Rubble Mound Breakwaters Research Articles

Bulk adhesion of ice to concrete–strength

This paper presents the results of a laboratory test program designed to investigate the adhesive effects of large-scale (bulk) ice on concrete. Medium-strength concrete cylinders were sawn into discs, and attached to a sample table. Freshwater ice samples, frozen using smaller, standard-sized concrete cylinders, were adhered to the concrete with both varying bond times and added weight during bonding. Shear strength tests were conducted at a set displacement rate, under a number of temperatures. The effect of these variables on the adhesive strength of ice to concrete was examined, as well as whether there was any noticeable removal of concrete cement paste or aggregate during testing. The tests indicate that the adhesive strength is negligible when the method of adhesion is “dry” (no liquid layer at the onset of adhesion). Tests with “wet” adhesion indicated a significantly higher strength. The nominal versus the apparent contact area had significant implications for the determination of the adhesive strength of the bond between the ice and the concrete. Removal of cement paste was evident in a number of tests, however the amount was not significant. The results have relevance for design of structures in a marine environment, such as revetement dams or rubblemound breakwaters, as well as for the standardization of adhesion tests with ice and concrete.

The influence of toe geometry on toe armour stability for conventional rubble mound breakwaters

Rubble mound breakwaters have toe structures that provide support to the armour layer and prevent damage to the structure from scouring. In this way, toe failure may be regarded as one of the primary failure modes of rubble mound breakwaters. Several formulae have been proposed for toe stability with conflicting results owing to the neglect of factors such as storm duration and armour layer slope. In the present study, a statically stable rubble mound with rock armoured toe was tested under irregular waves. The effects of wave height, wave period, armour layer slope, water height in front of the structure, water height above the toe, and toe width on the hydraulic stability of a toe structure are investigated. Toe width was found to be ineffective on toe stability, while the relative depth was found as an effective parameter for the toe stability. Greater toe damage was observed by increasing wave height and storm duration. The results highlighted the necessity for a new stability number involving the type of wave breaker on the armour slope with a critical Iribarren value of 4.1. Based on the data from the present study and the findings of another study, new formulae have been developed for estimating toe damage in rubble mound breakwaters. Evaluating the proposed formula using other researchers’ data and formulae highlighted its strengths.

Wave overtopping discharges at rubble mound structures in shallow water

Wave overtopping of coastal structures has been studied using physical model experiments with rubble mound breakwaters in shallow water. The mean overtopping discharge is determined for three different foreshore slopes and various hydrodynamic conditions. The hydrodynamic results confirm that energy is transferred to low-frequency waves in very shallow water and that the short waves are in phase with the lower-frequency waves in very shallow water. As a result, the extreme waves (e.g. 2% exceedance wave height) become relatively large in very shallow water due to the energy of the low-frequency waves affecting thereby the wave overtopping. To estimate the amount of energy at the low-frequency waves, an expression is derived which reasonably accurately predicts the low-frequency wave energy (RMSE of 0.06). Considering the non-dimensional overtopping discharge, the existing formulations for the non-dimensional mean wave overtopping discharge perform poorly to reasonably in shallow water with RMSLE ranging from 1.04 to 2.92. A parameter sensitivity study shows that the short-wave steepness, relative crest height and the low-frequency wave height are the most important parameters when predicting the mean overtopping discharge in shallow water. When including the short-wave steepness and relative crest height in an empirical formulation the RMSLE for the current dataset reduces to 0.69. A further increase in accuracy is found when the low-frequency wave height and 2% exceedance wave height are included (RMSLE 0.64).

Experimental investigation of equilibrium and temporal scour around the round head of the rubble mound breakwater in case of breaking and non-breaking waves

In this study, local sediment movements caused by regular wave effects around the head section of rubble mound breakwater were investigated experimentally. The experiments were performed in a wave channel with a length of 33 m, a width of 3.6 m and a depth of 1.2 m. A plunger-type wave generator was located at the offshore side, generating regular waves of varying heights and periods. Uniform sandbed material of three different grain sizes was used, alongside two distinct breakwater slopes and six different wave characteristics. Experiments were performed under both breaking and non-breaking wave conditions. Time-dependent scour depth and water level measurements were determined by using measuring equipment working with an ultrasonic method. In the paper, new empirical relations were derived for temporal relative scour depths and the non-dimensional time scale parameter and the non-dimensional time scale parameter. The determination coefficient and scatter index of these derived relations were calculated and evaluated statistically. It was observed that the relations are in good agreement with those obtained from experiments. Additionally, the findings indicate that wave breaking significantly reduces the equilibrium scour depth.

