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Flood Glossary


Convert Acre Feet To Gallons - link to the Irving Ranch Water District



Used to express volume of storage usually in a detention basin. One Acre-Foot is equal to one acre times a one-foot depth or 43,560 cubic feet (325,850 gallons).


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.

Alluvial Fan:

A geomorphologic feature characterized by a cone or fan-shaped deposit of boulders, gravel and fine sediments that have been eroded from mountain slopes, transported by flood flows and then deposited in the valley floors and which is subject to flash flooding, high velocity flows, debris flows, erosion, sediment movement and deposition and channel migration.

Area Drainage Master Plan (ADMP):

A plan which identifies the preferred alternatives of those identified in an ADMS. An ADMP provides minimum criteria and standards for flood control and drainage relating to land use and development.

Area Drainage Master Study (ADMS):

A study to develop hydrology for a watershed, to define watercourses, identify potential flood problem areas, drainage problems and recommend solutions and standards for sound floodplain and storm water management. The ADMS will identify alternative solutions to a given flooding or drainage problem.


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.

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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/100-Year Flood:

A flood having a 1 % chance of being equaled or exceeded in any given year. This flood is sometimes called the 1% or 100-year flood.

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.


The area of land that a river drains. This is used to determine how much water will enter a river after rainfall.

Benefit-to-Cost (b/c) Ratio:

Represents the overall efficiency of a plan. Determined by dividing the value of the annual benefit by the annual cost.


A horizontal ledge cut into or at the top or bottom of an earth bank or cutting, to ensure the safety of a long slope.


A cover of impermeable soil that rises from water pressure. If punctured, a blister can become a boil.


A concentration of seepage in one spot, usually caused by pressure from the river on a strata of coarse sand or gravel.

Braided Stream:

A stream whose flow is divided at normal stage by small islands.


The elimination of potential flood damages to houses or other types of structures by acquiring them and removing them.

Bypass Channel:

The construction of a new channel in order to convey stormwater runoff around an area. Usually required due to right-of-way considerations or to avoid environmentally sensitive areas.

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The measure of water capable of flowing through a channel, measured in cubic feet per second (CFS). Also the measure of how much water a stormwater detention facility holds, usually measured in acre-feet (AC-FT).

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 (C.F.S.), 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 C.F.S. is 7.5 gallons of water passing by you every second.


An open conveyance of surface storm water 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.

Channel Flow:

The amount of stormwater flowing through a channel, often measured in cubic feet (of stormwater) per second (or CFS).

Channel Modification:

A man-made change to a channel's characteristics, typically for the purposes of reducing flood damages by increasing its overall conveyance. This can be accomplished by widening and/or deepening the channel, reducing the friction by removing woody vegetation or by lining the channel with various materials.

CLOMR (Conditional Letter of Map Revision):

A Conditional Letter of Map Revision (CLOMR) is FEMA's comment on a proposed project that would affect the hydrologic and/or hydraulic characteristics of a flooding source and thus result in the modification of the existing regulatory floodway or effective Base Flood Elevations. There is no appeal period. The letter becomes effective on the date sent. This letter does not revise an effective National Flood Insurance Program map, it indicates whether a proposed project would produce a change in a Special Flood Hazard Area by FEMA if later submitted as a request for a Letter of Map Revision.

Cold Front:

The forward edge of a mass of cold air intruding into an area of warmer air. The cold front forces the warmer air aloft, where its moisture cools, condenses and forms rain.

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.


The intersection of two or more streams, or where one flows into another.


The ability of a channel or other drainage element to move stormwater.


The highest value of the stage or discharge attained by a flood; synonymous with Flood Peak, thus peak stage or peak discharge.


The point at which the water finally starts going down and people start cracking beers. Then it’s officially time for a neighborhood “crestival.”


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

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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, mining, dredging, filling, grading, paving, excavation, or drilling operations.  Fences or fence-type walls located within the floodplain are also included within this definition.


Somewhat the same as a levee. The Corps of Engineers prefers to use the term "levee" for flood protection projects, since dikes are also used in some parts of the world for coastal protection, and since the Corps also constructs pile dikes. The latter consists of timber piling extending out into a river which are built for river control and to reduce shoaling.


