The first term on the right side of the governing equation is the wave diffraction term formulated from a parabolic approximation wave theory (Mase 2001). In applications, the diffraction intensity parameter κ values (≥ 0) needs to be calibrated and optimized for structures. The model omits the diffraction effect for κ = 0 and calculates diffraction for κ > 0. Large κ (> 15) should be avoided as it can cause artificial wave energy losses (Mase 2001). In practice, values of κ between 0 (no diffraction) and 4 (strong diffraction) have been determined in comparison to measurements. A default value of κ = 2.5 was used by Mase et al. (2001, 2005a, 2005b) to simulate wave diffraction for both narrow and wide gaps between breakwaters. In CMSWave, the default value of κ assigned by SMS is 4, corresponding to strong diffraction. For wave diffraction at a semi-infinite long breakwater or at a narrow gap, with the opening equal or less than one wavelength, κ = 4 (maximum diffraction allowed in the model) is recommended. For a relatively wider gap, with an opening greater than on wavelength, κ = 3 is recommended. The exact value of κ in an application is dependent on the structure geometry and adjacent bathymetry, and should to be verified with measurements.