100-year flood: See one percent annual chance flood.
Accreditation and accredited levee system: A levee system for which evidence has been provided to the Federal Emergency Management Agency (FEMA) that demonstrates adequate design (i.e., certification), operation, and maintenance systems are in place to provide reasonable assurance that protection from the one percent annual chance flood exists. Design criteria, operation plans and criteria, and maintenance plans and criteria are discussed in the Title 44, Section 65.10, of the Code of Federal Regulations (44 CFR §65.10). See Appendix G.
Aleatory uncertainty: A type of irreducible uncertainty attributed to the inherent randomness of events in nature.
Base flood: See one percent annual chance flood.
Breach: Formation of a gap in the levee system through which water may flow uncontrolled onto the adjacent floodplain. A breach in the levee system may occur prior to or subsequent to overtopping.
Certification and certified levee system: A technical evaluation by a certificated professional engineer or federal agency provided to FEMA that demonstrates that the levee system meets the requirements of 44 CFR §65.10 to reduce risk from at least the one percent annual chance flood.
Digital Flood Insurance Rate Map (DFIRM): A Flood Insurance Rate Map that has been made available digitally.
Efficacy: The capacity or power for producing a desired outcome or effect.
Epistemic uncertainty: A type of uncertainty attributed to the lack of understanding about a physical process or system that must be modeled, referred to as knowledge-based uncertainty.
Executive Order 11988: Codified in 1977, the order requires federal agencies to “avoid to the extent possible the long and short term adverse impacts associated with the occupancy and modification of floodplains and to avoid direct or indirect support of floodplain developments wherever there is a practicable alternative.” It required each agency to “provide leadership and take action to reduce the risk of flood loss, to minimize the impacts of floods on human safety, health, and welfare, and to restore and preserve the natural and beneficial values served by floodplains in carrying out its responsibilities.”
FEMA Procedure Memorandum 34, Interim Guidance for Studies Including Levees: Provides FEMA staff, contactors, and mapping partners with guidance for the evaluation and mapping of levees and levee-affected areas as part of the FEMA Flood Map Modernization effort. See Appendix G.
FEMA Procedure Memorandum 43, Guidelines for Identifying Provisionally Accredited Levees (PALs): Provides a 2-year period for affected levee owners to complete those actions necessary to retain their accreditation. See Appendix G.
FEMA Procedure Memorandum 53, Guidelines for Notification and Mapping of Expiring Provisionally Accredited Levee (PAL) Designation: Provides guidance to FEMA field units regarding the steps that are to be taken if the required improvements to PALs have not been accomplished in the requisite 24-month period. See Appendix G.
Flood Hazard Boundary Map (FHBM): As defined by 44 CFR §59.1: “An official map of a community, issued by the Federal Insurance Administrator, where the boundaries of the flood, mudslide (i.e., mudflow) related erosion areas having special hazards have been designated as Zones A, M, and/or E.”
Flood Insurance Rate Map (FIRM): As defined by 44 CFR §59.1: “An official map of a community, on which the Federal Insurance Administrator has delineated both the special hazard areas and the risk premium zones applicable to the community.”
Flood insurance study (FIS): As defined by 44 CFR §59.1, (denoted as a flood elevation study therein): “an examination, evaluation, and determination of flood hazards and, if appropriate, corresponding water surface elevations, or an examination, evaluation and determination of mudslide (i.e., mudflow) and/or flood related erosion hazards.”
Flood or flooding: As defined by 44 CFR §59.1, flooding is:
(a) A general and temporary condition of partial or complete inundation of normally dry land areas from:
(1) The overflow of inland or tidal waters.
(2) The unusual and rapid accumulation or runoff of surface waters from any source.
(3) Mudslides (i.e., mudflows) which are proximately caused by flooding as defined in paragraph (a)(2) of this definition and are akin to a river of liquid and flowing mud on the surfaces of normally dry land areas, as when earth is carried by a current of water and deposited along the path of the current.
