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Thurston County Storm and Surface Water Utility

A Non-Engineers Guide to Stormwater Terms

Stormwater pollution: Stormwater is water from rain or melting snow that does not immediately soak into the ground. Instead stormwater flows across hard surfaces such as parking lots, streets, lawns and roofs and picks up pesticides and fertilizers, sediment, pet waste, cigarette butts, litter, oil and other pollutants.  This runoff flows into street drains and ditches. Eventually, the runoff ends up in rivers, streams and Puget Sound, or in the soil where it can seep into ground water (our source of drinking water).  The runoff does not enter a sewer-type treatment plant to be cleaned. raindrops
Storm drain:  A street grate that captures stormwater runoff. Catch basin:  A tank below a storm drain. Sediments settle to the bottom of the tank, and the cleaner water on top then flows through pipes either to a stormwater pond or directly into a body of water. catch basin
Swale: Wide, shallow ditches with gently-sloping sides and a flat bottom where stormwater runoff either infiltrates into the ground, flows to a stormwater pond, or travels to another location. A grass-lined "bioswale" operates in a similar manner, but also removes pollutants from runoff before discharging the runoff to a dry pond or another location. Also see "Swales: Go With the Flow." swale
Drywell: Drywells are open-ended cylinders with openings in the walls, or perforated pipes.  Drywells can be made from concrete, plastic, or other materials and come in a variety of widths and depths. Drywells are placed in a hole in the ground and surrounded by permeable soils or gravel. As water flows into the drywell, small holes in the side allow the water to infiltrate into the surrounding soil. Roof downspouts are often connected to drywells. Also see "Maintaining Your Catch Basins and Drywells." (PDF)   drywell
Dry stormwater pond:  An engineered depression in the land that gathers stormwater runoff until it can slowly seep into the ground. Most dry ponds infiltrate water into the ground within 72 hours. They often look like shallow, grass-lined bowls in the land.  The term "infiltration pond," refers to the same type of concept: an area where rainwater infiltrates back into the ground. Also see "Maintaining Your Neighborhood Stormwater Facilities" (PDF). dry pond
Wet stormwater pond:  A type of stormwater pond designed to remove pollutants. Unlike a dry pond, which lets water soak slowly into the ground, a wet pond allows the water to pool.  Pollutants settle to the bottom or stick to vegetation in the pond, and the cleaner top layers of water flow either to a dry pond, where it seeps into the soil, or into the nearest body of water. wet pond
Basin plans:  Basin plans are thorough investigations into the drainage problems and potential solutions within a given area.  Basin plans address issues such as flooding, poor water quality, erosion, and the degradation of aquatic habitat.  The plans are reviewed and approved by the elected officials of each participating agency involved. (Basin plans in Thurston County are usually written jointly by the county and cities.) A basin plan itself does not provide funding or authorization for construction of capital projects, however, Thurston County commissioners use the basin plans as a tool when deciding which stormwater construction projects to finance. To view basin plans in Thurston County, click here. basin plans

Contact Us

 Staff contact: Cathe Linn: (360) 754-4681. E-mail: stormwater@co.thurston.wa.us

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