A dike that separates a neighborhood, protecting some homes and leaving others on the wet side, creating a “dike-otomy.”

Diking Districts:

These districts are given responsibility over the approximately 80 miles of dikes and levees in Skagit County. These districts can assess those within the district that are receiving benefits as well as petition the county, state, and federal government for funding and assistance. Funds raised are used to construct and maintain dikes, levees, tide gates, keyways, and bank stabilization. These districts are administered by a board of commissioners which are elected but do not receive a salary. If you would like Dike District information please call Skagit County Surface Water Management at 336-9400.

Disaster Area:

When a disaster is beyond the capabilities of state and local government to respond, the Governor must make a formal request to the President to declare the affected region a "disaster area." When the presidential declaration is enacted, federal assistance is made available to public and certain non-profit entities, as well as to individuals who were adversely affected by the disaster. The assistance is available in many forms, including monetary, temporary housing, crisis counseling and even legal assistance. For more on the Disaster Declaration process, go to:


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 (C.F.S.).

Discrete Segment:

Unique term developed to describe the logical pieces of large, long range projects for determining Federal reimbursement to the local sponsor. Once a discrete segment of a project (e.g. defined element of channel or stormwater detention construction) is complete and functional, it qualifies for reimbursement.


A ridge or hump dividing the direction of surface runoff.


People who get hit by floods year after year.


Release of water from a storage reservoir.

Drainage Area:

The area (acres, square miles, etc.) from which water is carried off by a drainage system.

Drainage Basin:

That portion of the surface of the earth which is drained by a river and its tributaries, or which is occupied by a permanent body of water (lake, pond, reservoir) and all of its tributaries.

Alternatively, a geographical area which contributes surface water runoff to a particular point. The terms “drainage basin,” “tributary area,” and “watershed” can be used interchangeably.


The release of water from a reservoir for power generation, flood control, irrigation or other water management activity.


The scooping, or suction of underwater material from a harbor, or waterway. Dredging is one form of channel modification. It is often too expensive to be practical because the dredged material must be disposed of somewhere and the stream will usually fill back up with sediment in a few years. Dredging is usually undertaken only on large rivers to maintain a navigation channel.

Dry Well:

A deep hole, covered and designed to hold drainage water until it seeps into the ground.


The period of time in minutes or hours in which rainfall of a certain intensity (inches per hour) occurs, or the period of time in which a river is above zero damage or major damage stage.

Duplication of Benefits:

A situation in which benefits are derived from two federal government-sponsored programs for the same item. An example would be a homeowner collecting flood insurance to cover damage to the home, and then being paid full pre-flood value for the home without deducting the insurance proceeds. FEMA regulations prohibit duplication of benefits in a home buyout.

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Elevation Certificate:

The Elevation Certificate is an important administrative tool of the National Flood Insurance Program (NFIP). It is to be used to provide elevation information necessary to ensure compliance with community floodplain management ordinances, to determine the proper insurance premium rate, and to support a request for a Letter of Map Amendment or Revision (LOMA or LOMR-F).


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.

Erosion Hazard Zone:

Land adjacent to a watercourse regulated by Maricopa County that is subject to flood-related erosion losses.

Existing Capacity:

The measure of how much water a channel can currently carry, measured in cubic feet (of storm water) per second (CFS). Also the measure of how much water a stormwater detention facility can currently hold, usually measured in acre-feet (AC-FT) of volume.

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Fair Market Value:

The most probable price which a property should bring in a competitive and open market under all conditions requisite to a fair sale, the buyer and seller each acting prudently and knowledgeably, and assuming the price is not affected by undue stimulus.

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):

The federal agency responsible for providing leadership and support to reduce loss of life and property and to protect our institutions from all types of hazards. This is accomplished through a comprehensive, risk based, all hazards emergency management program consisting of mitigation, preparedness, response, and recovery. In relation to flooding hazards, FEMA is the federal agency responsible for administering the National Flood Insurance Program (NFIP).  Click here to visit the FEMA website.

FEMA donna:

Someone who thinks FEMA should cut them a check the minute their basement is damp. “DON’T YOU KNOW I’M A FLOOD VICTIM?!”

