(a) A county may issue bonds for a project under this chapter that are secured:
(1) solely by the pledge of the gross or net revenues of a project;
(2) by a pledge of:
(A) an ad valorem tax under § 9, Article VIII, Texas Constitution; or
(B) an unlimited ad valorem tax authorized by § 52, Article III, Texas Constitution;
(3) by designating part of the bonds to be secured solely by a pledge of project revenues and part of the bonds to be secured by pledge of the ad valorem tax; or
(4) by a combination of methods described by Subdivisions (1) and (2) with all of the bonds supported and secured by the ad valorem tax and the duty imposed on the county to collect tolls for use of the project facilities as long as the bonds are outstanding so that, as prescribed in the bond instrument, the amount of the tax may be reduced as the project revenues become sufficient to:
(A) meet the requirements for operation and maintenance; and
(B) provide money for the bonds.
(b) The commissioners court may secure bonds issued under this chapter through a trust indenture between the county and a corporate trustee. The corporate trustee may be any trust company or bank that has the powers of a trust company. The indenture may pledge or assign project tolls or revenues but may not convey or mortgage any part of the project.

(c) The bonds issued under this chapter may be authorized by bond resolution at one time or from time to time and shall mature on or before the 40th anniversary of their date.