Whenever in such county it is necessary to pay any construction costs and expenses in excess of the amounts paid by the federal government or to acquire any lands, rights-of-way, or easements under sections 23-320.01 to 23-320.06, the cost thereof and expenses connected therewith shall be defrayed by the issuance of general obligation bonds of the county, to be issued by the county board of such county without the necessity of an election, either in one issue or in separate issues from time to time as may be necessary and as determined by the county board of the county. The proceeds of the bonds shall be used for such purposes and no other except as otherwise provided in such sections. The aggregate of any such bonds so issued shall not be in excess of two-tenths of one percent of the taxable valuation of the county. All bonds issued under such sections shall mature in annual installments over a period of not more than twenty-five years, and it shall be the duty of the county board of such county to make an annual levy on all the taxable property in such county for the retirement of the principal and interest thereof as the same become due. The bonds provided for in such sections shall not be subject to nor included in any restrictions or limitations upon the amount of bonded indebtedness of the county contained in any other law affecting the county.