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Below are commonly used terms, phrases, and acronyms you will see in stormwater design, management, and planning. They are defined below to help provide a common understanding.

303(d) waterbody

A list of lakes, rivers, and streams that have been designated as impaired or threatened by a pollutant(s) for which one or more TMDL(s) are needed. Impaired means that the water is not meeting state water quality standards.


The section of the Federal Clean Water Act that deals with nonpoint pollution.



The penetration of a substance into or through another, such as the dissolving of a soluble gas in a liquid.


The adhesion of a substance to the surface of a solid or liquid; often used to extract pollutants by causing them to be attached to such adsorbents as activated carbon or silica gel. Hydrophobic, or waterrepulsing adsorbents, are used to extract oil from waterways when oil spills occur. Heavy metals such as zinc and lead often adsorb onto sediment particles.

Anadromous fish

Fish that ascend rivers from the sea for breeding.


All known, available, and reasonable methods of prevention, control, and treatment.

Algal Bloom


A geologic stratum containing groundwater that can be withdrawn and used for human purposes.



Water upstream from an obstruction which is deeper than it would normally be without the obstruction.


A device to deflect, check or regulate flow.


Any area draining to a point of interest. In this blog we are interested in all drainage areas which drain directly to Puget Sound.

Basin Plan

A plan that assesses, evaluates, and proposes solutions to existing and potential future impacts to the beneficial uses of, and the physical, chemical, and biological properties of waters within a basin. Basins typically range in size from 1 to 50 square miles. 

Beaver deceiver

A constructed flow control device that reduces beaver damming activities. It is a non-lethal beaver management technique.

Best management practice (BMP)

The schedules of activities, prohibitions of practices, maintenance procedures, and structural and/or managerial practices, that when used singly or in combination, prevent or reduce the release of pollutants and other adverse impacts to waters. Commonly used in surface water design manuals when referring to erosion control or low impact development methods.


A constructed barrier of compacted earth, rock, or gravel. In a stormwater facility, a berm may serve as a vertical divider typically built up from the bottom.


Bioengineering is the use of vegetation and other natural materials such as soil, wood and rock to stabilize soil, typically against slides and stream flow erosion. When natural materials alone do not possess the needed strength to resist hydraulic and gravitational forces, "bioengineering" may consist of the use of natural materials integrated with human-made fabrics and connecting materials to create a complex matrix that joins with in-place native materials to provide erosion control.


A designed treatment facility using a combined soil and vegetation system for filtration, infiltration, adsorption, and biological uptake of pollutants in stormwater when runoff flows over and through. Vegetation growing in these facilities acts as both a physical filter which causes gravity settling of particulates by regulating velocity of flow, and also as a biological sink when direct uptake of dissolved pollutants occurs. The former mechanism is probably the most important in western Washington where the period of major runoff coincides with the period of lowest biological activity.


The process of reducing pollutant concentrations in water by filtering the polluted water through biological materials.

Biofiltration swale or Bioswale

A long, gently sloped, vegetated ditch designed to filter pollutants from stormwater. Grass is the most common vegetation, but wetland vegetation can be used if the soil is saturated.


Engineered facilities that treat stormwater by passing it through a specified soil profile, and either retain or detain the treated stormwater for flow attenuation. Similar to a rain garden, but specifically designed to treat stormwater using specific soil mixes and plants. 

As an example, see BMP T7.30: Bioretention for Bioretention BMP types and design specifications in the 2019 Stormwater Management Manual for Western Washington.

Bioretention BMP

Engineered facilities that store and treat stormwater by passing it through a specified soil profile, and either retain or detain the treated stormwater for flow attenuation. Refer to Chapter 7 of Volume V for Bioretention BMP types and design specifications.


Best Management Practice. Also see Structural and Nonstructural BMPs.


