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Use the links below to navigate to the terms and acronyms that begin with that letter.

Numerical terms

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.


A geologic stratum containing groundwater that can be withdrawn and used for human purposes.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. Basins of interest to King County staff are those that drain either to the Cedar, Green, Snoqualmie, Skykomish, or White rivers, or the drainage areas which drain directly to Puget Sound.

Basin plan

A plan and all implementing regulations and procedures including but not limited to capital projects, public education activities, land use management regulations adopted by ordinance for managing surface and storm water management facilities, and features within individual subbasins.

Beaver deceiver

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

Read more about beaver-related problems and solutions


A benefit is a net project outcome and is usually defined in monetary terms. Benefits may include direct and indirect effects. For the purposes of benefit-cost analysis of proposed mitigation measures, benefits are limited to specific, measurable, risk reduction factors, including reduction in expected property losses (buildings, contents, and functions) and protection of human life

Benefit/Cost Analysis

A benefit/cost analysis is a systematic, quantitative method of comparing projected benefits to projected costs of a project or policy. It is used as a measure of cost effectiveness.


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.

View an example sketch of a bioswale (501KB)


A stormwater management system, often termed a rain garden, that uses soil and plants to reduce the volume of surface water runoff and to capture pollutants. Bioretention systems with liners and underdrains may more properly be called bio-detention, since they may not retain a significant fraction of the runoff.

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.



Concentrated Animal Feeding Operation

Catch Basin, Type I

An underground concrete water receiving inlet, rectangular in shape (approximately 3' x 2' x 4' deep) with a slotted iron grate on top to inlet water or a solid rectangular cover. Water may also enter/exit through culverts visible in the side walls of basin. Also referred to as Inlet.

View an example sketch of a Type I catch basin (444KB)

Catch Basin, Type II 

A round concrete underground basin (4'-8' diameter; 6' or greater deep); may contain Flow Restrictor/Oil Pollution control device. These basins are also required when larger diameter culverts are used. Also referred to as a Manhole or Control Manhole.

View an example sketch of a Type II catch basin (463KB)

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.


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.

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.

Channel stabilization

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

Channel storage

Water temporarily stored in channels while enroute to an outlet.

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

A naturally occurring, localized area of clay which acts as an impermeable layer to runoff infiltration.


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.


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 KCC 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.


Clean Water Act. The federal environmental law that includes the management of stormwater.

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


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.

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.

View an example sketch of a detention pond (770KB)

Detention tank

A type of detention facility.

View an example sketch of a detention tank (580KB)

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.

View an example sketch of a detention vault (506KB)

Determination of Non-Significance or DNS

The written decision by the responsible official of the lead agency that a proposal is not likely to have a significant adverse environmental impact per the SEPA process, and therefore an EIS is not required.

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.


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


Local Services - Permits


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 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.


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 storm water runoff generated by site development pursuant to the drainage requirements in King County Code Chapter 9.04. 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 a short period of time and then release it to the conveyance system.

Flow control standards

Section 3.1.2 of the 1998 King County SWDM describes Level 1, Level 2, and Level 3 flow control standards. The level of flow control required is based on the resource value of the receiving system in terms of its hydrology, ecology, geology and water quality. More....

Flow duration

The aggregate time that peak flows are at or above a particular flow rate of interest. For example, the amount of time that peak flows are at or above 50% of the 2-year peak flow rate for a period of record.

Flow frequency

The inverse of the probability that the flow will be equaled or exceeded in any given year (the exceedance probability). For example, if the exceedance probability is 0.01 or 1 in 100, that flow is referred to as the 100-year flow.

Flow path

The route that stormwater runoff follows between two points of interest.

Flow restrictor

A control device or a T section with a specifically sized orifice(s) to control release rates.

View an example sketch of a flow restrictor T section (631KB)


The vertical distance between the highest designed water surface elevation and the elevation of the crest of the facility. For example, in pond design, freeboard is the vertical distance between the emergency overflow water surface and the top of the pond embankment.


Flow Restrictor/Oil Pollution control device.



The slope of a road, channel, or natural ground. The finished surface of a canal bed, roadbed, top of embankment, or bottom of excavation; any surface prepared for the support of construction such as paving or the laying of a conduit.

Grassed waterway

A natural or constructed waterway, usually broad and shallow, covered with erosion-resistant grasses, used to conduct surface water from an area at a reduced flow rate. See also biofilter.

Green roof or vegetated roof

An engineered roof cover system that includes plants, a storage medium and a drainage layer. Green roof systems reduce runoff volume by evapotranspiration and provide detention storage that reduces flow rates.

Ground water

Water in a saturated zone or stratum beneath the land surface or a surface waterbody.

Ground water table

The free surface of the ground water, that surface subject to atmospheric pressure under the ground, generally rising and falling with the season, the rate of withdrawal, the rate of restoration, and
other conditions. It is seldom static.

Ground water recharge

Inflow to a ground water reservoir.


Groundwater is the water beneath the surface that often collects between soil and rock – it reappears on the surface through wells and springs.