Federal Energy Regulatory Commission (FERC):

The Commission issues and regulates licenses for construction and operation of non federal hydroelectric projects and advises federal agencies on the merits of proposed federal multipurpose water development projects.  Click here to visit the FERC website.


An earth embankment, i.e., a levee, highway, building foundation, or other raised area. The purpose of a fill may be to confine streamflow, raise ground surfaces above the waterline, or simplify transportation. All fills on flood plains create obstructions to some degree.

Fill Material:

Any material used for the primary purpose of replacing an aquatic area with dry land or for changing the bottom elevation of a waterbody. This includes both natural materials (silt, sand, gravel, rock, and wood) and manufactured materials (concrete, plastic, steel, treated wood).

Flash Flood:

A flood which follows within a few (usually less than six) hours of heavy or excessive rainfall, dam or levee failure, or the sudden release of water impounded by an ice jam.

Flash Flood Guidance (FFG):

An internal NWS product which indicates the amount of rainfall in a given amount of time (usually less than 6 hours) which most likely will produce flash flooding in a given area.

Flash Flood Statement (FFS):

A statement issued by the NWS which provides follow-up information on flash flood watches and warnings.

Flash Flood Warning (FFW):

A warning issued by the NWS to warn of flash flooding that is imminent or occurring.

Flash Flood Watch (FFA):

A statement issued by the NWS which alerts communities to the possibility of flash flooding in specified areas.


A flood is commonly interpreted as the temporary overflow of lands not normally covered by water, but which are used or usable by man when not inundated.

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 Crest/Crest:

The highest value of the stage or streamflow attained by a flood; it is the top of the flood wave.

Flood Damages:

Flood damages usually are classified as tangible or intangible. Tangible damages are the replacement costs or monetary loans resulting from the effects of floodwater and debris on crops, soil, buildings, furnishings, goods, roadways, utilities and levees; the added costs of protective efforts, evacuation and emergency care; and losses because of the interruption of commercial activities. Intangible damages are those which are difficult to measure in dollars, such as harm to life and health, inconvenience and discomfort.

Flood Damage Stage:

Generally comparable to "flood stage", but may be somewhat higher or lower than official flood stage designations; refers to the stage in a stream at which damage becomes significant at any specified location, whether caused by overflow or other causes.

Flood Duration:

Generally, the total length of time the stream is above "flood stage"; however, the term "flooding duration" may be used top designate the length of time a flood stage equals or exceeds any specified stage.

Flood Insurance:

The insurance coverage provided through the National Flood Insurance Program.

Flood Insurance Rate Map (FIRM):

Issued by FEMA, an official map of a community that has delineated both the special hazard areas and the risk premium zoned applicable to the community. FIRMs typically identify the elevation of the one percent annual chance/100-year flood and the areas that would be inundated by that level of flooding; they are used to determine flood insurance rates and for floodplain management. FIRM maps are available at the Skagit County Planning and Permit Center.

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.

Flood of Record:

The highest observed river stage or discharge at a given location during the period of record keeping. (Not necessarily the highest known stage.)

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.

Flood Stage:

The stage at which overflow of the natural banks of a stream begins to cause damage in the reach in which the elevation is measured.  The stage at which overflow of the natural banks of a stream begins to cause damage in the reach in which the elevation is measured.

Flood Statement:

A statement issued by the NWS which provides follow-up information on river flooding.

Flood Volume:

The total volume of runoff during a flood, which is equal to the average rate of flow multiplied by time (flood duration). The term "inches runoff" is sometimes used to designate flood volume, which means that the flood volume would cover the drainage area above the point of measurement to a uniform depth equal to the number of inches specified.

Flood Warning:

A warning issued by the NWS to warn of river flooding which is imminent or occurring. A flood warning is issued when a river first exceeds its flood stage, and it may be reissued if a new river forecast for a forecast point or reach is significantly higher than a previous forecast.

Flood Watch:

High flow or overflow of water from a river is possible in the given time period. It can also apply to heavy runoff or drainage of water into low-lying areas. These watches are generally issued for flooding that is expected to occur at least 6 hours after heavy rains have ended.

