Flood glossary
A little less jargon. A lot more clarity.
Plain-language explanations of 50 terms you may encounter in flood maps, policy documents, and coverage conversations.
1 Percent Annual Chance Flood (100-Year Flood)
A flood having a 1% chance of being equaled or exceeded in any given year. Also known as the base flood. This does not mean the flood will occur once every 100 years, but rather that there is a 1% probability of this flood occurring in any given year. Over a 30-year mortgage, there is approximately a 26% chance of experiencing a 100-year flood.
0.2 Percent Annual Chance Flood (500-Year Flood)
A flood having a 0.2% (1 in 500) chance of being equaled or exceeded in any given year. Areas between the 100-year and 500-year flood boundaries are considered moderate flood hazard areas. These areas are shown as Zone B or Zone X (shaded) on FIRMs.
Base Flood
A flood having a 1% chance of being equaled or exceeded in any given year. The base flood is the national standard used by the National Flood Insurance Program (NFIP) and all Federal agencies for the purposes of requiring flood insurance and regulating new development.
Base Flood Elevation (BFE)
The elevation of surface water resulting from a flood that has a 1% chance of equaling or exceeding that level in any given year. The BFE is shown on the Flood Insurance Rate Map (FIRM) for Zones AE, AH, A1-A30, AR, VE, and V1-V30. Buildings must be elevated to or above the BFE to reduce flood risk and insurance costs.
Base Flood Depth (BFD)
The depth shown on the Flood Insurance Rate Map (FIRM) for Zone AO that indicates the depth of water above the highest adjacent grade resulting from a flood that has a 1% chance of equaling or exceeding that level in any given year. Typically ranges from 1 to 3 feet.
Special Flood Hazard Area (SFHA)
An area having special flood, mudflow, or flood-related erosion hazards and shown on a Flood Insurance Rate Map as Zone A, AO, A1-A30, AE, A99, AH, AR, V1-V30, or VE. These are high-risk areas where mandatory flood insurance purchase requirements apply for federally backed mortgages.
Flood Insurance Rate Map (FIRM)
The official map of a community on which FEMA has delineated both the Special Flood Hazard Areas (SFHAs) and the risk premium zones applicable to the community. The FIRM shows base flood elevations, flood zones, and floodplain boundaries. Communities use FIRMs to regulate development in flood-prone areas.
FIRMette
A portion of a FIRM that can be electronically extracted from the National Flood Hazard Layer and printed. FIRMettes are often used for insurance applications, elevation certificates, and property transactions to show a property's flood zone.
Floodway
The channel of a river or stream and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height (typically one foot). Development is highly restricted in floodways because any obstruction could increase flood heights.
Flood Fringe
The portion of the floodplain outside the floodway. Development is allowed in the flood fringe with proper elevation and floodproofing, as it will not significantly increase base flood elevations. The flood fringe stores floodwater but is not part of the primary conveyance channel.
No Rise Certification
A certification from a licensed professional engineer that a proposed project in a regulatory floodway will not cause any increase in base flood elevations. This certification is required before development can occur in a floodway to ensure the project does not worsen flooding for neighboring properties.
Elevation Certificate (EC)
An administrative tool used to provide elevation information necessary to ensure compliance with floodplain management ordinances and to determine proper insurance premium rates. The EC must be completed by a licensed land surveyor or professional engineer and documents the building's lowest floor elevation relative to the base flood elevation.
Lowest Floor
The lowest floor of the lowest enclosed area, including a basement. An unfinished or flood-resistant enclosure usable solely for parking of vehicles, building access, or storage is not considered a building's lowest floor, provided it is properly designed with flood openings and not built to render the structure in violation of applicable requirements.
Freeboard
An additional amount of height above the base flood elevation (BFE) used as a factor of safety in determining the level at which a structure's lowest floor must be elevated to be in accordance with state or community floodplain management regulations. Freeboard compensates for factors such as wave action, debris, and hydrologic uncertainties. Common freeboard requirements range from 1 to 3 feet above BFE.
