Magnuson Park Wetlands and Active Recreation

Challenge

In 1922, the U.S. Navy began constructing an airfield on the Sand Point peninsula by leveling a peat bog and a forested hill. The ecologically bare landscape that remained had nearly no natural drainage and relied on sub-surface pipes to manage stormwater runoff by draining it directly into Lake Washington. The site operated as a Naval Air Station for nearly 50 years, resulting in some soil contamination and impenetrable soil conditions due to compaction. In 1975 much of the land was given to the City of Seattle and developed as a park. With additional land transfers and proposed park improvements, competing visions arose and the space was championed as a either a park for people and recreation or a place to restore ecological function. Surrounding property owners resisted changes to the park, specifically night lighting for playing fields, and took this issue to court after extensive community engagement.

Solution

Using a balanced cut and fill method, the design team engineered the site to maximize ecological value and performance for the lowest cost. Because remediating the soil compaction would have been cost-prohibitive, instead of trying to reduce the volume of stormwater runoff, the design focused on improving water quality before discharging it into Lake Washington. To do this, the site was graded so that water flows eastward across the site through a series of wetlands that filter the water. Instead of designing exclusively for active recreation or ecological function, the park incorporates both agendas. By limiting trails and access in habitat areas while prioritizing recreation in other areas of the park, a synergistic balance was struck between humans and nature. Legal objections were managed through probative evidence that their claims were invalid; previous opponents to the park are now strong allies.

• 10 acres of impervious surface were removed from the site. The park’s design created 10 acres of new wetlands and rehabilitated 4 acres of existing poorly-functioning wetlands.
• 5 distinct wetland systems were created: Northern Marsh, Entry Marsh, Marsh Ponds, Promontory Ponds, and Linked Marsh Ponds. The marsh ponds or rice paddies were designed to be seasonally hydrated to support growth of the Pacific chorus frog population in the park.
• The site can detain over 5 million gallons of water underneath the synthetic sports fields and in ponds and wetlands, which helps reduce the amount of non-point source pollution entering Lake Washington from the surrounding neighborhoods and roads. Previously, this runoff had been piped through the site and discharged directly into Lake Washington.
• The initial design scheme was modified to protect 8 groves of mature, native trees on the site by maintaining them as in-situ elements in the design.
• A total of 4,992 native plants were installed in the park, including 725 trees, 1,376 shrubs, and 2,891 transplants, livestakes, and rooted cuttings. Of the 30 species planted, the most common are Douglas fir, red-osier dogwood, willow, Pacific ninebark, and salmonberry.
• Industrial materials found on site were reused creatively to highlight the site’s history. For example, the wetland weir uses old drain pipes and tunnel sections as planters and structural elements. Concrete vault arches were repurposed as benches, and historic light poles from the airfield were reused as bollards.
• The Magnuson Outdoor Learning Lab is a hands-on science and service learning program with the Burke Museum that uses the park to teach environmental engagement and literacy to urban middle school students.

• By reusing 2,700 cu yds of concrete from the site as subbase for the synthetic turf fields, the project saved $95,000 in materials and delivery costs. This also served to avoid 3,600 vehicle miles and 11.8 tons of carbon emissions that would be produced in transporting this volume of gravel to the site.

• The exacting grading and drainage plan for the site, high levels of soil compaction, and design for specific animal species made it essential that all site grading be completed as specified. In the initial phases of the project, it became apparent that early, attentive communication with the contractor was required to achieve this. By catching possibly detrimental implementation in early site grading, later errors in crucial components of the site were avoided. The landscape architect noted how using those instances as a teaching opportunity to effectively communicate with the contractor became integral later in the project. Specifically, for the Phase II rice paddies, there is only an 3.5-in vertical change from one cell to the next on average. Any inaccuracy in grading on that portion of the site would make it impossible for the entire project to function and drain as designed. Explaining the key role of grading to the project’s success and functionality made a pivotal difference in the contractor’s buy-in on the importance of implementing the site grading as specified.
• Identifying a species to target for special attention early in the design process provided a framework for the entire design and created an opportunity for both the designers and community to understand how species-specific design could improve overall ecological diversity. Choosing to create habitat for the native Pacific chorus frog shaped the form and depth of the rice paddies and required a physical separation to limit colonization of the paddies by the invasive bullfrog.

Project Team

Landscape Architect: The Berger Partnership PS
Wetlands/Habitat Specialists: Dyanne Sheldon Ventures, Otak
Civil Engineer: Magnusson Klemencic Associates
Geotechnical: GeoEngineers
Athletic Field Design: D.A. Hogan & Associates
Lighting and Electrical: Sparling Electrical
Irrigation: Dragonfly Irrigation
General Contractor: Ohno Construction
Public Outreach: Norton-Arnold & Company

Role of the Landscape Architect

The landscape architect led a team of wetland, habitat, civil engineering, geotechnical, athletic field, lighting and electrical, irrigation, and public outreach specialists to create the master plan, develop the design, and construct the project.