Bioswale Design & Installation for Oregon Properties

What a Bioswale Does That a Plain Swale Doesn't

A standard drainage swale has one job: move surface water from point A to point B. A bioswale does that and more. It's a vegetated, engineered channel built to slow stormwater, filter pollutants out of it, and let a portion soak into the ground before the rest leaves the site. In a state where rain washes oil, sediment, and lawn chemicals straight toward streams, that treatment function matters.

Bioswales sit at the intersection of drainage and stormwater management. For Oregon properties — especially commercial sites facing stormwater requirements — they can solve a runoff problem and check a regulatory box at the same time. This guide covers how they're designed and built. For the broader picture, see our Oregon drainage guide.

Bioswale vs. Swale: The Real Difference

The two terms get used loosely, so here's the distinction that matters:

• A drainage swale is a shaped, often grassed channel that conveys water. Its purpose is conveyance — get the water moving and away. Our swale excavation in Oregon guide covers these.
• A bioswale is engineered for treatment. Beneath the surface it has a designed soil media — typically a sandy loam blend with compost — over a gravel drainage layer, and it's planted with deep-rooted, water-tolerant vegetation. Water ponds briefly, filters down through the media, and either infiltrates or collects in an underdrain.

Put simply: a swale moves water, a bioswale cleans and absorbs it on the way through.

Designing a Bioswale for Oregon Conditions

Good bioswale performance comes down to a handful of design decisions.

Sizing to the drainage area. The bioswale has to handle runoff from the surface it serves — a parking lot, a roof, a driveway. Larger contributing areas need longer or wider swales, or a deeper ponding zone. Undersize it and water overtops during the heavy PNW storms; oversize it and you've spent money on capacity you'll never use.

Longitudinal slope. Bioswales work best on a gentle grade — roughly 1 to 2 percent along their length. Too flat and water stagnates; too steep and it rushes through without time to filter, often scouring the channel. On steeper sites, check dams (small rock or vegetated steps) slow the flow and create treatment pockets.

Soil media. This is the engine. A free-draining engineered media lets water percolate at a useful rate while supporting plant life. Oregon's native clay won't do this on its own, so the existing soil is excavated and replaced with a designed blend over a gravel layer.

Plantings. Deep-rooted native and water-tolerant species hold the soil, take up water, and keep the media porous. Oregon-friendly choices include rushes, sedges, and tolerant native grasses that handle both wet winters and dry summers.

How a Bioswale Is Built

Installation follows a clear sequence:

1. Excavate the channel to the designed cross-section and grade.
2. Place the gravel drainage layer and, where needed, a perforated underdrain pipe to carry treated water to the outfall.
3. Add filter fabric between layers to keep fine soil from clogging the gravel.
4. Backfill with engineered media to the design depth.
5. Plant and mulch with the specified vegetation.
6. Stabilize the inflow points with rock so incoming water doesn't erode the channel.

Erosion control during and after construction is essential — a freshly built bioswale is vulnerable until plants establish. Our erosion control excavation guide covers the protective measures.

The DEQ and Stormwater Credit Angle

Bioswales are recognized stormwater management practices. On commercial and larger sites, an engineered bioswale can count toward stormwater treatment and infiltration requirements, helping a project meet local and state expectations. The exact credit depends on the jurisdiction and the project's stormwater plan, so design should be coordinated with the governing agency. The takeaway: a bioswale is often not just a nice-to-have, but a compliance tool.

When a Bioswale Makes Sense — and When It Doesn't

A bioswale fits when you have:

• Runoff carrying sediment or pollutants (parking lots, driveways)
• Room for a vegetated channel
• A reason to infiltrate or treat water on site
• A gentle slope to work with

It's less suited to tight lots with no space, sites with a very high water table (where infiltration can't happen), or situations where you simply need to move water fast and a plain swale or pipe does the job for less. For a cost comparison against simpler options, see our yard drainage cost guide.