Flood Openings (Vents)
Openings in foundation walls that allow floodwater to automatically enter and exit an enclosed area below the base flood elevation. Flood openings equalize hydrostatic pressure on foundation walls during flooding, preventing structural collapse. NFIP requires a minimum of two openings with a total net area of not less than one square inch for every square foot of enclosed area.
Substantial Damage
Damage of any origin sustained by a structure whereby the cost of restoring the structure to its before-damaged condition would equal or exceed 50% of the market value of the structure before the damage occurred. When a structure is substantially damaged, it must be brought into compliance with current floodplain management standards, which often means elevation or other flood protection measures.
Substantial Improvement
Any reconstruction, rehabilitation, addition, or other improvement of a structure, the cost of which equals or exceeds 50% of the market value of the structure before the start of construction of the improvement. When substantial improvement occurs, the entire structure must be brought into compliance with current floodplain management regulations.
Increased Cost of Compliance (ICC)
Coverage available under NFIP flood insurance policies that provides up to $30,000 to help pay for the cost of mitigation measures that will reduce flood risk to an NFIP-insured building that has been substantially damaged or repetitively damaged. Eligible activities include elevation, relocation, demolition, or floodproofing of non-residential buildings.
Community Rating System (CRS)
A voluntary program under the NFIP that recognizes and encourages community floodplain management activities that exceed the minimum requirements. Communities that participate in the CRS can receive discounts of 5% to 45% on flood insurance premiums for their residents. The program has 10 classes, with Class 1 representing the best practices and receiving the maximum premium discount.
Risk Rating 2.0
FEMA's updated flood insurance rating methodology that became effective October 1, 2021. Risk Rating 2.0 uses improved science and technology to calculate premiums based on individual property characteristics such as flood frequency, multiple flood types (river overflow, storm surge, coastal erosion, and heavy rainfall), distance to water source, and property elevation and type. This replaces the previous zone-based rating system.
Preferred Risk Policy (PRP)
A low-cost NFIP flood insurance option for buildings located in moderate-to-low risk areas (B, C, X zones). The PRP offers both building and contents coverage at reduced rates. Properties in high-risk areas that have been remapped to moderate or low-risk zones may also be eligible for PRP coverage.
Zone A
Areas with a 1% annual chance of flooding and a 26% chance of flooding over the life of a 30-year mortgage. These are high-risk Special Flood Hazard Areas where mandatory flood insurance purchase requirements apply. Because detailed analyses are not performed for Zone A areas, no base flood elevations (BFEs) are shown. Insurance rates are typically higher than in zones with determined BFEs.
Zone AE
The base floodplain where base flood elevations (BFEs) are provided. Zone AE replaced the old A1-A30 zone designations on modern FIRMs. This is a high-risk Special Flood Hazard Area with a 1% annual chance of flooding where mandatory flood insurance purchase requirements apply. Because BFEs are determined, more accurate insurance rating is possible compared to Zone A.
Zone AH
Areas with a 1% annual chance of shallow flooding, usually in the form of a pond, with an average depth ranging from 1 to 3 feet. These areas have a 26% chance of flooding over the life of a 30-year mortgage. Base flood elevations derived from detailed analyses are shown at selected intervals within Zone AH. This is a high-risk Special Flood Hazard Area where mandatory flood insurance purchase requirements apply.
Zone AO
River or stream flood hazard areas with a 1% or greater chance of shallow flooding each year, usually in the form of sheet flow, with an average depth ranging from 1 to 3 feet. These areas have a 26% chance of flooding over the life of a 30-year mortgage. Instead of base flood elevations, Zone AO maps show average flood depths. This is a high-risk Special Flood Hazard Area where mandatory flood insurance purchase requirements apply.
Zone VE
Coastal high hazard areas with a 1% or greater chance of flooding and additional hazards due to storm-induced velocity wave action (waves 3 feet or higher). The 'V' stands for velocity. Zone VE includes base flood elevations. This is the highest-risk flood zone where the most stringent building requirements apply, including elevation on pilings or columns with breakaway walls below. Buildings must be designed to withstand high winds and wave forces.
