Narrow Results

The reliability-based risk assessment and structural design model (REBAD) (Balas, 1998) introduced in this paper, is a worthwhile tool in the preliminary design of maritime structures which are portrayed by vast failure consequences and significant resource expenditures. REBAD model in which the Second-Order Reliability Method (SORM) is utilized together with a cost-optimization algorithm, is implemented to Mersin yacht harbor which is constructed near the city of Mersin located on the Turkish coast of Mediterranean Sea. SORM is established on a more correct approximation of the failure surface than the first-order method. The failure mode probability is predicted by approximating the failure surface by a quadratic surface with the identical curvature at the design point. First, REBAD is implemented to the main breakwater by utilizing the Hudson failure function to determine the size of armor units, then failure mode response functions were obtained for the fixed exceedance probability of several damage levels. Combining with the hydraulic model study carried out by Ergin and Özhan (1994) and REBAD outcome, the trunk section of main breakwater and design functions were determined. The second order reliability-based sensitivity study was also carried out to systematically compare limit state equations of Hudson and Van der Meer.

Authors concluded that, Van der Meer failure function is a more reliable design function for this case study which illustrated the affect of uncertainties encountered in Turkey on the design practice of rubble mound breakwaters. Since REBAD enabled the first application of reliability-based design practice for coastal structures in Turkey, it is the commencement for the development of a national reliability-based design standard of maritime structures.

An assumption of collinear straight coastlines in the far field, traditionally used in wave models for harbors, is a limitation to wave simulations in the domain extending to infinity, since this assumption is invalid for most real coastlines. By alleviating this limitation based on the geometric-optics approximation, functions of these coastal wave-deformation models are extended to be able to predict wave patterns around the semi-infinite breakwater and convex and concave coasts, such as bulkheads with discontinuous alignment. Mapped infinite elements are also formulated for exterior wave problems. A basic concept in developing is that the true decaying property of scattered waves, i.e. with the modes of r^-1/2, r^-3/2,…, where r is the radial distance, must be represented directly in infinite elements. To do so, a complementary element is introduced to the infinite element. Infinite element integrals can be expressed explicitly because of analytical integrability due to weak singularity in the infinite mapping. Three test problems for coastal wave-deformation models are solved combined with the cubic infinite element with r^-1/2 and r^-3/2 decays; this combination technique is found to be effective and accurate for problems of wave scattering.

The reflection and transmission characteristics of regular waves by a partially immersed curtain-type breakwater have been studied by the experiment and numerical model in the paper. Non-overtopping and overtopping of the breakwater by the incident wave were considered to compare different wave dissipation efficiencies. An incompressible Smoothed Particle Hydrodynamics (SPH) theory combined with a Large Eddy Simulation (LES) model was employed as the numerical tool. The SPH method is robust for tracking free surfaces without numerical diffusion and the LES model is capable of analyzing turbulence and eddy vortices during wave-breakwater interactions. A good agreement between computational and experimental wave profiles verifies the accuracy of the SPH-LES model. The computations also disclose that the wave energy dissipation is mainly attributed to the turbulence production and vortex shedding during the wave transmission and reflection processes.

In this study, the reliability design method developed by Shimosako and Takahashi in 2000 for the calculation of the expected sliding distance of the caisson of a vertical breakwater is extended to take into account the variability in wave direction. The effects of directional spreading and the variation of deepwater principal wave direction about its design value were found to be minor compared with those of the obliquity of the deepwater design principal wave direction from the shore-normal direction. Reducing the significant wave height at the design site by 6% to correct the effect of wave refraction when using Goda's model was found to be appropriate when the deepwater design principal wave direction was about 20 degrees. When we used the field data in a part of the east coast of Korea, taking the variability in wave direction into account reduced the expected sliding distance to about one third of that calculated without taking the variability in wave direction into account, and the required caisson width was reduced by about 10% at the maximum.

Climate change is expected to lead to increases in both sea level and typhoon intensity, which could threaten the stability of breakwaters in the future. In this study, calculations using the SWAN model showed that a 10% potential increase in the future wind speed of typhoons resulting from the warming of surface sea temperatures can lead to a 21% increase in the significant wave heights generated by these winds. To understand the effect that this would have on breakwater stability, the expected sliding distances for the breakwaters at Shibushi Ports in Japan were estimated using a probabilistic design method. The results show that in the future the expected sliding distances may become five times greater than at present, due to a combination of increases in sea level and wave height.