Flood Plain/Floodplain:

The lowland which borders a river, usually dry but subject to flooding. Also the portion of a river valley which has been inundated by the river during historic floods.

Floodplain Regulations:

A general term applied to the full range of codes, ordinances and other regulations pertaining to land uses and construction within flood plains.

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 complaint, protecting riparian habitat communities, and assuring effective maintenance and operation of flood control works.

Floodplain Use Permit:

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


Homeowners who allow their pampered teens to sit in their homes and play Wii while perfect strangers break their backs sandbagging their homes.


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.

Alternatively, for purposes of the National Flood Insurance Program (NFIP), a floodway is defined as the channel of a stream, plus any adjacent flood plain areas, that must be kept free of encroachment so that the 100-year flood can be carried without increasing the flood heights by more than 1.0 foot. This concept was designed for typical river valley situations, where the channel represents the lowest point in the flood plain and the most effective conveyance area is immediately adjacent to the channel.

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.


A line formed representing the lowest point in the bottom of and along a specified length of a channel.


(1) A narrow gorge, usually with a stream flowing through it. (2) An open artificial channel or chute carrying a stream of water, as for furnishing power, conveying logs, or as a measuring device.


The part of a dam's reservoir that is immediately upstream from the powerhouse.


Additional height of a levee above the design height to provide a factor of safety in the design.

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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.

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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.


The analysis of water or other liquid in motion, and its action.  Also a field of study dealing with the flow pattern and rate of water movement based on the principles of fluid mechanics.


The scientific analysis of rainfall and runoff, its properties, phenomena and distribution; as well as water dynamics below the ground and in the atmosphere.

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A measurement of water flow equivalent to 1,000 cubic feet of water passing a given point for an entire second.

Kilowatt (KW):

The electrical unit of power which equals 1,000 watts or 1.341 horsepower.

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Lateral Stream Migration:

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

Left Bank:

The land area to the left, adjacent to the river channel, looking downstream.


As defined by the Corps of Engineers, a levee is a compacted embankment built alongside a river for the purpose of preventing high water from flooding the adjoining land.

Alternatively a levee is an embankment constructed on the flood plain for the purpose of confining large flows to a comparatively narrow floodway while protecting the remainder of the flood plain from inundation. Similar structures built to protect lowlands from high tides or to give partial protection to a portion of the flood plain are usually called "dikes".

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/Low Water Channel:

A channel within a larger channel which typically carries low and/or normal flows.

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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.

Megawatt (MW):

A megawatt is one million watts or one thousand kilowatts, a measure of electrical power or generating capacity. A megawatt will typically serve about 1,000 people.


To offset the impact of one action by implementing another. Examples of various forms of mitigation, as used by the Harris County Flood Control District, include:

1. Offsetting the impacts from land development projects. This is usually in the form of a stormwater detention basin. The development area will drain into the stormwater detention basin, and ultimately, into a channel.

2. Offsetting the impacts of wetlands/habitat losses. State and Federal laws protect certain wetlands and habitat. Through a permit process, agencies require projects to "avoid, minimize and mitigate" any unavoidable losses. Mitigation is typically done through recreation of the affected wetlands or habitat areas. Certain wetlands losses can be mitigated with the District's wetlands mitigation banking.

3. Offsetting dollar losses from previous or expected flood damages. This can come in the form of buyout or even elevating a structure or dwelling. The Harris County Flood Control District may partner with FEMA or the U.S. Army Corps of Engineers in various buyout endeavors to eliminate the possibility of a structure flooding by purchasing it and demolishing it, thus removing it forever from a flood-prone area. A structure that no longer exists can no longer incur flood damages.

An individual homeowner can also mitigate financial losses caused by flood damage by purchasing a flood insurance policy.

Multi-Use Facility:

A detention or retention basin that provides additional benefits to its primary function of flood control. Such benefits include recreation, parking, visual buffers, or water harvesting.

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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.

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An object or condition in a river channel or flood plain which retards or impedes the flow of water.  All construction within the flood plain creates an obstruction to some degree.  Examples, are fills for buildings, roadways or bridges, utility intakes or outlets and levees.

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.