A designated area adjacent to and a part of a steep slope or landslide hazard area which protects slope stability, attenuation of surface water flows, and landslide hazards reasonably necessary to minimize risk; or a designated area adjacent to or a part of a stream or wetland that is an integral part of the stream or wetland ecosystem.


Catch Basin

Technical term that is commonly used to refer to drains in the street. A chamber or well, usually built at the curb line of a street, for the admission of surface water to a sewer or subdrain, having at its base a sediment sump designed to retain grit and detritus below the point of overflow.

Catch basin insert

A device installed underneath a catch basin inlet to treat stormwater through filtration, settling, absorption, adsorption, or a combination of these mechanisms. There are a number of shapes, sizes, and configurations of inserts available.

Do you have questions about the use and approval of catch basin inserts or other water quality treatment technologies in King County? Click here.

Cation Exchange Capacity (CEC)

The amount of exchangeable cations that a soil can absorb. Units are milli-equivalents per 100 g of soil, typically abbreviated simply as meq. Soil found to have a CEC of 5 meq at pH 7 will have CEC < 5 meq when pH < 7.

Certified Erosion and Sediment Control Lead (CESCL)

An individual who has current certification through an approved erosion and sediment control training program that meets the minimum training standards established by Ecology (see BMP C160: Certified Erosion and Sediment Control Lead). A CESCL is knowledgeable in the principles and practices of erosion and sediment control. 


A long, narrow excavation or surface feature that conveys surface water and is open to the air.


Channels or ditches constructed (or reconstructed natural channels) to convey surface water.


A channel which has occurred naturally due to the flow of surface waters; or a channel that, although originally constructed by human activity, has taken on the appearance of a natural channel including a stable route and biological community.

Channel stabilization

Erosion prevention and stabilization of velocity distribution in a channel using vegetation, jetties, drops, revetments, and/or other measures.

Check dam

Small dam constructed in a gully or other small watercourse to decrease the streamflow velocity, minimize channel scour, and promote deposition of sediment.

Chemical oxygen demand (COD)

A measure of the amount of oxygen required to oxidize organic and oxidizable inorganic compounds in water. The COD test, like the BOD test, is used to determine the degree of pollution in water.

Clay lens or Clay liner

A naturally occurring, localized area of clay which acts as an impermeable layer to runoff infiltration. Clay is also often used as a constructed impermeable layer to line berms or ponds to prevent infiltration.

Clean Water Act (CWA)

33 U.S.C. §1251 et seq. (1972) The Clean Water Act (CWA) establishes the basic structure for regulating discharges of pollutants into the waters of the United States and regulating quality standards for surface waters. The basis of the CWA was enacted in 1948 and was called the Federal Water Pollution Control Act, but the Act was significantly reorganized and expanded in 1972. "Clean Water Act" became the Act's common name with amendments in 1972.

Read the official text of the Clean Water Act at the Environmental Protection Agency

Closed depression

An area which is low-lying and either has no surface water outlet or has such a limited outlet that during storm events the area acts as a retention basin, with more than 5000 square feet of water surface area at overflow elevation.


The capacity of a soil to resist shearing stress, exclusive of functional resistance.

Combined sewer overflow (CSO)

Discharges of combined sewage and stormwater into water bodies during very wet or stormweather. These discharges occur to relieve the sewer system as it becomes overloaded with normal sewer flow and increased storm run-off. The term is also used to denote a pipe that discharges those overflows.


The densification, settlement, or packing of soil in such a way that permeability of the soil is reduced. Compaction effectively shifts the performance of a hydrologic group to a lower permeability hydrologic group. For example, a group B hydrologic soil can be compacted and be effectively converted to a group C hydrologic soil in the way it performs in regard to runoff. Compaction may also refer to the densification of a fill by mechanical means.


Organic material that has undergone biological degradation and transformation under controlled conditions designed to promote aerobic decomposition at a solid waste facility in compliance with the requirements of Chapter 173-350 WAC, or biosolids composted in compliance with Chapter 173-308 WAC. Composting is a form of organic material recycling. Natural decay of organic solid waste under uncontrolled conditions does not result in composted material. (Note: Various BMPs have restrictions on the percentage of biosolids in compost, or do not allow biosolids in compost.)

