Breaking criteria and energy losses for three‐dimensional wave breaking

Laboratory experiments were used to explore the influence of spatial focusing and diffraction on the evolution of unsteady, three‐dimensional, deep water wave packets with a constant‐steepness spectrum. The wave packets were generated by 13 independently programmed paddles and evolved to breaking near the midpoint of a 4 m × 11 m test section. Detailed measurements of surface displacements were made across the entire test section and were used to examine energy losses and breaking criteria. Three forms of breaking criteria were considered: (1) geometric criteria based on local and global wave steepness, (2) a kinematic criterion based on particle and phase velocities, and (3) a dynamic criterion based on higher harmonic energy evolution. The results indicate that directionality of the waves can either increase (focusing waves) or decrease (diffracting waves) the geometric breaking criterion as well as breaking severity. In contrast, the directionality of waves had little effect on the kinematic criterion. At breaking, the ratio of local particle velocity and phase velocity was shown to be larger than unity for both focusing and diffracting waves. Indeed, the robustness and simplicity of the kinematic criterion make it an excellent choice for field application. Finally, the directionality of waves did not alter the up‐frequency energy transfer associated with wave steepening. The three‐dimensional, spatially focusing and diffracting wave packets lost 34% and 18% of their energy, respectively, as a result of plunging breakers and lost 12% and 9%, respectively, as a result of spilling breakers. Comparable two‐dimensional breakers with the same spectral shape lost 16% for plunging and 12% for spilling.