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Seattle SEA Streets Drain Naturally

Credits: ©2011 City of Seattle

One of the main principles in green building is responsible site development, which includes dealing with rainwater runoff from buildings, sidewalks, streets and other impervious surfaces. The city of Seattle, Washington, addressed this issue in a pilot project by designing a street that provides drainage that more closely mimics the natural landscape prior to development than traditional piped systems. Known as the Street Edge Alternatives Project (SEA Streets), the pilot was completed in the spring of 2001 in a residential area in the north end of the city. After several years of monitoring, SEA Street proved that the natural stormwater treatment system reduced the total volume of stormwater leaving the street by 99 percent. The city accomplished this by reducing impervious surfaces to 11 percent less than a traditional street, providing surface detention in swales, and adding more than 100 evergreen trees and 1100 shrubs. (Scroll to bottom for additional resources)


Seattle SEA Streets Bioswale Cropped

The city of Seattle, Washington, designed streets that provide drainage to more closely mimic the natural landscape prior to development in its SEA Streets pilot project. Features include bioswales, no curbs, and angled parking. ©2011 Solaripedia

Making the Most of the Right-of-Way

Before it was transformed into SEA Street, this stretch of 2nd Avenue NW was covered in pavement and compacted gravel. When water flows over these hard surfaces, it picks up pollution and speed, often causing damage to creeks and lakes. The project designers have applied a Natural Drainage Systems (NDS) approach to managing the stormwater from the neighborhood. The NDS approach mimics nature by increasing the ability of the landscape to absorb rainwater with shallow depressions, amended soils, and plants - improving water quality and quantity, and reducing pollution and runoff speed.

The drainage goals for this project include conveyance, flood control, and minimizing the flow of stormwater off-site. The project team sculpted the project area to move water away from the roadway and homes and into planted swales along both sides of the road.

By retrofitting the street using an NDS approach, the project team supported the transportation needs of the neighborhood while simultaneously providing space for rainwater to return to the earth, rather than flowing rapidly across the paved landscape toward Pipers Creek.

Thinking Downstream

In addition to flooding and damaging flows to the creek, stormwater pollution is also a serious concern. As rain washes over the developed landscape, it may be contaminated by oil and grease, heavy metals, pet waste, sediments, chemical fertilizers and pesticides.

The SEA Street design cleanses stormwater through a series of natural functions. The plants and soils filter pollutants as rainwater moves through the swales, preventing them from traveling downstream to sensitive water bodies such as creeks and lakes. Bacteria within healthy soils can also help break down carbon-based pollutants like motor oil.

Natural Drainage Systems Plantings

Because space available in the right-of-way is limited, the project team for SEA Street created landscaping that is both useful and beautiful.

Grasses, sedges and rushes physically filter pollutants out of stormwater. The project designers followed the concept of "right plant, right place," selecting non-invasive species that can survive with little maintenance in our local climate. Trees with smaller root systems fit more easily within the right-of-way; plants that thrive in wetlands have been placed in the lower, moist areas of the stormwater swales and ponds.

The landscape design includes mostly native Pacific Northwest species and some ornamentals. Initially, over 100 deciduous and evergreen trees and 1100 shrubs were planted here. Nearly 100 percent of these plants have survived over the four year life of the project.

Beauty and Care, All Year Round

Perhaps one of the reasons that plant survival has been so successful on SEA Street is that the local residents have taken responsibility for maintenance. Neighbors have agreed to care for the plants within the right-of-way through weeding, mulching and mowing when necessary.

As we have learned on SEA Street, one of the keys to successful implementation of Natural Drainage Systems is designing the right-of-way to be beautiful as well as useful. If neighbors like the way the street looks, as it changes from season to season, they are more likely to help care for it.

The neatly trimmed grass strip along the roadway, shows the interest that neighbors take in caring for the landscaping. Within the context of the naturalistic planting pattern on the street, these grass strips create a consistent boundary between road and landscape.

Meeting Transportation Needs

The design of SEA Street helps slow traffic. The narrower driving lane and meandering shape of the roadway create visual interest and cause traffic to move at a safe pace. Accordingly, this is an ideal location for pedestrians and bicyclists.

While the street's shape and layout are unconventional, the SEA Street design team has ensured that large trucks and emergency vehicles can still safely access the entire street. Although they are not typically used for driving, the white strips, or "flat curbs," provide an additional 2 ft. of driving room on either side of the 14 ft. roadway, for a total of 18 ft. across (enough for two fire trucks to pass each other). In addition, the roadside grass-planted strips are structural grass, reinforced with a lattice of material that can handle occasional traffic.

Parking needs for neighbors are also met on SEA Street based on parking surveys conducted by project planners. The angled parking distributed along the roadway in clusters is unique to SEA Street. Other NDS projects employ alternative parking plans.

A Sense of Place

Accompanying the drainage improvements, this project has created a sense of place and community in the neighborhood. The addition of a sidewalk separates pedestrians from traffic and increases the feeling of safety in the neighborhood. "Walkability" from the sidewalk and the “garden-street” appeal makes this street a common destination for nearby residents.

The maintenance agreement between residents and Seattle Public Utilities encourages neighbors to get to know each other while they care for the plants in their shared right-of-way garden. Clusters of mailboxes create additional gathering spaces. The visual continuity of the street design and the street's unique appearance also subtly link people together along the corridor.

Finally, anyone who lives on or near SEA Street project is now aware of their place in the larger context of the local watershed. Many community members have become involved in efforts to improve water quality and stream health in Pipers Creek. SEA Street evokes environmental awareness and action… stewardship by design.

Learning from SEA Street

Many people visit SEA Street each year. They range from watershed residents taking a stroll, to university students (shown in photo), to engineers, designers and other professionals.