The specific area or environment in which a particular type of plant or animal lives and grows.

Hard surface

An impervious surface, a permeable pavement, or a vegetated roof.


A cemented or compacted and often clay-like layer of soil that is impenetrable by roots.

Harmful pollutant

A substance that has adverse effects to an organism including death, chronic poisoning, impaired reproduction, cancer, or other effects.

Heavy metals

Metals of high specific gravity, present in municipal and industrial wastes, that pose long-term environmental hazards. Such metals include:

  • Cadmium
  • Chromium
  • Cobalt
  • Copper
  • Lead
  • Mercury
  • Nickel
  • Zinc

Hog fuel

Wood-based mulch


Hydraulic residence time


Organic matter in or on a soil, composed of partly or fully decomposed bits of plant tissue or from animal manure.

Hydraulic conductivity

The quality of saturated soil that enables water or air to move through it. Also known as permeability coefficient.


Means the dynamic energy, force, or motion of fluids as affected by the physical forces acting upon those fluids.


A graph of runoff rate, inflow rate or discharge rate, past a specific point over time.

Hydrologic cycle

The circuit of water movement from the atmosphere to the earth and return to the atmosphere through various stages or processes such as precipitation, interception, runoff, infiltration, percolation, storage, evaporation, and transpiration.

Hydrologic soil groups

A soil characteristic classification system defined by the U.S. Soil Conservation Service in which a soil may be categorized into one of four soil groups (A, B, C, or D) based upon infiltration rate and other properties.

Type A: Low runoff potential. Soils having high infiltration rates, even when thoroughly wetted, and consisting chiefly of deep, well drained to excessively drained sands or gravels. These soils have a high rate of water transmission.

Type B: Moderately low runoff potential. Soils having moderate infiltration rates when thoroughly wetted, and consisting chiefly of moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission.

Type C: Moderately high runoff potential. Soils having slow infiltration rates when thoroughly wetted, and consisting chiefly of soils with a layer that impedes downward movement of water, or soils with moderately fine to fine textures. These soils have a slow rate of water transmission.

Type D: High runoff potential. Soils having very slow infiltration rates when thoroughly wetted, and consisting chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a hardpan, till, or clay layer at or near the surface, soils with a compacted subgrade at or near the surface, and shallow soils or nearly impervious material. These soils have a very slow rate of water transmission.1

1 Vladimir Novotny and Harvey Olem. Water Quality Prevention, Identification, and Management of Diffuse Pollution, Van Nostrand Reinhold: New York, 1994, p. 109.

Hydrologic Simulation Program-Fortan (HSPF)

A continuous simulation hydrologic model that transforms an uninterrupted rainfall record into a concurrent series of runoff or flow data by means of a set of mathematical algotrithms which represent the rainfall-runoff process at some conceptual level.

Hydrologically functional landscape

Term used to describe a design approach for the built environment that attempts to more closely mimic the overland and subsurface flow,  nfiltration, storage, evapotranspiration, and time of concentration characteristic of the native landscape of the area.


Hydrology is the analysis of waters of the earth. For example, a flood discharge estimate is developed by conducting a hydrologic study. Incorporated Areas. Incorporated areas are those areas that exist within a city or a city's jurisdiction. King County contains 39 whole incorporated cities and parts of two others.


Illicit discharges

Discharges of non-stormwater to the storm drainage system. Examples are discharges from internal floor drains, appliances, industrial processes, sinks, and toilets that are connected to the nearby storm drainage system. These discharges should be going to the sanitary sewer system, a holding tank, an on-site process water treatment system, or a septic system.


A surface which cannot be easily penetrated. For instance, rain does not readily penetrate paved surfaces.

Impervious surface

A hard surface area which either prevents or retards the entry of water into the soil mantle as under natural conditions prior to development; and/or a hard 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 surface and storm water runoff. Open, uncovered flow control or water quality treatment facilities shall not be considered impervious surfaces for determinations of thresholds. For the purpose of modeling though, onsite flow control and water quality ponds are modeled as impervious surface per Chapter 3 of the King County Surface Water Design Manual.


A natural or man-made containment for surface water.


Streets (with or without curbs or gutters), sidewalks, crosswalks, parking lots, water mains, sanitary and storm sewers, drainage facilities, street trees, and other appropriate items.


To allow water to soak into the ground (commonly referred to as percolation) to dispose of surface and storm water runoff.

Infiltration/Inflow (I/I)

Clean storm and/or groundwater that enters the sewer system through cracked pipes, leaky manholes, or improperly connected storm drains, down spouts and sump pumps. Most inflow comes from stormwater and most infiltration comes from groundwater. I/I affects the size of conveyance and treatment systems and, ultimately, the rate businesses and residents pay to operate and maintain them.

Learn more about infiltration and inflow

Infiltration basin

A type of infiltration facility.

View an example sketch of an infiltration basin (432KB)

Infiltration facility

A drainage facility designed to use the hydrologic process of water soaking into the ground (commonly referred to as percolation) to dispose of surface and storm water runoff.