Monsoonal patterns of wave reflection from rubble mound breakwater of Chabahar Bay

This paper presents the results of a one-year field study on the monsoonal reflective response of the rubble mound breakwater (RMB) of Chabahar Bay, located on the northern coast of the Gulf of Oman, Iran. Measurements show that, in general, the correlation between the reflection coefficient and Iribarren number during the winter monsoon period is more remarkable than that of the summer monsoon period. The difference in wave reflection behavior during monsoonal periods is mainly due to the energy proportion of incoming sea and swell waves. Various characteristic wave periods by means of power- and hyperbolic-law prediction functions are explored to enhance the wave reflection prediction, highlighting the significant performance of negative-moment spectral periods Tm−1 and Tm−2 compared with peak and mean spectral periods. Statistical comparison of the performance of Tm−1 and Tm−2 shows that Tm−2 considerably improves the prediction accuracy for moderated energy waves with bimodal sea and swell climates originating from different directions in the winter monsoon and pre-summer monsoon months. However, the prediction improvement is insignificant for unimodal energetic waves observed during the summer monsoon months. Generally, using Tm−2 increases the accuracy of the preexisting equations in predicting the observations of this study.

Influence of structure characteristics on wave transmission and shoreline response at a low-crested segmented breakwater in Little Egg Harbor Township, NJ

Graveling Point Beach, a public beach within Little Egg Harbor Township, New Jer- sey, has been experiencing long-term erosion. To address the issue, a living shoreline project consisting of four low-crested rubble-mound breakwaters and a beach nour- ishment was constructed beginning in fall 2022 (structures) and finishing in spring 2023 (beach nourishment). A variety of field methods were used to investigate the influence of the breakwater system on wave transmission and beach fill evolution. The observations are then compared to common empirical estimations. A median wave transmission coefficient of 0.4 was measured for incident waves with a median height and peak period of 9 cm and 2 sec, respectively. The observed transmission coefficient was higher than that predicted by models such as Briganti et al. (2003) and Van der Meer and Daemen (1994) suggesting other processes such as diffraction may be influencing the results. Structure from Motion (SfM) was used to generate a series of Digital Elevation Models (DEMs) which show salients forming behind the structures. The resulting salient growth was compared to the model of Suh and Dalrymple (1987), which was shown to overpredict the amplitude of the salient at Graveling Point. However, the expected relationship of breakwaters with increased offshore distance having shorter salient lengths was generally observed. The equation was refit using the field data at Graveling Point and laboratory data with similar wave heights and periods from Suh and Dalrymple to derive a modified model that may be more appropriate in low-energy environments

Impact of Rear Slope Variation on Rubble Mound Breakwater Stability Under Seismic Loading

This study aims to enhance the seismic stability of rubble mound breakwaters, crucial maritime structures, by examining how variations in the rear slope angle affect their response to seismic loads. Utilizing the Plaxis 2D software, a finite element method was employed to simulate the behavior of a conventional rubble mound breakwater under different seismic conditions. The analysis considered three different rear slope angles and subjected each to various seismic loads characterized by differing amplitudes and frequencies. Our findings indicate that the rear slope inclination significantly influences the seismic response of the breakwaters, notably affecting the displacements and deformations within the structure. The most optimal angle of inclination was identified, which minimized the seismic-induced deformations, thereby potentially improving the structural integrity and longevity of these maritime defenses. This investigation not only provides valuable insights into the design of more resilient maritime structures but also introduces an approach to optimize breakwater design for better performance under seismic conditions, marking a notable improvement in the field of maritime engineering. Doi: 10.28991/CEJ-SP2024-010-08 Full Text: PDF

