Bioinfiltration Swale

At Peninsula College, Port Angeles WA

A bioinfiltration swale is a heavily vegetated open channel that moves water slowly. It is specially engineered to convey stormwater. The grass-like plants help filter out oils, sediment, and other roadway contaminants.

Peninsula College is located on the Olympic Peninsula, nestled into the hillside above Port Angeles Harbor. With more than 35 inches of rain annually, stormwater runoff is an issue, especially because the terrain slopes down to the harbor. The parking lot was built decades ago and retrofitted with bioinfiltration swales around 2012.

Aerial - oblique of parking lot, city, harbor
Aerial showing Peninsula College with Port Angeles Harbor in the distance. Image courtesy of Google Earth.
Interpretive sign explaining bioinfiltration swales
This sign is installed at the site to provide more information when you visit. Created by Cameron Dunn. Left side graphics by Andrew Mack.

Planning and Design

The College wanted to manage rain runoff from the main parking lot which slopes downhill from south to north. They decided to build bioinfiltration swales throughout the existing 162,700 square foot parking lot. Their goal was to move the water slowly down gradient and allow it to infiltrate to remove pollutants.

They planned 600 feet of bioinfiltration swales along the main street running from west to east along the northside of the parking lot. Then they added 870 feet of smaller swales within the parking areas, running from south to north (downhill) toward the main swale. Together, these swales treat more than two million gallons of stormwater runoff, reducing flow and pollutants from entering Port Angeles Harbor.

construction diagram for biofiltration swales at Peninsula College
Construction drawing for the bioinfiltration swales at Peninsula College. Courtesy Peninsula College.

In this case, they used perforated underdrain pipes because of the large volume of water to avoid ponding in the swales. There are also notches in the curbs allow water to flow directly into the swales from many locations.

Here is an aerial view of the parking lot before and after installation.

Aerial of parking lot

Parking lot before bioinfiltration swales

Note the lack of swales or vegetation. Image courtesy of Google Earth archive 2000.
ariel photograph of parking lot after bioinfiltration swales were constructed

Parking lot after bioinfiltration swales constructed

Note the swales fitted in between the parking areas. There are four running south to north and two along the main street. Image from Google Earth archive 2013.


Bioinfiltration swales work, in part, because they are heavily vegetated. The grass-like plants used in a bioswale help filter out oils, sediment, and other roadway contaminants, by transporting them from impervious surfaces such as roads, driveways, and parking lots. Water is moved slowly along the swale, allowing water to filter through the plants and infiltrate into compost-amended soils below. In this case, Peninsula College chose to extensively landscape the entire area But the swales were originally planted (approximately 2011) with a bioinfiltration swale bottom seed mix that included:

  • 75% Festuca arundinacea – Tall Fescue
  • 15% Agrositis tenuis – Colonial bentgrass
  • 10% Agrositis alba – Redtop

The landscape plan notes that the plantings required irrigation and re-seeding should be done with native forb and wildflowers, not grass. Here are cross-sections of the landscape plan. Note they planned 12″ of amended soil and a 24″ planting depth.

View of cross section of swale landscaping

Landscape plan for plantings in the swales

Courtesy of Peninsula College.
View of cross section of swale

Landscape plan for swale along main street.

Courtesy of Peninsula College.


Today the trees along the bioinfiltration swales at Peninsula College are maturing and the swales are functioning as planned. Here’s what is looks like ten years after construction.

Aerial view of parking lot in 2021.
Aerial view of the parking lot in 2021. Courtesty Google Earth.

The swales are mowed regularly, as needed to remove biomass and allow for the continued flow of water.

Building your own

If you want to move water safely from one place to another on your property to alleviate ponding and improve water quality a bioinfiltration swale may work for you. While the bioinfiltration swale system at Peninsula College is very large, they can be built more simply and at a smaller scale. Please see the WSU Fact Sheet Bioinfiltration Swales (PDF) to learn more. View additional information on the Bioinfiltration Swale resources page.