Compost maturity

Term used to define the effect that compost has on plant growth. Mature compost will enhance plant growth; immature compost can inhibit plant growth.

Compost stability

Level of microbial activity in compost that is measured by the amount of carbon dioxide produced by a sample in a sealed container over a given period of time.

Constructed conveyance system facilities

  • Gutters
  • Ditches
  • Pipes
  • Channels
  • Most flow control and water quality treatment facilities.

Constructed wetland

Those wetlands intentionally createdon sites that are not wetlands for the primary purpose of wastewater or stormwater treatment and managed as such. Constructed wetlands are normally considered as part of the stormwater collection and treatment system.

Construction Stormwater Pollution Prevention Plan (CSWPPP)

A document that describes the potential for pollution problems on a construction project and explains and illustrates the measures to be taken on the construction site to control those problems.


An imaginary line on the surface of the earth connecting points of the same elevation.

Control Manhole 

See Catch Basin, Type II


A mechanism for transporting water from one point to another, including pipes, ditches, and channels.

Conveyance System

The drainage facilities, both natural and man-made, which collect, contain, and provide for the flow of surface and stormwater from the highest points on the land down to a receiving water. The natural elements of the conveyance system include swales and small drainage courses, streams, rivers, lakes, and wetlands. The human-made elements of the conveyance system include gutters, ditches, pipes, channels, and most retention/detention facilities.

Critical Drainage Area

An area with such severe flooding, drainage, and/or erosion/sedimentation conditions which have resulted or will result from the cumulative impacts of development and urbanization, that the area has been formally adopted as a Critical Drainage Area by rule under the procedures specified in King County Code 2.98.


Pipe or concrete box structure which drains open channels, swales, or ditches under a roadway or embankment typically with no catch basins or manholes along its length.


Dead storage

The volume available in a depression in the ground below any conveyance system, or surface drainage pathway, or outlet invert elevation that could allow the discharge of surface and stormwater

Debris Barrier

A metal trash rack


Reduction of nitrate (commonly by bacteria) to di-nitrogen gas.


The biochemical reduction of nitrates or nitrites in the soil or organic deposits to ammonia or free nitrogen.

Depression storage

The amount of precipitation that is trapped in depressions on the surface of the ground.

Design storm

A prescribed hyetograph and total precipitation amount (for a specific duration recurrence frequency) used to estimate runoff for a hypothetical storm of interest or concern for the purposes of analyzing existing drainage, designing new drainage facilities or assessing other impacts of a proposed project on the flow of surface water. (A hyetograph is a graph of percentages of total precipitation for a series of time steps representing the total time during which the precipitation occurs.)


The release of stormwater runoff from the site at a slower rate than it is collected by the stormwater facility system, the difference being held in temporary storage such as in a stormwater vault or pond.

Detention facility

A facility that collects water from developed areas and releases it at a slower rate than it enters the collection system. The excess of inflow over outflow is temporarily stored in a pond or a vault and is typically released over a few hours or a few days.

Detention pond

A type of detention facility. A dug hole in the ground to hold water. Components typically include an inlet, settling bay, treatment bay, and outlet.

Detention tank

A type of detention facility. Tanks are more commonly constructed in high-value areas due to the expense of digging the hole and the cost of materials. They are typically made of metal. New technologies are also using plastic chambers.

Detention time

The theoretical time required to displace the contents of a stormwater treatment facility at a given rate of discharge (volume divided by rate of discharge).

Detention vault

A type of detention facility.

Direct discharge

Undetained discharge from a proposed project to a major receiving water.


