In consideration of general benefits conferred on the city at large from the construction or installation of improvements in special or local improvement districts, the city council may levy annual taxes on the taxable property within the city, not exceeding two (2) mills in any one (1) year, to be disbursed, as determined by the city council, for the purpose of paying for such benefits, for the payment of any assessments levied against the city itself in connection with bonds issued for special or local improvement districts, and for the purpose of advancing money to maintain current payments of interest and equal annual payments of the principal amount of bonds issued for any special or local improvement district hereafter created. The proceeds of such taxes shall be placed in a special fund and shall be disbursed only for the purposes specified herein, provided however, that in lieu of such tax levies, the city council may annually transfer to such special fund any available money of the city, but in no event shall the amount transferred in any one (1) year exceed the amount which would result from a tax levied in such year as herein limited. As long as any bonds issued for special or local improvement districts hereafter organized remain outstanding, the tax levy or equivalent transfer of money to the special fund created for the payment of said bonds shall not be diminished in any succeeding year until all of said bonds and the interest thereon shall be paid in full, unless other available funds are on hand therefor, or such bonds and interest are paid by the city as required in Section 11-23 of this Charter.