A method is presented of estimating wave forces on a huge floating structure supported by a lot of float elements. We assume the structure's dimensions in the horizontal direction are comparable with the topographical scale, while each float-element's size is small. This method accounts for the effects of topography, such as bottom slope and wave breaking seashore. Wave motions a little away from each float-element affected by the bottom topography are described by the mild slope equation. They are matched with the solutions valid close to each float-element composing the huge structure. The wave breaking at seashore is incorporated in the mild slope equation.