Infiltration pond

A type of infiltration facility.

View an example sketch of an infiltration facility (608KB)

Infiltration rate

The rate, usually expressed in inches/hour, at which water moves downward (percolates) through the soil profile. Short-term infiltration rates may be inferred from soil analysis or derived from field
measurements. Long-term infiltration rates are affected by variability in soils and subsurface conditions at the site, the effectiveness of pretreatment or influent control, and the degree of long-term
maintenance of the infiltration facility.

Infiltration tank 

A type of infiltration facility

View an example sketch of an infiltration tank (665KB)


The points of access to and from a property


A form of connection between surface of the ground and a drain or sewer for the admission of surface and stormwater runoff.


See Catch Basin, Type I

In-line bioretention

Bioretention area that has a separate inlet and outlet.

Interception (hydraulics)

The process by which precipitation is caught and held by foliage, twigs, and branches of trees, shrubs, and other vegetation. Often used for "interception loss" or the amount of water evaporated from the precipitation intercepted.


That portion of rainfall that infiltrates into the soil and moves laterally through the upper soil horizons until intercepted by a stream channel or until it returns to the surface for example, in a roadside ditch, wetland, spring or seep. Interflow is a function of the soil system depth, permeability, and water-holding capacity.




King County Road Standards, which are available from the King County Department of Transportation.


Lag time

The interval between the center of mass of the storm precipitation and the peak flow of the resultant runoff.


An area permanently inundated by water in excess of two meters (7 ft) deep and greater than twenty acres in size as measured at the ordinary high-water mark.

Level spreader

A temporary ESC device used to spread out stormwater runoff uniformly over the ground surface as sheet flow (i.e., not through channels). The purpose of level spreaders is to prevent concentrated, erosive flows from occurring, and to enhance infiltration.

LID principles

Land use management strategies that emphasize conservation, use of onsite natural features, and site planning to minimize impervious surfaces, native vegetation loss, and stormwater runoff.

Low impact development (LID)

The use of site design and on-site Best Management Practices (BMPs) for the purpose of limiting surface water runoff and pollutant generation from a development site in order to more closely mimic the flow regime and water quality parameters found on an undisturbed site.

Low impact development best management practices (LID BMP)

Distributed stormwater management practices, integrated into a project design, that emphasize pre-disturbance hydrologic processes of infiltration, filtration, storage, evaporation and transpiration. LID BMPs include, but are not limited to:
  • Bioretention
  • Rain gardens
  • Permeable pavements
  • Roof downspout controls
  • Dispersion
  • Soil quality and depth
  • Minimal excavation foundations
  • Vegetated roofs
  • Water re-use.

Low permeability liner

A layer of compacted till, compacted clay, concrete, or a geomembrane.



Maintenance is the usual acts to prevent a decline, lapse or cessation from a lawfully established condition without any expansion of or significant change from that originally established condition. Activities within landscaped areas within areas subject to native vegetation retention requirements may be considered "maintenance" only if they maintain or enhance the canopy and understory cover. "Maintenance" includes repair work but does not include replacement work. When maintenance is conducted specifically in accordance with the Regional Road Maintenance Guidelines, the definition of "maintenance" in the glossary of those guidelines supersedes the definition of "maintenance" in this section.


See Catch Basin, Type II


Maximum extent practicable


Elements, such as mercury, lead, nickel, zinc, and cadmium, which are of environmental concern because they do not degrade over time. Although many are necessary nutrients, they are sometimes magnified in the food chain, and they can be toxic to life in high enough concentrations. They are also referred to as heavy metals.


The lower trophic levels of the soil food web. They are normally considered to include bacteria, fungi, flagellates, amoebae, ciliates, and nematodes. These in turn support the higher trophic levels, such as mites and earthworms. Together they are the basic life forms that are necessary for plant growth. Soil microbes also function to bioremediate pollutants such as petroleum, nutrients, and pathogens.


A layer of organic material or aggregate applied to the surface of soil.

Its purpose is any or all of the following:

  • To conserve soil moisture or temperature
  • To improve the fertility and health of the soil
  • To reduce weed growth
  • To hold fertilizer, seed, and soil in place
  • To enhance the visual appeal of the area

Types of mulches used in this manual include:

  • Chipped site
  • Vegetation
  • Compost
  • Hydromulch
  • Wood-based or wood straw
  • Wood strand
  • Straw
  • Aggregate


Municipal separate storm sewer systems. The regulatory requirements vary depending on the size of the system - small, medium, or large


A small MS4 is defined as any MS4 that is not a medium or large MS4 covered by Phase I of the NPDES Storm Water Program. Only some small MS4s are regulated.

Learn more about regulatory applicability from the Environmental Protection Agency


A medium MS4 is a system that is located in an area with a population between 100,000 - 249,999


A large MS4 is a system that is located in an area with a population of 250,000 or more. King County is a large MS4. So are the Cities of Tacoma and Seattle.


Other MS4s can be any size. These systems were designated by NPDES permitting authorities due to interrelationships with other MS4s. The Washington State Department of Transportation is an Other MS4.


