Many of the following definitions are derived from NFIP floodplain management; others are specific legal definitions; and yet others relate to NAI tools and approaches. This section is not all-inclusive of the flood risk management and legal terms used in this Guide; additional definitions may be provided elsewhere for ease of reference.
Base flood: The flood having a one percent probability of being equaled or exceeded in any given year (previously called the 100-year flood). This is the design flood for the NFIP and is used to map Special Flood Hazard Areas and to determine Base Flood Elevations. Modeling of the base flood uses historic flood data.
Base Flood Elevation (BFE): The modeled elevation of floodwater during the base flood. The BFE determines the level of flood protection required by NFIP floodplain development standards.
Building (structure): A walled and roofed building with two or more outside rigid walls and a fully secured roof that is affixed to a permanent site, as well as a manufactured home on a permanent foundation. The terms “structure” and “building” are sometimes used interchangeably in the NFIP. However, for NFIP floodplain management purposes, the term “structure” also includes a gas or liquid storage tank that is principally above ground.
Within the NFIP, residential and non-residential structures are treated differently. A residential structure built in a Special Flood Hazard Area must be elevated above the Base Flood Elevation. A non-residential structure may be elevated or dry floodproofed so that the structure is watertight to prevent the entry of water.
Climate change: Climate change refers to long-term shifts in temperatures and weather patterns. These shifts may be natural, such as through variations in the solar cycle. But since the 1800s, human activities have been the main driver of climate change, primarily due to the burning of fossil fuels like coal, oil and gas.[1]
Community: The NFIP definition of a community is a political subdivision that has the authority to adopt and enforce floodplain management regulations for the areas within its jurisdiction. The term usually means cities, villages, townships, counties, and Indian tribal governments. For the purposes of this Guide, a “community” also includes a neighborhood, unincorporated settlement, or other non-governmental subdivision where people live or work together.
Conservation Zone: An area indicated on a map or plan adopted by a local jurisdiction, municipality, or other governing body within which development is governed by special regulations in order to protect and preserve the quality and function of its natural environment.
Community Rating System (CRS): The NFIP Community Rating System is a program that provides reduced flood insurance premiums for policyholders in communities that go above and beyond the minimum NFIP criteria. For more information see https://www.fema.gov/floodplain-management/community-rating-system.
Federal Emergency Management Agency (FEMA): The federal agency under which the NFIP is administered.
Flood: A community may adopt a more expansive definition of “flood” than is used by the NFIP in order to include additional sources of water damage, such as groundwater flooding of basements or local washouts associated with a drainage ditch. The NFIP definition of a flood 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.
For NFIP flood insurance claims, a flood must inundate two or more acres of normally dry land area or two or more properties.
Flood Insurance Rate Map (FIRM): An official map of a community on which the Federal Emergency Management Agency has delineated the boundaries of Special Flood Hazard Areas. In some areas, FIRMS (with associated maps and studies) may also indicate Base Flood Elevations and regulatory floodways. FIRMs and other mapping products can be viewed and downloaded at FEMA’s Map Service Center ‒ https://msc.fema.gov/portal/home.
Floodplain: Nature’s floodplain is the land area susceptible to being inundated by water from any source. This includes:
- Special Flood Hazard Areas (SFHAs) mapped by FEMA for the NFIP program;
- Flood-prone areas near waterbodies for which SFHAs have not been mapped;
- Areas outside of the SFHA that are subject to inundation by larger flood events or floods that are altered by debris or other blockages;
- Areas subject to smaller, more frequent, or repetitive flooding;
- Areas subject to shallow flooding, stormwater flooding, or drainage problems that do not meet the NFIP mapping criteria;
- Areas affected by flood-related hazards, such as coastal and riverine erosion, mudflows, or subsidence; and
- Areas that will be flooded when future conditions are accounted for, such as climate-related issues, sea-level rise, and upstream watershed development.
The Special Flood Hazard Area mapped for the NFIP is only part of a community’s flood risk area, with 40 percent of flood insurance claims occurring outside of the SFHA.[2] To represent a community’s true flood risk, the term “floodplain” is used in this Guide instead of “SFHA.”
Floodplain stewardship: Caring for and protecting the beneficial biologic and hydrologic functions of areas where the risk of flooding is expected, while managing human uses to minimize the potential for adverse impacts and flood damage.
Floodproof: Floodproofing means any combination of structural and non-structural additions, changes, or adjustments to buildings or other structures that reduce or eliminate flood damage to real estate or improved real property, water and sanitary facilities, structures, and their contents. This term includes dry floodproofing, in which a structure is watertight, with walls substantially impermeable to the passage of water. NFIP development standards allow dry floodproofing of non-residential structures in lieu of elevating the lowest floor.
Freeboard: A factor of safety, usually expressed in feet above the Base Flood Elevation, that determines the required level of flood protection.
Future conditions flood: The flood having a one percent probability of being equaled or exceeded in any given year based on future-conditions hydrology. Also known as the “1%-annual-chance future conditions” flood.
Liability: A party is liable when they are held legally responsible for something. Unlike in criminal cases, where a defendant could be found guilty, a defendant in a civil case risks only liability.[3]
Mitigation: Hazard mitigation is any sustained action taken to reduce or eliminate any long-term risk to life or property from a hazard event. Mitigation is most often thought of as being applied to existing at-risk development. Examples of flood mitigation activities include: floodproofing, elevating, relocating or demolishing at-risk structures; retrofitting existing infrastructure to make it more flood resilient; developing and implementing Continuity of Operations Plans; structural mitigation measures such as levees, floodwalls and flood control reservoirs; detention/retention basins; and beach, dune, and floodplain restoration.
