The Tale of the Swale on Yale

South Lake Union is home to forward-thinking environmental design. One of the most innovative design features, a form of green infrastructure, is what’s known as the Swale on Yale. It’s two swales actually (one in the 400 block of Yale St. and the other parallel to it on Pontius St.), and it’s about to swell to two more (just south of both locations in the 300 block of both streets).

The swales give South Lake Union a bit of moorland feel, but beyond the aesthetics these stretches of grassland are working to treat Capitol Hill storm water roadway runoff before it reaches Lake Union.

Technically the swales are known as the Capitol Hill Water Quality Project, a public private collaboration between the city of Seattle and Vulcan Real Estate; KPFF Consulting Engineers also played a key role.

“When the swales were planned (in the early 2000s),” wrote Jason Sharpley of Seattle Public Utilities, in an email exchange, “there were no regional scale biofiltration swales treating stormwater from ultra-urban roadways that we were able to find.”

“Typically, swales were used on a more limited, roadway scale to treat and convey stormwater runoff from the adjacent roadway.”

The project was so unique that the Seattle Design Commission created a special award, an “In the Works “ Excellence Award, that they won in 2011. The swales came online in 2015.

Working swale on the 400 block of Pontius St.

“Since completing the first pair of swales there has been a lot of interest and there may be new systems in other cities.”

Seattle’s steep slopes helped propel the innovation. “We have the right topography for this,” says Dave Schwartz of KPFF Consulting Engineers. The slopes make it easier to divert water to where you want it, which makes cleaning it easier too. And that’s what the swales do, filter and clean. They’re made up of densely planted grasses, “a mixture of sedges, which have edges, and rushes, which are round,” says Schwartz describing his mnemonic means of distinguishing them. 

They clean roadway runoff that “includes everything that you see, and don’t see, that is on the roadway,” says Sharpley.  “This includes brake dust from cars that carries copper, dissolved metals from galvanized fences, and bacteria from wildlife and pet waste. The swales and pretreatment that make up the Swale on Yale system do a good job of removing a significant portion of the pollutants.”

The Swale on Yale couldn’t have been done without developer help, says Schwartz, stressing the huge role that Vulcan Real Estate played in making the public private partnership happen. Vulcan provided technical and profession assistance along with contributing about $1.3 million toward design and construction. Most critically they provided the easement to the city. Developers are playing key roles in creating environmental projects that provide a greater good, says Schwartz, noting another public private project under the Aurora Bridge, rain gardens catching bridge water runoff. “Not all try to just make money and destroy the world,” he added.

Planted in rain garden soil which is a bonus for filter, the grasses catch toxic sediment from the water as it takes its time to mosey through the swale, at least nine minutes.

But even before the water reaches the swales it’s run through a diversion tank that uses centrifugal force to flush out “floatables,” a nice name for trash such as cups, straws, and cigarette butts.

From the diversion tank, controlled amounts of water are released into the swales evenly so as not to overflow them and to keep their integrity intact.

The swales then drain into a discharge pipe and the water is released to the lake.  “The water is not drinkable,” says Sharpley, “but significantly cleaner than when it entered the swale.” 

New swale plantings on the 300 block of Pontius St.

The two new swales will come online once the plants mature. For now they look like woven works of art running between the sidewalk and roadway. Once they are put to work, the system will be able to treat the full design flow of 7.2 cubic feet of water per second, which is more than 3,000 gallons a minute. The older swales treat half that capacity today. The full swale build-out will treat 435 acres of storm water runoff from Capitol Hill’s 630-acre basin.

The Swale on Yale captures the dirtiest water from both small storms and the early runoff from larger storms. Thanks to this pioneering green infrastructure, Lake Union is much cleaner than it otherwise would be and could become cleaner still with even more projects like the Swale on Yale.

The Swale on Yale — 400 block, with city workers maintaining it.