(b) The collapse or subsidence of land along the shore of a lake or other body of water as a result of erosion or undermining caused by waves or currents of water exceeding anticipated cyclical levels or suddenly caused by an unusually high water level in a natural body of water, accompanied by a severe storm, or by an unanticipated force of nature, such as flash flood or an abnormal tidal surge, or by some similarly unusual and unforeseeable event which results in flooding as defined in paragraph (a)(1) of this definition.”
Flood risk analysis: A subset of a risk analysis, a flood risk analysis is an analytical process that provides information about or quantifies probabilities and consequences of a flood event.1
Flood risk management: Comprehensive efforts to continuously carry out analyses, assessments, and related activities to reduce flood risk.
Flood risk reduction: As defined by FEMA,2 “The goal of flood risk reduction is to reduce the risk to life and property, which includes existing structures and future construction, in the pre- and post-disaster environments. This is achieved through regulations, local ordinances, land use and building practices, and mitigation projects that reduce or eliminate long-term risk from flood hazards and their effects.”
Flood zone: A geographic area defined by FEMA according to risk and designated by a community’s FIRM.
Floodplain: As defined in 44 CFR §59.1 (also denoted as flood-prone area therein): “Any land area susceptible to being inundated by water from any source.”
1 Modified from NRC (2010)
Freeboard: As defined in 44 CFR §59.1, freeboard means “a factor of safety usually expressed in feet above a flood level for purposes of floodplain management. ‘Freeboard’ tends to compensate for the many unknown factors that could contribute to flood heights greater than the height calculated for a selected size flood and floodway conditions, such as wave action, bridge openings, and the hydrological effect of urbanization of the watershed.”
Hazard mitigation: see Flood risk mitigation Interior drainage: The drainage that is required to evacuate rainwater from areas behind levees. During periods when rivers are high on the levee, any accumulating rainfall must be pumped out of the leveed area or the area becomes subject to flooding.
Levee: As defined in 44 CFR §59.1: “A levee is a man-made structure, usually an earthen embankment, designed and constructed to contain, control, or divert the flow of water so as to provide protection from temporary flooding.”
Levee Analysis and Mapping Procedure (LAMP): A new approach to provide a more precise assessment of flood risk in areas impacted by levees. Developed and circulated for public comment in early 2012, the LAMP was designed to be implementable without statutory or regulatory changes.
Levee failure: Poor performance of a levee through a variety of occurrences including breach before or after overtopping and malfunction of system components (e.g., gates, pumping plants, or floodwalls).
Levee fragility curves: As defined in Schultz et al. (2010), levee fragility curves are intended to be “functions that describe the conditional probability of system failure over the full range of loads to which that system might be exposed.”
Levee reach: As defined in USACE (2010a): “A levee reach is a portion of a levee system (usually a length of a levee) that may be considered as a unit taken for analysis purposes to have approximately uniform representative properties. A levee reach will be the unique entity having properties different than other reaches of the levee system that may be used to evaluate the performance of a portion of the levee system. No maximum length is associated with a reach.”
Levee segment: A levee segment is a discrete portion of a levee system that is operated and maintained by a single entity. A levee segment may comprise one or more levee features.
Levee system: As defined in 44 CFR §59.1, a levee system is: “a flood protection system which consists of a levee, or levees, and associated structures, such as closure and drainage devices, which are constructed and operated in accordance with sound engineering practices.”
Leveed area: The lands from which floodwater is excluded by the levee system.
Level of protection: Normally seen as the flood recurrence interval against which a specific structure is designed to withstand.
Mitigation (structural and nonstructural): As defined by the U.S. Army Corps of Engineers (USACE, n.d.): “Structural measures such as dams, levees, and floodwalls alter the characteristics of the flood and reduce the probability of flooding in the location of interest. Nonstructural measures alter the impact or consequences of flooding and have little to no impact on the characteristics of the flood.”
One percent annual chance flood: A term commonly used to describe a hydrologic event that has in any year a 1 in 100 chance of being equaled or exceeded.
Overtopping: The condition that occurs when floodwaters reach the crest of the levee and exceed its height. Overtopping might or might not be in conjunction with levee failure.
