The Boeuf Floodway, authorized by the provisions adopted in section 702a of this title, shall be abandoned as soon as the Eudora Floodway, provided for in Flood Control Committee Document Numbered 1, Seventy-fourth Congress, first session, is in operative condition and the back protection levee recommended in said document, extending north from the head of the Eudora Floodway, shall have been constructed.