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Glossary of Terms


100-Year (or Base) Flood:
A flood event that statistically has a 1 out of 100 (or one percent) chance of being equaled or exceeded on a specific watercourse in any given year. A flood event of this magnitude is often used to determine if flood insurance is either advisable or required on a property.

A progressive buildup or raising of the channel bed due to sediment deposition. Permanent or continuous aggradation is an indicator that a change in the stream's discharge and sediment characteristics is taking place.

Surfacing of channel bed, banks, or embankment slope to resist erosion

As Built Plans:
A community may require submission of "as-built" plans to certify that a project was built in accordance with the permit. A registered professional architect or engineer certifies the actual construction.


The placement of fill material within a specified depression, hole or excavation pit below the surrounding adjacent ground level, as a means of improving flood water conveyance, or to restore the land to the natural contours existing prior to excavation.

Base Flood Elevation:
A base flood elevation (BFE) is the height of the base flood, usually in feet, in relation to the National Geodetic Vertical Datum of 1929, the North American Vertical Datum of 1988, or other datum referenced in the Flood Insurance Study report, or the depth of the base flood, usually in feet, above the ground surface.


Catch Basin:
A chamber or well, usually built at the curb line of a street, for the admission of surface water to a storm sewer or sub-drain

The measuring unit of cubic feet per second (CFS), which is used to quantify the amount of flow in a wash. A cubic foot is equivalent to 7.5 gallons of water. Thus, 1 CFS is 7.5 gallons of water passing by you every second.

An open conveyance of surface stormwater having a bottom and sides in a linear configuration. Channels can be natural or man-made. Channels have levees or dikes along their sides to build up their depth. Constructed channels can be plain earth, landscaped, or lined with concrete, stone, or any other hard surface to resist erosion and scour.

Channel Failure:
Sudden collapse of a channel due to an unstable condition.

CLOMR (Conditional Letter of Map Revision):
A Conditional Letter of Map Revision. A letter that indicates that the Federal Emergency Management Agency (FEMA) will revise base flood elevations, flood insurance rate zones, flood boundaries, or floodways as shown on an effective FIRM, after the record drawings are submitted and approved.

Community Rating System:
A program administered by the Federal Emergency Management Agency (FEMA) that recognizes and rewards communities working to reduce flood damages through a variety of approved floodplain management and flood awareness activities. Through the program, a community can reduce the flood insurance premiums that floodprone property owners pay.

A hydraulically short conduit which conveys surface water runoff through a roadway embankment or through some other type of flow obstruction.


Defining the physical boundaries of a stream, floodplain, jurisdictional wash, etc.

Something dropped or left behind by moving water, as sand or mud.

Design Discharge:
The nth-year storm for which it is expected that the structure or facility is designed to accommodate.

Detention Basin:
A basin or reservoir where water is stored for regulating a flood. It has outlets for releasing the flows during the floods.

A man-made change to property, such as buildings or other structures with, mining, dredging, filling, grading, paving, excavation, or drilling operations.

The amount of water that passes a specific point on a watercourse over a given period of time. Rates of discharge are usually measured in cubic feet per second (CFS).

Drainage Basin:
A geographical area which contributes surface water runoff to a particular point. The terms "drainage basin", "tributary area", and "watershed" can be used interchangeably.


A man-made earth structure constructed for the purpose of impounding water.

Emergency Spillway:
An outflow from a detention/retention facility that provides for the safe overflow of floodwaters for large storms that exceed the design capacity of the outlet or in the event of a malfunction. The emergency spillway prevents the water from overtopping the facility.

The result of placing a building, fence, berm or other structure in a floodplain in a manner that obstructs or increases the depth (or velocity) of flow on a watercourse.

The wearing away of land by the flow of water.


Federally-Mapped Floodplain:
A floodprone area that has been mapped and accepted by FEMA as the result of a flood insurance study (FIS) for a watercourse and surrounding areas. Mapped floodplains are used for flood insurance needs and for other regulatory purposes.

FEMA (Federal Emergency Management Agency):
An independent federal agency established to respond to major emergencies that state and local agencies don't have the resources to handle. FEMA seeks to reduce the loss of life and protect property against all types of hazards through a comprehensive, risk-based emergency management program. Click here to visit the FEMA web site.

Flood Control:
Various activities and regulations that help reduce or prevent damages caused by flooding. Typical flood control activities include: structural flood control works (such as bank stabilization, levees, and drainage channels), acquisition of floodprone land, flood insurance programs and studies, river and basin management plans, public education programs, and flood warning and emergency preparedness activities.

Flood Insurance Rate Map (FIRM):
Issued by FEMA, these maps show special hazard areas, including the 100-year floodplain. They also show flood insurance risk zones and other flood-related information applicable to a community.

