Biofiltration Swale

Biofiltration swale

A biofiltration swale uses grass or other dense plants to filter out sediment and oily materials. Swales often look like flat-bottomed channels with grass growing in them. A swale is usually dry, but after a storm, the runoff moves through it slowly and at a shallow depth. As stormwater passes through the plants, pollutants are removed by the combined effects of filtration, infiltration, and settling. Any standing water should drain fairly quickly.

Biofiltration swales provide treatment for pollution, but do not control the amount of stormwater passing through them. 

Virtual tour 

Click on the image below to get inside of a biofiltration swale and learn about common components and maintenance tasks.

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Maintenance needs commonly associated with biofiltration swales

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Sediment trap

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Trash screen 

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Mowing patterns 

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Inlets and outlets 

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Pet waste 

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Field inlet 

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Catch basin 

Maintenance sheets


Maintenance is needed if you see these signs

overgrown vegetation Problem Biofiltration Swale
Problem: Noxious weeds have taken over 
this biofiltration swale.

Bare, exposed soil
Clogged inlet and outlet pipes
Bottom of swale is eroded 
Sediment buildup, usually near inlet
Unhealthy or dead vegetation
Blackberries or other problem vegetation
Overgrown vegetation
Leaves, trash and other debris


Tips for fixing problems and general maintenance

Remove weeds such as blackberries and English ivy, then check for them on a regular basis and remove new vines.  Contact Clark County Vegetation Management for questions about controlling weeds at (360) 397-6140 or email 

Remove all trees and saplings that block facility elements.

Mow grass to keep height at 4 to 6 inches and remove clippings.

Replace vegetation damaged or removed during maintenance.

Avoid using fertilizers, herbicides, or pesticides within or near the facility.  These chemicals pollute the water and can cause unwanted plant growth.

Control erosion by reseeding areas where soil is exposed, especially on slopes.

Fill in eroded areas, and seed with grass.

Removing sediment is best done in dry months before winter rains set in.

If needed, seed and water during dry months to re-establish grass.

Pick up leaves before rains begin.

Inspect and remove debris regularly, particularly after storms.

Remove any material clogging drain, outfalls and channels.

Check roads and fencing in the facility.  They must be maintained to allow easy access.