Additional funds which shall be known as "Special Funds" shall be created by ordinance to provide for monies held for depreciation and obsolescence of structures, machinery and equipment, for debt service, for special services and for local improvements. The ordinance creating a special fund shall specify its source of income and the purpose for which expenditures from the special fund shall be made. Special funds created by ordinance may be abolished by ordinance, subject to existing trusts, and the remaining assets of any special fund shall be transferred to the general fund. The ordinances creating special funds may provide for annual periodic or continuing appropriations for such funds.