Pressures on a rubble-mound breakwater crown-wall for tsunami impact
Introduction
The hydraulic performance of marine structures subjected to tsunamis is essential to mitigate their onshore consequences. Despite their relative low occurrence frequency, large tsunamis can definitively test marine structures’ resisting capabilities, given the extraordinary loads that these extreme events can imply. A proper design of marine defenses and their components can contribute to reducing the risk in the areas they protect (Takagi et al., 2014).
In the literature, several studies on the performance of vertical breakwaters under tsunami actions attack can be found, i.e., Kato et al. (2006), Jayaratne et al. (2016), Palermo et al. (2013) and Esteban et al. (2016), among others. However, the stability of rubble-mound breakwaters (RMBs) under tsunami actions has not been sufficiently addressed. Some authors, such as Esteban et al., 2014, Hanzawa et al., 2012, and Aniel-Quiroga et al. (2018), studied the effect of tsunamis on the stability of RMB armors based on laboratory experiments or in-field observations.
RMBs are commonly constructed with a parapet or crown-wall at the crest. These superstructures have several functions, such as allowing rolled access for functional or maintenance operations, overtopping reduction or back slope armor protection. The crown-wall stability design depends on the expected storm wave load history along its service life cycle. It is common to link these loads to a design-storm. However, tsunami loads in the crown-wall can exceed the ones considered for storm waves.
Regarding the design of crown-walls loaded by wind waves, there are several methods to calculate the forces on crown-walls. A detailed enumeration and comparison of these methods can be found in Negro et al. (2013). In general, these methods split the effect of the impinging wave into two components: a dynamic part considering the impact of the wave on the crown-wall and a pseudo-hydrostatic part. This separation of components was upgraded first, by Pedersen and Burcharth (1992), and second, by Martín et al.(1995 and 1999). More recently, following a different approach, Molines et al. (2018) also calculated pressures on crown-walls by means of the overtopping rate.
These existing methods are developed to design crown-walls exposed to wind-waves. As a consequence, tsunamis fall out of their application range, due to their characteristics, such as an extremely long wave length and period, and to be closer to a transient effect.
Harbitz et al. (2016) and Guler et al., 2015, Guler et al., 2018, conducted laboratory experiments on RMBs with crown-walls under tsunami-like actions. The former used solitary waves and bores to model tsunamis, and the latter used solitary waves and a constant flow to model overflow. However, in their experiments, the crown-wall was only laid on the breakwater and not fixed, and consequently, it slid or turned owing to the tsunami and overflow loads. Although the real effect was qualitatively well represented, the pressure records on the crown-wall were incomplete. This is precisely the gap that this study aims to cover. Focused on gaining a better understanding of the interaction between tsunamis and RMBs, laboratory experiments on rubble-mound breakwaters under tsunami-like actions were conducted. In Aniel-Quiroga et al. (2018), the experiments were presented and a stability analysis was conducted. This research paper now presents a second part of the analysis of those laboratory experiments, focused on understanding the pressures that crown-walls of RMB must support due to tsunami-like actions. In these experiments the crown-wall was fixed to the structure, and thus, it was not allowed to slide or turn, providing the complete recording of the loadings or pressures in the crown-wall. In addition, this setup avoids a lack of representativeness of the crown-wall foundations friction, what could trigger unreal sliding and failure mechanisms.
In this study, one of the referred methods, Martin et al. (1999), is extended to calculate the tsunami loads on crown-walls, based on the results of laboratory experiments that are also presented. Given the infeasibility of simulating the whole tsunami length in the laboratory at an adequate scale, the tsunami action was reproduced by being split into two parts. The loadings of the first impact of the tsunami were simulated by means of solitary waves, and the overflow was simulated by creating a current over the crown-wall. The modeled section reproduced a representative marina or fishing port breakwater in the Spanish Mediterranean coast on a 1:20 scale (see Aniel-Quiroga et al. (2018)). This scale allows accurate measurement of crown-wall pressures and tsunami generation.
The rest of this paper is structured as follows: Section 2 describes the experimental setup, summing up the details presented in Aniel-Quiroga et al. (2018), focusing on the crown-wall; section 3 presents the results of the experiments; and section 4 focuses on the extension of existing stability design methods to a tsunami case. Section 5 discusses the results, and finally, in section 6, some conclusions are presented.
Section snippets
Experimental setup
Physical experiments were conducted in the wave-current and tsunami flume (COCOTSU) of the University of Cantabria Environmental Hydraulics Institute (IHCantabria). This flume is 52 m long, 2 m wide and 2.5 m high (see Fig. 1), and it has a 2 m-stroke piston wavemaker and a pumping system that allows the fast recirculation of water to generate currents.
The instrumentation consisted of free surface, pressure and velocity sensors. Nine capacitive wave gauges were installed in the flume, 6 of them
Analysis of results
The loads that the crown-wall supports, and, consequently its hydraulic performance, varied from solitary waves to overflow currents tests. In the following subsections, the calculated variables and their characterization are approached for each action type.
Crown-wall overflow loading analysis
In general terms, the pressures measured in the overflow experiments were lower than those measured in the solitary wave experiments. However, tsunamis do not always generate solitons, and a gradual and continue level rising leads directly to the overtopping (Shuto, 1985). In those cases, values of the maximum overflow peak, Hp, higher than those tested in this study could produce pressures that may come to be higher than first impact pressures. On account of this possibility, a methodology to
Summary
In this study, the pressures induced by tsunami-like waves on the crown-wall of rubble-mound breakwaters were measured and analyzed by means of laboratory experiments on a scale model. The tsunami action was split into 2 actions: the first impact of the wave in the crown-wall was simulated by means of solitary waves, and the overflow was simulated by means of a steady current overtopping the crown.
In the laboratory experiments, the crown-wall of the structure was fixed, not allowing sliding or
Conclusions
Regarding the pressures produced by a tsunami in the crown-wall of an RMB, the following conclusions were drawn:
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The largest horizontal pressures in the crown-wall come from the initial dynamic impact of the solitary wave.
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For tsunami-like solitary waves:
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The pressure in the part of the crown-wall vertical face that is not protected by the berm is three times the pressure in the protected part.
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The horizontal force time series present two clear peaks. The first one is related to the initial dynamic
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Acknowledgments
The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP7/2007–2013) under grant agreement n°603839 (Project ASTARTE - Assessment, Strategy and Risk Reduction for Tsunamis in Europe).
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