For the purpose of paying the cost of improvements to be made in an improvement district created pursuant to section 16-617 or 16-624, the mayor and city council shall have power and may, by ordinance, cause to be issued bonds of the city payable in not exceeding twenty years from date of the original issue of such bonds, and bearing interest, payable as provided in the ordinance authorizing such bonds. In such cases the mayor and city council shall also provide that any special taxes and assessments imposed pursuant to section 16-622 shall constitute a sinking fund for the payment of the bonds.