Stormwater Glossary


Impervious Surface: Hard, manmade surfaces which stormwater cannot infiltrate. This lack of natural infiltration leads to the need for stormwater quality facilities, flow control facilities, and infiltration facilities.

Stormwater: Any precipitation or snowmelt which runs off impervious surfaces such as rooftops, paved streets, gravel, and parking lots. Stormwater often picks up pollutants from these surfaces before being discharged to local waterbodies, which is why it is regulated and requires treatment.

Storm Drain (aka Catchbasin, Inlet): A grated opening through which stormwater drains from a street or parking lot to a pipe system, which is often connected to a treatment facility. Storm drains are susceptible to clogging and must have regular sediment and debris removal in order to prevent localized flooding. These features also usually have a fish plaque or fish stencil to discourage pollution and remind the public that they convey stormwater to natural creeks, rivers, wetlands, and groundwater sources.

Urban Stream: Any creek or river which travels through a developed city or town. Urban streams face many water quality and flooding challenges that streams in undeveloped, natural areas do not.

Watershed (aka Basin, Drainage Basin, Catchment): Any area of land which drains to a common creek, river, or lake. Watersheds are based on stream order and can be broken into smaller subwatersheds— depending on the first stream the precipitation enters—or larger regional basins, based on which main river the precipitation eventually travels to.


Bioswale (aka Biofiltration Swale):  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. The only difference between a bioswale and a ditch is that a bioswale is engineered to suspend water for a specific period of time so that pollutants can settle out of the water naturally.

Filter System:  An engineered, flow-through stormwater quality facility. Filter systems are comprised of rechargeable cartridges and work by conveying stormwater through granular media in the cartridges, which traps particulates and absorbs pollutants such as dissolved metals and oils.

Mechanical Stormwater Quality Facility:  Any mechanical structure which utilizes engineering and hydraulics to remove pollutants from stormwater before conveying the treated water to a piped system, infiltration facility, or natural waterway. Mechanical stormwater quality facilities are usually located underground in manholes, vaults, or storm drains.

Oil Water Separator:  An engineered structure which treats stormwater by trapping oil at the surface of the water and sediment at the bottom, allowing treated water to pass through.

Planter (aka Stormwater Planter, Planter Box, Flow-Through Planter):  A contained, vegetated area that collects and filters water through various layers of vegetation and soils. Planters are commonly seen in cities along streets and sidewalks. The main difference between a bioswale and a planter is that a planter is enclosed by concrete.

Sedimentation Manhole:  A manhole which is constructed with its own sump in order to capture sediment and pollutants from stormwater as it travels through the piped system.

Stormwater Quality Facility:  Stormwater infrastructure that is designed to remove pollutants from stormwater before it is discharged to a local stream or groundwater source. A stormwater quality facility can be vegetated, like a bioswale, or mechanical, like an underground filter vault. All stormwater quality facilities are designed to slow and filter water for a specific period of time, during which pollutants and sediment can settle out before the treated water is discharged to local waterways. Stormwater quality facilities must be inspected annually by cities in Washington.

Vegetated Stormwater Quality Facility (aka Green Stormwater Infrastructure, Low Impact Development):  An area which is designed to mimic natural landscapes using grasses, wetland plants, and soils to absorb stormwater or capture pollutants through filtering processes.

Vortex System (aka Hydrodynamic Separator):  An engineered structure consisting of a cylindrical vessel in which stormwater spirals like a whirlpool, causing heavier particles to settle out in separate compartments.


Detention Pond (aka Dry Pond):  An excavated basin installed to ensure treatment of polluted stormwater, to protect against urban streambank erosion, and to minimize urban flooding risk. Detention pond inlets and outlets are engineered to slowly release stormwater into treatment facilities or urban streams, and generally only fill with stormwater in large precipitation events.

Detention Vault (aka Inline Storage, Detention Chamber):  A large tank or series of tanks which are installed underground in order to slow and regulate the flow of stormwater. Detention vaults function as enclosed, underground detention ponds in a sense.

