Skip to contentSkip to navigation

Floodplain Concepts and Terms

Allowable Rise - The Allowable Rise in a floodplain is simply the amount of rise in floodwater that FEMA or a local municipality will allow as caused by man-made structures, placement of fill material, etc.  The allowable rise is used as the basis for delimiting a floodway.

Base Flood Elevation (BFE) -  Defined by FEMA as the height of the base (100-year) flood in relation to a specified datum, usually the National Geodetic Vertical Datum of 1929 or North American Vertical Datum of 1988. Generally speaking, this is the elevation of the 100-year floodwaters relative to "mean sea level". BFE is not depth of flooding. To determine depth of flooding, you would need to subtract the lowest elevation of a particular property from the BFE. For example, if the property's foundation was at an elevation of 125 feet and the BFE was 131 feet, then one might infer that the 100-year depth of flooding would be approximately 6 feet.

Base Flood Approximation (BFA) - Flood mapping that shows the approximate outline of the base floodplain. An approximate study does not produce a base flood elevation.

Bench Marks - Monuments on the ground that show the elevation of the spot above sea level.

Building (Structure) - A walled and roofed structure that is principally above ground.  The term includes manufactured homes, mobile homes and gas or liquid storage tanks.

Conveyance - Conveyance is a term commonly referring to the amount of area water needs to maintain its flow.  Specific to floodplains, conveyance is the area that flood flows require to pass the peak flood flow rate.  If, say, conveyance is increased by digging a larger channel for flood flows to pass through, it would likely result in lowered flood elevations and/or lowered water velocities.

Conditional Letter of Map Amendment - CLOMA.  A statement from FEMA that if a project is constructed as planned, a letter of map amendment can be issued later.

Conditional Letter of Map Revision - CLOMR.  A statement that if a project is constructed as planned, a letter of map revision can be issued later.

Federal Emergency Management Agency - FEMA.  FEMA manages the National Flood Insurance Program (NFIP). In 2003, FEMA became part of the U.S. Department of Homeland Security. 

Flood Insurance Rate Map- FIRM.  The FIRM is where to look for flood information on a property or region.  The FIRM is a map provided by the National Flood Insurance Program (NFIP) showing floodplain boundaries and sometimes floodway boundaries, depending on the level of study that an area has received.   Some of the older FIRM’s do not have floodway mapping, and separate Floodway maps provided by the NFIP must be obtained.

Floodplain - The floodplain line represents the extents to which flood waters would encroach in a specified flood event.  Most commonly, the 100-year flood event is shown on flood map-ping done by FEMA and local municipalities.  Also, the 500-year flood event is typically shown on flood mapping done in more populated areas.

Floodway - The floodway is a theoretical line which represents the limits to which the flood-plain could be constrained (filled in, blocked, etc.) before a specified allowable rise in flood elevation occurs.  Commonly, the allowable rise ranges from 6- to 12- inches. 

Flood Fringe - The flood fringe is the area between the floodplain line and the floodway line.  As discussed above, the floodway represents the limits to which the floodplain could be filled in.  Accordingly, the flood fringe is the area of the floodplain that can be filled with the assurance that the allowable rise in flood elevation will not be exceeded.

Flood Insurance Rate Map - FIRM.  An official map of a community, on which the Federal Insurance Administration has delineated both the Special Flood Hazard areas and the risk premium zones applicable to the community.

Flood.  A general and temporary condition of partial or complete inundation of normally dry land areas.

Flood Fringe.  The portion of the floodplain lying on either side of the floodway.

Flood Hazard Mitigation.  All actions that can be taken to reduce property damage and the threat to life and public health from flooding.

Flood Mitigation Assistance.  A grant program funded by the National Flood Insurance Program.

Floodplain.  Any land area susceptible to being inundated by flood waters from any source.

Floodproofing. Protective measures added to or incorporated in a building that is not elevated above the base flood elevation to prevent or minimize flood damage.  "Dry floodproofing" measures are designed to keep water from entering a building.  "Wet floodproofing" measures minimizes damage to a structure and its contents from water that is allowed into a building.

Floodway.  The stream channel and that portion of the adjacent floodplain which must remain open to permit passage of the base flood.

Geographic Information System - GIS.  Computer based map system that allow the user to keep a map updated easily and to correlate geographic information with other data, such as tax records on properties.

Letter of Map Amendment - LOMA.  The LOMA process is administered by FEMA and can be utilized to amend/revise a Flood Insurance Rate Map (FIRM).  The LOMA is very similar to a LOMR-F, and is used to show that due to the existence of high ground or fill material, an area or structure should be removed from the floodplain.  The LOMA has certain restrictions applied to the timing of placement of fill.  In short, a LOMA can only be used if fill was placed prior to the date of the first FIRM identifying the floodplain to be amended. 

