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Stormwater facility in North Bend. Stormwater pond signage in front and trees and mountains in the background.

Stormwater facility in North Bend.

What are stormwater facilities?

Stormwater facilities carry storm runoff, remove pollutants, and control the flow of water. Stormwater facilities include:

  • Pipes
  • Ditches
  • Swales
  • Filters
  • Ponds
  • Underground tanks
  • Vaults

What are the benefits of stormwater facilities?

Stormwater facilities help prevent flooding and erosion. Another key function is to protect water quality. These facilities filter and remove sediments, excess nutrients, and toxic chemicals.

Why is stormwater runoff a problem?

Many of our actions change how water soaks into the ground. Cutting down trees, clearing land, paving roads and parking lots, and constructing buildings all affect runoff. These changes mean that rain has fewer places to slowly soak into the ground. Rain pools on hard surfaces more quickly when there are less places to soak into the ground. Increased runoff can make flooding, erosion, and water pollution worse. These issues lead to the destruction of stream habitats.

Types of stormwater facilities

Stormwater flow control facilities

A flow control facility is designed to capture, store, and slowly release stormwater runoff downstream or into the ground. These facilities include:

  • Ponds
  • Underground tanks
  • Vaults
  • Infiltration systems

How do flow control facilities work?

There are two types of flow control facilities, detention and retention.

Detention ponds store stormwater runoff and slowly release it downstream. In some cases, detention ponds are combined with water quality treatment ponds. Those ponds store some water on a permanent basis.

Retention, or infiltration, ponds collect stormwater and allow water to soak into the soil. This process helps recharge groundwater. Infiltration tanks or pipes are located underground and release water into the soil to be absorbed.

Underground tanks and vaults function like detention ponds. Rainwater from storms are stored in these facilities and slowly released downstream. Tanks and vaults are often located in places like a road right-of-way, parking lots, and easement on private property.

Water quality facilities

Water quality facilities protect water quality by removing pollutants. These facilities are often combined with flow control facilities. Water quality facilities include:

  • Vegetated swales
  • Settling ponds or vaults
  • Sand filters
  • Compost filters
  • Oil-water separators
  • Constructed wetlands

Water draining out of a stormwater pipe onto small rocks.

End of a pipe discharging stormwater.

Who maintains stormwater facilities?

We own and maintain many stormwater facilities in unincorporated King County. We maintain facilities located in the road right-of-way and county-owned tracts. These facilities mostly serve public roads and single-family subdivisions.

Stormwater facilities on private properties like multi-family and commercial developments are often privately owned and maintained.