Flow Control Facility:  An engineered structure which is designed to significantly slow the flow of stormwater in large precipitation events, decreasing erosion, pollution, and potential flooding in urban streams and on nearby properties. In a natural setting, less than 10% of all precipitation reaches a stream; in a town or city, however, over 40% of total precipitation can be rapidly discharged to a stream, due to impervious surfaces such as streets and roofs that prevent infiltration. Flow control facilities can be vegetated or mechanical, and must be inspected annually by cities in Washington. 

Mechanical Flow Control Facility (aka Flow Control Manhole):  An engineered, underground facility which slows the flow of stormwater through a treatment system. This is usually accomplished by installing outlet pipes that have smaller diameters than inlet pipes.

Retention Pond (aka Wet Pond):  Similar to a detention pond, except that a retention pond holds a permanent or semi-permanent pool of water. When precipitation occurs and new water flows into the facility, the water that was previously retained is discharged from the facility.

Vegetated Flow Control Facility:  Any facility which utilizes soil, grading, and vegetation to slow the flow of stormwater to natural waterways. The slowing of stormwater flow reduces flooding and stream erosion, and ensures the treatment of polluted stormwater before it is discharged to a natural waterway.


Drywell (aka Injection Well, Underground Injection Control Well, UIC):  A porous-walled, underground structure that allows surface runoff and stormwater to soak into the surrounding ground. Drywells are subject to registration and regulation in Washington.

Infiltration Facility:  Any vegetated or mechanical structure which is designed to convey stormwater to underlying soils, water tables, or aquifers. Infiltration facilities aid in groundwater recharge and lessen flooding risks by reducing surface water runoff in large precipitation events. Infiltration facilities must be inspected annually by cities in Washington.

Infiltration Pond (aka Infiltration Basin):  A vegetated, open depression in the landscape which is engineered to soak up stormwater and reduce urban flooding risk. The main difference between an infiltration pond and a rain garden is size; infiltration ponds are generally much larger in area.

Infiltration Trench:  A shallow, excavated trench filled with gravel or stone that is designed to absorb and convey stormwater to underlying soils and groundwater sources.

Permeable Pavement (aka Pervious Pavement, Porous Pavement, Paver Blocks, Permeable Asphalt, Permeable Concrete):  Pavement which is engineered to absorb stormwater and convey it below to soil and groundwater sources. Common subtypes include pervious concrete and pervious asphalt, through which stormwater flows directly, and paver blocks, which are installed with spaces between blocks, through which stormwater flows.

Rain Garden (aka Bioretention Cell):  A designed, depressed area in a landscape that soaks up stormwater and conveys it to underlying soils and groundwater sources. Rain gardens often use engineered bioretention soil to achieve higher infiltration rates.


Clean Water Act (CWA):  Comprehensive federal legislation passed in 1972 in order to protect and improve water quality in the United States. It is the overarching law through which most federal, state, and local water quality regulations are enforced, including the NPDES.

Critical Drainage Area (CDA):  An area that has been designated by the State of Washington to have severe flooding, drainage, erosion, and/or sedimentation conditions which have resulted or will result from the cumulative impacts of development and urbanization.

Endangered Species Act (ESA):  Federal legislation passed in 1973 which provides a program for the conservation of threatened and endangered plants and animals.

Fish Bearing Stream:  Washington State Department of Ecology classification used to denote the presence of native fish species and provide additional land use protections to a particular stream or stream reach. Every named creek or river in the City of Washougal is a Fish Bearing Stream.

Illicit Discharge:  Any discharge of non-stormwater or solid debris to the stormwater drainage system. Illicit discharges can include illegal dumping of chemicals or pollutants into the stormwater system, or can include discharges from internal floor drains, appliances, industrial processes, sinks, and toilets.

Municipal Separate Storm Sewer System (MS4):  A municipal stormwater system which is not connected to a municipal sewer system. MS4’s are more regulated than combined systems because stormwater is not treated at a central treatment plant—as it is in a combined system—and is discharged to streams, lakes, and groundwater sources.

