1-percent annual chance floodplain

This is the boundary of the flood that has a 1-percent chance of being equaled or exceeded in any given year. Also known as the 100-year floodplain.

1-percent annual chance water-surface elevation

The height, in relation to the National Geodetic Vertical Datum of 1929 (or other datum, where specified), of the flood having a 1-percent chance of being equaled or exceeded in any given flood year (also known as the 100-year flood or the base flood).

100-year flood

The flood having a 1-percent chance of being equaled or exceeded in any given year; also known as the base flood. The 1-percent annual chance flood, which is the standard used by most Federal and state agencies, is used by the National Flood Insurance Program (NFIP) as the standard for floodplain management and to determine the need for flood insurance. A structure located within a special flood hazard area shown on an NFIP map has a 26 percent chance of suffering flood damage during the term of a 30-year mortgage.

100-year floodplain

This is the boundary of the flood that has a 1-percent chance of being equaled or exceeded in any given year. Officially termed the 1-percent annual chance floodplain.

500-year floodplain

This is the boundary of the flood that has a 0.2-percent chance of being equaled or exceeded in any given year. Officially termed the 0.2-percent annual chance floodplain.

Backwater

The effect of downstream flow on the water-surface profile.

Base Flood

The flood having a 1-percent chance of being equaled or exceeded in any given year; also known as the 100-year flood. The base flood, which is the standard used by most Federal and state agencies, is used by the National Flood Insurance Program (NFIP) as the standard for floodplain management and to determine the need for flood insurance. A structure located within a special flood hazard area shown on an NFIP map has a 26 percent chance of suffering flood damage during the term of a 30-year mortgage.

Base Flood Elevation (BFE)

The height of the base flood, usually in feet, in relation to the National Geodetic Vertical Datum of 1929, the North American Vertical Datum of 1988, or other datum referenced in the Flood Insurance Study report, or depth of the base flood, usually in feet, above the ground surface.

Channel Bank Stations

Points that identify the extreme limits of the natural stream channel. These stations are typically assigned at locations along a cross section where a relatively flat area exists outside of the channel.

Community

Any State or area or political subdivision thereof, or any Indian tribe or authorized tribal organization, or Alaska Native village or authorized native organization, which has authority to adopt and enforce flood plain management regulations for the areas within its jurisdiction.

Confluence

A location where two streams or rivers meet.

Contour

A line on a map joining points of equal altitude.

Critical Depth

The depth of flow at which, for a given discharge at a given location, the total energy is the minimum value possible for flow to occur.

Cross Section

A line developed from topographic information across a floodplain at which a computation of flood flow has been made to establish a potential flood elevation. Cross sections are shown on the Flood Boundary and Floodway Map, Flood Insurance Rate Map, and/or Flood Profiles of a Flood Insurance Study.

Cubic feet per second (cfs)

Typical units used to express the rate of flow of surface water in open channels. One cubic foot is approximately equal to 7.5 gallons per second.

Datum

FEMA's Flood Insurance Rate Maps (FIRMs) reference the elevation datum used to compute flood elevations. In completing elevation certificates, the same elevation datum as that shown on the FIRM must be used to compute lot and/or structure elevations and to compute flood elevations that are not given on the FIRM. The National Geodetic Vertical Datum (NGVD) is the national standard reference datum for elevations, formerly referred to as Mean Sea Level (MSL) of 1929. NGVD is used as the reference datum on most FIRMs.

Depth

Maximum depth of water in the cross section as measured below the water-surface elevation.

Digital Flood Insurance Rate Map (DFIRM)

As part of FEMA`s Map Modernization Objectives, the Digital Flood Insurance Rate Map (DFIRM) product was developed. The DFIRM product includes a spatial database with options that can be invoked depending on the available data. The DFIRM spatial database includes certain standard features and meets minimum mapping requirements.

Discharge

The volume of water that passes a given location within a given period of time. Usually expressed in cubic feet per second (cfs).

Federal Emergency Management Agency (FEMA)

An independent agency of the Federal government, founded in 1979, which reports directly to the President. FEMA is responsible for identifying and mitigating natural and man-made hazards. The agency's mission is: to reduce loss of life and property and protect our nation's critical infrastructure from all types of hazards through a comprehensive, risk-based, emergency management program of mitigation, preparedness, response and recovery.

Flood (also Flooding)

A general and temporary condition of partial or complete inundation of 2 or more acres of normally dry land areas. For flood insurance claim purposes, two or more properties must be inundated before flood damage will be covered.

Flood Boundary and Floodway Map (FBFM)

A pre-Map Initiatives floodplain management map that delineates the 100-year (1% annual chance) and 500-year (0.2% annual chance) floodplains, floodway, and cross sections.

