Various theories and wave transformation models have been devised to describe nearshore wave fields. Each of the theories and models has certain advantages and limitations with respect to its applicability. In order to estimate the wave conditions, the appropriate model has to be selected depending on the relative importance of the various physical processes and the target coastal area. Wave prediction models based on the energy balance equation are suitable for applications involving large sea areas because they require little computational power. These models are used for wave forecasting and hindcasting. The calculated quantities are the spectral energy densities, which are phase averaged quantities slowly varying over several wavelengths. Originally, phase averaging wave models did not account for wave diffraction. Recently, however, attempts have been made to introduce wave diffraction effects by Resio (1988), Booij et al. (1997), Rivero et al. (1997) and Mase (2001). Mase (2001) directly introduced a diffraction term, formulated from a parabolic approximation wave equation, into the energy balance equation and employed the first order upwind difference scheme, which is numerically stable due to numerical diffusion. The numerical diffusion term represented by the second derivative is similar to the wave diffraction term.
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