Symbiotic association of the mycelium of a fungus with the roots of a seed plant.


Natural conveyance system elements

Swales and small drainage courses, streams, rivers, lakes, and wetlands.

Natural onsite drainage feature

A natural swale, channel, stream, closed depression, wetland, or lake.


Neighborhood Drainage Assistance Program.

Learn more about NDAP

Nitrate (NO3)

A form of nitrogen which is an essential nutrient to plants. It can cause algal blooms in water if all other nutrients are present in sufficient quantities. It is a product of bacterial oxidation of other forms of nitrogen, from the atmosphere during electrical storms and from fertilizer manufacturing.


The biochemical oxidation process by which ammonia is changed first to nitrites and then to nitrates by bacterial action, consuming oxygen in the water.


Usually ammonium, nitrite, and nitrate ions, and certain simple amines available for plant growth. A small fraction of organic or total nitrogen in the soil is available at any time.


Notice of intent

Nonpoint source (NPS) pollution

NPS pollution occurs when rainfall, snowmelt, or irrigation runs over land or through the ground, picks up pollutants, and deposits them into rivers, lakes, and coastal waters or introduces them into ground water.

Learn more from the Environmental Protection Agency factsheet

Nonstructural BMP

A preventative action to protect receiving water quality that does not require construction. Nonstructural BMPs rely predominantly on behavioral changes in order to be effective. Major categories of non-structural BMPs include education, recycling, maintenance practices and source controls.


National Pollutant Discharge Elimination System. The part of the Clean Water Act which requires point source discharges to obtain permits. These permits, referred to as NPDES permits, are administered by the Washington State Department of Ecology.


Essential chemicals needed by plants or animals for growth. Excessive amounts of nutrients can lead to degradation of water quality and algal blooms. Some nutrients can be toxic at high concentrations.


Off-line bioretention

Bioretention area where water enters and exits through the same location.

Off-line facilities

Water quality treatment facilities to which stormwater runoff is restricted to some maximum flow rate or volume by a flow-splitter.

Oil/water separator

A vault, usually underground designed to provide a quiescent environment to separate oil from water. Floatables (e.g., styrofoam) are also removed.

On-line facilities

Water quality treatment facilities which receive all of the stormwater runoff from a drainage area. Flows above the water quality design flow rate or volume are passed through at a lower percent removal

On-site stormwater management BMPs

As used in this manual, a synonym for Low Impact Development best management practices.


Operation and maintenance

Organic matter

Organic matter as decomposed animal or vegetable matter. It is measured by ASTM D 2974. Organic matter is an important reservoir of carbon and a dynamic component of soil and the carbon cycle. It improves soil and plant efficiency by improving soil physical properties including drainage, aeration, and other structural characteristics. It contains the nutrients, microbes, and higher-form
soil food web organisms necessary for plant growth. The maturity of organic matter is a measure of its beneficial properties. Raw organic matter can release water-soluble nutrients (similar to chemical fertilizer). Beneficial organic matter has undergone a humification process either naturally in the environment or through a composting process.


A point where collected and concentrated surface and storm water runoff is discharged from a pipe system or culvert.


A pipeline or conduit device, together with an outlet pipe, that provides for the discharge of portions of combined sewer flows into receiving waters or other points of disposal, after a regular device has allowed the portion of the flow which can be handled by interceptor sewer lines and pumping and treatment facilities to be carried by and to such water pollution control structures.

Overflow rate

Detention basin release rate divided by the surface area of the basin. It can be thought of as an average flow rate through the basin.


To flow over the limits of a containment or conveyance element.



The movement of water through soil

Percolation rate

The rate, often expressed in minutes/inch, at which clear water, maintained at a relatively constant depth, will seep out of a standardized test hole that has been previously saturated. The term
percolation rate is often used synonymously with infiltration rate (short-term infiltration rate).

Permanent Stormwater Control (PSC) Plan

A plan which includes permanent BMPs for the control of pollution from stormwater runoff after construction and/or land disturbing activity has been completed.

Permeable pavement

Pervious concrete, porous asphalt, permeable pavers, or other forms of pervious or porous paving material intended to allow passage of water through the pavement section. It often includes an aggregate base that provides structural support and acts as a stormwater reservoir.

Permeable soils

Soil materials with a sufficiently rapid infiltration rate so as to greatly reduce or eliminate surface and stormwater runoff. These soils are generally classified as SCS hydrologic soil types A and B.

Pervious surface

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


Related to the size and continuity of void spaces in soils; related to a soil's infiltration rate.

Phase 1 Stormwater Permit Program

The Phase I program addressed sources of storm water runoff that had the greatest potential to negatively impact water quality. Under Phase I, EPA required NPDES permit coverage for storm water discharges from "medium" and "large" municipal separate storm sewer systems (MS4s) located in incorporated places or counties with populations of 100,000 or more; and eleven categories of industrial activity, one of which is construction activity that disturbs five or more acres of land.

