Drainage System
Everyone in Seattle relies on the drainage system to prevent flooding and keep our local waters clean. This system affects how Seattle looks, feels and functions as water flows in, out, and around our city.
SPU builds and manages the publicly owned drainage system. Private drainage systems often connect to the publicly owned drainage system. To see a map of where SPU drainage assets are located in your area use the DSO Water & Sewer Map.
What is Stormwater?
Stormwater is rain and melting snow that runs off rooftops, lawns, streets, sidewalks, bridges, and parking lots, and into storm drains. Most stormwater travels through the drainage system and is eventually released into creeks, lakes or waterbodies that surround Seattle. In some parts of the city, stormwater enters Seattle’s sewer system.
- Storm drains: Inlets that take in stormwater from streets, parking lots and other surfaces
- Catch basins: A structure near an inlet that captures sediment, debris and pollutants so they do not enter and clog the pipes
- Maintenance holes: An opening for people to access underground pipes for maintenance, inspections and cleaning.
- Ditches: Open channels dug into the ground to carry water along a road
- Culverts: Pipes or enclosed channels that carry water under roads, trails, railroads or land areas. Creek culverts carry creeks under roads and land areas.
- Pipes: Large concrete, clay, plastic, or metal circular tubes that carry stormwater between two points
- Rain gardens: Shallow, planted basins that collect and absorb stormwater and can filter out pollutants
- Bioretention facilities: Basins with special soil mixes and plants that capture and clean stormwater
- Ponds: large natural or created basins that collect and retain (permanently hold) or detain (temporarily hold) stormwater
- Detention tanks: underground or above ground structures that temporarily store stormwater
- Swales: Shallow, sloped, planted channels that filter and move stormwater—often on the side of a road.
- Outfalls: Points where stormwater exits the drainage system into a body of water
Ditches, rain gardens, bioretention facilities, swales, and ponds are all forms of Green Stormwater Infrastructure.
Maintaining your property's drainage system is your responsibility.
- Visually inspect catch basins, gutters, downspouts and other parts of your system regularly. Make sure retaining walls are in good condition, and there are no signs of unstable slopes and/or erosion. Hire a professional to regularly "video inspect" your underground drainage system and contact a soils engineer if you see signs of erosion.
- Clean gutters and downspouts at least twice per year.
- Direct downspout flows at least 5 feet away (10 feet is better!) from your foundation, without sending flows to adjacent properties or over the edge of a steep hill. Coordinate with the Seattle Department of Construction and Inspections for approved points to discharge water.
- Create “flow paths” on your property where water can flow during a storm without damaging your neighbors’ properties. Consider using sandbags (SPU has community sandbag sites) to redirect flows.
- Visit Federal Emergency Management flood risk website to help assess your property’s flood risk and learn about FEMA’s Homeowner's Guide to Retrofitting: Six Ways to Protect Your House from Flooding (PDF).