Tuesday, March 26, 2019

Green Line, Green Spaces

Now that Swift Green Line is operational, you may have noticed a "green" area behind the northbound station on 128th St. SE at 3rd Ave. SE.

A Swift Green Line bus pulls into the northbound station on 128th St. SE at 3rd Ave. SE, just in front of a Native Growth Protection Area and wetlands.
At first glance, it appears to be just a bunch of wild brush, but every leaf, stone, log, and blade of grass was actually placed there for a specific purpose. It's a Native Growth Protection Area (NGPA) that is intended to filter and clean water runoff from 128th St. before it joins the nearby wetlands to the east, and, eventually, North Creek.

Runoff from 128th St. comes into the area via street-level storm drains, filters through the vegetation and soil, and eventually seeps through holes in a pipe buried a few feet underground. It should be a slow process that often leaves the area damp and wetland-ish, but three years ago, the NGPA -- and the wetlands it drains into -- was drying up and dying.

Snohomish County, which is responsible for these areas, granted Community Transit special permission to rebuild the NGPA at its own expense in order to expedite the process and have the work complete in time for the opening of Swift Green Line.

The Native Growth Protection Area under construction in January 2018, before the Swift station was built.
After months of heavy construction to replace the underground stormwater drainpipe it was time to re-plant the area with native flora.

That's where Curtis LaPierre comes in. He's a senior landscape architect with Otak, the engineering firm contracted by Community Transit to rebuild the NGPA.

"We started from scratch to create a constructed biofiltration rain garden," Curtis said.

Landscape architect Curtis LaPierre reviewing design plans for the site.
Curtis and his team designed the area to slope like a natural swale (shallow ditch) then added layers of gravel, mulch and bioretention soil specially formulated to help clean the water before it enters the underground pipe.

Flood plants -- sedges, grasses and bulrushes -- were planted on the bottom of the swale. These native plants will thrive in the wet, marshy ground.

Sedges, grasses and bulrushes thrive in wet soil.
At strategic locations about midway up the swale wall, Curtis placed several logs, what landscape architects call "large woody debris."

"They function like natural shorelines," Curtis explained. "They will only occasionally be underwater -- perfect for amphibians and insects and the like."

Several strategically placed logs will become homes for frogs, insects and other damp-loving fauna.
The top of the slopes feature flowering shrubs like salmonberry and snowberry, and native trees like vine maple, hemlock and Douglas fir. They provide stability to the slope and, eventually, a visual screen to the residents of the condominiums on the other side.

The next time you're at the northbound Swift Green Line station on 128th St. SE at 3rd Ave. SE, take a peek at the rain garden. It's beautiful and functional, helping to clean stormwater runoff and protect nearby wetlands and streams -- all part of what makes Swift Green Line green.

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