Now a days, the NOWT-PARI is widely applied to calculation of wave transformation in shallow water area. It is, however, difficult for such a depth-integrated wave model to timate distribution of wave pressures and splash of wave overtopping on a seawall with complex cross-section. On the other hand, CADMAS-SURF, which is one of CFD, is employed for reproducing such phenomena though its target duration and sphere are restricted due to the limitation for computation capacity. Therefore, a coupling of them is going to be discussed to obtain an effective solution on numerical calculation for hydrodynamics on wave transformations.
　By the way, in order to conduct a coupling between a wave equation and the Navier-Stokes equation, the vertical distribution of horizontal velocities should be estimated at the connecting boundary of them with the variables calculated in the wave equation.The FFT method which is applied to the wave train obtained only after wave calculation can be used to that, though it requires larger number of component waves to approximate the original wave train with higher wave nonlinearity.
　In this paper, a new method, which estimates the vertical distribution of horizontal velocities under wave calculation, is proposed with applying each nonlinear wave theory to approximation of the variables of wave calculation respecting the relative water depth. And an applicability of a one-way coupling from NOWT-PARI to CADMAS-SURF/2D with the present approximation method, called as Single-component wave approximation, is verified. Comparing the FFT method, the present method can estimate appropriate horizontal velocities distributed in vertical for more nonlinear wave profiles effectively with better accuracies. Finally, some other problems to conduct both one-way and two-way couplings between NOWT-PARI and CADMAS-SURF/3D are discussed in order to calculate the multi-directional random wave transformations.