Flood Hazard Boundary Map (FHBM)

Initial map issued by FEMA to identify approximate Special Flood Hazard Areas (SFHAs) within a community.

Flood Insurance Rate Map (FIRM)

A map on which the 100-year (1% annual chance) and 500-year (0.2% annual chance) floodplains, Base Flood Elevations, and risk premium zones are delineated to enable insurance agents to issue accurate flood insurance policies to homeowners in communities participating in the National Flood Insurance Program.

Flood Insurance Study (FIS)

An examination, evaluation, and determination of flood hazards and, if appropriate corresponding water-surface elevations. The resulting reports are used to develop Flood Insurance Rate Maps. Also known as a flood elevation study.

Flood Profile

A cross-sectional drawing showing the contiguous cross sections along a stream, with ground elevations and potential flood elevations plotted. Floodplain or Flood-Prone Area Any land area susceptible to inundation by water from any source.

Floodplain Management

The operation of a program of corrective and preventative measures for mitigating flood damage, including, but not limited to, emergency preparedness plans, flood-control works, and floodplain management regulations.

Floodway

Channel of a stream plus any adjacent floodplain areas that must be kept free of encroachment so that the 100-year flood discharge can be conveyed without increasing the elevation of the 100-year flood by more than a specified amount (1 foot in most states).

HEC-2 and HEC-RAS

Are step backwater programs developed by the US Army Corps of Engineers Hydrologic Engineering Center for use in calculating water-surface profiles for steady, gradually varied flow in natural or man-made channels.

Hydraulic Radius

Equal to (Area of cross section / Wetted Perimeter)

Lacustrine

Relating to or associated with lakes.

Letter of Map Change (LOMC)

A LOMC is a letter which reflects an official change to an effective Flood Insurance Rate Map (FIRM). LOMCs are issued in response to a request for FEMA to amend or revise its effective flood map to remove a property or reflect changed flooding conditions on the effective map. When certain technical or scientific evidence is available that proves that a property has been inadvertently mapped in a Special Flood Hazard Area (SFHA), or flood hazard data needs to be revised due to man-made changes, a request to FEMA for a LOMC may be submitted. For more information on LOMCs, select this link.

Manning’s “n” Roughness Coefficient

Coefficient used to account for the friction caused by earthen, vegetative, and/or man-made surfaces within a floodplain cross-section. The coefficient, n, is commonly used to represent flow resistance for hydraulic computations of flow in open channels. The procedure for selecting n values is subjective and requires judgment and skill that is developed primarily through experience. The expertise necessary for proper selection of n values can be obtained in part by examining characteristics of channels that have known or verified roughness coefficients. A table of Manning` values is available from the pull-down menu in the Quick-2 program.

Map Initiatives

A Flood Insurance Rate Map format developed in 1985 that incorporates information formerly shown on the separately printed Flood Boundary and Floodway Map (FBFM) (e.g., floodways and cross-sections). FIRM A Flood Insurance Rate Map format developed in 1985 that incorporates information formerly shown on the separately printed Flood Boundary and Floodway Map (FBFM) (e.g., floodways and cross-sections). Also referred to as combined Flood Insurance Rate Map/FBFM.

Map Repository

The location where a community's flood maps are kept; usually the local zoning and planning office.

Map Service Center (MSC)

The Map Service Center (MSC) distributes National Flood Insurance Program (NFIP) products including: Digital Flood Insurance Rate Maps (DFIRM), Flood Insurance Rate Maps (FIRM), Flood Insurance Studies (FIS), Digital Q3 flood data, Community Status Book, Flood Map Status Information Service (FMSIS), and Letters of Map Change (LOMC).

National Flood Insurance Program (NFIP)

Federal insurance program under which flood-prone areas are identified and flood insurance is made available to residents of participating communities that agree to adopt and enforce floodplain management ordinances to reduce future flood damage.

Normal Depth

The depth expected for a stream when the flow is uniform, steady, one dimensional, and is not affected by downstream obstructions or flow changes. This is the usual calculation that is utilized to determine Base Flood Elevations for property or structures in approximate Zone A) areas.

Overbank

The area of a cross section that is found outside of the channel bank stations on either side of the stream channel.

Peak Discharge

The peak volume of water that passes a given location within a given period of time. Usually expressed in cubic feet per second.

Rating Curve

A curve showing the relationship between depth of flow and the discharge of a stream at a given location.

Scale

A representative fraction of map distance to ground distance. Example: 1:12,000 is the representative fraction in which one unit of measure on the map is equal to 12,000 of the same units of measure on the ground. Federal Emergency Management Agency map scales are expressed in a ratio of map distance equal to a given number of feet on the ground.