The 2011 Great East Japan Earthquake and Tsunami caused devastation all along the coast of eastern Japan. Ishinomaki City was one of the most severely damaged municipalities, though the height of the tsunami in this city was smaller than that in Iwate and northern Miyagi. A large difference in the extent of building damage was found comparing two areas of Ishinomaki: one not protected by breakwaters, and the other behind the breakwaters of a large Fishery port. In this paper, the authors perform numerical simulations to determine whether these general breakwaters, which were designed not to block tsunamis but wind waves, reduced the tsunami's energy and contributed to a reduction in inundation of the areas behind the port. Before assessing the effectiveness of breakwaters against tsunamis, simulated inundation heights in each of these two areas were compared with heights measured by The 2011 Tohoku Earthquake Tsunami Joint Survey Group. It was found that a simulation with larger Manning's n value (n = 0.30) can evaluate inundation more precisely than when this value is small (n = 0.06), as often used by Japanese coastal engineers. Comparing the region protected by breakwaters with the unsheltered area, the results of a 2D shallow water equation model do not show a significant difference in inundation mainly because the tsunami intrudes through the openings in the breakwater and fills up the port basin with seawater in a very short time. Therefore, the effectiveness of general breakwaters in reducing tsunami impact should not be overestimated. However, a hypothetical study shows that water levels could be greatly reduced if the port were fully enclosed by breakwaters. This implies that a port could substantially reduce tsunami inundation if the breakwater openings were equipped to be closed before tsunami arrival. The authors also demonstrate that the difference in the extent of building damage in the two areas of Ishinomaki considered can be explained by the difference in drag forces due to the topography as well as the difference in land use, rather than by the presence of breakwaters.

This contribution presents a new Artificial Neural Network (ANN) tool that is able to predict the main parameters describing the wave-structure interaction processes: the mean wave overtopping discharge ($q$), the wave transmission and wave reflection coefficients ($K_t$ and $K_r$). This ANN tool is trained on an extended database (based on the CLASH database) of physical model tests, including at least one of the three output parameters, for a total number of nearly 18,000 tests. The selected 15 nondimensional ANN input parameters represent the most significant effects of the structure type (geometry, amour size and roughness) and of the wave attack (wave steepness, breaking, shoaling, wave obliquity). The model can be used for design purposes, leading to a greater accuracy than existing formulae and similar tools for complex geometries for the prediction of $K_r$ and $K_t$, and it has a similar accuracy as the CLASH ANN for predicting $q$.

According to the main recommendations and technical criteria along the world, the design project of a breakwater must address the verification of the different failure modes that can affect the breakwater stability. This research focuses on the estimation of the statistical characteristics of the wind waves interacting with different breakwater types and their evolution from the toe of the structure to the toe of the crown. This knowledge is essential to calculate the reliability of the structure in its useful life. Partial standing wave patterns are likely to occur in front and along the section of the breakwater depending on their typology. Based on Rice’s theory of envelope amplitude, we present an approximate solution of the total wave height distribution in front of the breakwater. The experimental results in 2D confirmed that the incident, the reflected and the total wave height in front of the breakwater followed a Rayleigh distribution in which the parameter is the root-mean-square total wave height. Its value depends on the modulus and phase of the reflection coefficient. The probability density function of the total wave height evolves from a Rayleigh to a Weibull distribution, whose scale and shape parameters vary from the toe to the crown of the structure, and depend on the breakwater type, the relative grain diameter, and the relative water depth. The largest deviation from the Rayleigh distribution occurs at the toe of the crown. These findings are weakly dependent on the incident wave steepness, and are valid for narrow-banded incident wave trains, impinging perpendicularly on nonovertopped breakwaters with a steep frontface, which ensures the rapid evolution of the wave train. Then, each mode of failure that might occur in the breakwater section can be formally checked against the same incident wave height, but verified (calculated) with the actual wave height, locally transformed by the specific typology.

Experimental and numerical studies were conducted to assess the stability of the breakwater of Fujairah Port on the east coast of the United Arab Emirates against tsunamis. Experiments were conducted on a 1:50 scale and numerical analysis was done with the formulas of tsunami-breakwater interaction. The experimental observations corroborated the numerical results. The breakwater was safe up to 3m of tsunami height, but when the tsunami height increased from 3m to 6m, the percentage damage to the leeside (portside) of the breakwater increased compared to the seaside. The leeside of the breakwater was deformed by tsunami overtopping and seepage — the breakwater cross-section reshaped from trapezoidal to quasi-triangular at 6m tsunami impact, however, the breakwater was not breached. Rubbles of the breakwater were transported due to sliding, rolling, and saltation by overtopping and seepage. For 6m tsunami height, rubbles from the seaside rolled over up with saltation and transported to the leeside. A large number of rubbles were transported landward from the leeside of the breakwater but a few were displaced offshore from the seaside.

