble mound breakwaters with a berm was investigated for irregular and regular placement methods. Cubes are simple and cheap in production. However, the design conditions for the upper slope and lower slope may differ depending on the water level of the dominant storm conditions. For the upper slope, relatively higher water levels are dominant. The berm has a reducing effect on the wave loading on the upper slope and therefore, it may be relevant to use cubes in the lower slope, in combination with rock in the upper slope. In this study, a regular placement technique named “the double pyramid placement method” was applied for cubes placed on the lower slope and the berm. This placement technique was compared with the irregular placement technique by performing experiments in a wave flume. As a result of this investigation, the regular placement method was found to be superior for the stability of cubes in the lower slope and berm of rubble mound breakwaters with a berm.

Investigation on hydrodynamic performance of an environmentally friendly pile breakwater

In this paper, a type of pile breakwater is studied in a configuration that can provide an effective shore protection structure. While, the rubble mound breakwaters are the most common type, but they suffer from a number of shortcomings including the interruption of water that may lead to deterioration of water quality and corresponding environmental problems. The breakwater of this study with rows of piles can be used against sea waves, as an alternative. Its hydraulic performance, including wave reflection and transmission, shear stresses, pressure measurements, drag and inertia coefficients are investigated. In the first step, the hydrodynamics around the mono-piles with square cross-sections are inspected. The parametric equations are developed to measure the optimal orientation of the piles. As evidenced in the literature, relatively limited works have been carried out on this kind of piles. Also, the performance of the considered pile breakwaters are evaluated. To do this, the numerical results are calibrated and validated by experimentation. It is observed that the group piles with the square cross-section in optimum conditions, could have a higher value of reflection almost between 7 even to almost 30 percent, than other considered conditions with mild and short waves. It can be related to the effect of the square piles edges on the separation process of the flow.

Numerical investigation of dynamic soil response around a submerged rubble mound breakwater: II. Loose sandy seabed

Low-crested structures such as submerged rubble mound breakwaters (RMB) are commonly used as successful examples of coastal protection measures. This study addresses foundation issues related to the performance of a submerged RMB constructed on a liquefiable site where the foundation soils have been loosely deposited; these issues were investigated using an integrated numerical model proposed by Zhao et al. (2018). Unlike previous work in Zhao et al. (2018), the dynamic features of loose sand deposits (i.e. build-up of pore water pressures, development of plastic strains, degradation of soil stiffness) due to the fluid–seabed-structure interactions, accompanied by the onset of liquefaction, were simulated. The reliability of this model to predict the liquefaction susceptibility of loose sand deposits was validated against the experimental results available in literature. This study shows that the submerged RMB constructed on a liquefiable seabed experienced progressive and asymmetric downward settlement under successive loading cycles. The opposing currents in the wave field tended to enhance this asymmetric settlement whereas the following currents did the opposite. Regardless of changes to the magnitude and direction of current velocities, ignoring the currents in the wave field may lead to an overestimation of the increased rate of residual pore water pressures in the region underneath the breakwater, particularly in shallow soil layers where the maximum relative differences can reach up to 120% of (p̄current−p̄no-current)∕P0 at t∕T=25.

Assessing failure probability of coastal structures based on probabilistic representation of sea conditions at the structures’ location

In the present paper a thorough probabilistic methodology is presented, aiming at estimating the reliability of coastal structures, such as rubble mound breakwaters during their lifetime, based on the probabilistic representation of load environmental and resistance parameters. One of the innovative points and main objectives of this study is the estimation of the failure probability of a coastal structure based on the long-term wave climate at the structure's location, usually met in intermediate waters, using wave observations or measurements in deeper waters. This task is accomplished by applying a wave propagation statistical model in order that the joint probability density function of all random load parameters be estimated at the structure's location. Moreover, a relation between an event-based extreme value analysis and an analysis on sea-state conditions within storm events is derived in order that both of these two approaches could be compared estimating the same kind of failure probability; an unconditional one. The latter is properly defined here as the percentage of the structure's Lifetime that the structure will be in a failure situation. This unconditional failure probability provides direct information on the time period that the structure will be in a situation of failure (since it considers the total range of loadings and total lifetime), and thus can be incorporated more efficiently in an integrated risk analysis with consideration of social and economic costs. Besides, another specific issue of scientific originality could be considered the investigation on the proper time step denoting the sea state in the sea state analysis applied. In this manner, the actual history and shape of each storm event is taken into consideration. Furthermore, it is shown that these two approaches could be incorporated in the design of a coastal structure. Moreover, two different fully probabilistic methods, Direct Integration Method and Monte Carlo Method, were applied (and compared) by using a combination of variables with zero and non-zero hazard rate, referred here as a combined time-invariant and time-variant analysis. Finally, the effect of considering additional and different types of parameters as random variables on the assessed failure probability of the structure has been investigated. The aforementioned methodology has been applied to a sample of wave and sea level data at the structure's location generated for this purpose. The original wave data were derived from measurements in deeper waters than the structure's location, covering a period of 8 years, obtained from an oceanographic buoy, located in the western Mediterranean off Malaga, Spain. Sea level data were also obtained from a tide gauge, located in Malaga's harbor. Finally, it is shown that the methodology derived from this study could be incorporated into a coastal structure's design process to meet specific safety requirements.

