For water purification of man-made lakes and reservoirs, we propose a method to make an one directional flow by making use of natural wave energy. A devised artificial floating structure has a dam with a slope to activate the wave overtopping and to gain effectively the head difference. How much the volume of overtopping is gained has been investigated. The volume obtained and sent to a bottom layer is estimated briefly for the water purification. It is shown that the wave energy in man-made lakes seems to be large enough to be used for the water purification.