Said bonds shall mature in series amortized in a definite schedule during a period not to exceed sixty years from the date of their issuance, shall be in such denominations and form including bearer bonds or registered bonds as provided in RCW
39.46.030, and shall be payable, with annual or semiannual interest at a rate or rates the board shall provide: PROVIDED, That such bonds may also be issued in accordance with chapter
39.46 RCW.