Visit the Environmental Protection Agency for more information about the Phase 1 Stormwater Permit Program

Phase 2 Stormwater Permit Program

The Phase II Program requires NPDES permit coverage for storm water discharges from certain regulated small municipal separate storm sewer systems (MS4s); and construction activity disturbing between 1 and 5 acres of land.

Visit the Environmental Protection Agency for more information about the Phase 2 Stormwater Permit Program

Point discharge

The release of collected and/or concentrated surface and storm water runoff from a pipe, culvert, or channel.

Point source pollutant

Storm water discharges are generated by runoff from land and impervious areas such as paved streets, parking lots, and building rooftops during rainfall and snow events that often contain pollutants in quantities that could adversely affect water quality. Most storm water discharges are considered point sources and require coverage by an NPDES permit. The primary method to control storm water discharges is through the use of best management practices.


Contamination or other alteration of the physical, chemical, or biological properties, of waters of the state, including change in temperature, taste, color, turbidity, or odor of the waters, or such
discharge of any liquid, gaseous, solid, radioactive or other substance into any waters of the state as will or is likely to create a nuisance or render such waters harmful, detrimental or injurious to the public health, safety or welfare, or to domestic, commercial, industrial, agricultural, recreational, or other legitimate beneficial uses, or to livestock, wild animals, birds, fish, or other aquatic life.

Pollution-generating hard surface (PGHS)

Those hard surfaces considered to be a significant source of pollutants in stormwater runoff. See the listing of surfaces under pollution-generating impervious surface.

Pollution-generating impervious surface (PGIS)

An impervious surface considered to be a significant source of pollutants in surface and storm water runoff.. Such surfaces include those subject to vehicular use or storage of erodible or leachable materials, wastes, or chemicals, and which receive direct rainfall or the run-on or blow-in of rainfall. Thus, a covered parking area would be included if runoff from uphill could regularly run through it or if rainfall could regularly blow in and wet the pavement surface. Metal roofs are also considered pollution-generating impervious surface unless they are treated to prevent leaching.

Pollution-generating pervious surface (PGPS)

A non-impervious surface with vegetative ground cover subject to use of pesticides and fertilizers. Such surfaces include, but are not limited to, the lawn and landscaped areas of residential or commercial sites, golf courses, parks, and sports fields.


Publicly owned treatment works

Predeveloped 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.


The three elements in an ecological system that interact to produce value, which is defined as benefits to people and other organisms. In general, the flow of impacts goes from process through structure to function, but there can be feedback loops between each part. See the discussion in the introduction of section 2 and the accompanying figure.


Rain garden

A non-engineered shallow, landscaped depression, with compostamended native soils and adapted plants. The depression is designed to pond and temporarily store stormwater runoff from adjacent areas, and to allow stormwater to pass through the amended soil profile.

Rainwater harvesting

A system for collecting, storing, treating and using rainwater.


A length of channel with uniform characteristics.

Receiving waters

Bodies of water or surface water systems receiving water from upstream man-made or natural systems.


The flow to groundwater from the infiltration of surface and stormwater runoff.

Replaced hard surface

For structures, the removal and replacement of hard surfaces down to the foundation. For other hard surfaces, the removal down to bare soil or base course and replacement.

Replaced impervious surface

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.

Resource stream

A stream section mapped and rated by King County as being a regionally significant stream reach that harbors significant concentrations of fish for some period in their life cycle.


The process of collecting and holding surface and storm water runoff with no surface outflow.

R/D Facility

Retention and detention facility. A type of drainage facility 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 surface and storm water runoff for a short period of time and then release it to the surface and storm water conveyance system. Also called flow control facilities.

Learn more about stormwater facilities


Pertaining to the banks of rivers and streams, and sometimes also wetlands, lakes, or tidewater.


A facing layer or protective mound of stones placed to prevent erosion or sloughing of a structure or embankment due to the flow of surface and storm water runoff.


A vertical pipe extending from the bottom of a pond BMP that is used to control the discharge rate from a BMP for a specified design storm.


Water originating from rainfall and other precipitation that ultimately flows into drainage facilities, rivers, streams, springs, seeps, ponds, lakes, and wetlands as well as shallow groundwater.



A member of the fish family Salmonidae. In King County salmonid species include Chinook, Coho, chum, sockeye, and pink salmon; cutthroat, rainbow, and brown trout and steelhead; Dolly Varden, brook trout, char, kokanee, and whitefish.

Sand filter

A man-made depression or basin with a layer of sand that treats stormwater as it percolates through the sand and is discharged via a central collector pipe.

Saturated hydraulic conductivity

Ability of a fluid to flow through a porous medium under saturated conditions; is determined by the size and shape of the pore spaces in the medium, their degree of interconnection, and by the viscosity of the fluid. Hydraulic conductivity can be expressed as the volume of fluid that will move in unit time under a unit hydraulic gradient through a unit area measured at right angles to the direction of flow.

Saturation point

In soils, the point at which a soil or an aquifer will no longer absorb any amount of water without losing an equal amount.