In this study, the performance-based design method developed for a conventional solid-wall caisson breakwater is extended to a perforated-wall caisson breakwater. First, to verify the mathematical model to calculate the sliding distance of a perforated-wall caisson, hydraulic experiment is conducted. A good agreement is shown between the model and experimental results. The developed performance-based design method is then compared with the conventional deterministic method in different water depths. Both the expected sliding distance and the exceedance percentage of total sliding distance during the structure lifetime decrease shorewards outside the surf zone, but they increase again toward the shore inside the surf zone. The performance-based design method is either more economical or less economical than the deterministic method depending on which design criterion is used. If the criterion for expected sliding distance or exceedance percentage is used in the ultimate limit state, the former method is less economical than the latter outside the surf zone, whereas the two methods are equally economical inside the surf zone. However, if the breakwater is designed to satisfy the criterion for exceedance percentage in the repairable limit state, the former method is more economical than the latter in all water depths.

Some neural network stability models for rubble mound breakwaters are proposed and analyzed. The proposed models give the more reliable results than the well known van der Meer’s formula. Among them, the neural network model having the slope angle and the wave steepness as independent inputs shows the best performance. But the neural network model having independent input parameter for significant wave height, significant period is found not to be useful for the design of rubble mound breakwater because the design parameters exceed the training data ranges. In addition, a reliability analysis technique using the trained neural network model is proposed. Based on two analysis examples, it was found that van der Meer’s formula gives the larger or smaller failure probabilities than the neural network models. Therefore, one should be very careful in making a decision whether the designed armor units are safe or not if he/she uses only empirical formula. To avoid this situation, it is heavily recommended that more advanced models such as the neural network model proposed here should be simultaneously considered.

A tsunami force has unknown features due to its complexity, and makes it difficult to be predicted accurately. As a typical example, in the 2011 off the Pacific coast of Tohoku earthquake with huge tsunami, many breakwaters slid or fell down by the unexpectedly terrible damages caused by the tsunami. For effective design of coastal structures, achieving a correct prediction of the tsunami pressure should be an urgent subject. In the existing studies, the tsunami force F with an overflow is estimated by using a compensation coefficient α based on the hydrostatic pressure as: F = αρgh. The compensation coefficient α is usually set as α = 1.1 for the front of the caisson and 0.9 for the rear of that constantly. However, in recent studies, it was found that Parameter α can be easily and randomly changed depending on the boundary condition, and unfortunately, estimation method of its effective has not been developed yet. To resolve this problem, in this study, hydraulic experiments targeting on tsunamis with their overflows on a breakwater are implemented to examine the variability of the compensation coefficient α. And from the experimental results, a simple and effective estimation method for the compensation coefficient α is newly proposed.

After field surveys on the Tohoku earthquake tsunami in 2011, coastal engineers recognized the importance of persistent coastal structure to reduce the tsunami damages. It is important to understand the characteristics of tsunami flow formed around the breakwater and rubble mound with three-dimensional configuration. The applicability of CADMAS-SURF has been investigated mainly in two-dimensional wave deformation problems and few investigation has been done in the case of three-dimensional tsunami flow problems. CADMAS-SURF/3D was employed to simulate tsunami flow over the breakwater. In this study, a numerical half-basin domain was considered in 3D simulations. Hososhima Yojima breakwater was selected as the prototype structure. This is one of the breakwaters that are needed to be reinforced to realize a persistent breakwater against a supposed L2 tsunami attack on the eastern side of Kyushu Island. This study firstly discusses the applicability of CADMAS-SURF to the three-dimensional tsunami flow problem. Some hydraulic experiments were conducted to verify the validity of numerical results. This study also discussed some characteristics of tsunami flow over the breakwater such as tsunami flow passes through the gap between caissons, the effect of the partial subsidence of caissons on tsunami flow above the rubble mound, and the characteristics of tsunami flow in around a breakwater.

Many caissons of breakwaters were slid or overturned due to tsunami overflow pressure caused by 2011 Tohoku earthquake. To prevent this sliding failure, the pressure estimation method under tsunami overflow was introduced in the new design guideline of breakwater against tsunami in 2015. In this guideline, the uplift and the overburden pressure are not considered, instead only buoyancy force acting on the caisson is considered. However, under tsunami overflow, the pressure difference between the bottom and the top of breakwater caisson, especially the caisson having a large parapet, can be extremely larger than the buoyancy force. In order to examine this excess uplift force, a series of hydraulic experiments were conducted. The experiment was conducted in an experimental flume in which the large pump was installed to produce tsunami overflow. Pressure gauges, water level gauges and velocity meter were installed at the top and the bottom of the caisson model. Through the experiments, it was clarified that the large uplift pressure and the small overburden pressure cause the upward force larger than the buoyancy force. This upward force reduces the stability of caisson, especially the caisson having the large parapet.