Letter of Map Revision - LOMR.  The LOMR process is administered by FEMA and can be utilized to revise a Flood Insurance Rate Map (FIRM).  The LOMR is typically used to show more complex changes to the floodplain than simply the placement of fill, as with the LOMR-F.  The LOMR can be used to show corrections to floodplain and floodway mapping, changes in regional hydrology (determination of flood flows), addition of bridges or structures within a waterway, etc.

Letter of Map Revision based on Fill - LOMR-F.  The LOMR-F process is administered by FEMA and can be utilized to revise a Flood Insurance Rate Map (FIRM).  The LOMR-F is used to show that due to the placement of fill, an area or structure has been raised out of the floodplain.

Market Value.  The price a willing buyer and seller agree upon.

National Flood Insurance Program - NFIP.  The NFIP is the federal agency which oversees the nation’s database of Flood Insurance Rate Maps (FIRM), in addition to serving many other flood related functions.

Non-structural Flood Protection Measures.  Administrative tools for controlling flooding and flood damage, including regulations on development, building codes, property acquisition and structure relocation, and modification of existing buildings.

No-Rise Certification.  A certification by an engineer that a project will not cause a set increase in flood heights.

Ponding. Runoff that collects in depressions and cannot drain out, creating a temporary pond.

Post-FIRM Building.  For insurance rating purpose, a post-FIRM building was constructed or substantially improved after December 31, 1974, or after the effective date of the initial Flood Insurance Rate Map of a community, whichever is later. A post-FIRM building is required to meet the National Flood Insurance Program's minimum Regular Program flood protection standards.

Pre-FIRM Building.  For insurance rating purposes, a pre-FIRM building was constructed or substantially improved on or before December 31, 1974, or before the effective date of the initial Flood INsurance Rate Map of the community, whichever is later.  Most pre-FIRM buildings were constructed without taking the flood hazard into account.

Probability.  A statistical term having to do with the size of a flood and the odds of that size of flood occurring in any year.

Regular Program.  Also called the Regular Phase. The phase of community participation in the National Flood Insurance Program that begins on the date of the Flood Insurance Rate Map or when the community adopts an ordinance that meets the minimum requirements of the NFIP and adopts the technical data provided with the FIRM, whichever is later.  Nearly all communities participating in NFIP are in the Regular Program.

Runoff.  Rainfall and snowmelt that reaches a stream.

Special Flood Hazard Area  -SFHA.  Land areas that are at high risk for flooding.

Stormwater Management. Efforts to reduce the impact of increased runoff that results from new development.

Substantial Damage.  Damage of any origin sustained by a structure whereby the cost of the re-storing the structure to its undamaged condition would equal or exceed 50% of the market value of the structure before the damage occurred.

Substantial Improvement.  Any reconstruction, rehabilitation, addition or other improvement to a structure, the total cost of which equals or exceeds 50% of the market value of the structure before the start of construction of the improvement.  The definition of "substantial improvement" includes buildings that have been repaired after suffering substantial damage. [Special exceptions apply minimum code requirements and listed properties on an historic register.]

Watershed. An area that drains into a lake, stream or other body of water.

Wet Floodproofing. Protecting a building from flood damage by using flood-resistant materials below the flood level and elevating things subject to flood damage above the flood level.

Zones.  Special Flood Hazard Areas are designated by zones.

Flood Zones
 Five types of AASFHA where no base flood elevation is provided. This link, Managing Floodplain Development in Approximate Zone A Areas - A Guide for Obtaining and Developing Base (100 yr) Flood Elevations,  will help engineers, land surveyors and architects determine a base flood elevation when no study was performed for the flood area.
 A#Numbered A Zones (e.g., A7 or A14), SFHA where the FIRM shows a base flood elevation in relation to NGVD.
 AESFHA where base flood elevations are provided. AE Zone delineations are now used on new FIRMs instead of A# Zones.
 AHAreas of 1 percent annual chance shallow flooding with constant water-surface elevation (usually areas of ponding) where average depths are between 1 and 3 feet.
 AOSFHA with sheet flow, ponding, or shallow flooding. Base flood depths (feet above grade) are provided.
Zone B Area of moderate flood hazard, usually depicted on Flood Insurance Rate Maps as between the limits of the base and 500-year floods. B Zones are also used to designate base floodplains of little hazard, such as those with average depths of less than 1 foot.
Zone C Area of minimal flood hazard, usually depicted on Flood Insurance Rate Maps as above the 500-year flood level. B and C Zones may have flooding that does not meet the criteria to be mapped as a Special Flood Hazard Area, especially ponding and local drainage problems.
Zone D Area of undetermined but possible flood hazard.
Zone V The Special Flood Hazard Area subject to coastal high hazard flooding. There are three types of V Zones:  V, V#, and VE, and they correspond to the A Zone designations.
Zone X Newer Flood Insurance Rate Maps show Zones B and C (see above) as Zone X.