An outfall is simply the pipe, channel, or opening where water "falls out" and then into another body of water, typically a drainage channel. In a typical stormwater detention basin, the outfall is at or connected to the lowest point of the basin so that detained water drains completely.


The condition in which the water level of a channel rises above the top of its banks and spills into the surrounding land area.

Overland Flow (Sheet Flow) Flooding:

Flooding that occurs when intense local rainfall flows overland to reach a channel. Frequently, this conditions exists when runoff exceeds storm sewer or roadside ditch capacity, and the water can "pond" in the streets deep enough to flood residences that are not even near a creek of bayou. The water will seek a path to the channel by flowing overland (Sheet Flow). When residences and other structures are in that path, flooding occurs and this type of flooding is not identified on the Flood Insurance Rate Maps.


Generally, a U-shaped bend or meander in a channel. Oxbows are sometimes "cut off" and abandoned when a channel is straightened. This can occur both naturally or by man-made means.

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Pay-As-You-Go refers to using current income (cash) instead of relying on debt (e.g. bonds) as a way to fund projects. Cash funding avoids long-term debt and its associated interest payments.

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.

Phase 1 Flooding:

Phase 1 floods inundate low areas near the Skagit River, may cover a few small sections of roads, and occur every few years on the average.  These floods generally do not cause significant damage in the Skagit River Valley.  A large phase 1 flood occurred in December 1989.

Phase 2 Flooding:

Phase 2 floods inundate a wider area and may cause significant damage.  A large phase 2 is approximately what occurred in December 1975 which was estimated to be a 10-year flood event (the magnitude of a flood that would have a 10% chance of occurring on any given year.)

Phase 3 Flooding:

Phase 3 floods can cause catastrophic damage in the valley.  A very large phase 3 flood would be considered a 100-year flood which means the probability of a flood of this magnitude would have approximately a 1% chance of occurring in any given year.

Physical Weathering:

Breaking down of rock into bits and pieces by exposure to temperature and changes and the physical action of moving ice and water, growing roots, and human activities such as farming and construction.

Pre-Flood Fair Market Value:

"Pre-flood" is a term relating to the buyout program which establishes the value prior to recent flood damages.

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.


The process, occurring after a rainfall, when water gathers in low lying areas throughout a watershed. Frequently referring to water standing in the streets when the capacity of the storm sewer is exceeded.

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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.

Alternatively, the length of a river between two gaging stations.


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 (C.F.S.) or greater. Regulatory floodplains also include areas which are subject to sheet flooding, or areas on existing recorded subdivision plats mapped as being flood prone.

Repetitive Loss Property:

Homes that have received more than $1,000 of flood insured damage two or more times in the last ten years will appear on the National Flood Insurance Program (NFIP) repetitive loss database and receive higher priority for certain types of buyout.

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.

Return Period:

The average time interval between occurrences of a hydrological event of a given or greater magnitude, usually expressed in years.


A facing of stone, concrete, or even such materials as tires, placed on a riverbank or levee to protect them from erosion.

Right Bank:

The land area to the right, adjacent to the river channel, looking downstream.


Rocks or broken pieces of concrete often placed in areas where the flow of stormwater is expected to cause erosion. The riprap serves as "armor" for areas of channels and detention basins to minimize the occurrence of erosion.

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.

River Forecast Center (RFC):

A division of the National Weather Service which provides river forecasts for rivers within its area of responsibility. There are 13 RFCs in the United States. Their areas are demarked by hydrologic boundaries and watersheds rather than political boundaries.


Surface water resulting from rainfall or snowmelt that flows overland to streams, usually measured in acre-feet (the amount of water which would cover an acre one foot deep). Volume of runoff is frequently given in terms of inches of depth over the drainage area. One inch of runoff from one square mile equals 53.33 acre-feet.

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Determines how much rainfall can be absorbed by soil. Rainfall on ground that is completely saturated turns immediately to runoff.


Erosion caused by rapid flow of water.


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.


A periodic oscillation of the water in a lake usually caused by strong winds or barometric pressure changes; stratified layers may be mixed and shoreline habitats flooded or exposed.


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 Flooding:

A condition where storm water runoff forms a sheet of water to a depth of six inches or more. Sheet flooding is often found in areas where there are no clearly defined channels.

