In the event bonds are to be issued for any local improvement, the assessment unpaid after the payment period provided by law shall be divided into equal annual installments, the number of which shall be two less than the number of years the bonds shall run, which assessment shall bear interest at the rate fixed by the ordinance ordering the improvement or general ordinance, from first day after lapse of payment period aforesaid, the first installment to become due and payable one year from such date. (1957 code § 8.02.060.)