Abstract: Wave diffraction will happen when it encounters the island in the wave propagation process. Based on the modified slope equation, the hybrid element method was introduced to solve the equation, and the analytical solutions proposed by KUO et al was used to verify the present numerical solutions. Wave diffraction from a 3D circular island for engineering scale was conducted. The ratio of the wave height to the incident wave height (defined as the relative wave height) along the section of the wave propagation direction and around the island coastline was calculated for different incident wave periods, shoal shape parameters and island sizes. The results indicate that with the decrease in the incident wave period and the size of island, as well as the increase in the shoal shape parameters, the oscillation amplitude of the relative wave height at the head wave side of the circular island, the value of the relative wave height at the back wave side of the circular island, and both the oscillation amplitude and the value of the relative wave height around the island coastline all increase. However, for the different cases, the majority of the maximal relative wave heights happen at the head wave point, and several maximal relative wave heights happen at about 20-25 degrees on the both sides of the head wave point. The minimum relative wave heights are located about 30 degrees on the both sides of the back wave point.

Key words: modified mild slope equation, hybrid element method, circular shoal, island, diffraction