The Study Of Oblique And Multidirectional Irregular Wave Action On Vertical Breakwater

Vertical breakwater is one of typical maritime sheltering structures, which is mainly subjected to wave forces. Traditionally, the design of breakwater is based on the formulae developed for head-on unidirectional waves, very little attention has been addressed so far to the effects of wave obliquity and multi-directionality on the wave forces on vertical breakwaters. However, real sea waves are multidirectional and irregular, and usually attack a breakwater obliquely. It has been found that oblique waves may exert larger forces than standing waves, and under oblique and multidirectional waves attack, long structures are not simultaneously subjected to the peak wave forces, a reduction of the total load on long structures can be expected, but the regularity of reduction is still uncertain. In order to carry out safe and economical design, an accurate estimation of wave forces acting on breakwater becomes very important. In this paper, the influences of wave obliquity and multi-directionality on wave forces acting on vertical breakwaters are investigated by a 3-D hydraulic model test and the numerical simulation.The physical model test was carried out in the multidirectional basin at the State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology. The JONSWAP spectrum suggested by Goda was used to simulate irregular waves, and directional spreading function of Guangyi-type was used in the test. Angles of incidence varied from 0° to 60°, and three spreading parameters, s=10, 40,∞, three values of wave steepnesses and wave heights were tested.The variation of horizontal and uplift forces acting on the unit length of breakwater with attack angle is presented, and the effects of multi-directionality, wave steepness, and relative wave height on wave forces are discussed. The longitudinal distribution of wave forces and the longitudinal load reduction are analyzed, and the influences of wave obliquity, multi-directionality and wave parameters are discussed. By use of the Ordinary Least Square Estimation (OLSE), the empirical formulae of longitudinal distribution coefficients and longitudinal load reduction coefficients are presented for theoretical study and practical use. The analytical method used in this paper is justified by comparing with others' results.A numerical model based on the potential theory is established to simulate head-on and oblique irregular waves acting on vertical breakwater. A source term is used inside the domain to generate the waves, and outgoing waves are dissipated by sponge layers. Numerical tests show that incident wave train can be continuously generated from the source region without being affected by reflected waves. The non-linearity is considered enough by using theoriginal fully nonlinear free surface boundaries without being simplified. The σ-coordinate transformation is introduced to map the irregular physical domain with wavy free surface and uneven bottom to a regular one, and the numerical model is solved by the Finite Difference Method. The numerical results agree well with the experimental ones.