Hydraulic Model Experiments and Performance Analysis of Existing Empirical Formulas for Overtopping Discharge on Tetrapod Armored Rubble Mound Structures with Low Relative Freeboard

In coastal structure design incorporating revetments, the assessment of wave overtopping discharge relies on hydraulic model experiments. Numerous empirical formulas have been developed to predict overtopping discharge based on quantitative data from these experiments. Typically, for revetment structures aimed at mitigating wave overtopping, crest height is determined by considering the maximum amplitude of the design wave, resulting in a relatively high freeboard compared to wave heights. However, achieving complete prevention of all wave overtopping would require the crown wall to have substantial crest heights, rendering it economically impractical. Therefore, the concept of limiting discharge has been introduced in the design of revetment structures, aiming to restrict wave overtopping discharge to an acceptable level. Consequently, many coastal structures in real-world settings feature relatively lower freeboard heights than incident wave heights. This study investigated wave overtopping discharge on rubble-mound breakwaters with relatively low freeboard heights through hydraulic model experiments. Furthermore, it conducted a comparative analysis of the predictive capabilities of existing empirical formulas for estimating overtopping discharge using experimental data.

Hydrodynamic Assessment of A New Nature-Based Armour Unit on Rubble Mound Breakwater for Coastal Protection

Hydrodynamic Assessment of A New Nature-Based Armour Unit on Rubble Mound Breakwater for Coastal Protection

Experimental Evaluation of the deep toe stability of rubble mound breakwater with Accropod concrete armor

Experimental Evaluation of the deep toe stability of rubble mound breakwater with Accropod concrete armor

Analisis Of Upgrading Low-Crested Structures As An Adaptation Measure To Climate Change For Coastal Protection: A Hybrid Approach

Coastal zones have consistently been among the most appealing settlement areas due to their proximity to the sea, rich natural resources, and the high quality of life they offer (Lamberti et al., 2005). However, these regions are affected by climate change impacts, such as sea-level rise, storm surges, and an increased intensity of extreme weather events (Burcharth et al., 2014). Traditional low-crested rubble mound breakwaters are commonly used to protect coastal areas from wave damage. However, it is expected that the variability of climate conditions will induce a loss of functionality and structural integrity in the coming decades. Coastal communities and coastal managers, within the framework of sustainable development, are demanding new approaches that include not only the preservation of the hydraulic performance of the breakwaters, but also other factors, including social and environmental impacts. These requests significantly affect the traditional way to conceive those structures to be integrated into the coastal landscape. To cope with the variation of climate drivers, existing low-crested breakwaters must be adapted to accommodate social demands and environmental issues. Therefore, upgrading and maintaining the existing rubble-mound breakwater is a hot topic in coastal engineering, and deserves special attention due to the possible intensification of external loads resulting from the impacts of climate change (Stagnitti et al., 2023). Upgrading can be done by modifying the structure profile and/or adding structure elements (Burcharth et al., 2014). In 2011, Cappietti provided curves for the functional design of submerged breakwaters to be used in place of preexisting emergent breakwaters. Burcharth et al. (2014) explained that the best way to improve the structure is to put an additional layer of protection on the front slope, as long as the foreshore has a mild slope of around 1:100. But if the foreshore becomes steeper because of erosion, then a front berm will also be needed. Stagnitti et al. (2022) introduced a novel methodology based on the calculation of the failure probability during a lifetime due to independent failure modes. Their method was employed to evaluate the performance of upgraded breakwaters in response to climate change. Estimating the wave overtopping of both existing and upgraded breakwaters is essential for designing upgrade options that can ensure the safety of port operations. Stagnitti et al. (2023) applied the numerical model IH2VOF, which was calibrated using experimental data, to study the wave overtopping of damaged and upgraded rubble-mound breakwaters. The present work examines the feasibility of converting emerged rubble-mound breakwaters into submerged breakwaters to minimize their detrimental environmental effects. To achieve this goal, an investigation was conducted to evaluate the hydraulic performance of submerged breakwaters that are created by lowering the crest of existing emerged breakwaters. Additionally, two-dimensional numerical simulations were performed using IH2VOF to investigate wave interactions with the structures.