Safe Drinking Water Act


State Environmental Policy Act.

Septic system

An onsite wastewater collection system

Sewer system

The system of pipes and pump stations that collect and transport wastewater from homes and businesses to a wastewater treatment plant.

Sheet flow

Runoff that flows over the ground surface as a thin, even layer, not concentrated in a channel.

Shoreline development

The proposed projects regulated by the Shoreline Management Act. Usually this includes the construction over water or within a shoreline zone (generally 200 feet landward of the water) of structures such as buildings, piers, bulkheads, and breakwaters, including environmental alterations such as dredging and filling, or any project which interferes with public navigational rights on the surface waters.


The area defined by the legal boundaries of a parcel or parcels of land that is (are) subject to new development or redevelopment. For road projects, the length of the project site and the right-of-way boundaries define the site.


Degree of deviation of a surface from the horizontal; measured as a numerical ratio, percent, or in degrees. Expressed as a ratio, the first number is the horizontal distance (run) and the second is the vertical distance (rise), as 2:1. A 2:1 slope is a 50 percent slope. Expressed in degrees, the slope is the angle from the horizontal plane, with a 90° slope being vertical (maximum) and 45° being a 1:1 or 100 percent slope.


The unconsolidated mineral and organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants. See also topsoil, engineered soil/landscape system, and properly functioning soil system.

Soil amendment

The addition of compost and/or other soil materials to on-site soils to improve the drainage characteristics and pollutant removal capacity. Soil amendment can also reduce the need for water, fertilizer and pesticides.

Soil bulk density

Ratio of the mass of a given soil sample to the bulk volume of the sample.

Soil group—hydrologic

A classification of soils by the Soil Conservation Service into four runoff potential groups. The groups range from A soils, which are very permeable and produce little or no runoff, to D soils, which are not very permeable and produce much more runoff.

Soil horizon

A layer of soil, approximately parallel to the surface, which has distinct characteristics produced by soil-forming factors.

Soil permeability

The ease with which gases, liquids, or plant roots penetrate or pass through a layer of soil.

Soil profile

A vertical section of the soil from the surface through all horizons,
including C horizons.

Soil staibilization

The use of measures such as rock lining, vegetation or other engineering structures to prevent the movement of soil when loads are applied to the soil.

Soil stratigraphy

Sequence, spacing, composition, and spatial distribution of sedimentary deposits and soil strata (layers).

Soil structure

The relation of particles or groups of particles which impart to the whole soil a characteristic manner of breaking; some types are crumb structure, block structure, platy structure, and columnar structure.

Soil texture class

The relative proportion, by weight, of particle sizes, based on the USDA system, of individual soil grains less than 2 mm equivalent diameter in a mass of soil. The basic texture classes in the approximate order of increasing proportions of fine particles include: sand, loamy sand, sandy loam, loam, silt loam, silt, clay loam, sandy clay, silty clay, and clay.


The physical or chemical binding of pollutants to sediment or organic particles.

Sphagnum bog wetlands

Unique wetlands having a predominance of sphagnum moss creating a substrate upon which a distinctive community of plants is established. Some of these include ledum groenlandicum (Labrador tea), Kalmia occidentalis (bog laurel), Drosera rotundifolia (sundew), and Vaccinium oxycoccos (cranberry). Stunted evergreen trees are also sometimes present. In addition to a distinctive plant community, the water chemistry of sphagnum wetlands is also unique. It is characterized by acidic waters (pH 3 to 5.5), low nutrient content, low alkalinity, and a buffering system composed predominantly of organic acids. In the Puget Sound area, mature sphagnum bog wetlands are typically very old, often dating back thousands of years.

Steep slope

Slopes of 40 percent gradient or steeper within a vertical elevation change of at least ten feet. A slope is delineated by establishing its toe and top, and is measured by averaging the inclination over at least ten feet of vertical relief. For the purpose of this definition: The toe of a slope is a distinct topographic break in slope that separates slopes inclined at less than 40% from slopes 40% or steeper. Where no distinct break exists, the toe of a steep slope is the lowermost limit of the area where the ground surface drops ten feet or more vertically within a horizontal distance of 25 feet;


The top of a slope is a distinct topographic break in slope that separates slopes inclined at less than 40% from slopes 40% or steeper. Where no distinct break exists, the top of a steep slope is the uppermost limit of the area where the ground surface drops ten feet or more vertically within a horizontal distance of 25 feet.

Storage routing

A method to account for the attenuation of peak flows passing through a detention facility or other storage feature.

Storm drain system

The system of gutters, pipes, streams, or ditches used to carry surface and storm water from surrounding lands to streams, lakes, or Puget Sound. Also see Conveyance System.

Storm drains

The enclosed conduits that transport surface and stormwater runoff toward points of discharge (sometimes called storm sewers).

Storm frequency

The time interval between major storms of predetermined intensity and volumes of runoff for which storm sewers and other structures are designed and constructed to handle hydraulically without surcharging and backflooding; e.g., a 2-year, 10-year or 100-year storm.