Runoff leaving a new development or redevelopment via overland flow, built conveyance systems, or infiltration facilities. A hydraulic rate of flow, specifically fluid flow; a volume of fluid passing a point per unit of time, commonly expressed as cubic feet per second, cubic meters per second, gallons per minute, gallons per day, or millions of gallons per day.

Discharge point

The location where a discharge leaves the Permittee’s MS4 through the Permittee’s MS4 facilities/BMPs designed to infiltrate.

Dispersed discharge

Release of surface and storm water runoff from a drainage facility system such that the flow spreads over a wide area and is located so as not to allow flow to concentrate anywhere upstream of a drainage channel with erodible underlying granular soils or the potential to flood downstream properties.


Release of surface and stormwater runoff such that the flow spreads over a wide area and is located so as not to allow flow to concentrate anywhere upstream of a drainage channel with erodible underlying granular soils. Example see BMP T5.10B: Downspout Dispersion Systems.


A long narrow excavation dug in the earth for drainage with its top width less than 10 feet at design flow.


A change in the natural discharge location or runoff flows onto or away from an adjacent downstream property.


Department of Natural Resources and Parks


A buried pipe or other conduit (closed drain). A ditch (open drain) for carrying off surplus surface water or ground water.


Refers to the collection, conveyance, containment, and/or discharge of surface and stormwater runoff.

Drainage area or Drainage basin

An area draining to a point of interest.

Drainage facility

A constructed or engineered feature that collects, conveys, stores or treats surface and storm water runoff. Drainage facilities shall include but not be limited to all constructed or engineered streams, pipelines, channels, ditches, gutters, lakes, wetlands, closed depressions, flow control or water quality treatment facilities, erosion and sedimentation control facilities, and other drainage structures and appurtenances that provide for drainage.

Drainage pathway

The route that surface and stormwater runoff follows downslope as it leaves any part of the site.


As a natural condition of the soil, soil drainage refers to the frequency and duration of periods when the soil is free of saturation; for example, in well-drained soils the water is removed readily but not rapidly; in poorly drained soils the root zone is waterlogged for long periods unless artificially drained, and the roots of ordinary crop plants cannot get enough oxygen; in excessively drained soils the water is removed so completely that most crop plants suffer from lack of water. Strictly speaking, excessively drained soils are a result of excessive runoff due to steep slopes or low available water-holding capacity due to small amounts of silt and clay in the soil material. The following classes are used to express soil drainage:

  • Well drained - Excess water drains away rapidly and no mottling occurs within 36 inches of the surface.
  • Moderately well drained - Water is removed from the soil somewhat slowly, resulting in small but significant periods of wetness. Mottling occurs between 18 and 36 inches.
  • Somewhat poorly drained - Water is removed from the soil slowly enough to keep it wet for significant periods but not all of the time. Mottling occurs between 8 and 18 inches.
  • Poorly drained - Water is removed so slowly that the soil is wet for a large part of the time. Mottling occurs between 0 and 8 inches.
  • Very poorly drained - Water is removed so slowly that the water table remains at or near the surface for the greater part of the time. There may also be periods of surface ponding. The soil has a black to gray surface layer with mottles up to the surface.

Dry Season

May 1 to September 30.


Earth material

Any rock, natural soil or fill and/or any combination thereof. Earth material shall not be considered topsoil used for landscape purposes. Topsoil used for landscaped purposes shall comply with ASTM D 5268 specifications. Engineered soil/landscape systems are also defined independently.


The legal right to use a parcel of land for a particular purpose. It does not include fee ownership, but may restrict the owners use of the land.

Effective impervious area (EIA)

Subset of total impervious area that is hydrologically connected via sheet flow or discrete conveyance to a drainage system or receiving body of water. The Washington State Department of Ecology considers impervious areas in residential development to be ineffective if the runoff is dispersed through at least 100 feet of native vegetation using approved dispersion techniques.

