23 CFR § 650.113 - Only practicable alternative finding.
(a) A proposed action which includes a significant encroachment shall not be approved unless the FHWA finds that the proposed significant encroachment is the only practicable alternative. This finding shall be included in the final environmental document (final environmental impact statement or finding of no significant impact) and shall be supported by the following information:
(1) The reasons why the proposed action must be located in the flood plain,
(2) The alternatives considered and why they were not practicable, and
(3) A statement indicating whether the action conforms to applicable State or local flood-plain protection standards.