Where a subdivision or land development is traversed by a watercourse, there shall be drainage easement or right-of-way provided to the Township conforming substantially with the line of such watercourse and of such width as will be adequate to preserve natural drainage but not less than 20 feet, or as may be required or directed by the Township and/or the Department of Environmental Protection. The owner shall properly grade and seed slopes and fence any open ditches when deemed necessary by the Township.