Search Results (10)

Over the past three decades, the development and testing of various overtopping wave energy converters (OWECs) have highlighted the importance of accurate wave run-up and overtopping predictions on those devices. This study systematically reviews the empirical formulas traditionally used for predicting overtopping across different types of breakwaters by assessing their strengths, limitations, and applicability to OWECs. This provides a foundation for future research and development in OWECs. Key findings reveal that empirical formulas for conventional breakwaters can be categorized as mild or steep slopes and vertical structures based on the angle of the slope. For the same relative crest freeboards, the dimensionless average overtopping discharge of mild slopes is larger than that of vertical structures. However, the formula features predictions within a similar range for small relative crest freeboards. The empirical formulas for predicting overtopping in fixed and floating OWECs are modified from the predictors developed for conventional breakwaters with smooth, impermeable and linear slopes. Different correction coefficients are introduced to account for the effects of limited draft, inclination angle, and low relative freeboard. The empirical models for floating OWECs, particularly the Wave Dragon model, have been refined through prototype testing to account for the unique 3D structural reflector’s influence and dynamic wave interactions. Full article

Low-crested and submerged breakwaters are frequently employed as coastal defence structures. Their efficiency is governed by wave energy dissipation, and the wave transmission coefficient can evaluate this parameter. The current study conducts experimental investigations on both low-crested and submerged breakwaters exposed to different wave conditions to compare their performance with that of emerged breakwaters. The current study provides a comprehensive review of existing formulae and highlights the impact of design variables. To evaluate the reliability of each existing formula, four “reference” configurations are used. Having these structures at the same overall volume, the results also provide a useful tool for engineers involved in the lowering operation of existing breakwaters. Nature and magnitude of governing parameters are investigated, and some points of criticism are outlined. The comparison results show that few of the existing equations give reliable estimates of the transmission coefficient for all the models tested in this study. Higher values of root mean square error are related to the emerged breakwater rather than the submerged ones. To obtain information about the transmitted wave energy, spectral analysis is applied as well. Different behaviours of the transmitted spectrum, n terms of shape and peak frequency, are highlighted. The results improve the overall knowledge on formulae that are in the literature, in order to make the user more aware. Full article

Submerged and low-crested breakwaters are nearshore barriers with an underwater or slightly emergent crest, designed to reduce the energy of wave attacks and, consequently, to protect the coast from erosion and flooding. Their performance in reducing the wave energy can be evaluated by the value of the wave transmission coefficient, which thus requires accurate prediction. In the last few decades, several experimental investigations allowed the development of several formulae to predict this coefficient that agreed well within the given range of validity. In the present study, a comprehensive review of the existing formulae has been reported and the influence of input design variables has been highlighted. Moreover, an extensive set of experimental data has been collected and critically examined and re-analyzed to obtain a homogenous up-to-date database. Special attention has been addressed to the assessment of the reliability of each existing formula for and to evaluate its performance beyond the validity limits for which it was developed. Full article

Despite all efforts and massive investments, the restoration of mangroves has not always been successful. One critical reason for this failure is the vulnerability of young mangroves, which cannot grow because of hydrodynamic disturbances in the shallow coastal water. For a comprehensive study bridging ecological and engineering principles, a portable community-based reef is proposed to shield mangroves from waves during the early stages of their growth. A series of field observations were conducted on Amami Oshima Island (Japan), to observe the growth of young mangroves and their survival rate under moderate wave conditions. The evolution of young mangroves was also observed in the laboratory under a controlled indoor environment. At the research site, it was confirmed that, after six months of germination, young mangroves could withstand normal high waves. Laboratory-grown plants were lower in height and had fewer leaves compared with the native mangroves on Amami. Based on these results, an economical reef system was designed. For this purpose, the Ahrens formula for the design of a low-crested reef breakwater was revisited. The results showed that a 50-cm-high reef constructed with 15-kg stones can protect mangroves that are a few months old and effectively promote early mangrove growth. Full article

A numerical study for the effect of crest width, breaking parameter, and trunk permeability on hydrodynamics and flow behavior in the vicinity of rubble-mound, permeable, zero-freeboard breakwaters (ZFBs) is presented. The modified two-dimensional Navier-Stokes equations for two-phase flows in porous media with a Smagorinsky model for the subgrid scale stresses were solved numerically. An immersed-boundary/level-set method was used. The numerical model was validated for the cases of wave propagation over a submerged impermeable trapezoidal bar and a low-crested permeable breakwater. Five cases of breakwaters were examined, and the main results are: (a) The size of the crest width, B, does not notably affect the wave reflection, vorticity, and currents in the seaward region of ZFBs, while wave transmission, currents in the leeward side, and mean overtopping discharge all decrease with increasing B. A non-monotonic behavior of the wave setup is also observed. (b) As the breaking parameter decreases, wave reflection, transmission, currents, mean overtopping discharge, and wave setup decrease. This observation is also verified by relevant empirical formulas. (c) As the ZFB trunk permeability decreases, an increase of the wave reflection, currents, wave setup, and a decrease of wave transmission and mean overtopping discharge is observed. Full article

