An indenture may include a clause relating to the bonds issued thereunder which limits, restricts, or prohibits any right, power or privilege of the board to mortgage or otherwise encumber, sell, lease or dispose of any improvements constructed from the proceeds of the bonds, or to enter into any lease or agreement which impairs or impedes the operation of such improvement, or any part thereof, necessary to secure adequate revenues or which otherwise impairs or impedes the rights of the holders of the bonds with respect to such revenues.