If bonds under this chapter are payable in whole or in part from project revenue, the county shall impose tolls and charges that are, together with other money or revenues available for the project, including ad valorem tax, sufficient to:

1. (1) pay the maintenance and operating expenses of the project;
2. (2) pay the principal of, premium of, if any, and interest on the bonds when due;
3. (3) establish a reserve for payment of bond principal, premium, and interest; and
4. (4) establish an adequate fund for project depreciation and replacement.

Acts 1995, 74th Leg., ch. 165, Sec. 1, eff. Sept. 1, 1995.