Seattle Public Utilities - Street Edge Alternatives

In 2001, Seattle Public Utilities completed construction of its Seattle Street Edge Alternatives (SEA Streets) project, in which a single residential block was retrofitted with vegetated swales and rain gardens. SEA Streets was a pilot demonstration project designed to return drainage and vegetation in the area to a natural systems approach - providing community and street level aesthetic benefits, as well as contributing to the management of rainfall with green alternatives to stormwater drainage. 

Seattle used methods to maximize the stormwater time of concentration and the sites detention volume, without compromising homeowner access and parking needs on the street. The installation of the swales and rain gardens reduced that street’s impervious surfaces to 11 percent less than a traditional street, and added over 100 evergreen trees and 1100 shrubs. After two years, monitoring data showed that the project reduced the total volume of stormwater by 99 percent. Though the cost of the project was comparable to that of traditional street design, new projects constructed since have benefited from the knowledge gained from the pilot and generally cost 15 to 25 percent less than traditional street development.

One of the primary drainage goals was to use surface retention or detention to reduce 2-year, 24-hour storm event peak runoff rate and volume to pre-developed condition. Detention volume achieved by the swales was 2,500 cubic feet; 37% more volume than would have been required by their drainage ordinance.

Another priority of the project was to reduce the impacts of the urban environment on streams inhabited by salmon, so SEA streets 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. The landscape design complements the drainage system function and focuses on native and salmon-friendly plantings.

Some of the additional successful components of SEA Streets include: 

There was an emphasis on retaining existing large-scale trees and relocating vegetation to meet homeowner needs and project goals. Trees planted help to restore more of the evaporation and transpiration that was present before development.

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. 

Native soil from excavations was mixed with organic compost to provide rich topsoil and reduce water and fertilizer needs. 

Clay was the preferred liner material for swales, to help 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.





Publication Date: 2011

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