In order to measure the drainage and water quality results of the project, monitoring equipment has been placed at the downstream (south) end of the project. Data collected with these instruments are used in an ongoing study being conducted by SPU and the University of Washington. The first three years of project monitoring has shown that 98% of wet-season and 100 percent of dry-season stormwater runoff has been eliminated by the project. For additional results, please see the links to Hydrologic Monitoring on the SEA Street main page.

Learning from Natural Drainage Systems

SEA Street was the first project of its kind. It serves as a benchmark for continual learning. The design for SEA Street has helped inspire and inform subsequent Natural Drainage System designs in Seattle and in other cities. (For local examples, please see the 110th Cascade and Broadview Green Grid projects within the Natural Drainage Systems website.)

Compare the current appearance of SEA Street to this pre-construction photo. How does each street seem to function in terms of mobility, safety, walkability, and visual appeal?

SEA Street offers clear evidence that alternative street design can meet neighborhood street design goals, while making significant improvements in drainage and water quality, providing community amenities and educational benefits.

Landscape Architecture

The landscape elements serve an important role in both providing an aesthetic benefit as well as contributing to the management of rainfall. Trees will help to restore more of the evaporation and transpiration that was present before development.

The other vegetation in the drainage swales will also help to filter and slow the flow of storm water. Over 100 deciduous and evergreen trees and 1100 shrubs were added as part of the project.

Design involvement

The landscape architect was involved more during the design than a typical project. The most apparent example of this is a sidewalk design that not only serves but attracts pedestrians.

There also was an emphasis on retaining existing large-scale trees and relocating vegetation to meet homeowner needs and project goals. The swales and surrounding areas are artfully graded and planted with native wetland and upland plant species. Granite boulders and various sizes of washed river rock provide both function and beauty.

Landscape design

The landscape design complements the drainage system function and focuses on native and salmon-friendly plantings. The system is unique in its use of grading, soil engineering, plant selection and layout as components that function together -- much as they do in a natural ecosystem.

Natural systems approach

Returning drainage and vegetation in the area to a natural systems approach is an important element of this demonstration project.

For example, native soil from excavations was mixed with organic compost to provide rich topsoil and reduce water and fertilizer needs.

As another example, clay was the preferred liner material for swales. This helps to ensure vegetation can survive in the summer months by allowing moisture to move up through the soil. A liner fabric would be less effective in this role.


The cost of this project was $850,000 (in 2001). This included an extensive design and communications budget due to the need to work closely with residents on the design. Future projects will cost less than traditional street improvements.


The project helps creeks by reducing stormwater at the source. As Seattle has developed, stormwater running off impervious surfaces has had a major impact on our creeks and wildlife.

SEA Streets provides an example of the environmental benefits that can be realized with natural systems instead of traditional systems, especially in areas which are installing new infrastructure.

Residents of this neighborhood enjoy walking along SEA Streets because it is a natural, soft-edged environment, in contrast to the hard edges of traditional linear streets. Also, more tree cover helps reduce summer heat while absorbing air pollutants and rainfall.

Drainage Improvements

The drainage design represents a unique hybrid, combining hydraulic engineering with soil science and botany to create a more natural system.

Hydraulic engineering required strict control of elevations utilizing various aggregates and soil mixes below grade. Drainage improvements combine contoured swales with traditional drainage infrastructure (culverts, catch basins, flow control structures and slotted pipe) to regulate the flow and discharge of storm water.

Project goals

One of our primary drainage goals was to use surface retention or detention to reduce 2-year, 24-hour storm event (1.68-inches) peak runoff rate and volume to pre-developed conditions.

Since our goal was developed to reduce the impacts of our urban environment on streams inhabited by salmon, we chose to detain stormwater for the entire contributing drainage area (street right-of-way and properties along the east side, total 2.3 acres), not just new or redone impervious area.

Also, we worked to meet the goal of conveying the site’s 25-year, 24-hour storm event, which is the design requirement for all City of Seattle drainage systems.

Methods used

The method we used for achieving this goal was to maximize the stormwater time of concentration and the sites detention volume, without compromising homeowner access and parking needs on the street.

Using a long flow path length and high surface roughness along the flow path increased the basin's time of concentration. All areas within the right-of-way without hard surfaces were used toward detention.

Some additional considerations

Our original hope for retaining flows and allowing infiltration into the native soils throughout the length of the block was not possible because some homes had an existing groundwater intrusion problem. To limit the potential for stormwater to adversely impact the residences of concern, our geotechnical engineers identified some swales that needed an impermeable liner – for example, a six inch depth of natural clay material was used as the preferred material.

Sizing the swales to detain the pre-developed two-year, 24-hr storm event, while not allowing infiltration, resulted in the need for very small outlet orifice diameters. Due to maintenance concerns, we wanted outlet orifice diameters greater than ½-inch. To reduce potential for these small orifices to clog, we also provided a sump in the flow control structure. Working with the site’s flat topography (1% longitudinal slope) and the minimum orifice size restraint, we chose to use only 3 flow control facilities on the block.

We hydraulically connected the swales into three groups, with each group controlled by a flow control structure. Detention volume achieved by swales was 2,500 cubic feet; 37 percent more volume than would have been required by our drainage ordinance.

The project is located on 2nd Ave NW, between NW 117th and 120th Streets in Seattle, Washington.

View map


  Seattle SEA Streets Presentation Slides (8,952 kb)

  Seattle SEA Street Alternatives Handouts (821 kb)

  Seattle SEA Street Construction Plans (4,220 kb)

  Seattle SEA Streets Case Study (5,955 kb)

  Seattle SEA Street One Page Case Study (57 kb)

  Seattle SEA Street Watershed Map (11,133 kb)