(a) The district may retire the original cost of construction of its improvements or pay for the cost of construction by borrowing money and pledging and mortgaging land, wharves, docks, warehouses, grain elevators, bunkering facilities, belt railroads, floating plants, lighterage, towing facilities, and other facilities or aids incident or necessary to the operation or development of ports or waterways.(b) The district may issue its debentures or other evidences of debt secured by a mortgage for the length of time and a rate of interest of not more than eight percent a year. In addition, the district may secure the debentures, notes or other evidences of debt with bonds of the district. Acts 1971, 62nd Leg., p. 110, ch. 58, Sec. 1, eff. 8/30/1971.