23 C.F.R. § 650.105 – Definitions.

(a) Action shall mean any highway construction, reconstruction, rehabilitation, repair, or improvement undertaken with Federal or Federal-aid highway funds or FHWA approval.
(b) Base flood shall mean the flood or tide having a 1-percent chance of being exceeded in any given year.
(c) Base flood plain shall mean the area subject to flooding by the base flood.
(d) Design Flood shall mean the peak discharge, volume if appropriate, stage or wave crest elevation of the flood associated with the probability of exceedance selected for the design of a highway encroachment. By definition, the highway will not be inundated from the stage of the design flood.
(e) Encroachment shall mean an action within the limits of the base flood plain.
(f) Floodproof shall mean to design and construct individual buildings, facilities, and their sites to protect against structural failure, to keep water out or to reduce the effects of water entry.
(g) Freeboard shall mean the vertical clearance of the lowest structural member of the bridge superstructure above the water surface elevation of the overtopping flood.
(h) Minimize shall mean to reduce to the smallest practicable amount or degree.
(i) Natural and beneficial flood-plain values shall include but are not limited to fish, wildlife, plants, open space, natural beauty, scientific study, outdoor recreation, agriculture, aquaculture, forestry, natural moderation of floods, water quality maintenance, and groundwater recharge.
(j) Overtopping flood shall mean the flood described by the probability of exceedance and water surface elevation at which flow occurs over the highway, over the watershed divide, or through structure(s) provided for emergency relief.
(k) Practicable shall mean capable of being done within reasonable natural, social, or economic constraints.
(l) Preserve shall mean to avoid modification to the functions of the natural flood-plain environment or to maintain it as closely as practicable in its natural state.
(m) Regulatory floodway shall mean the flood-plain area that is reserved in an open manner by Federal, State or local requirements, i.e., unconfined or unobstructed either horizontally or vertically, to provide for the discharge of the base flood so that the cumulative increase in water surface elevation is no more than a designated amount (not to exceed 1 foot as established by the Federal Emergency Management Agency (FEMA) for administering the National Flood Insurance Program).
(n) Restore shall mean to reestablish a setting or environment in which the functions of the natural and beneficial flood-plain values adversely impacted by the highway agency action can again operate.
(o) Risk shall mean the consequences associated with the probability of flooding attributable to an encroachment. It shall include the potential for property loss and hazard to life during the service life of the highway.
(p) Risk analysis shall mean an economic comparison of design alternatives using expected total costs (construction costs plus risk costs) to determine the alternative with the least total expected cost to the public. It shall include probable flood-related costs during the service life of the facility for highway operation, maintenance, and repair, for highway-aggravated flood damage to other property, and for additional or interrupted highway travel.
(q) Significant encroachment shall mean a highway encroachment and any direct support of likely base flood-plain development that would involve one or more of the following construction-or flood-related impacts:
1. (1) A significant potential for interruption or termination of a transportation facility which is needed for emergency vehicles or provides a community's only evacuation route.
2. (2) A significant risk, or
3. (3) A significant adverse impact on natural and beneficial flood-plain values.
(r) Support base flood-plain development shall mean to encourage, allow, serve, or otherwise facilitate additional base flood-plain development. Direct support results from an encroachment, while indirect support results from an action out of the base flood plain.