A new method is developed to evaluate radiation force acting on a large-scale platform that is composed of many floating elements. The order of the size of the platform is almost equal to the one of topographical feature change. Mild slope equation is used in order to taking into the influence of the topographical effect. The interaction between floating elements is also taken into consideration. Energy absorption at the seashore is modeled by two methods. Added-mass and damping derived by this coastal model is compared with those of flat bottom.