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UPDATED: Will rain gardens on Pacific Avenue in downtown Tacoma be sidewalk swamps? Designers say no

Post by Kathleen Cooper / The News Tribune on Nov. 17, 2011 at 10:10 am |
November 21, 2011 12:55 pm

Downtown Tacoma observer and community activist Erik Bjornson wrote a letter to the editor that we published today about the rain gardens planned for downtown. He says they’re “little more than ditches that accumulate garbage, stagnant water, algae and require high maintenance” and suggests Tacoma is headed toward a boondoggle like Ballard.

I thought I’d share a little of what I learned at the last two Pacific Avenue open houses about the rain gardens, because I too wondered about the water accumulation. I wondered if they’d be a sidewalk swamp, and I wondered about clumsy people (like me) stumbling into them, or dogs, ahem, taking advantage of them. At the first meeting, in October, I hadn’t heard of the Ballard issue but someone brought it up.

A mockup of what a raingarden might look like near the Chamber building, across the street from the Washington Building (Design credit: AHBL)
This mockup shows a rain garden near Pacific Plaza. Credit: AHBL Inc.

Doreen Gavin, a principal at AHBL Inc., the Tacoma firm working on the rain garden design, said the rain gardens in Ballard were poorly designed. The ones in Ballard didn’t have a drain. They were designed to completely absorb all stormwater into the ground, preventing extra water from flowing through the stormwater system. They didn’t work. They turned swampy. Click here to read a thorough story by the SeattlePI.com on the issue.

Gavin said the rain gardens in Tacoma will be much different. The Tacoma rain gardens won’t try to absorb all water.

“We’re filtering (stormwater) through eighteen inches of special soils, then it’s picked up in a pipe” and carried away, she said. They also will be filled with plants. (I know nothing of plants so I’m trying to get more detail on exactly what kind we’re talking about here, and why they would be chosen. Here’s a slide from the presentation in October with some examples. This rain garden would go on the north end of Pacific Avenue, I believe near Old City Hall.)

Concept for downtown Tacoma rain garden with plants. Credit: AHBL Inc.

I’ve called Gavin for even more detail on the rain gardens, and I’ll update this post when I learn more.

UPDATE, 1:50 p.m.: I dove deep into the details with Gavin. Here’s what I learned.

In Ballard, the rain gardens had two jobs: to filter out the bad stuff in the storm water, and to help control the flow into the city’s storm water system, which was overflowing a lot. So the goal was to soak the water into the ground and keep it out of the treatment system all together. In Tacoma, the rain gardens’ only job is to help clean the storm water before it flows into the city’s system.

“We know that the soils under downtown aren’t very good,” Gavin said. “We know that we would not have success trying to infiltrate.”

Rain gardens are “are one of the most effective techniques” to filter storm water, Gavin said. Other cities agree. They’re popping up across the country, from Lansing, Mich., to St. Louis, which finished one in its downtown just this week. And they’re all over Portland, a city that has so many different environmentally friendly ways to treat storm water, they almost blend into the landscape there.

Gavin estimates the Tacoma gardens will suck in 91 percent of the rainwater on Pacific Avenue, and more than 80 percent of typical urban pollutants will be removed.

But wait, you might say, what does stormwater cleaning have to do with this? I thought this project was about improving downtown. It is, but a lot of the money is coming from sources intended to improve stormwater runoff. Our stormwater runs into Commencement Bay, which has been quite polluted, and there’s a commitment to make it better.

So, back to the swamp issue. Tacoma’s rain gardens will drain three ways: First, a perforated pipe under the soil, called an underdrain, will pull in the water and whisk it away. Second, if it rains so quickly that water builds up past 3 inches, it will flow into a separate grate along the surface. Finally, in a gully-washer, the water will run out of the garden, down the gutter and into the grate at the end of the block, like all the water does now.

The soil is 18 inches of sand and compost – “a special mix that helps filter the pollutants out of the storm water,” Gavin said. It’s good for growing things, too. Some of the illustrations I’m running here show a golf-course grassy look, but my understanding is it will be more rugged than that. That’s important for many reasons, not the least of which is to make it discouraging to dogs.

How many will there be? This phase of Pacific Avenue streetscaping handles the area from South 7th to 17th streets. There will be about two rain gardens per block, Gavin said, ranging in size from 200-500 square feet. (Remember downtown has double-blocks, so we’re talking about a dozen rain gardens.) Their design is site-specific, so they’ll all be different. I’m working on getting a map of all the locations.

What about maintenance? Gavin said they’re designed to be easy, especially after the vegetation has grown in. Key issues are trash, animals and weeds. Gavin said the City of Tacoma and the downtown Business Improvement Area, which represents downtown property owners, still are working on dividing maintenance duties, but it’s logical that the city would maintain parts of the gardens consistent with their stormwater system upkeep, and the BIA would handle the immediate issues like trash.

I’m still working to connect with another person at AHBL who is the plant expert, and I’ll post more when I know more.

UPDATE, 3:15: Meghan Montgomery, landscape architect project manager at AHBL, just talked plants with me. Here’s what I learned.

Outside of the rain gardens, about 80 new trees will be planted along the blocks from South 7th to South 17th streets. They will include varieties of oaks, elms, sweetgum (that don’t produce spiny ball-like fruit), sour gum, sycamore, honey locust and ginkgo (only the male trees, because the female trees create smelly seed shells). The City of Tacoma’s arborist has approved the plan. Update, Nov. 21: I got this wrong. The arborist has not approved the final plan, though she has reviewed preliminary ideas and requested changes including the removal of honey locusts.

Back to original post: The trees will be planted quite large, with the goal being little-to-no obstruction of ground-floor businesses along the route. And as they grow, the lower limbs can be removed, shifting the canopy higher.

In the rain garden, low-maintenance native plants are the key.

“We’re looking at this in terms of what will look interesting in all seasons and have texture, and also will look great in a minimal maintenance situation. So it will be big blocks of planting. It doesn’t need to be staked. It doesn’t need to be pruned,” Montgomery said. The plants chosen will include the red-twigged dogwood, the dwarf variety, which is red in the winter; has a bloom in the spring, and lots of leaves in the summer. Other plantings will include native plants such as the Quartz Creek Rush grass, which is blue-green, and a native Iris, which has many different varieties.

“This isn’t something that visually you’ll want to step foot on,” Montgomery said. It will be densely planted.

The rain garden soil also will have about 6 inches of drain rock under it.

“These have excellent drainage. We’ve looked at the City of Shoreline – they just did a rain garden system,” she said. “It’s not visually similiar, but in terms of draininge and function, it’s similiar. And Portland has so many rain garden systems, they’re really setting the pace.”

The rain gardens planned for downtown Tacoma is one of the larger settings for urban rain gardens, Montgomery said, but “we’re definitely not breaking new ground.”

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