The council shall, prior to delivery of any bonds payable from the consolidated improvement bond debt service fund, estimate the amount of special assessments to be levied for each improvement financed by such bonds and the amount, if any, to be used with bond proceeds to defray expenses of the improvement and pay principal and interest on the bonds prior to completion of the improvement. After considering the rate of interest to be charged on deferred installments of assessments and the expected times of collection thereof, the council shall by resolution levy, if necessary, a direct general ad valorem tax upon all taxable property in the city, to be spread upon the tax rolls for each year of the term of the bonds in such amounts for all years that if collected in full, they, together with estimated collections of all taxes heretofore levied and appropriated to the consolidated improvement bond debt service fund, the estimated collections of special assessment levied for each improvement financed by such bonds (other than special assessments estimated to be required to defray expenses of the improvement and pay principal and interest on the bonds prior to completion of the improvement), and the estimated collections of all other special assessments theretofore pledged to the consolidated improvement bond debt service fund, will produce at least five percent (5%) in excess of the amounts needed to meet, when due, the principal and interest payments on such bonds and all other then outstanding bonds payable from the consolidated improvement bond debt service fund. Such resolution shall irrevocably appropriate the taxes so levied and the special assessments so pledged to the consolidated improvement bond debt service fund. All such tax levies shall be irrepealable; except, that if the council in any year makes an irrevocable appropriation to the sinking fund of monies actually on hand or if there is on hand any excess amount in the consolidated improvement bond debt service fund, the council may cause the clerk to certify to the county auditor the fact and amount thereof and request that the auditor reduce by the amount so certified the amount otherwise to be included in the rolls next thereafter prepared. (1981 Code 110 § 4)