In issuing general obligation bonds at any time after February 20, 1970 for the purpose of providing all or part of the cost and expense of planning and design, establishing, acquiring, developing, constructing or improving the county capital purposes authorized by this chapter and RCW 86.12.020, the board of county commissioners may provide that such bonds also be made payable from any otherwise unpledged revenue which may be derived from the ownership or operation of any such properties or facilities.