Effectiveness Of Stilling Wave Basins In Reducing Wave Overtopping On Dikes And Rubble Mound Breakwaters

Motivated by the impacts of Climate Change (CC), such as rising sea levels and the increased intensity and frequency of storms, the importance of adapting our coastal defense structures has never been more important (Toimil, 2020). With CC causing coastal structures to be exposed to more extreme hydraulic conditions than originally designed for, there is an increased risk of these structures failing in terms of stability and hydraulic performance, such as wave overtopping. To reduce wave overtopping, structural modifications of existing coastal structures are often adopted and in this work the analysis of wave overtopping discharge at slope structures equipped with Stilling Wave Basins (SWB) at their crest (Figure 1) is investigated.

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.

Experimental Comparison Of The Hydraulic Performance Of Overhanging And Vertical Parapets Under Limited Wave Breaking Conditions: The Case Of The New Offshore Ravenna Lng Terminal (It)

Harbors play a pivotal role in global trade, serving as vital gateways for the transportation of goods, fostering economic growth. These essential coastal infrastructures are subjected to relentless forces (e.g. wave action, storm surges, and sea level variation induced by climate change) which can jeopardize their functionality and safety. Safe working conditions are mandatory for the operability of all kinds of harbors. However, particular attention must be paid in the case of terminals dedicated to dangerous goods, as for example Liquefied Natural Gas (LNG). Composite vertical breakwaters may be efficiently used to protect this kind of terminal and have both advantages and disadvantages over rubble mound breakwaters. This work presents the results of a 2D experimental campaign carried out to optimize, from a hydraulic point of view, the parapet wall of the new composite vertical breakwater that will be built to protect the new offshore LNG terminal located in in the North Adriatic Sea in the South-East of the Port of Ravenna (Italy). In particular, two types of parapets walls have been compared: one overhanging (recurved in the shape of one fourth of a circumference with a ray of one meter) and one vertical. For each type of parapets, three different heights were evaluated for a total of six different configurations. The typical water depth in the area where the new terminal will be built is approximately -15.0 m with respect to the MSL and the total range of the astronomical tides does not exceed one metre. The terminal is located about 8.0 km far from the coast and is positioned on a very mild seabed foreshore slope of less than 0.05°

LOW-CRESTED AND EMERGENT BREAKWATERS WITH ECO-FRIENDLY ARMOUR UNITS

Artificial coastal defense systems (such as concrete armour units) frequently host less diverse aquatic populations than natural environments and feature higher concentrations of invasive species (Dafforn et al., 2009). Therefore, coastal structures need to be designed or refitted to achieve sustainable goals through the application of ecological engineering solutions applied to coastal defence structures and for the ecosystem, marine habitat, and biodiversity improvement. Ecological engineering, which integrates ecosystems with engineering principles to construct coastal structures that benefit both humans and the ecosystem, is growing as a means of reducing the adverse ecological impacts of coastal infrastructure (Mitsch & Jorgensen, 2003). Thus, creating new eco-friendly breakwater design guidelines is critical and will benefit from the multidisciplinary involvement of marine biologists and ecologists. The purpose of this experimental modelling study was to provide data on the hydraulic performance and stability of low-crested and emergent rubble mound breakwaters (RMBW) constructed using ecologically friendly armour units under various wave conditions. The University of Ottawa, the National Research Council of Canada (NRC), and ECOncrete collaborated to develop and conduct the physical testing program. Several breakwater models were tested to evaluate their performance, as the idea of an eco-friendly breakwater is still a new area of research. Thus, this experimental program is essential to promote environmentally-friendly armour units in the design of new coastal structures (such as Baker et al., 2018), as well as ecological retrofitting of existing coastal structures. The physical tests were conducted between June 2023 and August 2023 in the Large Wave Flume of NRC’s Ocean, Coastal, and River Engineering Research Center in Ottawa, Canada. ECOncrete's Coastalock armour units were tested in various configurations at a 1/15 scale using two-dimensional low-crested and emergent RMBW models. The hydraulic performance and failure mechanisms of these environmentally-friendly breakwater models were tested under severe wave conditions.

