When an ordinance proposed by petition is to be submitted to a vote of the electors, the committee of the petitioners shall certify that fact and the proposed ordinance to the Clerk of the Council within thirty (30) days after the final action on such proposed ordinance by the Council and also file with the Clerk a supplemental petition asking that the proposed ordinance be submitted to popular vote, which petition shall be signed by ten (10) percent of the qualified electors of the City in addition to those who signed the original petition. The sufficiency in valid signatures of any such supplemental petition shall be determined, and it may be amended in the manner provided for original petitions for proposing ordinances to the Council.