Stormwater Facilities

What are Stormwater Facilities?

Stormwater facilities are engineered facilities that are designed to convey storm runoff, remove pollutants and to control flow rates. These facilities include pipes, ditches, swales, filters, ponds, underground tanks and vaults. These system specifically designed to capture, treat, store and then slowly release stormwater runoff downstream or into the ground.

Why is Stormwater Runoff a Problem?

As we cut woodlands, clear land, pave roads and parking lots, and construct houses and buildings, we change the permeability of the ground. Falling rain has fewer places to soak in gradually. Runoff on hard surfaces occurs faster and in greater volumes. Increased stormwater runoff can worsen flooding, erosion, and water pollution and destroy stream habitat.

What are the Benefits of Stormwater Facilities?

In addition to helping prevent flooding and erosion, stormwater facilities help to protect water quality by incorporating features that filter or remove sediments, excess nutrients and toxic chemicals.

What is a Stormwater Flow Control Facility?

A Flow Control (FC) facility can be either a pond, an underground tank or vault, or an infiltration system specifically designed to capture, store and then slowly release stormwater runoff downstream or into the ground. King County inspects about two thousand FC facilities built to hold the large volumes of water accumulated during rainy weather.

How Do FC Facilities Work?

Flow Control facilities can be either detention or retention.

A detention pond stores accumulated stormwater runoff and slowly releases it downstream. A flow control structure regulates the release rate of the stored water. Some detention ponds are combined with water quality treatment ponds (i.e., wetponds) which are intended to have some water in them on a permanent basis.

A retention or infiltration pond collects stormwater and allows the water to soak into the soil. This infiltration process helps recharge groundwater.

Underground tanks and vaults function the same way detention ponds do: storing rainwater from a storm and then slowly releasing the water downstream. Tanks and vaults can be located underground in the road right-of-way, parking lots, easements on private property, or any designated tract of land. Infiltration tanks or pipes are located underground and release water into the soil where it is absorbed.

What is a Water Quality Facility?

Water Quality facilities use, singly or in combination, vegetated swales, settling ponds or vaults, sand filters, compost filters, oil-water separators and constructed wetlands. These facilities protect water quality by removing pollutants. Water Quality facilities are often combined with FC facilities for more efficient use of space.

Who Maintains Stormwater Facilities?

Many stormwater facilities in unincorporated King County are owned and maintained by the County, and some are privately owned and maintained. King County maintains facilities that are in road right-of-way and county-owned tracts. Generally, these facilities serve public roads and single-family subdivisions. Facilities serving multi-family and commercial development are on private property, and are typically owned and maintained by the property owners.

Facilities are inspected periodically to ensure they are safe and function properly. The County also relies on citizens to report problems that arise between inspection dates.

If you have a problem or question about a FC facility, Contact King County Water and Land Resources (WLR), Stormwater Services Section, (206) 296-1900, or after hours and weekends call (206) 296- 8100, (1-800-KC-ROADS). To expedite your call, if the pond has a sign with an identification number, give the number to the King County staff.

For questions about the Stormwater Web Site, please contact Mary Lear.