Tenn. Code § 69-5-901
If the county legislative body determines that the estimated cost of reclamation and improvement of the district of land or levee or drainage district is greater than should be levied in a single year upon the lands benefited, it may fix the amount that shall be levied and collected each year, and may issue drainage bonds of the county, bearing interest payable semiannually, and may devote such bonds at par, with accrued interest, to the payment of the expenses and work as it progresses, or may sell the bonds at not less than par, with accrued interest, and devote the proceeds to such payment, and if, in the sale of the bonds, a premium is received, such premium shall be credited to the drainage fund. If the cost of such work exceeds the estimate, a new apportionment of the assessment may be made and levied and other bonds issued and sold in like manner, but in no case shall the bonds run longer than twenty (20) years.
T.C.A. § 69-5-901