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Thornton Creek Water Quality Channel

Landscape Performance Benefits

Environmental

  • Designed to remove an estimated 40-80% of total suspended solids from 91% of the average volume of annual stormwater runoff from the 680-acre drainage basin.
  • Created new habitat within this heavily paved commercial area. Within one month after opening, native birds were observed at the project. A variety of desirable native volunteer plants have migrated into the site and begun to establish.
  • Increased open space within the Northgate Urban Center by about 50%.
  • Reduced impervious surfaces by 78%.

Social

  • Provides pedestrian links from adjacent commercial and residential neighborhoods. Shortened walking access by 50%.

Economic

  • Catalyzed $200 million in adjacent private residential and commercial development.

At a Glance

  • Designer

    SvR Design Company

  • Project Type

    Park/Open space
    Stormwater management facility
    Stream restoration

  • Former Land Use

    Greyfield

  • Location

    NE Thornton Place
    Seattle, Washington 98125

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  • Climate Zone

    Warm-summer Mediterranean

  • Size

    2.7 acres treating runoff from 680 acres

  • Budget

    $14.7 million

  • Completion Date

    2009

Carved out of an abandoned parking lot, the Thornton Creek Water Quality Channel treats urban stormwater runoff from 680 acres within a necklace of channels, pools, and terraces designed to mimic the performance of a natural creek. Its lush plantings, overlooks, and paths have added 2.7 acres of public open space to the Northgate UrbanCenter and catalyzed surrounding redevelopment. The facility is a model for how multi-functional landscapes can be integrated into the dense urban fabric.

  • The project uses a tiered system to treat stormwater flows. The Upper Cascade Swale receives runoff from 20 acres. The Lower Channel receives runoff from 660 acres diverted from the 60-inch pipe running under the site.
  • A modified water quality channel design was developed to meet hydraulic flow and residence time parameters. In constrast to a standard bioswale design that sends shallow flows across 3-6 inch tall grasses, the project’s channel handles a greater water volume by sending deeper flows through wide densely vegetated terraces with a minimum width of 30 feet.
  • All flows pass through a series of channels, sediment pools, and planted bioswale terraces that provide flow attenuation and water quality treatment.
  • Mechanically stabilized earth walls support the site’s steep topography and provide substrate for “painted walls” of plants.
  • 85% of the project’s plant palette are native species, including 172 native trees, 1,792 native shrubs, and 49,000 native perennials, herbs, grasses, rushes, and sedges.
  • The planting mix and the channel’s alignment will be allowed to evolve over time.

Challenge

The asphalt parking lot covered a 60-inch storm pipe that channeled untreated runoff from 680 highly urbanized acres into Thornton Creek, a critical salmon-bearing stream that has been in decline due to urbanization. Conflict over whether to exhume the storm pipe to create a constructed creek bed and park or to develop the site for mixed use had left the site in limbo for decades.

Solution

Working with community stakeholders, private developers, and Seattle Public Utilities, the design team developed a solution that created an open, planted channel to treat the pipe’s flows while allowing development to occur on the site. The project’s channel design served as the focal point for adjacent commercial and residential development and also contributed much-needed public open space to this urban neighborhood.

The project was intended to serve multiple functions: improve water quality, provide public open space and native vegetation, and facilitate economic development within the Northgate area by integrating the project’s design with adjacent private development. These functions were integrated in the design, so that the higher capital costs were distributed across multiple public benefits.

  • Plants must be carefully sourced and inspected before installation to reduce invasive species introduction.
  • It is essential to have an adaptive management plan in place.
  • Engaging maintenance staff in the design process helps ensure the project can be maintained over the long term.
  • Pre-construction investigation of the site reduces unforeseen complications.

Project Team

Client: City of Seattle, Seattle Public Utilities
Citizen Oversight Committee: 
Northgate Stakeholders Group
Civil Engineering & Landscape Architecture: SvR Design Company
Structural and Electrical Engineering: HDR, Inc.
Geotechnical Engineering: Associated Earth Sciences, Inc.
Concept Design: GAYNOR, Inc. in collaboration with PACE Engineers, Inc.
Project Artist: Benson Shaw
Educational Signage: Seattle Public Utilities and Whiting Design
Hydraulic Modeling and Monitoring Services: Herrera Environmental Consultants, Inc.
Engineering Resource: Rich Horner, Associate Professor, University of Washington
Plant Ecology Resource: Kern Ewing, Professor, University of Washington
Soil Specialty Consultants: The Hendrikus Group/Soil Dynamics, HWA Geo Sciences, Inc. 
General Contractor/Construction Manager: Walsh Construction
Site Work and Utility Contractor: Gary Merlino Construction Company, Inc.
Landscape Contractor: Cerna Landscape Inc.
Other Agencies and Offices: City of Seattle – Mayor’s Office, Department of Planning and Development (DPD), Department of Transportation (SDOT), Department of Executive Administration; Washington Department of Ecology; Washington Department of Fish and Wildlife
Adjacent Property Owners/Developers: Northgate South Commons LLC , a joint venture between Lorig Associates and Stellar International Holdings; Aljoya Thornton Place managed by Era Living, LLC

Role of the Landscape Architect

Led a collaborative and integrated team of landscape architects, civil engineers, structural and electrical engineers, artist, hydraulic modelers, plant ecologists, and soil specialists in the development of this high performance open space and water quality channel.

Topics

Stormwater management, Water quality, Habitat creation, preservation & restoration, Economic development, Bioretention, Native Plants, Trees, Play

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