Breakwater Behaviour

Breakwaters are characteristic maritime works and provide basic infrastructure for artificial sea and land areas such as ports. Given their importance, it is of special interest to study in depth the hydrodynamic, structural and constructive behaviours of breakwaters and to analyse their functions as shelter infrastructures for port areas. In the last decade, breakwater design has moved from traditional deterministic design to probabilistic design, in which concepts such as uncertainty and risk can be quantified. Thus, this Special Issue promotes contributions including, but not limited to, the following topics: coastal structures, hydrodynamic behaviour, hydraulic stability, wave overtopping, damage evolution and modelling, risk evaluation and probabilistic design, and physical and numerical modelling.

This Special Issue comprises 13 papers that address different topics related to wave and breakwater interactions, including several challenges that maritime works are currently facing: scouring, load over walls, dissipation of wave energy, run-up and overtopping, damage to rubble mound breakwaters, damage to main layer units, performance of artificial reefs, efficiency of combined WEC-breakwater structures, and offshore structures.

Books regarding breakwater behaviour, in principle, focus on vertical caissons and rubble mound breakwaters. Thus, eight of the thirteen papers included in this Special Issue are devoted to these two typologies: vertical or caisson breakwaters [1,2,3] and rubble-mound breakwaters [4,5,6,7,8]. Breakwaters are designed not only to create sheltered areas, but also to protect the coast. To address these issues, one paper analyses the performance of an artificial reef [9]. In addition, breakwater design reflects advances in wave energy technology developed by maritime engineers. Hence, one analyses the performance of an oscillating water column (OWC) device [10] and another investigates an array of OWC devices [11] in front of a breakwater. Finally, two review papers are devoted to offshore structures.

The goal of this Special Issue is to improve breakwater performance knowledge to better prevent the main modes of failure, namely, seabed scouring, sliding and overturning in vertical breakwaters and armour damage, run-up and overtopping in rubble-mound breakwaters.

The presence of a breakwater in a maritime environment causes a change in the flows around the structure, which can lead to seabed scouring. The scouring process phenomenon results from the complexity of the flows in front of the breakwater comprising waves, currents or a combination of waves and currents. Seabed scouring causes the structure to fail or hinders port operations. To improve knowledge of the processes that create seabed scouring in front of a vertical breakwater, a broad study including field measurements, physical experiments and numerical simulations has been performed via a real case study [1]. The results from these three approaches confirmed that the scouring mechanism is caused by a combination of the geometry of the breakwater geometry, the sand source, seasonal changes in sea current directions, tidal current effects, and wave directions. On the other hand, another phenomenon that can cause scouring is a tidal bore, which is the front of flood tide waves that are impacted by rapidly rising tidal levels and changes in velocities. A tidal bore can generate scouring problems mainly in groins or spur dikes. In [2], the reader can develop a better understanding of the hydrodynamic response of spur dikes. The fitting formulas can estimate the maximum scour depth around the spur dike by considering the influence of the spur dike geometry, hydrodynamic forces, and sedimentary environment.

The main failure modes in vertical caissons are sliding and overturning. Traditionally, deterministic design methods have incorporated a safety factor to account for uncertainty. However, reliability design methods now prevail because they quantitatively consider uncertainty, although their inherent difficulties currently prevent their generalized use. A method proposed in [3] incorporates a reliability analysis that obtains the sensitivity of the limit state function to the design variables on a vertical caisson. The partial safety factors of the design variable were also obtained in this manner, then the load resistance factor was calculated, thus allowing for the calculation of a reliability index.

On the other hand, for rubble-mound breakwaters, the main failure mode is armour damage. Hence, the hydraulic stability of the armour layer of mound breakwaters has been widely studied. Armour layers of mound breakwaters are typically designed using formulas in the literature for nonovertopped mound breakwaters in nonbreaking wave conditions, although overtopped mound breakwaters in the depth-induced breaking wave zone are common design conditions. Based on physical model tests, armour damage on overtopped breakwaters is described in [5], which concludes with new design formulas for these breakwater typologies. In addition, the structural strength of armour units is explained by means of a numerical methodology in [7]. This work notes that its numerical model can potentially give more insight into the physical response of drop tests, for which the processes are difficult to objectively control, than into that of field tests.

Flooding risk in coastal urban fronts is mainly dominated by strong wave action. Sea level rise increases this risk, making it an urgent concern. In [6], different physical and numerical tools are used to analyse run-up and overtopping in smooth impermeable dikes with promenades, revealing some discrepancies. The results show that some cases demonstrate very complex wave–structure interactions involving plunging breaking waves followed by significant and intense turbulent air/water mixture flows. In these cases, the models reach their application limits, resulting in substantial differences between the numerical models and experimental tests. Other typologies suitable for protection under flooding are breakwaters with fence revetments, which effectively weaken the impact of waves. The fence plate can effectively dissipate energy and reduce the overtopping volume by generating eddy current in the cavity. Considering the stability and energy dissipation capacity of the fence plate, it is suggested that a gap ratio of 50% is reasonable [4].

If the objective of the maritime structure is to protect the coastline, a suitable solution could be a submerged breakwater. Submerged breakwater include artificial coral reefs, i.e., submerged structures that are deliberately placed on the seabed to mimic specific characteristics of a natural reef. These breakwaters can be constructed of modular concrete elements, depending on the aim of the reef; some design lines are included in [9].

All the previous works are based on experimental physical tests [1,2,4,5,6,9], numerical modelling [1,4,6,7] or field campaigns [1,2]. In particular, physical tests have typically been used to research breakwater performance; however, virtual level wave gauges, which are proposed in one paper as an alternative to these physical tests, offer a way to investigate the interaction of waves with coastal structures in wave flume experiments [8].

Additionally, analytical methods can be used to study breakwater performance, as in [10,11], where the efficiency of a single oscillating water column (OWC) device [10] or an array of OWC devices [11] in front of vertical breakwaters are analysed. This concept entails exploiting the anticipated amplification of the scattered and reflected wave fields that originates from the vertical walls to increase the wave power absorbed by the OWC in response to the walls’ wave reflections. The results in [10,11] prove that the amount of the wave power harvested by the OWC in front of a vertical breakwater is amplified compared to the wave power absorbed by the same OWC in the open sea.

Finally, two reviews of offshore maritime structures are included [12,13], revealing the fundamentals of many types of offshore structures (fixed and floating). These studies also present various design parameters for state-of-the-art offshore platforms. Sustainable design approaches and project management for these structures are also analysed. These reviews could serve as reference data sources for designing new offshore platforms and related structures.