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Section 2: Definitions

Floodplain encroachment, as it applies to TxDOT, is any construction, replacement, or extension of a bridge, culvert, low-water crossing, or storm drain outfall in a floodplain whether the structure interferes with flood waters or not. Encroachments can also be bridge widening, pavement overlays, modification or addition of bridge rails and traffic median barriers, and safety end treatments (SETs). Longitudinal encroachments are roads or walkways constructed in the floodplain paralleling a watercourse instead of crossing it. For the purposes of this chapter, the term “encroachment includes structures over any waters of the U.S., whether in a FEMA mapped floodplain or not.

Minor structures are smaller culverts under driveways in ditches or under the roadway connecting two ditches, but not conveying waters of the U.S.

Mapped Floodplain or Special Flood Hazard Area (SFHA) is an area that FEMA has designated as having a probability of inundation during a 1% AEP (Annual Exceedance Probability) or 100-year flood, usually shown on a Flood Insurance Rate Map (FIRM). Most of the mapped floodplains or SFHAs are riverine designated Zone A, AE, or A1-30; other types are playas (AH), flatlands with standing waters (AO), and coastal floodplains, (V, VE, or V1-30). The accepted definitions of various risk zones, including SFHAs, are listed below (see Types of Flood Zones).

Flood Insurance Rate Map (FIRM) is a graphical representation of SFHAs and floodways, flood hazard risk zones, base flood elevations, 0.2% AEP (500-year) floodplain areas, and other flood-related information.

Effective Map is the latest FIRM issued by FEMA, which is in effect as of the date shown in the title box of the FIRM as “Effective Date,” “Revised,” or “Map Revised.”

Base Flood Elevation (BFE) is the 1% AEP flood water surface elevation. BFEs are usually given in feet above mean sea level. BFEs are determined through a hydrologic and hydraulic study of the area or the waterway as a whole, not a small area or isolated reach of stream.

Flood Insurance Study (FIS) is the final report which summarizes the results of the detailed studies on which a Zone AE is based. The FIS usually includes an appraisal of a community's flooding problems, engineering methodologies, flood discharges, flood profiles, and floodplain/floodway technical data.

Floodway is the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without increasing the water-surface elevation more than a designated height.

Flood fringe is the area within the floodplain but outside of the floodway.

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Types of Flood Zones (Risk Flood Insurance Zone Designations)

ZONE A: SFHAs inundated by the 1% AEP (100-year) flood. No BFEs determined.

ZONE AE (formerly A1-30): SFHAs inundated by the 1% AEP (100-year) flood. BFEs deter mined by detailed study.

ZONE AO: SFHAs inundated by 1% AEP (100-year) flood depths of 1 to 3 feet, usually sheet flow on sloping terrain. Depths shown are average. For areas of alluvial fan flooding, velocities are also determined. See 44CFR59, “ Area of shallow flooding".

ZONE AH: SFHAs inundated by 1% AEP (100-year) flood depths of 1 to 3 feet, usually areas of ponding. BFEs are determined. In Texas, Zone AH usually applies to playas, low areas with no outflow. The only escapes are infiltration and evaporation.

ZONE AR: SFHAs that result from the decertification of previously accredited flood protection systems that are in the process of being restored to provide a 1% AEP (100-year) or greater level of flood protection.

ZONES AR/A1-30, AR/AE, AR/AH, AR/AO and AR/A (Dual Zones): SFHAs that result from the decertification of previously accredited flood protection systems that are in the process of being restored to provide a 1% AEP (100-year) or greater level of flood protection. After restoration is complete, these areas will still experience residual flooding from other flooding sources.

ZONE A99: SFHAs inundated by the 1% AEP (100-year) flood to be protected from the 1% AEP flood by a Federal flood protection system under construction. No BFEs determined.

ZONE V: SFHAs in coastal areas with velocity hazards (wave action) inundated by the 1% AEP (100-year) flood; no BFEs determined.

ZONE VE (formerly V1-30): SFHAs in coastal areas with velocity hazards (wave action) inundated by the 1% AEP (100-year) flood. Base flood elevations determined by detailed study.

ZONE B and ZONE X (shaded): Areas of 0.2% AEP (500-year) flood; areas subject to the 1% AEP (100-year) flood with average depths of less than 1 foot or with contributing drainage area less than 1 square mile; areas protected by levees from the base flood.

ZONE C and ZONE X (unshaded): Areas determined to be outside the 0.2% AEP (500-year) floodplain.

ZONE D: Areas in which flood hazards are undetermined.

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