Special Flood Hazard Area (SFHA)

Area inundated by the base (1-percent annual chance) flood, identified on the Flood Insurance Rate Map as Zones A, AE, AH, AO, AR, V, VE, or A99.

Step-Backwater Analysis

Method used in Quick-2 (and other modeling programs) to analyze multiple cross-sections. Water-surface elevations are determined for all sections based on a given discharge. The initial water-surface elevation is automatically determined by the normal depth method or by a direct input of a water-surface elevation or depth.

Water-Surface Elevation

The height, in relation to the National Geodetic Vertical Datum of 1929 (or other datum, where specified) of floods of various magnitudes and frequencies in the identified floodplains of coastal or riverine areas.

ZONE A

Areas subject to inundation by the 1-percent-annual-chance flood event generally determined using approximate methodologies. Because detailed hydraulic analyses have not been performed, no Base Flood Elevations (BFEs) or flood depths are shown. Mandatory flood insurance purchase requirements and floodplain management standards apply. Most current Zone A definition available at https://www.fema.gov/zone

Zone A99

Areas subject to inundation by the 1-percent-annual-chance flood event, but which will ultimately be protected upon completion of an under-construction Federal flood protection system. These are areas of special flood hazard where enough progress has been made on the construction of a protection system, such as dikes, dams, and levees, to consider it complete for insurance rating purposes. Zone A99 may only be used when the flood protection system has reached specified statutory progress toward completion. No Base Flood Elevations (BFEs) or depths are shown. Mandatory flood insurance purchase requirements and floodplain management standards apply. Most current Zone A99 definition available at https://www.fema.gov/zone-a99

Zone AE and A1-A30

Areas subject to inundation by the 1-percent-annual-chance flood event determined by detailed methods. Base Flood Elevations (BFEs) are shown. Mandatory flood insurance purchase requirements and floodplain management standards apply. Most current Zone AE and Zones A1-30 definitions available at https://www.fema.gov/zone-ae-and-a1-30

Zone AH

Areas subject to inundation by 1-percent-annual-chance shallow flooding (usually areas of ponding) where average depths are between one and three feet. Base Flood Elevations (BFEs) derived from detailed hydraulic analyses are shown in this zone. Mandatory flood insurance purchase requirements and floodplain management standards apply. Most current Zone AH definition available at https://www.fema.gov/zone-ah

Zone AO

Areas subject to inundation by 1-percent-annual-chance shallow flooding (usually sheet flow on sloping terrain) where average depths are between one and three feet. Average flood depths derived from detailed hydraulic analyses are shown in this zone. Mandatory flood insurance purchase requirements and floodplain management standards apply. - Some Zone AO have been designated in areas with high flood velocities such as alluvial fans and washes. Communities are encouraged to adopt more restrictive requirements for these areas. Most current Zone AO definition available at https://www.fema.gov/zone-ao

Zone AR, AR/A1-30, AR/AE, AR/AH, AR/AO, and AR/A

Areas that result from the decertification of a previously accredited flood protection system that is determined to be in the process of being restored to provide base flood protection. Mandatory flood insurance purchase requirements and floodplain management standards apply. Most current Zone AR, AR/A1-30, AR/AE, AR/AH, AR/AO, and AR/A definition available at https://www.fema.gov/zone-ar

Zone D

Designation on National Flood Insurance Program maps used for areas where there are possible, but undetermined, flood hazards. In areas designated as Zone D, no analysis of flood hazards has been conducted. Mandatory flood insurance purchase requirements do not apply, but coverage is available. The flood insurance rates for properties in Zone D are commensurate with the uncertainty of the flood risk.

Zone V

Areas along coasts subject to inundation by the 1-percent-annual-chance flood event with additional hazards associated with storm-induced waves. Because detailed hydraulic analyses have not been performed, no Base Flood Elevations (BFEs) or flood depths are shown. Mandatory flood insurance purchase requirements and floodplain management standards apply. Most current Zone V definition available at https://www.fema.gov/zone-v

Zone VE and Zones V1-30

Areas subject to inundation by the 1-percent-annual-chance flood event with additional hazards due to storm-induced velocity wave action. Base Flood Elevations (BFEs) derived from detailed hydraulic analyses are shown. Mandatory flood insurance purchase requirements and floodplain management standards apply. Most current Zone VE and Zones V1-30 definitions available at https://www.fema.gov/zone-ve-and-v1-30

Zone B, C, and X

Zones B, C, and X are the flood insurance rate zones that correspond to areas outside the 100-year floodplains, areas of 100-year sheet flow flooding where average depths are less than 1 foot, areas of 100-year stream flooding where the contributing drainage area is less than 1 square mile, or areas protected from the 100-year flood by levees. No Base Flood Elevations or depths are shown within this zone.