A stormwater retrofit makes space for a new park on Puget Sound.

By Betsy Anderson, Associate ASLA

A ferry ride away from Seattle, Washington’s Kitsap Peninsula is a deeply lobed fjordscape whose 216 miles of shoreline have provided the perfect laboratory for development of so-called end-of-pipe stormwater solutions: space-saving treatment occurring at the bottom of the watershed, near aging municipal outfalls. These stormwater retrofits were most recently tested in Manchester, a close-knit village piled up at the peninsula’s eastern edge. Here, a former gas station site was recruited to treat 100 million gallons of polluted stormwater each year, before it enters Puget Sound.

Tucked between homes and businesses and adjacent to a busy swimming beach, the half-acre parcel now known as Manchester Stormwater Park inspired the project team—a cadre of Kitsap County staff plus engineers and landscape architects from Parametrix, N. L. Olson & Associates, and GeoEngineers—to take a multifunctional approach. “People really wanted a green space here,” says Jens Swenson, ASLA, a landscape architect for Parametrix who led the hardscape, lighting, and plant design for the park.

The green space was hard-won. Funded by a million-dollar grant from the Washington State Department of Ecology, the facility could only implement state-approved treatment methods. At the time, the only sanctioned end-of-pipe treatment method was a sand filter, “which would have covered the whole lot and treated only 30 acres, and not the full 100,” says Tim Beachy, Kitsap County’s project manager for the park.

“We couldn’t have a stormwater prison here, with a chain-link fence around a big sand field,” says Chris May, director of the Stormwater Division for Kitsap County Public Works. “That was when someone said, ‘Those Filterra tree box systems work really well—couldn’t we scale up that concept?’” The team convinced the state to take a risk on a super-sized application of the engineered filter media, which had never been attempted.

Designed for a 32-inch-per-hour infiltration rate and to accommodate flows in excess of 2,000 gallons per minute, the concrete cells—though technically a retrofit—are sized to meet state stormwater treatment requirements for new development, capturing 91 percent or more of the basin’s runoff volume. In a storm event, runoff enters the four cells through conveyance piping and is distributed into the filtration media via 18 channels. The seven-foot-deep cells wrap the site, leaving enough space to stage farmers’ markets and other events.

Anchoring the northwest corner of the park is a concrete spiral, likened by community members to a giant nautilus. Here water emerges at intervals and is cleaned while it travels through a coil of bioretention cells. More than a splashy feature, the structure relieves the treatment cells of the constant low flow of groundwater that crosses the site. This was crucial because the expensive treatment cells need to dry out between storms to ensure optimal performance.

Imagined by Swenson as a functional answer to the community’s desire for a fountain, the spiral playfully connects Manchester to its watershed. It also reflects a change in the approach to stormwater treatment along the Kitsap Peninsula. Instead of an unseen, untreated outfall extending deep into Puget Sound, water now runs a short distance under a walkway and is released across a pebble-strewn beach, where children safely play. “Our mandate now is that we don’t look at projects solely as stormwater projects,” May says. “It’s got to treat the water, that’s the first thing, but can we do something for the community at the same time?”