The multi-grid method is a well-known iterative technique to enhance computational efficiencies. Although there have been a number of investigations on it, dependencies of the computational costs on the presence of free surface and turbulent intensity seem to be still required in terms of computations in coastal engineering.
The multi-grid method was applied to two and three-dimensional simulations for water waves on various topographies, and the efficiency in terms of computation costs was discussed in this paper. A purpose of this paper is to reduce the computational costs required by the numerical investigations by introducing the multi-grid method, and to evaluate the efficiency of this numerical method for practical use in the actual wave field. The computing time and number of iterations in the multi-grid method is quantitatively compared with the conventional Gauss-Seidel method. Most interests in this paper are to identify the cost dependencies of this method on the presence of free surface and turbulent intensity. It is found that introducing the multi-grid method can save the computing time 80 - 97 % for two-dimensional simulations and 65 - 80 % for three-dimensional simulations.