US Army Corps of Engineers
Institute for Water Resources

Flood Risk Management Program

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Terminology

Below are some common terms used in Corps flood risk management studies and projects. For commonly used acronyms, please see the Acronyms FAQ.

Accredited Levee System

A system that FEMA has determined can be shown on a Digital Flood Insurance Rate Map as providing a 1-percent-annual-chance or greater level of flood protection. This determination is based on the submittal of data and documentation required by 44 CFR 65.10.

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Annual Exceedance Probability (AEP)

The probability that flooding will occur.

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Authorized Project

A project specifically authorized by Congress for construction, generally, through language in an authorization or appropriation act, or a project authorized pursuant to Section 201 of the Flood Control Act of 1965 (ER 1105-2-100, appendix G).

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Base Flood

The regulatory standard under the National Flood Insurance Program for a flood having a one percent chance of being equaled or exceeded in any given year. It is also referred to as the “100-year flood.” The base flood is the national standard used by the NFIP and all Federal agencies for the purposes of requiring the purchase of flood insurance and regulating new development. Base Flood Elevations (BFEs) are typically shown on Flood Insurance Rate Maps (FIRMs).

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Base Flood Plain

The one-percent-chance floodplain (ER 1165-2-26).

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Channel

A natural or artificial watercourse of perceptible extent with a definite bed and banks to confine and conduct continuously or periodically flowing water (ER 1165-2-26).

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Critical Action

Any activity for which even a slight chance of flooding would be too great. The critical action flood plain is defined as the 500-year flood plain (0.2 percent chance flood plain) (ER 1165-2-26).

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Discharge

The volume of water passing a specific point for a given time interval. For example, 2,000 cubic feet per second (CFS). Also referred to as flow.

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Exceedence Flow

Excess flow that emerges on the surface once the conveyance capacity of a drainage system is exceeded.

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Exceedance Probability Event

The probability that a specific event will occur in any given year. For example, the 0.01 exceedance probability event has one chance in a hundred or a one percent chance of occurring in any given year.

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Federal Authorized Levee

A levee designed and built by USACE or authorized to be part of the USACE federal program, but it is locally operated and maintained in accordance with standards established by USACE.

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Flood Frequency Analysis

Uses a probability of a given magnitude flood peak that may be expected to occur for a given return period expressed in years. For example, the “1 in 100 year” flood would have a probability of 0.01 or one (1) percent of being equaled or exceeded in one (1) year.

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Flood Fringe

That portion of the flood plain outside of the regulatory floodway (often referred to as "floodway fringe") (ER 1165-2-26).

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Flood Peak

The highest value of the stage or discharge attained by a flood, thus, the peak stage or peak discharge.

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Floodplain

The lowland and relatively flat area adjoining inland and coastal waters including flood prone areas of offshore islands and including at a minimum, that area subject to a one-percent chance of flooding in any given year (E.O. 11988).

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Floodplain Management (FPM)

A continuing process, involving both federal and non-federal actions that seek a balance between use and environmental quality in the management of the inland and coastal flood plains as components of the larger human communities. The flood damage reduction aspects of flood plain management involve modifying floods and modifying the susceptibility of property to flood damages. The former embraces the physical measures commonly called "flood control;" the latter includes regulatory and other measures intended to reduce damages by means other than modifying flood waters. By guiding flood plain land use and development, flood plain regulations seek to reduce future susceptibility to flood hazards and damages consistent with the risk involved and serve in many cases to preserve and protect natural flood plain values (EP 1165-2-1).

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Flood Risk Management

Flood risk management seeks to reduce flood risks by managing the floodwaters to reduce the probability of flooding (including by levees and dams) and by managing the floodplains to reduce the consequences of flooding. Flood risk management requires integrating and synchronizing programs at various levels of government designed to reduce flood risk.

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Flood Stage

The stage at which overflow of the natural banks of a stream begins to cause damage in the reach in which the elevation is measured.

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General Reevaluation

A study to affirm, reformulate or modify a plan, or portions of a plan, under current planning criteria. This study may be similar to a feasibility study (ER 1105-2-100).

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General Reevaluation Report (GRR)

A separate report that documents the analyses undertaken in the general reevaluation study (ER 1105-2-100).

