The bonds shall be secured by a pledge of and lien upon the revenues of the project described in the indenture authorizing the issuance of the bonds, and such revenues may be used only as provided in said indenture. The revenues of the project include revenues from improvements to or additions to or extensions thereof later constructed or made. The bonds shall also be secured by additional revenues, including existing funds, if any, of the entity from other sources, to the extent set forth in the indenture.