National Flood Insurance Program (NFIP): Federal program that maps flood hazard areas and provides flood insurance in participating communities that agree to regulate new construction in mapped high flood hazard areas. Most community floodplain maps and floodplain management standards have been adopted to meet the NFIP’s criteria. Learn more at www.fema.gov.
Natural floodplain functions: The functions associated with the natural or relatively undisturbed floodplain that moderate flooding, maintain water quality, recharge groundwater, reduce erosion, redistribute sand and sediment, and provide fish and wildlife habitat. One goal of NAI floodplain stewardship is to preserve and protect these functions, in addition to protecting human development.
Police powers: Police powers are the fundamental ability of a government to enact laws to coerce its citizenry for the public good, although the term eludes an exact definition. The term does not directly relate to the common connotation of police as officers charged with maintaining public order, but rather to broad governmental regulatory power. Berman v. Parker, a 1954 U.S. Supreme Court case, stated that “[p]ublic safety, public health, morality, peace and quiet, law and order. . . are some of the more conspicuous examples of the traditional application of the police power;” while recognizing that “[a]n attempt to define [police power’s] reach or trace its outer limits is fruitless.”[4]
Regulatory floodway: The channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood (with a 1% annual probability) without cumulatively increasing the water surface elevation more than a designated height.
Resilience: “The ability to prepare for and adapt to changing conditions and withstand and rapidly recover from disruptions,” as defined in FEMA’s National Disaster Recovery Framework.
Riparian buffer: Zone of variable width along the banks of a stream, river, lake, or wetland that provides a protective natural area adjacent to the waterbody.
Sovereign immunity: Sovereign immunity refers to the fact that the government cannot be sued without its consent.[5]
Special Flood Hazard Area (SFHA): An area mapped on the NFIP FIRM that shows the area subject to inundation by the base flood (with a one percent or greater probability of flooding in any given year). SFHAs have been mapped for flooding caused by rivers, lakes, oceans, and other larger sources of flooding.
Standard of care: The watchfulness, attention, caution, and prudence that a reasonable person in the circumstances would exercise. If a person’s actions do not meet this standard of care, then their acts fail to meet the duty of care, which all people (supposedly) have toward others.[6]
Substantial damage: Damage of any origin sustained by a structure (building) whereby the cost of restoring the structure to its before damaged condition would equal or exceed 50 percent of the market value of the structure before the damage occurred.
Substantial improvement: Any reconstruction, rehabilitation, addition, or other improvement of a structure (building), the cost of which equals or exceeds 50 percent of the market value of the structure before the start of construction for the improvement. This term includes structures that have incurred substantial damage, regardless of the actual repair work performed. NFIP development standards require that a substantially improved building be regulated as new construction.
Sustainable: Able to “meet the needs of the present without compromising the ability of future generations to meet their own needs,” as defined by the United Nations.
Takings: A taking is when the government seizes private property for public use. A taking can come in two forms. The taking may be physical, meaning the government physically interferes with private property; or the taking may be constructive (also called a regulatory taking), meaning that the government restricts the owner's rights to such an extent that the governmental action becomes the functional equivalent of a physical seizure.[7]
Tort: A tort is an act or omission that gives rise to injury or harm to another and amounts to a civil wrong for which courts impose liability. In the context of torts, "injury" describes the invasion of any legal right, whereas "harm" describes a loss or detriment in fact that an individual suffers.[8]
Watershed: The land area that channels rainfall and snowmelt to creeks, streams, and rivers, and eventually to outflow points, such as reservoirs, bays, and the ocean. Also known as a basin or catchment area.
[1] Source: United Nations, “What is Climate Change?” webpage, accessed March 2023, https://www.un.org/en/climatechange/what-is-climate-change.
[2] Federal Emergency Management Agency, 2021, “Myths and Facts About Flood Insurance,” https://www.fema.gov/fact-sheet/myths-and-facts-about-flood-insurance-1.
[3] Source: Cornell Law School, Legal Information Institute, https://www.law.cornell.edu/wex/liability. Liability is “[t]he quality or state of being legally obligated or responsible.” Black’s Law Dictionary: New Pocket Edition (1996).
[4] Source: Cornell Law School, Legal Information Institute, https://www.law.cornell.edu/wex/police_powers. Police power has also been defined as “1. [a] state’s Tenth Amendment right, subject to due process and other limitations, to establish and enforce laws protecting the public’s health, safety, and general welfare, or to delegate this right to local governments. 2. Loosely, the power of the government to intervene in privately owned property, as by subjecting it to eminent domain.” Black’s Law Dictionary: New Pocket Edition (1996).
[5] Source: Cornell Law School, Legal Information Institute, https://www.law.cornell.edu/wex/sovereign_immunity.
[6] Source: Law.com Dictionary, https://dictionary.law.com/Default.aspx?selected=2002.
[7] Source: Cornell Law School, Legal Information Institute, https://www.law.cornell.edu/wex/takings. A taking may also be defined as “[t]he government’s actual or effective acquisition of private property either by ousting the owner and claiming title or by destroying the property or severely impairing its utility.” Black’s Law Dictionary: New Pocket Edition (1996).
[8] Source: Cornell Law School, Legal Information Institute, https://www.law.cornell.edu/wex/tort.