Storm sewer

A sewer that carries stormwater and surface water, street wash and other wash waters or drainage, but excludes sewage and industrial wastes. Also called a storm drain.


Stormwater is the water that runs off surfaces such as rooftops, paved streets, highways, and parking lots. It can also come from hard grassy surfaces like lawns, play fields, and from graveled roads and parking lots.

Stormwater drainage system

Constructed and natural features which function together as a system to collect, convey, channel, hold, inhibit, retain, detain, infiltrate, divert, treat or filter stormwater.

Stormwater facility

Facilities that control the discharge of stormwater and that remove pollutants make up the bulk of the structural solutions applied to surface water problems in King County. Stormwater facilities included storage facilities (ponds, vaults, underground tanks, and infiltration systems); water quality facilities (wetponds, biofiltration swales, constructed wetlands, sand filters, and oil/water separators); and conveyance systems (ditches, pipes, and catchbasins).

These systems are most often built-in conjunction with new development, but include regional facilities designed and constructed by the Department of Natural Resources.

Once constructed, stormwater facilities require on-going maintenance to ensure they continue to perform as intended. Maintenance of storage facilities typically includes the removal of accumulated sediment and debris, routine mowing, and minor repairs to mechanical appurtenances. Management of water quality facilities is more complex, requiring intensive vegetation management, inspection and maintenance of flow control features, and restoration or replacement of filter media. King County plays an active role in the management of three categories of stormwater facilities: residential, commercial, and regional. These three terms are defined in the following paragraphs.

Stormwater Facility, Regional

In King County, regional stormwater facilities are constructed and/or managed by DNR's WLR Division. They typically serve large areas with a variety of land uses, and are intended to address problems resulting from large storm events. Examples of regional facilities include pump stations, regional storage facilities, sedimentation ponds, and enclosed drainage systems. These facilities are inspected annually and maintained by DNR's WLR Division.

Stormwater Facility, Residential

As defined by King County, residential stormwater facilities typically serve all or part of a single development and are built on a tract dedicated to this purpose. While the design and construction of these facilities is the responsibility of the developer, King County ultimately assumes responsibility for their long-term operation and maintenance.

Stormwater Facility, Commercial

In King County, commercial developments (which include businesses, apartments, and condominiums) are subject to stormwater management regulations that are similar to those applied to residential developments. However, unlike stormwater facilities in single family residential neighborhoods, commercial facilities remain the property and responsibility of the commercial landowner or manager.

King County staff conduct annual inspections of commercial facilities to identify maintenance needs for the property managers. In return for completion of the necessary maintenance, property owners receive a discount on their annual Surface Water Management (SWM) fees. Without this inspection service, commercial facilities often do not receive adequate maintenance.

Stormwater facilities regulated by the Permittee

Permanent stormwater treatment and flow control BMPs/facilities located in the geographic area covered by the permit and which are not owned by the Permittee, and are known by the permittee to discharge into MS4 owned or operated by the Permittee.

Stormwater Management

The application of site design principles and construction techniques to prevent sediments and other pollutants from entering surface or ground water; source controls; and treatment of runoff to reduce pollution.

Stormwater site plan

The comprehensive report containing all of the technical information and analysis necessary for regulatory agencies to evaluate a proposed new development or redevelopment project for compliance with stormwater requirements. Contents of the Stormwater Site Plan will vary with the type and size of the project, and individual site characteristics. It includes a Construction Stormwater Pollution Prevention Plan (Construction SWPPP) and a Permanent Stormwater Control Plan (PSC Plan). Guidance on preparing a Stormwater Site Plan is contained in Chapter 3 of Volume I.

Stormwater treatment and flow control BMPs/facilities

Detention facilities, treatment BMPs/facilities, bioretention, vegetated roofs, and permeable pavements that help meet minimum requirement #6 (treatment), #7 (flow control), or both.


Stormwater Management Program

Storm drain system

The system of gutters, pipes, streams, or ditches used to carry surface and storm water from surrounding lands to streams, lakes, or Puget Sound. Also see Conveyance System.

Structural BMP

Constructed facilities or measures to help protect receiving water quality and control stormwater quantity. Examples include storage, vegetation, infiltration, and filtration.


A layer soil used as the underlying base for a BMP.


The B horizons of soils with distinct profiles. In soils with weak profile development, the subsoil can be defined as the soil below the plowed soil (or its equivalent of surface soil), in which roots normally grow. Although a common term, it cannot be defined accurately. It has been carried over from early days when "soil" was conceived only as the plowed soil and that under it as the "subsoil."


The natural soil base underlying a BMP.


The base on which an organism lives.

Surface Water Design Manual

The manual (and supporting documents as appropriate) describing surface and storm water design and analysis requirements, procedures, and guidance which has been formally adopted by rule under the procedures specified in KCC 2.98. The Surface Water Design Manual is available from the King County Department of Development and Environmental Services or the Department of Natural Resources.

Suspended solids

Organic or inorganic particles that are suspended in and carried by the water. The term includes sand, mud, and clay particles (and associated pollutants) as well as solids in stormwater.