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Limited Reevaluation

A study to provide an evaluation of a specific portion of a plan under current policies, criteria, and guidelines, and it may be limited to economics or environmental effects (ER 1105-2-100).

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Limited Reevaluation Report (LRR)

A separate report that documents the analyses undertaken in a limited reevaluation study (ER 1105-2-100).

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Natural and Beneficial Values

Include but are not limited to water resources values (natural moderation of floods, water quality maintenance, and ground water recharge), living resource values (fish, wildlife and plant resources), cultural resource values (open space, natural beauty, scientific study, outdoor education and recreation) and cultivated resource values (agriculture, aquaculture and forestry) (ER 1165-2-26).

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Nonstructural Approaches

Nonstructural approaches to flood proofing are intended to reduce damage from encroaching flood water by altering the property. These include acquiring and/or relocating a building, preparing emergency measures, such as sandbagging, and flood proofing structures. See additional nonstructural terms below.

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Peak Flow

The maximum rate of runoff that occurs from a watershed during a storm event.

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Post Authorization Change (PAC) Report

Recommended changes to authorized but unconstructed projects may require a post authorization change (PAC) report. Guidance on post authorization change reports is in Appendix G, Section III, of ER 1105-2-100.

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Practicable

Capable of being done within existing constraints. The test of what is practicable depends upon the situation and includes consideration of the pertinent factors, such as, environment, cost or technology (ER 1165-2-26).

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Preserve

To prevent adverse modification to the existing flood plain environment or to maintain it (ER 1165-2-26).

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Provisionally Accredited Levee (PAL) System

The PAL designation may be used for a levee system that FEMA has previously accredited with providing one-percent-annual-chance flood protection on an effective FIRM/DFIRM, and for which FEMA is awaiting data and or documentation that will show the levee system is compliant with 44 CFR 65.10. Before FEMA will apply the PAL designation to a levee system, the community or levee owner will need to sign and return an agreement indicating the data and documentation required will be provided within a specified timeframe.

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Reach

A length of stream that has generally similar physical and biological traits.

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Regulatory Floodway

The area regulated by federal, state or local requirements. It is the channel of a river or other watercourse and the adjacent land areas that must be reserved in an open manner, i.e., unconfined or unobstructed either horizontally or vertically to provide for the discharge of the base flood so the cumulative increase in water surface elevation from encroachment does not exceed one foot as set by the National Flood Insurance Program (ER 1165-2-26).

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Residual Risk

The flood risk that remains if a proposed flood damage reduction project is implemented. Residual risk includes the consequence of capacity exceedance as well (ER 1105-2-101).

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Restore

To reestablish a setting or environment in which the natural functions of the flood plain can again operate (ER 1165-2-26).

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Risk

The measure of the probability and severity of undesirable consequences. Risk = (Frequency of an event) x (Probability of occurrence) x (Consequences) (EC 1110-2-6067).

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Risk Analysis

An approach to evaluation and decision making that explicitly and, to the extent practical, analytically incorporates considerations of risk and uncertainty in a flood damage reduction study (ER 1105-2-101).

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Special Flood Hazard Area (SFHA)

As defined by the National Flood Insurance Program (NFIP), SFHA is the land area covered by the floodwaters of the base flood on NFIP maps. The SFHA is the area where the NFIP’s floodplain management regulations must be enforced and the area where the mandatory purchase of flood insurance applies.

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Stage

The vertical distance in feet (or meters) above a local or national datum. For example, 480.0 feet NAVD88.

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Storm of Record

The largest flood stage or discharge event that occurred at a river gage.

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Structural Approaches

Structural approaches to flood proofing are intended to prevent flooding by altering the flow of floodwater; these include constructing levees or dams, or modifying a waterway’s channel.

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Uncertainty

A measure of imprecision of knowledge of parameters and functions used to describe the hydraulic, hydrologic, geotechnical and economic aspects of a project plan (ER 1105-2-101).

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NONSTRUCTURAL MEASURES TERMINOLOGY

Acquisition

This nonstructural technique consists of buying the structure and the land. The structure is either demolished or is sold to others and relocated to a site external to the floodplain. Development sites, if needed, can be part of a proposed project in order to provide locations where displaced people can build new homes within an established community.

