Any local authority may from time to time enlarge lessen alter the course of cover in or otherwise improve any sewer belonging to them, and may discontinue close up or destroy any such sewer that has in their opinion become unnecessary, on condition of providing a sewer as effectual for the use of any person who may be deprived in pursuance of this section of the lawful use of any sewer : Provided that the discontinuance closing up or destruction of any sewer shall be so done as not to create a nuisance.