Effective impervious surface

Those impervious surfaces that are connected via sheet flow or discrete conveyance to a drainage system. Impervious surfaces are considered ineffective if: 1) the runoff is dispersed through at least one hundred feet of native vegetation in accordance with BMP T5.30 – “Full Dispersion” as described in Chapter 5 of Volume V; 2) residential roof runoff is infiltrated in accordance with Downspout Full Infiltration Systems in BMP 5.10A Volume III; or 3) approved continuous runoff modeling methods indicate that the entire runoff file is infiltrated.


Environmental Impact Statement. A document that discusses the likely significant adverse impacts of a proposal, ways to lessen the impacts, and alternatives to the proposal. It is required by the national and state environmental policy acts when projects are determined to have the potential for significant environmental impact.


A structure of earth, gravel, or similar material raised to form a pond bank or foundation for a road.

Emerging plants

Aquatic plants that are rooted in the sediment but whose leaves are at or above the water surface. These wetland plants often have high habitat value for wildlife and waterfowl, and can aid in pollutant uptake.


Environmental Protection Agency


Endangered Species Act

Energy Dissipater

Any means by which the total energy of flowing water is reduced. In stormwater design, they are usually mechanisms that reduce velocity prior to, or at, discharge from an outfall in order to prevent erosion. They include rock splash pads, drop manholes, concrete stilling basins or baffles, and check dams.

Engineered soil—landscape system

This is a self-sustaining soil and plant system that simultaneously supports plant growth, soil microbes, water infiltration, nutrient and pollutant adsorption, sediment and pollutant biofiltration, water interflow, and pollution decomposition. The system shall be protected from compaction and erosion. The system shall be planted and/or mulched as part of the installation. The engineered soil/plant system shall have the following characteristics:

A. Be protected from compaction and erosion.

B. Have a plant system to support a sustained soil quality.

C. Possess permeability characteristics of not less than 6.0, 2.0, and 0.6 inches/hour for hydrologic soil groups A, B, and C, respectively (per ASTM D 3385). D is less than 0.6 inches/hour.

D. Possess minimum percent organic matter of 12, 14, 16, and 18 percent (dry-weight basis) for hydrologic soil groups A, B, C, and D, respectively (per ASTM D 2974)."


To raise value, desirability, or attractiveness of an environment associated with surface water.


The detachment and transport of soil or rock fragments by water, wind, ice, etc.

Erosion and Sedimentation Control

Any temporary or permanent measures taken to reduce erosion; control siltation and sedimentation; and ensure that sediment-laden water does not leave the site.


Erosion and Sediment Control

Evapotranspiration (ET)

The collective term for the processes of evaporation and plant transpiration by which water is returned to the atmosphere.


A condition of a water body in which excess nutrients, particularly phosphorous, stimulates the growth of aquatic plant life usually resulting in the depletion of dissolved oxygen. Thus, less dissolved oxygen is available to other aquatic life.


Refers to the process where nutrient over-enrichment of water leads to excessive growth of aquatic plants, especially algae.


The mechanical removal of earth material.


The downward movement of runoff through the bottom of an infiltration BMP into the soil layer or the downward movement of water through soil.



Federal Emergency Management Agency.


Any material or mixture used to supply one or more of the essential plant nutrient elements.


A deposit of earth material placed by artificial means.

Flow control BMP (or facility)

A drainage facility designed to mitigate the impacts of increased surface and stormwater runoff flow rates generated by development. Flow control facilities are designed either to hold water for a considerable length of time and then release it by evaporation, plant transpiration, and/or infiltration into the ground, or to hold runoff for a short period of time, releasing it to the conveyance system at a controlled rate.

Flow Control Facility

A drainage facility designed to mitigate the impacts of increased surface and stormwater runoff flow rates generated by development. Flow control facilities are designed either to hold water for a considerable length of time and then release it by evaporation, plant transpiration, and/or infiltration into the ground, or to hold runoff for a short period of time, releasing it to the conveyance system at a controlled rate. The size of the facility is required to be large enough to hold water from an area based on a specified design storm.