Whenever any water impoundment structure is approved pursuant to sections 39-1817 to 39-1820, it shall be the responsibility of the county board to erect at both ends of the portion of the road subject to such inundation permanent warning devices providing notice of the potential hazard. Such warning devices shall conform to the United States Department of Transportation’s Manual of Uniform Traffic Control Devices, shall have printed thereon the words FLOOD AREA, and shall indicate the distance from such sign to the opposite extreme of the flood hazard area. The county board shall exercise reasonable care in maintaining such warning devices.