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Stormwater services

Managing stormwater and waterborne pollution

Find comprehensive services, data and information related to stormwater and non-point source pollution in King County, Washington.

To offer a suggestion or report an error on the King County stormwater services and information website, please contact Fred Bentler, webmaster.

Stormwater Services and Information for King County, Washington

Stormwater glossary of terms and abbreviations

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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.

319

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

Anadromous fish

Fish that ascend rivers from the sea for breeding.

Aquifer

A geologic stratum containing groundwater that can be withdrawn and used for human purposes.

Backwater

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

Baffle

A device to deflect, check or regulate flow.

Basin

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. More....

Berm

A constructed barrier of compacted earth.

Biofiltration swale or Bioswale sketch (PDF, 400-800kb)

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.

BMP

Best Management Practice. Also see Structural and Nonstructural BMPs.

Buffer

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.

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CAFO

Concentrated Animal Feeding Operation

Catch Basin, Type I sketch (PDF, 400-800kb)

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.

Catch Basin, Type II sketch (PDF, 400-800kb)

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.

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.

Channel

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

Channel, constructed

A channel or ditch constructed to convey surface water; also includes reconstructed natural channels.

Channel, natural

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.

CSO

Combined Sewer Overflow

Constructed conveyance system facilities

Gutters, ditches, pipes, channels, and most flow control and water quality treatment facilities.

Control Manhole sketch (PDF, 400-800kb)

See Catch Basin, Type II

Conveyance System

Drainage facilities and features that collect, contain, and provide for the flow of surface and storm water from the highest points on the land down to a receiving water. Conveyance systems are made up of natural elements and of constructed 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.

Culvert

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.

CWA

Clean Water Act. The federal environmental law that includes the management of stormwater. (See EPA resources and the text of the law).

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DDES

King County Department of Development and Environmental Services

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 storm water runoff.

Debris Barrier

A metal trash rack

DNRP

Department of Natural Resources and Parks

Depression storage

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

Detention

Release of surface and storm water runoff from the site at a slower rate than it is collected by the drainage 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 sketch (PDF, 400-800kb)

A type of detention facility.

Detention Tank sketch (PDF, 400-800kb)

A type of detention facility.

Detention Vault sketch (PDF, 400-800kb)

A type of detention facility.

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.

Discharge

Runoff, excluding offsite flows, leaving the proposed development through overland flow, built conveyance systems, or infiltration facilities.

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.

Ditch

A constructed channel with its top width less than 10 feet at design flow.

Diversion

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

Drainage

The collection, conveyance, containment, and/or discharge of surface and storm water 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.

Dry Season

May 1 to September 30.

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EIS

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.

Embankment

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

EPA

Environmental Protection Agency

ESA

Endangered Species Act

Energy Dissipater

A rock pad constructed at inlets/outlets to prevent erosion, or a constructed percolation trench to disperse outletting flows over a large area, or a catch basin used to slow fast flowing runoff. Catch basins may be a part of the dispersion trench.

Erosion

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

ESC

Erosion and Sediment Control

Eutrophic

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.

FEMA

Federal Emergency Management Agency.

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 Restrictor sketch (PDF, 400-800kb)

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

Freeboard

The vertical distance between the design water surface elevation and the elevation of the structure or facility which contains the water.

FROP

Flow Restrictor/Oil Pollution control device.

Groundwater

Underground water usually found in aquifers. Groundwater usually originates from infiltration. Wells tap the groundwater for water supply uses.

Habitat

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

Hardpan

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.

HRT

Hydraulic residence 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.

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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.

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.

Impoundment

A natural or man-made containment for surface water.

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. See King County I/I Program Information.

Infiltration Basin sketch (PDF, 400-800kb)

A type of infiltration facility

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 sketch (PDF, 400-800kb)

A type of infiltration facility

Infiltration Tank sketch (PDF, 400-800kb)

A type of infiltration facility

Inlet sketch (PDF, 400-800kb)

See Catch Basin, Type I

KCRS

King County Road Standards, which are availabe from the King County Deparment of Transportation.

Lake

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.

Manhole sketch (PDF, 400-800kb)

See Catch Basin, Type II

MEP

Maximum extent practicable

MS4

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

Small MS4s

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. See EPA information on regulatory applicability.

Medium MS4s

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

Large MS4s

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

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.

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Natural conveyance system elements

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

NDAP

Neighborhood Drainage Assistance Program. More ....

NOI

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. (See USEPA 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.

NPDES

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.

Natural onsite drainage feature

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

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.

Outfall

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

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. (See USEPA Information).

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. (See USEPA Phase 2 Information).

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.

Pollution-generating impervious surface

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

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.

POTW

Publicly owned treatment works

Reach

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.

Recharge

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

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.

Retention

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. More....

Riparian

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

Riprap

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.

Runoff

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.

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Salmonid

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.

SDWA

Safe Drinking Water Act

SEPA

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.

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.

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.

Stormwater

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 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 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.

SMP (or SWMP)

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.

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.

Swale

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

SWMP (or SMP)

Stormwater Management Program

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Tightline

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

TMDL

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.

Toxic

Poisonous, carcinogenic, or otherwise directly harmful to life.

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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 (PDF file, 246kb). 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.

WDOE

Washington State Department of Ecology

WDOH

Washington State Department of Health

Wet Season

October 1 to April 30.

Wetland

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.

Wetpond sketch (PDF, 400-800kb)

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.

Wetvault sketch (PDF, 400-800kb)

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.

WLR

Water and Land Resources Division

WQMA

Water Quality Management Area

WRIA

Water Resource Inventory Area

For questions about the Stormwater Web Site, please contact Dale Nelson,Engineer II, King County Stormwater Services Section.