Municipal Stormwater Permit:  A combination of state and federal water quality regulations through which cities and counties are required to regulate discharges from MS4’s to receiving waters. There are separate permits for Eastern and Western Washington, and separate permits for large (Phase I, 100,000+ people) and small (Phase II, 10,000+ people) municipalities and counties. The City of Washougal is a Phase II Western Washington Permittee.

National Pollutant Discharge Elimination System (NPDES):  A permit-based program which arose from the Clean Water Act, designed to regulate point sources that discharge pollutants to the waters of the United States. The City of Washougal is both a Municipal Stormwater and Municipal Wastewater Permittee.

Total Maximum Daily Load (TMDL):  A regulatory term describing the maximum amount of a certain pollutant that an impaired waterbody can receive and still meet water quality standards.

Water Quality Standards for Surface Waters (aka WAC 173-201A):  State of Washington Administration Code which defines surface water quality standards. The Washington State Department of Ecology administers and enforces this legislation along with the NPDES program through Municipal Stormwater Permits.

Wellhead Protection Area (WPA):  A surface and subsurface land area regulated by the State of Washington to prevent contamination of wells or well-fields supplying a public water system.

UIC Program:  State of Washington program which is designed to protect groundwater by regulating the discharge of fluids from UIC injection wells (primarily drywells).


Aquifer:  A contained, underground, water-bearing rock formation.

Best Management Practice (BMP):  A term for stormwater quality and flow control infrastructure. BMP’s vary significantly and can target specific pollutants, a range of pollutants, and/or reduce flooding and stream erosion risks.

Ecosystem:  A community of living organisms and their physical environment.

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

Erosion:  Any surface process that removes soil, rock, or dissolved material from one location.

Fish Passage Barrier:  A manmade structure which stops or impedes aquatic species from moving upstream, often to formerly natural spawning grounds.

Floodplain:  An area of low-lying ground adjacent to a river, formed mainly of river sediments and subject to natural flooding.

Groundwater:  Water present beneath Earth’s surface in soil pore spaces, below soils (water tables), or in the fractures of rock formations (aquifers).

Low Impact Development (aka LID, Green Stormwater Infrastructure):  Small-scale, engineered hydrologic systems designed to mimic pre-development conditions by infiltrating or filtering stormwater. These systems are often designed to be vegetated to promote filtration and pollutant removal. The State of Washington recently prioritized LID stormwater design and construction and required municipalities to integrate LID practices into their city codes.

Manhole:  An underground concrete chamber which can contain mechanical stormwater quality infrastructure.

Nonpoint Source Pollution (NPSP):  A combination of pollutants from a large area, rather than from a specific, identifiable source. In most cases, urban stormwater is considered NPSP and is therefore regulated by federal and state agencies.

Outfall:  The point at which stormwater leaves a piped system and is discharged to a stream, lake, or groundwater source. Outfalls must be inspected annually by cities in Washington.

Point Source Pollution (PSP):  Pollution which comes from a single point, such as a factory or wastewater treatment plant. Point source pollution is regulated by federal and state agencies.

Receiving Waters:  Any creek, river, wetland, pond, lake, or aquifer which receives stormwater runoff or treated wastewater.

Riparian Zone (aka Riparian Area, Stream Buffer):  The interface between land and a stream or wetland. These areas can be dry, partially saturated, or underwater, depending on the local topography, ecosystem, climate, and season.

Sedimentation:  Any surface process that deposits soil, rock, or dissolved material in one location.

Vault:  An underground concrete chamber which can contain mechanical stormwater quality infrastructure. Vault chambers are larger than manhole chambers, and usually contain more than one manhole lid for access.

Wetland:  A distinct ecosystem that is flooded by water, either permanently or seasonally, and contains hydric soils and/or aquatic vegetation.

Wetland Buffer:  A setback area between a stream, river, or wetland and a developed area. Wetland buffers are designated by the Washington State Department of Ecology and help maintain natural vegetation and wildlife along waterways.