(a) Rates fixed precedent to the issuance of bonds shall not be reduced until all of the bonds shall have been fully paid and, whenever necessary, may be increased in amounts sufficient to provide for the payment of the bonds, both principal and interest, and to provide proper funds for the depreciation account and operation and maintenance charges.
(b)
(1) If any surplus shall be accumulated in the operating and maintenance fund which shall be in excess of the costs of maintaining and operating the port during the remainder of the fiscal year then current, and the costs of maintaining and operating the port during the fiscal year ensuing, then any excess may be transferred to either the depreciation account, or to the bond and interest redemption account as may be provided.
(2) If any surplus shall be accumulated in the depreciation account over and above that which may be found necessary for the proper replacements which may be needed during the present fiscal year and the ensuing fiscal year, then the excess may be transferred to the bond and interest redemption account.
(3) If any surplus shall exist in the bond and interest redemption account, it shall be applied, insofar as possible, in the purchase or retirement of outstanding revenue bonds payable from the account, and for that purpose the bonds not yet due may be purchased in the open market at not more than the fair market value of them.