Commonly used terms in stormwater design, management, and planning are defined below to help provide a common understanding of these technical terms and phrases.


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

Algal Bloom

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. 

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.


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.

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.

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. 

Clean Water Act

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.

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.

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.


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.


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.

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.


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

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.

Frequency of Storm (design storm frequency)

The anticipated period in years that will elapse, based on average probability of storms in the design region, before a storm of a given intensity and/or total volume will recur; thus a 10-year storm can be expected to occur on the average once every 10 years, or has a 10% chance of occurring in any year. Sewer pipes designed to handle flows from a 10-year storm would be expected to backup by any storms of greater amount or intensity.

Green Roofs (also known as vegetated roofs)

Bioretention systems placed on roof surfaces that capture and temporarily store rainwater in a soil medium. They consist of a layered system of roofing designed to support plant growth and retain water for plant uptake while preventing ponding on the roof surface.

Illicit Discharge

All non-stormwater discharges to stormwater drainage systems that cause or contribute to a violation of state water quality, sediment quality or ground water quality standards, including but not limited to sanitary sewer connections, industrial process water, interior floor drains, car washing, and greywater systems.

Impervious Surface 

A non-vegetated surface area which either prevents or retards the entry of water into the soil mantle as under natural conditions prior to development. A non-vegetated surface area which causes water to run off the surface in greater quantities or at an increased rate of flow from the flow present under natural conditions prior to development. Common impervious surfaces include, but are not limited to, roof tops, walkways, patios, driveways, parking lots or storage areas, concrete or asphalt paving, gravel roads, packed earthen materials, and oiled, macadam or other surfaces which similarly impede the natural infiltration of stormwater.

Infiltration Basins/Facilities/Ponds

Shallow depressions filled with grass or other natural vegetation that capture runoff from adjoining areas and allow it to infiltrate into the soil. They provide storage volume and additional time for captured runoff to infiltrate the native soil below.

Low Impact Development (LID)

A stormwater and land use management strategy that strives to mimic natural (pre-disturbance) hydrologic processes of infiltration, filtration, storage, evaporation and transpiration by emphasizing conservation, use of on-site natural features, site planning, and distributed stormwater management practices that are integrated into a project design. Stormwater LID options include dispersion trenches, infiltration trenches, and bioretention.

Native Vegetation (Western Washington)

Vegetation comprised of plant species, other than noxious weeds, that are indigenous to the coastal region of the Pacific Northwest and which reasonably could have been expected to naturally occur on the site. Examples include trees such as Douglas fir, western hemlock, western red cedar, alder, big-leaf maple, and vine maple; shrubs such as willow, elderberry, salmonberry and salal; and herbaceous plants such as sword fern, foam flower, and fireweed. King County native plants information, start at the King Conservation District.

Porous Pavement (aka pervious pavement)

Excavated areas filled with gravel and paved over with a porous concrete or pervious asphalt mix or with modular porous blocks. Normally all rainfall will immediately pass through the pavement into the gravel storage layer below it where it can infiltrate at natural rates into the site's native soil. Example see BMP T5.15: Permeable Pavements.

Pervious Surface

Any surface material that allows stormwater to infiltrate into the ground. Examples include lawn, landscape, pasture, native vegetation areas, and permeable pavements.

Pre-developed Condition

The native vegetation and soils that existed at a site prior to the influence of Euro-American settlement. The pre-developed condition shall be assumed to be forested land cover unless reasonable, historic information is provided that indicates the site was prairie prior to settlement.

Rain Gardens

Hallow depressions filled with a soil mix that supports vegetative growth. They provide opportunity to store and infiltrate captured runoff and retain water for plant uptake. They are commonly used on individual home lots to capture roof runoff. Example, see BMP T5.14: Rain Gardens.

Rainwater Harvesting (Rain Barrels or Cisterns)

Containers that collect roof runoff collect runoff from rooftops and convey it to a tank where it can be used for non-potable water uses and onsite infiltration. Cisterns may be located above or below ground and have a greater storage capacity than rain barrels.

Replaced Impervious Surface

Stormwater management is required not only for new impervious areas, but also for replaced areas. Municipalities may exempt a project from requiring stormwater management if the amount of impervious (new or replaced) is under 2,000 square feet, but several jurisdictions have lowered this threshold. For structures, the removal and replacement of impervious surfaces down to the foundation. For other impervious surfaces, the removal down to bare soil or base course and replacement.


The renovation of an existing structure or facility to meet changed conditions or to improve performance. This is becoming a more common approach to help improve stormwater designs of older developments when they either did not have any formal stormwater controls, or used older design standards (typically considered pre-1998 development standards for flow control, or 2007 for low impact development features).

Rooftop (Downspout) Disconnection

This practice allows rooftop rainwater to discharge to pervious landscaped areas and lawns instead of directly into storm drains. It can be used to store stormwater and/or allow stormwater to infiltrate into the soil.


Water originating from rainfall and other precipitation that is found in drainage facilities, rivers, streams, springs, seeps, ponds, lakes and wetlands as well as shallow ground water.


That portion of precipitation that does not naturally percolate into the ground or evaporate, but flows via overland flow, interflow, pipes and other features of a stormwater drainage system into a defined surface waterbody, or a constructed stormwater facility.

Stormwater Facility

A constructed component of a stormwater drainage system, designed or constructed to perform a particular function, or multiple functions. Stormwater facilities include, but are not limited to, pipes, swales, ditches, culverts, street gutters, detention ponds, retention ponds, constructed wetlands, infiltration devices, catch basins, oil/water separators, and biofiltration swales.


Dispersion or scattering of light in a liquid, caused by suspended solids and other factors; commonly used as a measure of suspended solids in a liquid.

Water Quality

A term used to describe the chemical, physical, and biological characteristics of water, usually in respect to its suitability for a particular purpose.

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