Centrifuge study of the mechanisms of rubble mounds sinking into soft clay seabeds in the construction stage

The construction of a rubble mound breakwater over a soft clayey seabed is a challenging issue in marine engineering. Excessive settlement and the sinking of such structures into the seabed sludge impose high costs on a project and can cause operational problems. The present study evaluated the mechanisms contributing to the large deformation that can occur during the construction of a breakwater using the results of four centrifuge tests. The result showed that the penetration of individual pieces of rubble and the flow of soft clay into the rubble pores were the leading mechanisms contributing to material loss during the construction of the breakwater. According to the conducted tests, the amount of material loss contributing to the penetration of individual rock particles for Cu = 3.2, 4.1, 11.2 kPa, equal 10.3, 7.6 and 2% of the initial breakwater section respectively. Also, the amount of material loss caused by the flow of sludge into rock pores for the three mentioned undrained shear strengths are equal to 15.1, 10.4 and 5.9% of the initial section respectively. So current study indicates that failing to account for these two mechanisms can lead to considerable underestimation of the required material for construction.

The Practical Simplified Equation for Settlement Evaluation of Counter Facility in Soft Ground Centering on Rubble Mound

The Practical Simplified Equation for Settlement Evaluation of Counter Facility in Soft Ground Centering on Rubble Mound

Image-Based Measurement of Wave Interactions with Rubble Mound Breakwaters

Over the past decade, the use of imaging devices to perform quantitative measurements has seen wide-scale adoption and has become integral to the emerging fields of research, such as computer vision and artificial intelligence. Recent studies, published across a wide variety of fields, have demonstrated a vast number of ways through which image-based measurement systems can be used in their respective fields. A growing number of studies have demonstrated applications in coastal and ocean research. Edge detection methods have been used to measure water surface and bedform elevation from recorded video taken during wave flume experiments. The turbulent mixing of air and water, induced by the breaking waves and the runup processes, poses a particular problem for the edge-detection methods, since they rely on a sharp contrast between air and water. In this paper, an alternative method for tracking water surface, based on color segmentation, is presented. A set of experiments were conducted whereby the proposed method was used to detect water surface profiles for various types of breaking waves interacting with a rubble mound breakwater. The results were further processed to compute the surface velocity during runup. The time-history of surface velocity is shown to closely parallel the point measurements taken nearby the instrumented armor unit. These velocities can potentially serve as boundary conditions for determining the dynamic loads exerted on the armour units. Further, the image processing results are used to remove the time-varying buoyant force from the measured force acting on an individual armour unit, providing additional insight into how the forces develop over time.

The Analytical Solution of Waves Transmission Coefficient on Semi Fixed Floating Breakwater

One type of breakwaters that can be used to protect port basin is floating breakwaters. The use of floating breakwaters is suitable for relatively large water depths. In deep waters, the floating breakwaters volume is smaller than rubble mounds breakwater. Consequently, floating breakwaters are cheaper than rubble mounds breakwater. Unfortunately, a practical approach to analysing the performance of floating breakwater in reducing wave height is unknown. Floating breakwater performances indicated by wave transmission coefficient. Therefore, in this study an analytical wave transmission coefficient was formulated. The simple formula can be used to estimate high of wave at behind of floating breakwater. Based on these equations, the wave transmission coefficient influenced by the wave parameters and structural parameters. Wave parameters represents by wave high, wave period, water depth and wave length while structure parameters represents by both of width and draft of structure. Wave transmission coefficient also influenced by the wave frequency and having motion frequency.

Random Wave-Induced Momentary Liquefaction around Rubble Mound Breakwaters with Submerged Berms

The effects of submerged berms in attenuating the momentary liquefaction beneath rubble mound breakwaters under regular waves were investigated in a recent study. The present work aims to investigate the momentary liquefaction probabilities around and beneath breakwaters with submerged berms under random waves. The interaction between waves and breakwaters with submerged berms has been simulated through a phase-resolving numerical model. The soil response to the seabed pressure induced by random waves has been investigated using a poro-elastic soil solver. For three different breakwater configurations, the liquefaction depths under random wave conditions have been compared with those cases under representative regular waves. In the present study, the offshore spectral wave height ( H m 0 ) and the peak period ( T p ) of irregular waves are used as representative regular wave parameters. Results reveal the importance of considering random waves for a safe estimation of the momentary liquefaction probability. Indication about the minimum number of random waves, which is required to properly catch the liquefaction occurrences, has been also addressed.

Damage in Rubble Mound Breakwaters. Part I: Historical Review of Damage Models

The term “damage” in rubble mound breakwaters is usually related to the foremost failure mode of this kind of coastal structures: their hydraulic instability. The characterization of the breakwater response against wave action was and will be the goal of hundreds of studies. Because of the large amount of information, the present review on damage in rubble mound breakwaters is divided in two papers, which are closely linked but conceptually different; whereas Part II is focused on the various approaches for defining and measuring damage, Part I summarizes the diverse strategies for modelling damage development and progression. The present paper compiles 146 references on this topic, chronologically discussed over almost a century of history: from 1933 to 2020. It includes 23 formulations of hydraulic stability models and 11 formulations of damage progression models, together with main advances and shortcomings up to date. The future of rubble mound design is linked to risk-based tools and advanced management strategies, for which deeper comprehension about the spatial and temporal evolution of damage during the useful life of each particular structure is required. For this aim, damage progression probabilistic models, full-scale monitoring and standardization will presumably be some of the key challenges in the upcoming years.