No land shall be considered as benefited by the construction or maintenance of the works embraced in a project or unit, nor shall any land be assessed for the expense of the construction or maintenance of the project or unit, because the land has been or may be first endangered or flooded, or the natural drainage thereof obstructed by the construction or maintenance of any part or portion of the works embraced in the project or unit prior to the completion of the construction of the entire works embraced in the project or unit.