(a) Any bonds, including refunding bonds, authorized by this subchapter, and not payable wholly from ad valorem taxes, may be additionally secured by a trust indenture under which the trustee may be a bank having trust powers which may be situated either inside or outside the State of Texas.
(b) The trust indenture may contain provisions prescribed by the governing body for the security of the bonds and the preservation of its properties, contracts, and rights. It may contain a provision for the amendment or modification of the trust indenture in the manner which it prescribes.

(c) Without limiting the generality of the provisions which may be contained in the indenture, it may provide that the district shall comply with the requirements of designated consulting engineers for the proper maintenance and operation of the district’s properties and for the fixing of adequate tolls, charges, and rates, to assure proper maintenance and operation, and to provide proper debt service for the outstanding bonds in the manner prescribed in the resolution authorizing the issuance of the bonds or in the trust indenture securing the bonds.