Flood Insurance Study (FIS):
Hydrologic and Hydraulic studies that identify a flood hazard area, flood insurance risk zones and other flood data such as flood depths and velocities.

The area adjoining a watercourse that may be covered by floodwater during a flood. Storm runoff and flood events may cause alterations in the floodplain in certain areas.

Floodplain Management:
A program that uses corrective and preventative measures to reduce flood and erosion damage and preserve natural habitat and wildlife resources in floodprone areas. Some of these measures include: adopting and administering floodplain regulations, resolving drainage complaints, protecting riparian habitat communities, and assuring effective maintenance and operation of flood control works.

Floodplain Regulations:
Adopted policies, codes, ordinances, and regulations pertaining to the use and development of lands that lie within a regulatory floodplain.

Floodplain Use Permit:
An official document which authorizes specific activities within a regulatory floodplain or erosion hazard area.

Flood Proofing:
Any combination of changes to a structure or property using berms, flood walls, closures or sealants, which reduces or eliminates flood damage to buildings or property.

The channel of a watercourse and portion of the adjacent floodplain that is needed to convey the base or 100-year flood event without increasing flood levels by more than one foot and without increasing velocities of flood water.

Floodway Fringe:
The areas of a delineated floodplain adjacent to the Floodway where encroachment may be permitted.

Flowage Easement:
Legal right to allow water to flow across someone's property


Grade Control Structure:
A structure used across a stream channel placed bank to bank to control bed elevation, velocity, pressure, etc.

Disturbance of existing land contours

Water within the earth that supplies wells and springs; water in the zone of saturation where all openings in rocks and soil are filled, the upper surface of which forms the water table.


Habitat Mitigation:
The compensation for the removal of natural vegetation during the construction of a flood control project by establishing new vegetation elsewhere.

Hydraulic Structures:
The facilities used to impound, accommodate, convey, or control the flow of water, such as dams, intakes, culverts, channels, and bridges.

A field of study dealing with the flow pattern and rate of water movement based on the principles of fluid mechanics.

A field of study concerned with the distribution and circulation of surface water, as well as water dynamics below the ground and in the atmosphere.


Lateral Stream Migration:
Change in position of a channel by lateral erosion of one bank and simultaneous deposition on the opposite bank.

A man-made structure, usually an earthen embankment often reinforced with soil cement, that is designed to contain or divert the flow of water.

LOMA (Letter of Map Amendment):
An official amendment of a current Flood Insurance Rate Map (FIRM) accepted by FEMA for a property or a structure. The LOMA verifies that the structure or portions of the property have been removed from a designated-floodplain area.

LOMR (Letter of Map Revision):
An official revision of a current Flood Insurance Rate Map (FIRM) accepted by FEMA, which reflects changes in mapped areas for flood zones, floodplain areas, floodways and flood elevations.

Low Flow Channel:
A channel within a larger channel which typically carries low and/or normal flows


Map Repository:
An agency or entity designated to maintain official FEMA flood insurance rate maps for the community as well as LOMAs and LOMRs to those maps.


National Flood Insurance Act of 1968:
An Act passed by Congress that established the National Flood Insurance Program as a means of mitigating flood damages. The Act makes flood insurance available to communities that adopt and enforce measures to reduce flood losses. Prior to the Act, property owners in floodprone areas typically were not able to obtain this coverage through private insurance companies.

National Flood Insurance Program (NFIP):
A federal program that allows property owners to purchase insurance protection against losses due to flooding. In order to participate in this program, local communities must agree to implement and enforce measures that reduce future flood risks in special flood hazard areas.


Outlet Structure:
A hydraulic structure placed at the outlet of a channel, spillway, pipe, etc., for the purpose of dissipating energy and providing a transition to the channel or pipe downstream.


Peak Flow:
The maximum rate of flow through a watercourse for a given storm

The movement of water through the subsurface soil layers, usually continuing downward to the groundwater or water table reservoirs.

Probable Maximum Flood:
The flood runoff that may be expected from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible in the region.


A term used to describe a specific length of a stream or watercourse. For example, the term can be used to describe a section of a stream or watercourse between two bridges.

Subject to the control of or required to follow rules set forth by a governmental agency. With respect to washes or streams it refers to those areas where the federal government restricts the use or development of areas it has deemed to be "Waters of the U.S." These regulations are part of the Clean Water Act.

Regulatory Flood Elevation:
The elevation which is one foot above the base flood elevation for a watercourse. Where a floodway has been delineated, the base flood elevation is the higher of either the natural or encroached water surface elevation of the 100-year flow.

