What is a Detention Pond?

It’s a bowled section of land that is a helpful stormwater treatment system that helps prevent flooding and polluted stormwater runoff from entering the Puget Sound.

How do detention ponds work?

Some of the pollutants in stormwater are like superfoods for plants, but in a natural stream or wetland, they can cause problems by encouraging too much plant growth.

By capturing stormwater, fewer of these nutrients enter natural streams and wetlands and instead go to plants in the detention pond. Bugs and microorganisms living in the mud then help cleanse the stormwater of pollutants.

How are pond levels controlled?

There’s a special restrictor device that’s located downstream from the pond. The restrictor, a small hole for water to filter through, is similar to an overflow drain for a bathtub.

When the pond fills with more water than can pass through the restrictor, the water backs up and is stored in the pond until it stops raining. Then, it should empty to its normal level.

Why detention pond maintenance is important:

Plants: Plant control, including weeding, mowing, and other maintenance, can ensure that the pond works as intended. Poor management can result in clogged pipes, stagnant water, and overgrowth.

Soils: Detention pond soils are designed to clean and soak in water. If the soils are not cared for, they can become clogged with pollutants.

Control Structures: Control structures are manholes at the pond outlet. They control how fast water leaves the pond, so that nearby creeks do not get flooded during a storm. Control structures need to be maintained by an expert, to make sure the complicated pipe system inside does not become plugged.

Maintenance Hazards: Maintaining a stormwater pond has significant risks, and may result in injury if not done properly. Please work with your local municipal stormwater authority and a licensed contractor to carry out maintenance in a safe and effective manner.

Who maintains detention ponds?

If you are a member of a Homeowners Association (HOA), you do! In order for the detention pond to work correctly, it needs regular maintenance. The city will regularly inspect public and private drainage facilities like this to determine whether it meets performance standards.

You are not alone in this work!

Please reach out to the City of Granite Falls Public Works - charles.white@ci.granite-falls.wa.us or Snohomish Conservation District - outreach@snohomishcd.org - for assistance.