Sheet Flow:

Very shallow overland discharge.


Those who like to make predictions about flooding, based on snowmelt, warming trends and what their buddy Earl said at coffee yesterday.

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.


The water-surface height of a stream, usually registered in feet and tenths of a float on a fixed staff gage.


Water naturally or artificially stored in surface or underground reservoirs.

Storm Drainage System:

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

Storm Water/Stormwater:

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

Substantially Damaged Property:

Flood damage to a structure where the cost to repair equals or exceeds 50% of the value of the structure, excluding the land value.

Surface Water:

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

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


The line of maximum depth in a stream. The thalweg is the part that has the maximum velocity and causes cutbanks and channel migration.


Bottom of levee slope.


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.

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Urban Flooding:

The inundation of streets, basements, ground level floors of buildings, et cetera in urban areas.

U.S. Army Corps of Engineers/Corps:

U.S. Army Corps of Engineers, also USACE. The federal agency authorized to partner with local governments (such as the District) to conduct major water resources projects. The Corps operates nationally and evaluates funding requirement for all projects. The Corps also supports U.S. military operations. For more general info on the Corps, go to

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Valley Storage:

Natural storage of flood water in adjacent areas when a stream overflows its banks, measured in acre-feet.


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

Virgin flow:

The streamflow which exists or would exist if man had not modified the conditions on or along the stream or in the drainage basin.

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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.

Watercourse Master Plan (WCMP):

A hydraulic plan for a watercourse that examines the cumulative impacts of existing development and future encroachment in the floodplain and future development in the watershed on potential flood damages, and establishes technical criteria for subsequent development so as to minimize potential flood damages for all flood events up to and including the one hundred-year flood.

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.

Water Year

A water year is the 12-month period of October 1 through September 30 and is designated by the calendar year in which it ends.

Wave Wash / Wake Wash:

Erosion caused by waves.


A structure typically constructed to control the timing and amount of stormwater flowing into an adjacent detention basin. As the stormwater level in the channel increases, water flows into the basin over the weir. The lower a weir, the sooner the rising stormwater enters the basin. The longer a weir, the greater the flow of stormwater entering the basin.


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.

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Zero Damage Stage:

Usually considered the same as bankfull stage, the height at which the river is just beginning to overtop its banks.

Zero-Rise Floodway:

Means the channel of the stream and that portion of the adjoining floodplain which is necessary to contain and discharge the base flood flow without any measurable increase in flood heights. A measurable increase in base flood height means a calculated upward rise in the base flood elevation, equal to or greater than .01 foot, resulting from a comparison of existing conditions and changed conditions directly attributable to development in the floodplain. This definition is broader than that of the Federal Emergency Management Agency floodway, but would always include the FEMA floodway. The boundaries of the 100-year floodplain as shown on the Flood Insurance Study are considered the boundaries of the zero-rise floodway unless otherwise delineated by a special sensitive areas study.

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.


The division of an entire area, such as a county or municipality, into zones, with the type of construction and use allowable in each zone fixed by law. Zoning is carried out under the provisions of a State zoning enabling law.

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This glossary is an aggregate of the following: March 6, 2011 Inforum Column by Tammy Swift: Flood Fight has its Own Language, Center for Columbia River History, Oregon; FEMA Severe Weather Watches And Warnings Definitions; Glossary; Flood Control District of Maricopa County, Arizona; Harris County Flood Control District, Texas; National Weather Service - West Gulf Forecast Center - D; Skagit County Flood Advisory Conditions, Washington; StreamNet Glossary of Dam Related Terms; and USGS An Overview of the Stream-Gaging Program. Also Flood Plain Information Study, Skagit River Basin, Washington, Technical Report, U.S. Army Engineer District, Seattle, Washington, April 1967; Skagit River, Levee & Channel Improvements, Public Brochure, Seattle District, Corps of Engineers, March 1978; Example Flood Damage Prevention Ordinance, Federal Emergency Management Agency, 1985; Skagit County Flood Damage Prevention Ordinance No. 11888, Adopted December 28, 1988;  King County Sensitive Areas Ordinance No. 9614, adopted August 29, 1990

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