Whenever there shall be a change in population in any of the districts heretofore established, evidenced by a federal census of population, or by virtue of a substantial change in the corporate limits, there shall be a reapportionment of the council districts in the manner hereinafter provided:
(1) The mayor shall, within six months after the publication of each federal census of population for the municipality or within six months after there shall have been any substantial change in the corporate limits of the municipality, file with the council a report containing a recommended plan for reapportionment of the council district boundaries to comply with the following specifications:
a. Each district shall be formed of contiguous and, to the extent reasonably possible, compact territory, and its boundary lines shall be the center lines of streets or other well-defined boundaries;
b. Each district shall contain as nearly as is possible the same population, but not more than five per centum more or less than the average.
(2) The council shall enact a redistricting ordinance within six months after receiving such report. If the council fails to enact the redistricting ordinance within the said six months, the redistricting plan submitted by the mayor shall become effective without enactment by the council, as if it were a duly enacted ordinance.
(3) Such redistricting ordinance shall not apply to any primary or regular or special election held within six months after its becoming effective. No incumbent member of the council shall be deprived of his unexpired term of office because of such redistricting.