Scour Hole Evolution Near A Detached Low-Crested Rubble-Mound Breakwater

Low-crested detached breakwaters are affected by coastal hydro- and morphodynamics, due to complex wave-structure- seabed interactions, which impact their stability, namely due to scour effects. The scour phenomenon has been under study in several mobile-bed physical model research studies (e.g., Fredsøe et al., 1997, Sumer et al., 2000 and Sumer et al., 2005). However, scour phenomenon near detached rubble-mound breakwaters is still far from being well understood. Therefore, the present research work comprises an innovative and comprehensive analysis of the local morphodynamics around a detached low-crested rubble-mound breakwater, with a special focus on the quantitative characterization of scour phenomena, namely the scour depth.

A New Wave Breaking Benchmark On Rubble Mound Breakwaters

The potential of predictive maintenance for critical port infrastructure, reducing risks and extending its life cycle, remains to be unlocked by suitable quantitative information that fosters predictive maintenance and sustained investments towards progressive climate-proofing of the infrastructure. The PI-BREAK (Predictive Intelligent system for BREAKwater maintenance) project will apply state-of-the-art monitoring and modelling techniques to the maintenance of the Punta Lucero rubble mound breakwater in the port of Bilbao, Spain. This critical infrastructure is crucial to the welfare of coastal communities and the project aims to demonstrate how predictive maintenance can enhance port safety and efficiency, while also supporting economic recovery through low carbon footprint adaptation to future scenarios. By combining short-term risk reduction (for service limit states) with long-term structural reliability (for ultimate limit states) PI-BREAK will develop a sequenced maintenance model to progressively extend the breakwater life span with controlled risk levels and impacts.

Rocking Of Single Layer Armour Units Measured By Embedded Sensors

Single layer randomly placed armour units are used in many rubble mound breakwaters around the world. For these armour layers, breakage of armour units due to rocking could be a major damage mechanism, but no good methods exist to evaluate and quantify rocking. The aim of the study is to quantify the rocking impact velocities for irregularly placed single layer armour units. This study utilizes embedded Rocking Sensors to obtain the first measurements of rocking impact velocities of single layer armour units. More generally, the paper (Hofland et al. 2023) shows how novel measurement techniques can be used for the investigating the stability of single layer armour layers.

Effects of Soil Foundation and Mesh Size on Rubble Mound Breakwater Stability under Seismic Loading

Effects of Soil Foundation and Mesh Size on Rubble Mound Breakwater Stability under Seismic Loading

Hybrid Modelling Of Wave Overtopping At Rubble Mound Breakwaters

Wave overtopping at rubble mound structures is one of the most important phenomena affecting the hydraulic performance of these coastal structures. In addition to the design of coastal structures, also the climate adaptation of coastal structures has become more important due to sea level rise. Adding a crest wall to an existing structure, increasing the height of a crest wall, adding a berm, or increasing the width or height of a berm, can be effective measures to account for effects of sea level rise. For this purpose, the individual effects of a crest walls and a berm need to be predicted, but also the combination of both (see for instance Van Gent, 2019, and Van Gent and Teng, 2023). Wave overtopping estimates are generally based on physical modelling in wave flumes and wave basins. Numerical modelling of wave overtopping provides additional opportunities to examine wave overtopping for a wide variety of structure geometries. The combination of physical modelling with numerical modelling is referred to as hybrid modelling. To provide design guidelines for rubble mound structures with a crest wall and for structures with a berm in the seaward slope, Van Gent et al (2022) provides design guidelines based on physical model tests. Numerical modelling provides opportunities to examine wave overtopping at structures with a crest wall and a berm to further extend guidelines for the design and (climate) adaptation of rubble mound structures. In Irías Mata and Van Gent (2023) guidelines based on physical modelling have been extended based on numerical modeling with OpenFOAM to examine the influence of several aspects such as the wave steepness, crest wall and recurved parapet, berm, and structure slope on wave overtopping at rubble mound breakwaters. Although the present work focusses on wave overtopping, also forces on crest walls have been examined using the applied numerical model, see for instance Jacobsen et al, 2018, and Irías Mata et al, 2023.