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Berms and Floodwalls

This nonstructural technique is applicable on a small-scale basis. As nonstructural measures, berms and floodwalls should be constructed to no higher than 6 feet above grade and should not be considered for certification through the NFIP, meaning that flood insurance and floodplain management requirements of the NFIP are still applicable in areas were these berms or floodwalls are constructed. These nonstructural measures are intended to reduce the frequency of flooding but not eliminate floodplain management and flood insurance requirements. These measures can be placed around a single structure or a small group of structures. Since application of these measures are considered nonstructural in nature, they cannot raise the water surface elevation of the 100-year flood by any more than 0.00 feet.

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Dry Floodproofing

This nonstructural technique consists of waterproofing the structure. This can be done to residential homes as well as commercial and industrial structures. This measure achieves flood risk reduction but it is not recognized by the NFIP for any flood insurance premium rate reduction if applied to a residential structure. Based laboratory tests, a “conventional” built structure can generally only be dry flood proofed up to 3-feet in elevation. A structural analysis of the wall strength would be required if it was desired to achieve higher protection. A sump pump and perhaps French drain system should be installed as part of the measure. Closure panels are used at openings. This concept does not work with basements nor does it work with crawl spaces. For buildings with basements and/or crawlspaces, the only way that dry floodproofing could be considered to work is for the first floor to be made impermeable to the passage of floodwater.

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Elevation

This nonstructural technique lifts an existing structure to an elevation which is at least equal to or greater than the 1% annual chance flood elevation. In many elevation scenarios, the cost of elevating a structure an extra foot or two is less expensive than the first foot, due to the cost incurred for mobilizing equipment. Elevation can be performed using fill material, on extended foundation walls, on piers, post, piles and columns. Elevation is also a very successful technique for slab on grade structures.

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Fill Basement with Main Floor Addition

This nonstructural technique consists of filling in the existing basement without elevating the remainder of the structure. This could occur if the structure’s first floor was located above the base flood elevation or above the design elevation, whichever is higher. With this measure, placing an addition on to the side of the structure could compensate for the lost basement space to the owner. If the addition could not be done because of limited space within the lot or because the owner did not want it, compensation for the lost basement space would be in order to the owner. This measure would only be applicable where the design flood depth is relatively small and the first floor elevation is already located above the design depth.

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Flood Emergency Preparedness Plans

Local governments, through collaboration with USACE, FEMA and other interested federal partners, are encouraged to develop and maintain a Flood Emergency Preparedness Plan (FEPP) that identifies flood hazards, risks and vulnerabilities, identifies and prioritizes mitigation actions, and encourages the development of local mitigation. The FEPP should incorporate the community’s response to flooding, location of evacuation centers, primary evacuation routes, and post flood recovery processes.

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Flood Warning System

This nonstructural technique relies upon stream gage, rain gages, and hydrologic computer modeling to determine the impacts of flooding for areas of potential flood risk. A flood warning system, when properly installed and calibrated, is able to identify the amount of time available for residents to implement emergency measures to protect valuables or to evacuate the area during serious flood events.

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Land Use Regulations

Land use regulations within a designated floodplain are effective tools in reducing flood risk and flood damage. The basics principles of these tools are based nationally in the National Flood Insurance Program (NFIP) which requires minimum standards of floodplain regulation for those communities that participate in the NFIP. For example, land use regulations may identify where development can and cannot occur, or to what elevation structures should locate their lowest habitable floor to.

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Relocation

This nonstructural technique requires physically moving the at-risk structure and buying the land upon which the structure is located. It makes most sense when structures can be relocated from a high flood hazard area to an area that is located completely out of the floodplain.

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Wet Floodproofing

This nonstructural technique is applicable as either a stand-alone measure or as a measure combined with other measures such as elevation. As a stand-alone measure, all construction materials and finishing materials need to be water resistant and all utilities must be elevated above the design flood elevation. Wet floodproofing is quite applicable to commercial and industrial structures when combined with a flood warning and flood preparedness plan. This measure is generally not applicable to large flood depths and high velocity flows.

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