Zone X
Areas of moderate to minimal flood hazard. Zone X (shaded) represents areas between the 100-year and 500-year floodplain with a 0.2% annual chance of flooding. Zone X (unshaded) represents areas of minimal flood hazard outside the 500-year floodplain or areas protected by levees from the 100-year flood. Flood insurance is available but not required by federal lending regulations for Zone X properties.
Letter of Map Amendment (LOMA)
An official amendment by FEMA to an effective Flood Insurance Rate Map (FIRM) that establishes that a property is not located in a Special Flood Hazard Area (SFHA). A LOMA is based on technical data showing the property was inadvertently mapped as being in the floodplain but is actually on naturally higher ground above the base flood elevation. Once issued, flood insurance is typically no longer required.
Letter of Map Revision (LOMR)
An official amendment to the currently effective FEMA Flood Insurance Rate Map. It is issued by FEMA and changes flood zones, delineations, and elevations. LOMRs are typically based on fill placement, channel improvements, or other physical changes to the floodplain. Unlike LOMAs which are based on naturally occurring ground elevations, LOMRs are based on man-made changes.
Conditional Letter of Map Revision (CLOMR)
A formal review and comment by FEMA on a proposed flood protection project or proposed changes to the base flood elevations shown on a Flood Insurance Rate Map. The CLOMR does not revise an effective FIRM; it indicates whether the project, if built as proposed, would be recognized by FEMA. A CLOMR is typically followed by a LOMR after project construction is completed and certified.
Dry Floodproofing
A floodproofing method that involves making a building watertight below the base flood elevation through sealants, coatings, shields, or other measures to prevent floodwater from entering. Dry floodproofing is only recognized by NFIP for non-residential buildings. All openings below the floodproofing level must be sealed, including doors, windows, and vents. Structural walls must be able to withstand hydrostatic and hydrodynamic loads.
Wet Floodproofing
A floodproofing method that allows floodwater to intentionally enter a building to equalize hydrostatic pressure on exterior walls and foundation. The structure must be built with flood-resistant materials and must have openings (flood vents) that allow water to automatically flow in and out. Wet floodproofing is recognized by NFIP for accessory structures and certain enclosures below elevated buildings.
Breakaway Walls
Walls that are not part of the structural support of a building and are designed to break away under specific flood loads without damaging the elevated portion of the building or its foundation. Breakaway walls are typically used for enclosures below elevated buildings in coastal high hazard areas (V zones). They must be designed to collapse from a water load of less than 10 or more than 20 pounds per square foot.
Coastal A Zone
Coastal areas that have flood risks associated with storm surge, waves, and erosion but are mapped as Zone A or AE instead of V zone. Coastal A zones typically experience wave heights between 1.5 and 3 feet during the base flood. Some communities require more stringent building requirements in Coastal A zones, similar to V zone construction standards, to account for the additional wave hazards.
Storm Surge
An abnormal rise in sea level accompanying a hurricane or other intense storm, caused primarily by high winds pushing water toward shore. Storm surge can cause extensive flooding in coastal areas and is one of the most dangerous aspects of hurricanes. The surge is measured as the height of water above the normal predicted astronomical tide. In major hurricanes, storm surge can reach 15-25 feet or more.
Stillwater Elevation
The elevation of the water surface without the effects of waves. In coastal areas, the stillwater elevation during a flood event is the combination of the astronomical tide plus storm surge. Base flood elevations in older V zone maps (pre-1981) show only stillwater elevations and do not account for wave heights, which must be added separately.
Wave Runup
The rush of water up a beach or structure following the breaking of a wave. Wave runup elevation is the maximum vertical extent of wave uprush on a beach or structure above the stillwater level. In coastal areas, wave runup can cause flooding significantly higher than the base flood elevation and can damage structures even if they are elevated above the BFE.
Erosion Setback
A line landward of the shoreline or riverbank established to ensure that buildings and structures are set back a safe distance from areas likely to be affected by erosion over a specified period (typically 30 to 60 years). Erosion setbacks are used by coastal and riverine communities to prevent structures from being undermined or destroyed by shoreline erosion.
