Let Me Tell You About My Bioswales: Managing Stormwater in Portland Gardens

Bioswale & Rain Garden Design

You guys: stormwater management is no joke.  As the climate changes, and we have to deal with more frequent and stronger storm surges, it’s something all of us will need to think about in our own gardens.  I’m reading a great book on it right now, actually, Sustainable Stormwater Management: A Landscape-Driven Approach to Planning and Design. The author, Tom Liptan, worked via the Bureau of Environmental Services for the City of Portland on lots of innovative stormwater management techniques starting many years ago.  He writes about the theory behind them, the different methods of implementation, how they benefit the environment, and provides lots of great examples.  Recently, in the Tabor-to-the-River project here in Portland, our neighborhood added lots of bioswales to city streets to help mitigate runoff for our sewer system and act as a natural filtration system.  They’re quite beautiful!: https://www.portlandoregon.gov/bes/45386

So, what’s a bioswale?  It’s a purposeful depression in the ground that filters stormwater on its way back to the water table.  The term is often used interchangeably with “rain garden” — and I understand the colloquial difference to be that rain gardens tend to be smaller, and more residentially-focused, while bioswales are focused on managing stormwater for larger, impervious surfaces (like a parking lot).  Personally, I tend to refer to rain gardens as more shallow depressions where water collects from either the rest of the garden or a downspout, and bioswales as features that carry water over some distance and have a deeper depression at the one end with an overflow.

Here at our house, we have roughly 1800 square feet of roof, resulting in LOTS of stormwater needing to be managed on our property.  When we moved in about 9 years ago we had downspouts connected to buried, flexible, perforated drain pipe, and we weren’t sure where they went or how long they had been there.  Over time we realized they were clogged with debris, and didn’t extend far enough from the house, so they weren’t doing anyone any good.  You can read all about the saga of our basement water issues here because of this inadequate system, but the result of many years of work is that we now have an entirely new foundation, new gutters & downspouts, solid 4′ ABS downspout extensions, and two bioswales on the property that seem to be doing the job! *knocking on wood*

One we added many years ago that collects water from 2 downspouts on the north side of the house, which travels under ground via ABS pipe to an outlet just next to a garden path.  You can see where it empties in the photo below where I circled the area.

Portland Bioswale / Rain Garden

Portland Bioswale / Rain Garden

There is an initial depression in the soil that’s filled with river rocks, and then it overflows right down across the garden to the front of the property where there’s a much larger depression also filled with river rocks.  The edges of the water’s path are planted with (mostly) natives, including Snow Berry / Symphoricarpos, Dwarf Red-Osier Dogwood / Cornus sericea ‘Kelseyi’, Sword Fern / Polystichum munitum, Licorice Fern / Polypodium glycyrrhiza, Kinnikinnick / Arctostaphylos uva-ursi, Beach Daisy / Erigeron glaucus, and Oregon Grape / Mahonia repens.

Portland Bioswale / Rain Garden

Portland Bioswale / Rain Garden

The photo below shows where the water ends up in heavy rains, right near the retaining wall at the front of our property.  In winter, the path of the water is much more visible, and in summer it’s mostly covered in foliage.  There’s nothing under the river rocks that carries the water other than weed barrier, so water can percolate into the soil the whole way.

Portland Bioswale / Rain Garden

On its way through the downspouts, the water is filtered by these downspout filters that we spray painted to match our gutters.  They are sized to fit our larger, 3×4″ downspouts without constricting the flow of the water.  The filter is primarily a piece of open sponge-like material at the bottom, and you can easily remove them to empty them & keep them clean.

Portland Bioswale / Rain Garden

Portland Bioswale / Rain Garden

More recently, on the other side of the property, we added a second bioswale.  This one manages water from the downspouts on the south side of the house.  And as part of this process we added a rain chain.  I’ve wanted one for a long time because I think they’re beautiful, and because it’s visible from the front windows of our house.  This means you can sit inside in winter in front of the fire, and watch the rain as it cascades down.

Portland Bioswale / Rain Garden

We added a collection bowl at the bottom to keep the chain from swaying around too much in the wind, and some stones behind to manage splashing.

Portland Bioswale / Rain Garden

Here (below) you can see it from the front path.  The water is carried by a shallow rill that’s lined with pond liner & covered in stones, around the corner of our driveway and to the bioswale itself, which is unlined to allow water to percolate.

Portland Bioswale / Rain Garden

You can also see our rain barrels, which are now connected to a downspout just behind our fence.  The rain chain actually replaced a downspout that used to travel along the corner front porch column, which I hated to look at, so we just moved the downspout a little further back so we wouldn’t lose capacity and cause our gutters to overflow. (Please ignore the fact that our house needs to be painted — we know & we’re saving up to do that soon)

Portland Bioswale / Rain Garden

Like the other bioswale, there’s an outlet from buried 4″ ABS carrying water from the back corner of the house, which combines with the outlet from the overground water from the rain chain.  I’ve circled below where the ABS outlet is placed.

Portland Bioswale / Rain Garden

Below you can see the same outlet (right), and also where the overflow from this bioswale (left) carries water through a garden bed, through our driveway wall, and into a dry well in our driveway, which itself has an overflow into the sewer.  It basically acts like a bathroom sink or tub where once the basin is close to overflowing, the overflow drain carries the excess water away.  And when the ground is really saturated, and it’s raining buckets, the entire system is in use & it’s quite the spectacle.  I’ve been known to go outside in a rain coat to watch it all happening. At night. With a head lamp.

Next spring we’ll be adding more plants in and around this bioswale to help filter the water and make it more beautiful in all seasons. AND we’re gonna build a bridge over it that will become part of a garden path — stay tuned for that excitement!

Portland Bioswale / Rain Garden

Maintenance for these bioswales is minimal — we need to make sure the water is filtered on its way in & that those filters stay clear, we remove leaves/debris when it builds up in the basins, and make sure that the overflow path for the water (buried or over ground) makes for easy & efficient water flow without obstruction so it doesn’t back up.

Depending on your needs, these ideas can be utilized in any garden, of any size.  You can scale the concept up/down successfully.  Rain gardens & bioswales don’t have to be complicated — they need a place for water to enter, a way for that water to be filtered by plants, and a way for that water to make its way back into the water table away from structures where it can do damage.  If you have a shovel and some gumption, you can have one too!