Small slope approximation modeling of scattering from a spilling breaker wave crest

The second-order small-slope approximation (SSA) model of the electromagnetic rough surface has been applied to a series of surface profiles representing the time-evolution of the crest of a spilling-breaker wave. The crest profiles were measured directly in a wave tank using an optical measurement technique. The SSA results were compared directly with reference scattering found using a moment-method based numerical technique and the standard two-scale model (TSM) at various angles of incidence. Both SSA and TSM give excellent results with profiles where no post-breaking steep features remain. As the incidence angle increases both SSA and TSM lose accuracy as expected. Both models give fair accuracy at vertical polarization with 80° incidence (10° grazing). However, at horizontal polarization SSA dramatically underpredicts the scattering during most of the measurement time. TSM shows better performance under these conditions, provided that the appropriate scale-separation threshold is used.