Alluvial Fan Flooding
Flooding occurring on the surface of an alluvial fan or similar landform which originates at the apex and is characterized by high-velocity flows, active processes of erosion, sediment transport and deposition, and unpredictable flow paths. Alluvial fan flooding is common in arid and semi-arid mountainous regions of the western United States and presents unique hazards due to rapidly shifting channels and debris flows.
Debris Flow
A moving mass of rock, soil, and water that flows down slopes and stream channels. Debris flows, also called mudslides or mudflows, typically occur in steep terrain after heavy rainfall, rapid snowmelt, or following wildfires that destabilize slopes. They are extremely dangerous due to their speed (up to 35 mph), volume, and ability to carry large objects including boulders, trees, and vehicles.
Rain on Snow
A flood-producing event that occurs when warm, heavy rainfall falls on existing snowpack, accelerating snowmelt. The combined runoff from the rainfall and rapid melting can exceed the runoff from rain alone. Rain on snow events are particularly common in mountainous regions and can cause severe flooding, especially in late winter and early spring.
Ice Jam
An accumulation of ice in a river channel that restricts or blocks water flow, causing water levels to rise rapidly upstream of the blockage. When the ice jam breaks apart suddenly, it can release large volumes of water downstream, causing flash flooding. Ice jams are most common during spring thaw but can occur during winter warm spells. They are a significant flood hazard in northern climates.
Flash Flood
A rapid and extreme flow of high water into a normally dry area, or a rapid rise in water level in a stream or creek above a predetermined flood level, beginning within six hours of the causative event (heavy rainfall, dam failure, ice jam, etc.). Flash floods are characterized by a rapid rise in water levels and high velocity flows. They are the most dangerous type of flood and are the leading cause of flood-related deaths.
Pluvial Flooding
Flooding that occurs when heavy rainfall creates a flood event independent of an overflowing water body. Also called surface water flooding or rainfall flooding. Pluvial flooding happens when the volume of rainfall exceeds the capacity of drainage systems (sewers, storm drains) or when the ground cannot absorb water quickly enough. This type of flooding can occur in any area regardless of proximity to a mapped floodplain.
Fluvial Flooding
Flooding that occurs when water levels in rivers, streams, or other watercourses exceed their banks. Also called riverine flooding or overbank flooding. Fluvial flooding is the most common type of flooding and typically develops over hours or days, allowing some time for warnings. It occurs when prolonged or heavy rainfall, snowmelt, or dam releases exceed the channel capacity.
Backwater Flooding
Flooding that occurs when downstream conditions (such as a high river stage, tidal surge, or obstruction) cause water to back up into tributary streams or drainage systems. Backwater flooding can affect areas that are not adjacent to the main water body and may not be shown on standard flood maps. It commonly occurs where smaller streams join larger rivers during flood events.
Hydrograph
A graph showing the rate of water flow (discharge) versus time past a specific point in a river or channel. Hydrographs are used to analyze flood events and illustrate how quickly water levels rise (rising limb), peak, and recede (falling limb). Flash floods have steep rising limbs with rapid peak flow, while riverine floods show more gradual increases and decreases.
Return Period
The average time interval between flood events of a given magnitude. For example, a 100-year flood has a return period of 100 years, meaning that statistically, a flood of that magnitude has a 1% chance of occurring in any given year. Return periods are a statistical concept and do not mean floods occur at regular intervals; multiple 100-year floods can occur in successive years, or none may occur for hundreds of years.
Design Storm
A rainfall or storm event with a specified intensity, duration, and return period used for the design of drainage systems, flood control structures, and other hydraulic facilities. For example, a 24-hour, 100-year design storm represents the amount of rainfall expected to occur over 24 hours in a 100-year flood event. Design storms are used by engineers to size culverts, storm sewers, retention basins, and other infrastructure.
Detention Basin and Retention Basin
Stormwater management facilities that temporarily store runoff to reduce downstream flooding and erosion. A detention basin (or dry pond) temporarily holds water and slowly releases it through a controlled outlet, remaining dry between storm events. A retention basin (or wet pond) maintains a permanent pool of water and provides storage volume above the pool for flood control. Both reduce peak flow rates and can improve water quality by allowing sediments to settle.
No term matched that search. Try a shorter word.