Regulatory Floodplain:
A portion of the geologic floodplain that may be inundated by the base flood where the peak discharge is 100 cubic feet per second (CFS) or greater. Regulatory floodplains also include areas which are subject to sheet flooding, or areas on existing recorded subdivision plats mapped as being floodprone.

Retention Basin:
A basin or reservoir where water is stored for regulating a flood. Unlike a detention basin, it does not have outlets for releasing the flows, the water must be disposed by draining into the soil, evaporation, or pumping systems.

Riparian Habitat:
Plant communities that occur in association with any spring, cienega, lake, watercourse, river, stream, creek, wash, arroyo, or other body of water. Riparian habitats can be supported by either surface or subsurface water sources.

Riparian Zone:
A stream and all the vegetation on its banks.

The portion of precipitation on land that ultimately reaches streams, especially water from rain or melted snow that flows over ground surface.


Soil particles, sand, and minerals washed from the land into aquatic systems as a result of natural and human activities.

A large scale water treatment process where heavy solids settle out to the bottom of the treatment tank after flocculation.

The minimum distance required between a man-made structure and a watercourse. This distance is measured from the top edge of the highest channel bank or the edge of the 100-year flood water surface elevation.

Sheet Flow:
Very shallow overland discharge.

Soil Erosion:
The processes by which soil is removed from one place by forces such as wind, water, waves, glaciers, and construction activity and eventually deposited at some new place.

An outlet pipe or channel serving to discharge water from a dam, ditch, gutter, or basin.

Storm Drainage System:
A drainage system for collecting runoff of stormwater on highways and removing it to appropriate outlets. The system includes inlets, catch basins, storm sewers, drains, reservoirs, pump stations, and detention basins.

Precipitation from rain or snow that accumulates in a natural or man-made watercourse or conveyance system.

Surface Water:
Water that flows in streams and rivers and in natural lakes, in wetlands, and in reservoirs constructed by humans.


The water surface elevation in the channel downstream of a hydraulic structure

A metal bar or grate located at the outlet structure of a detention or retention basin which is designed to prevent blockage of the structure by debris.

A stream that contributes its water to another stream or body of water.


Legal permission to build a structure in a manner that would otherwise be prohibited by an ordinance.


Water quality standards:
Laws or regulations, promulgated under Section 303 of the Clean Water Act, that consist of the designated use or uses of a waterbody or a segment of a waterbody and the water quality criteria that are necessary to protect the use or uses of that particular waterbody. Water quality standards also contain an antidegradation statement. Every State is required to develop water quality criteria standards applicable to the various waterbodies within the State and revise them every 3 years.

Water table:
Level below the earth's surface at which the ground becomes saturated with water. The surface of an unconfined aquifer which fluctuates due to seasonal precipitation.

Any minor or major lake, river, creek, stream, wash, arroyo, channel or other topographic feature on or over which waters flow at least periodically. Watercourse includes specifically designated areas in which substantial flood damage may occur.

Waters of the U.S.:
All waters which are currently used, were used in the past, or may be susceptible to use in interstate or foreign commerce.

An area from which water drains into a lake, stream or other body of water. A watershed is also often referred to as a basin, with the basin boundary defined by a high ridge or divide, and with a lake or river located at a lower point.

Those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas.


Zone A (unnumbered):
Zone A is a Special Flood Hazard Area identified by FEMA that is subject to inundation from a 100-year flood event. Because detailed hydraulic analyses have not been performed, no base flood elevation or depths are shown. Mandatory flood insurance requirements apply.

Zone AE and A1-30:
Special Flood Hazard Areas subject to inundation by the 100-year flood determined by a Flood Insurance Study (FIS). Base flood elevations are shown within these zones and mandatory flood insurance requirements apply. (Zone AE is used on newer maps in place of Zones A1-30.)

Zone AH:
Special Flood Hazard Areas subject to inundation by 100-year shallow flooding (usually areas of ponding) with average depths between one and three feet. Base flood elevations derived from detailed hydraulic analyses are shown in this zone. Mandatory flood insurance requirements apply.

Zone AO:
Special Flood Hazard Areas subject to inundation by 100-year shallow flooding, usually resulting from sheet flow on sloping terrain, with average depths between one and three feet. Average flood depths derived from detailed hydraulic analyses are shown within this zone. Mandatory flood insurance requirements apply.

Zone B, C and X:
Areas that have been identified in a community flood insurance study as having moderate or minimal hazard from flooding. Buildings or other improvements in these zones could be flooded by severe, concentrated rainfall, in the absence of adequate drainage systems. Flood insurance is available in participating communities, but it is not required in these zones. (Zone X is used on newer maps in place of Zones B and C.)

Zone D:
Unstudied areas where flood hazards are undetermined but where flooding is possible. No mandatory flood insurance requirements apply, but coverage is available in participating communities.