A shallow drainage conveyance with relatively gentle side slopes, generally with flow depths less than one foot.


Stormwater Management Program



Typically, a continuous length of pipe used to convey flows down a steep or sensitive slope with appropriate energy dissipation at the discharge end.

Tile drainage

Land drainage by means of a series of tile lines laid at a specified depth and grade.


Pipe made of burned clay, concrete, or similar material, in short lengths, usually laid with open joints to collect and carry excess water from the soil.


A layer of poorly sorted soil deposited by glacial action that generally has very low infiltration rates.


A TMDL or Total Maximum Daily Load is a calculation of the maximum amount of a pollutant that a waterbody can receive and still meet water quality standards. Water quality standards identify the uses for each waterbody, for example, drinking water supply, contact recreation (swimming), and aquatic life support (fishing), and the scientific criteria to support that use. The Clean Water Act, section 303, establishes the water quality standards and TMDL programs.


The upper portion of a soil, usually dark colored and rich in organic material. It is more or less equivalent to the upper portion of an A horizon in an ABC soil.

Total dissolved solids

The dissolved salt loading in surface and subsurface waters.

Total impervious area (TIA)

Total area of surfaces on a developed site that inhibit infiltration of stormwater. The surfaces include, but are not limited to, conventional asphalt or concrete roads, driveways, parking lots, sidewalks or alleys, and rooftops.

Total suspended solids

That portion of the solids carried by stormwater that can be captured on a standard glass filter.


Poisonous, carcinogenic, or otherwise directly harmful to life.

Travel time

The estimated time for surface water to flow between two points of interest.

Treatment BMP or facility

A BMP that is intended to remove pollutants from stormwater. A few examples of treatment BMPs are Wetponds, oil/water separators, biofiltration swales, and constructed wetlands.

Treatment liner

A layer of soil that is designed to slow the rate of infiltration and provide sufficient pollutant removal so as to protect ground water quality.

Treatment train

A combination of two or more treatment facilities connected in series.


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.



Plastic pipes with holes drilled through the top, installed on the bottom of an infiltration BMP, which are used to collect and remove excess runoff.

Undisturbed buffer

A zone where development activity shall not occur, including logging, and/or the construction of utility trenches, roads, and/or surface and stormwater facilities.

Undisturbed low gradient uplands

Forested land, sufficiently large and flat to infiltrate surface and storm runoff without allowing the concentration of water on the surface of the ground.


Vegetated roof

See green roof.


All organic plant life growing on the surface of the earth.


Water quality

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

Water Quality Ordinance

The King County Code that provides King County staff the authority to regulate stormwater quality in King County. The King County Code reference is K.C.C. 9.12. The code makes it unlawful for any person to discharge any contaminant into surface and storm water or ground water. Discharge is broadly defined to include indirect discharges associated with storm water runoff, and direct discharges through spills, dumping or other releases of contaminants. Any illicit connection to the storm sewer system or a water body is also prohibited. This section of the code, however, also lists ten actions or substances as being allowable discharges, such as runoff from lawn watering, potable water line flushing, and residential car washing.

Water quality treatment facility

A drainage facility designed to reduce pollutants once they are already contained in surface and storm water runoff. Water quality treatment facilities are the structural component of best management practices (BMPs); when used singly or in combination, WQ facilities reduce the potential for contamination of surface and/or ground waters.
Do you have questions about water quality treatment technology approval and usage in King County? Click here.


Washington State Department of Ecology


Washington State Department of Health

Wet Season

October 1 to April 30.


An area inundated or saturated by ground or surface 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 (U.S. Army Corps of Engineers Regulation 33 CFR 328.3 (1988)).

Wetlands in King County include all area waterward from the wetland edge. Where the vegetation has been removed, a wetland shall be determined by the presence of hydric soils, as well as other documentation of the previous existence of wetland vegetation such as aerial photographs.


Drainage facilities for water quality treatment that contain a permanent pool of water. They are designed to optimize water quality by providing long retention times (on the order of a week or more) to settle out particles of fine sediment to which pollutants such as heavy metals adsorb, and to allow biologic activity to occur that metabolizes nutrients and organic pollutants. For wetvaults, the permanent pool of water is covered by a lid which blocks sunlight from entering the facility, limiting light-dependent biologic activity.

View an example sketch of a wetpond (614KB)


A pond or constructed wetland that stores runoff temporarily and whose normal discharge location is elevated so as to maintain a permanent pool of water between storm events.


Drainage facilities for water quality treatment that contain a permanent pool of water. They are designed to optimize water quality by providing long retention times (on the order of a week or more) to settle out particles of fine sediment to which pollutants such as heavy metals adsorb, and to allow biologic activity to occur that metabolizes nutrients and organic pollutants. For wetvaults, the permanent pool of water is covered by a lid which blocks sunlight from entering the facility, limiting light-dependent biologic activity.

View an example sketch of a wetvault (517KB)


Water and Land Resources Division


Water Quality Management Area


Water Resource Inventory Area