Numerical Analysis of Vertical Breakwater Stability under Extreme Waves

The purpose of this study is to perform a numerical simulation of caisson breakwater stability concerning the effect of wave overtopping under extreme waves. A numerical model, which solves two-dimensional Reynolds-averaged Navier–Stokes equations with the k−ε turbulence closure and uses the volume of fluid method for surface capturing, is validated with the laboratory observations. The numerical model is shown to accurately predict the measured free-surface profiles and the wave pressures around a caisson breakwater. Considering the dynamic loading on caisson breakwaters during overtopping waves, not only landward force and lift force but also the seaward force are calculated. Model results suggest that the forces induced by the wave overtopping on the back side of vertical breakwater and the phase lag of surface elevations have to be considered for calculating the breakwater stability. The numerical results also show that the failure of sliding is more dangerous than the failure of overturning in the vertical breakwater. Under extreme waves with more than 100 year return period, the caisson breakwater is sliding unstable, whereas it is safe in overturning stability. The influence of wave overtopping on the stability analysis is dominated by the force on the rear side of the caisson and the phase difference on the two ends of caisson. For the case of extreme conditions, if the impulse force happens at the moment of the minimum of load in the rear side, the safety factor might decrease significantly and the failure of sliding might cause breakwater damage. This paper demonstrates the potential stability failure of coastal structures under extreme sea states and provides adapted formulations of safety factors in dynamic form to involve the influence of overtopping waves.

Performance of an Array of Oscillating Water Column Devices in Front of a Fixed Vertical Breakwater

The present study explores the performance of an array of cylindrical oscillating water column (OWC) devices, having a vertical symmetry axis, placed in front of a bottom seated, surface piercing, vertical breakwater. The main goal of this study is the investigation of a possible increase in the power efficiency of an OWC array by applying, in the vicinity of the array, a barrier to the wave propagation, aiming at amplifying the scattered and reflected waves originating from the presence of the devices and the wall. To cope with the set goal, a theoretical analysis is presented in the framework of linear potential theory, based on the solution of the proper diffraction, and pressure-radiation problems in the frequency domain, using the image theory, the matched axisymmetric eigenfunction expansion formulation, and the multiple scattering approach. Numerical results are presented and discussed in terms of the expected power absorption by the OWCs comparing different array’s characteristics i.e.,: (a) angle of incidence of the incoming wave train; (b) distances from the breakwater; and (c) geometric characteristics of the different arrangements. The results show that compared to the isolated OWC array (i.e., no presence of the wall), the power efficiency of the OWCs in front of a breakwater is amplified at specific frequency ranges.

Mean Drift Forces on Vertical Cylindrical Bodies Placed in Front of a Breakwater

This paper presents a numerical and experimental investigation of the second-order steady horizontal and vertical drift forces acting on cylindrical bodies in regular waves. The examined bodies are either kept restrained in front of a vertical breakwater or are considered free- floating when alone in the wave field. Two principally different approaches for mean drift forces determination are described: the momentum conservation principle and the direct integration of all pressure contributions upon the instantaneous wetted surface of the bodies, whereas, for the solution of the associated diffraction and motion radiation problems, analytical and panel methodologies are applied. The hydrodynamic interaction phenomenon between the bodies and the adjacent breakwater are taken into account by using the method of images. Theoretical and numerical results, concerning the horizontal and the vertical drift forces, are presented and compared with each other. Furthermore, additional comparisons are made with experimental data obtained during an experimental campaign at French research institute for exploitation of the sea (IFREMER), in France.

Evaluation of the Hydraulic Performance of a Rear-Parapet Vertical Breakwater under Regular Waves through Hydraulic Experiments

Climate change has resulted in increased intensity and frequency of typhoons and storm surges. Accordingly, attention has been paid to securing the breakwater’s stability to protect the safety of the port. Herein, hydraulic model experiments were conducted to evaluate the hydraulic performance of a vertical breakwater having a rear parapet. For comparison, cases in which the parapet was placed on the seaside, the harborside, and at the center of the breakwater were considered. Regular waves were used for convenient performance analysis. Five wave gauges and nine pressure transducers were installed to secure physical data for hydraulic performance evaluation. Results showed that a rear parapet can reduce the maximum wave force acting on the breakwater. Even though impulsive pressure was generated, it did not affect the stability of the breakwater owing to the phase difference between the maximum wave pressures acting on the caisson and parapet. By decreasing the maximum wave force, the required self-weight that satisfies the safety factor of 1.2 was reduced by up to 82.7%; the maximum bearing pressure was reduced by up to 47.6% compared with that of the parapet located on the seaside. Thus, the rear parapet was found to be more suitable for actual applications.

Wave Power Absorption by Arrays of Wave Energy Converters in Front of a Vertical Breakwater: A Theoretical Study

The present paper deals with the theoretical evaluation of the efficiency of an array of cylindrical Wave Energy Converters (WECs) having a vertical symmetry axis and placed in front of a reflecting vertical breakwater. Linear potential theory is assumed, and the associated diffraction and motion radiation problems are solved in the frequency domain. Axisymmetric eigenfunction expansions of the velocity potential are introduced into properly defined ring-shaped fluid regions surrounding each body of the array. The potential solutions are matched at the boundaries of adjacent fluid regions by enforcing continuity of the hydrodynamic pressures and redial velocities. A theoretical model for the evaluation of the WECs’ performance is developed. The model properly accounts for the effect of the breakwater on each body’s hydrodynamic characteristics and the coupling between the bodies’ motions and the power take-off mechanism. Numerical results are presented and discussed in terms of the expected power absorption. The results show how the efficiency of the array is affected by (a) the distance between the devices and the wall, (b) the shape of the WEC array configuration, as well as (c) the angle of the incoming incident wave.