Understanding the dynamics of sediment transport and erosion-deposition patterns in the locality of a coastal structure is vital to evaluating the performance of coastal structures and predicting the changes in coastal dynamics caused by a specific structure. The nearshore hydro-morphodynamic responses to coastal structures vary widely, as these responses are complex functions with numerous parameters, including structural design, sediment and wave dynamics, angle of approach, slope of the coast and the materials making up the beach and structures. This study investigated the sediment transport and erosion-deposition patterns in the locality of a detached low-crested breakwater protecting the cohesive shore of Carey Island, Malaysia. The data used for this study were collected from field measurements and secondary sources from 2014 to 2015. Sea-bed elevations were monitored every two months starting from December 2014 to October 2015, in order to quantify the sea-bed changes and investigate the erosion-deposition patterns of the cohesive sediment due to the existence of the breakwater. In addition, numerical modelling was also performed to understand the impacts of the breakwater on the nearshore hydrodynamics and investigate the dynamics of fine sediment transport around the breakwater structure. A coupled two-dimensional hydrodynamics-sediment transport model based on Reynolds averaged Navier-Stokes (RANS) equations and cell-centered finite volume method with flexible meshing approach was adopted for this study. Analysis of the results showed that the detached breakwater reduced both current speed and wave height behind the structure by an average of 0.12 m/s and 0.1 m, respectively. Also, the breakwater made it possible for trapped suspended sediment to settle in a sheltered area by approximately 8 cm in height near to the first main segment of the breakwater, from 1 year after its construction. The numerical results were in line with the field measurements, where sediment accumulations were concentrated in the landward area behind the breakwater. In particular, sediment accumulations were concentrated along the main segments of the breakwater structure during the Northeast (NE) season, while concentration near the first main segment of the breakwater were recorded during the Southwest (SW) season. The assessment illustrated that the depositional patterns were influenced strongly by the variations in seasonal hydrodynamic conditions, sediment type, sediment supply and the structural design. Detached breakwaters are rarely considered for cohesive shores; hence, this study provides new, significant benefits for engineers, scientists and coastal management authorities with regard to seasonal dynamic changes affected by a detached breakwater and its performance on a cohesive coast. Full article

Coastal defense strategies based on structures are increasingly unpopular as they are costly, leave lasting scars on the landscape, and sometimes have limited effectiveness or even adverse impacts. While a clear improvement concerning aesthetic considerations using soft submerged breakwater is undeniable, their design has often focused on wave transmission processes across the crest of the structure, overlooking short- to medium-term morphodynamic responses. In this study, we used a time- and depth-averaged morphodynamic model to investigate the impact of the implementation of a submerged breakwater on surf zone sandbar dynamics at the beach of Sète, SE France. The hydrodynamic module was calibrated with data collected during a field experiment using three current profilers deployed to capture rip-cell circulation at the edge of the structure. The model showed good agreement with measurements, particularly for the longshore component of the flow (RMSE = 0.07 m/s). Results showed that alongshore differential wave breaking at the edge of the submerged breakwater drove an intense (0.4 m/s) two-dimensional circulation for low- to moderate-energy waves. Simulations indicated that inner-bar rip channel development, which was observed prior to the submerged reef implementation, was inhibited in the lee of the structure as rip-cell circulation across the inner bar disappeared owing to persistently low-energy breaking waves. The cross-shore sandbar dynamics in the lee of the structure were also impacted due to the drastic decrease of the offshore-directed flow over the inner-bar during energetic events. This paper highlights that implementation of a submerged breakwater results in larges changes in nearshore hydrodynamics that, in turn, can affect overall surf zone sandbar behavior. Full article

Low-crested detached breakwaters (LCDBs) have been widely employed as a mitigation measure against beach erosion. However, only a few studies have assessed their performance in sea-breeze-dominated environments. This work investigates the beach morphodynamics behind LCDBs deployed on a micro-tidal sea-breeze-dominated beach. The study area, located in the northern Yucatán peninsula, is characterized by low-energy, high-angle waves, which drive a persistent (westward) alongshore sediment transport (O(10⁴) m³/year). High-resolution real-time kinematics global positioning system (GPS) beach surveys were conducted over a one-year period (2017–2018) to investigate the performance of LCDBs at three sites. Moreover, unmanned aerial vehicle flights were employed to evaluate far-field shoreline stability. Field observations revealed a distinct behavior in the three study sites, dependent on the breakwaters’ transmission characteristics, geometry, stability, and shoreline orientation. Impermeable LCDBs, made of sand-filled geosystems, induced significant beach accretion (erosion) in up-(down-)drift areas. On the other hand, permeable LCDBs, made of Reef Ball™ modules, induced moderate beach changes and small erosion in down-drift areas owing to higher transmission coefficients. Measurements of LCDBs’ freeboard height show that sand-filled geosystems’ breakwaters presented a significant loss of sand during the study period, which explains the unexpected beach morphodynamic response on the lee side of the structure. Observations suggest that the study area is highly sensitive to the presence of LCDBs with low transmissivity. Full article

Mound breakwaters with significant overtopping rates in depth-limited conditions are common in practice due to social concern about the visual impact of coastal structures and sea level rise due to climatic change. For overtopped mound breakwaters, the highest waves pass over the crest producing armor damage, not only to the front slope, but also to the crest and the rear slope. To guarantee the breakwater stability, it is necessary to limit the armor damage in the three parts of the structure: Front slope, crest, and rear slope. This paper describes the hydraulic stability of the armor layer of medium and low-crested structures in wave breaking conditions. Small-scale physical model tests were carried out with different relative crest freeboards and three armor units: Rocks, cubes, and Cubipods. The armor damage progression in the front slope, crest, and rear slope was analyzed using the Virtual Net method to consider the heterogeneous packing and porosity evolution along the armor slope. A comparison is provided between the hydraulic stability of the different armors and their relationship with the measured overtopping volumes. Full article

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. Full article