Institute for Water Resources


Flood Risk Management Program

Frequently Asked Questions


“Accredited Levee System” An accredited levee system is a system that FEMA has determined can be shown on a DFIRM 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. 

 “Annual Exceedance Probability (AEP)” is the probability that flooding will occur.

“Authorized Project”  An authorized project is defined as 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).

"Base Flood" Base flood is 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).

"Base Flood Plain" is the one-percent-chance floodplain (ER 1165-2-26).

"Channel" is 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).

"Critical Action" is 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).

“Engineering Documentation Report (EDR)”  An engineering documentation report is prepared to support the project cooperation agreement when there are only minor changes in design and costs from the authorizing reports.  An engineering documentation report may also be used for individual projects, which have been authorized as part of a large system study.  In these cases, the report serves to define the specific design concept and to firmly establish the baseline cost estimate.  The engineering documentation report can also be used for projects authorized by Congress without a feasibility report when only technical decisions are required.  Guidance on engineering documentation reports is in ER 1110-2-1150.

“Federal Authorized Levee” A Federal authorized levee is 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.

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

"Flood Plain" is 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).

“Flood Plain Management” (FPM) is 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).

 “General Design Memorandum (GDM)”  A general design memorandum is an obsolete document and will not be prepared for any project.  Either a limited or general reevaluation report shall be prepared in lieu of a general design memorandum (1110-2-1150).

“General Reevaluation”  A general reevaluation is 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).

“General Reevaluation Report (GRR)”  A general reevaluation report is a separate report that documents the analyses undertaken in the general reevaluation study (ER 1105-2-100).

“Limited Reevaluation”  A limited reevaluation is 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).

“Limited Reevaluation Report (LRR)”  A limited reevaluation report is a separate report that documents the analyses undertaken in a limited reevaluation study (ER 1105-2-100).

"Minimize" is to reduce to the smallest possible amount or degree (ER 1165-2-26).

"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).

“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.

“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.

"Practicable" is 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).

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

“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.

"Regulatory Floodway" is 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).

“Residual Risk” is 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).

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

“Risk” is 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).

“Risk Analysis” is 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).

"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.

"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.

 “Uncertainty” is 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).

Flood Risk Management Program