Practices: Methods or techniques used to implement policies.
Policies: Rule or principle used to guide decision making, typically defined in guidance documents.
Residual risk: The risk that remains after considering the mitigating effects of structural, nonstructural, and other risk reduction measures is known as residual risk. Residual risk is always present behind a levee, because no levee is failsafe.
Risk: The potential for adverse effects from the occurrence of a particular hazardous event, which is derived from the probable combination of physical hazards (physical characteristics), the exposure and vulnerabilities of people and property subject to danger or damage from the hazard, and the consequences (impact or damage) caused by the hazard. In insurance nomenclature, risk is the term used to designate the loss consequence of the realization of the uncertain peril, and it may be financial or nonfinancial in nature (e.g., reputational loss). The causes of loss are called a peril, and a condition or situation that increases either the likelihood or severity of the peril occurring is called a hazard.
Risk analysis: A detailed examination to understand the nature of adverse consequences from a particular event to human life, property, or the environmental; an analytical process that provides information about or quantifies probabilities and consequences of an unwanted event. Often, broad definitions of risk analysis include examination of risk communication, risk perception, and risk management alternatives.3
Severe repetitive loss properties: Section 1361A of the National Flood Insurance Act, as amended, 42 U.S.C. 4102a, defines it as a residential property that is covered under an NFIP insurance policy and has at least four claim payments and for which two of these payments in total have exceeded the market value of the property.
Special Flood Hazard Area (SFHA): As defined in 44 CFR §59.1 (denoted as an area of special flood hazard therein), the SFHA is: “the land in the flood plain within a community subject to a 1 percent or greater chance of flooding in any given year.”
Standard project flood: The standard project flood is a derived discharge estimate that represents a flood that can be expected from the combination of the most severe meteorologic and hydrologic condition that are considered reasonable characteristic of a region (USACE, 1965).
Tolerable risk guidelines: As defined in USACE (2010b), tolerable risk guidelines “categorize the nature of risks in ways that can assist in assessing their acceptability or nonacceptability, and in prioritizing actions for reducing risks.”
Uncertainty: As discussed in NRC (2010), uncertainty is “always present in our ability to predict what might occur in the future, and is present as well in our ability to reconstruct and understand what has happened in the past. This uncertainty arises from missing or incomplete observations and data; imperfect understanding of the physical and behavioral processes that determine the response of natural and built environments and the people within them; and our inability to synthesize data and knowledge into working models able to provide predictions where and when we need them.”
Vulnerability: As defined in NRC (2012), vulnerability is “the potential for harm to the community and relates to physical assets (building design and strength), social capital (community structure, trust, and family networks), and political access (ability to get government help and affect policies and decisions). Vulnerability also refers to how sensitive a population may be to a hazard or to disruptions caused by the hazard.”
NRC (National Research Council). 2010. Review of the Department of Homeland Security’s Approach to Risk Analysis Washington, DC: The National Academies Press.
NRC. 2012. Disaster Resilience: A National Imperative. Washington, DC: The National Academies Press.
Schultz, M. T., B. P. Gouldby, J. D. Simm, and J. L. Wibowo. 2010. Beyond the Factor of Safety: Developing Fragility Curves to Characterize System Reliability. Washington, DC: U.S. Army Corps of Engineers.
3 Modified from NRC (2010).
USACE (U.S. Army Corps of Engineers). 1965. Standard Project Flood Determinations, EM 110-2-14-11. Washington, DC: USACE.
USACE. 2010a. USACE Process for the National Flood Insurance Program (NFIP) Levee System Evaluation. Available online at http://publications.usace.army.mil/publications/eng-circulars/EC_1110-2-6067.pdf. Accessed November 27, 2012.
USACE. 2010b. Proceedings of the Workshop Exploration of Tolerable Risk Guidelines for the USACE Levee Safety Program. Report 10-R-9. Washington, DC:
USACE. USACE. n.d. National Nonstructural Flood Proofing Committee. Available online at http://www.usace.army.mil/Missions/CivilWorks/ProjectPlanning/nfpc.aspx. Accessed November 15, 2012.