The proceeds derived from the operation of each unit of development acquired or constructed pursuant to the provisions of this article shall be applied for the following purposes in the following order: (1) Payment of the expenses of operating dock facilities acquired, constructed or operated pursuant to the provisions of this article; (2) payment at their respective maturities of the principal of and interest on any bonds issued pursuant to the provisions of this article; (3) establishment and maintenance of a sinking fund for retirement of any bonds issued pursuant to the provisions of this article, into which there shall be paid monthly, beginning at the end of the first 12 months following the completion of each unit of development acquired or constructed hereunder, one half of one per cent of the cost of each such unit of development, not exceeding, however, one half of the gross income of such unit of development received during the month preceding the making of such monthly payment. The proceeds from each unit of development acquired, constructed or operated pursuant to the provisions of this article remaining after compliance with the foregoing sentence shall be used for such of the following purposes as may from time to time be designated by the department: (1) Payment of expenses of operating any dock facility or facilities constructed pursuant to the provisions of the 1957 Docks Amendment; and (2) payment at their respective maturities of the principal of and interest on any bonds at any time issued pursuant to the 1957 Docks Amendment. With respect to the revenues from any dock facilities, the acquisition, construction or operation of which was financed in part with bonds issued under the 1957 Docks Act and in part with bonds issued under this article, the provisions of this section shall be subject to the provisions of Sections 33-2-9 and 33-2-10. The retention by the department of such revenues as may be reasonably required for operating capital for dock facilities acquired, constructed or operated pursuant to the provisions of this article shall be deemed a retention of moneys for operating expenses within the meaning of this section. Moneys in the sinking fund provided for in this section shall be used only for the purpose of retirement of the bonds, either by payment of the principal thereof or the interest thereon at their respective maturities, redemption of the bonds prior to maturity pursuant to any redemption provisions that may be contained in the order or resolution whereunder such bonds are issued, or purchase of any of the bonds for cancellation and retirement at a purchase price not exceeding the face value thereof plus a premium equal to 12 months' interest thereon; provided, that pending use of moneys in said sinking fund for the purposes specified in this sentence such moneys may, in the discretion of the department with the approval of the Governor, be invested in securities which are direct general obligations of the United States.