Biofiltration swale

Biofiltration Swale
Good: Well maintained 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. 




Maintenance sheets



Basic biofiltration swale
Wet biofiltration swale
Field inlet
Sediment trap
Energy dissipater
Access road

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.


Additional elements to swales

Field inletField inlet square med.jpg

A field inlet is a concrete structure that collects stormwater and routes it through underground pipes. Fitted with a slanted, slotted grate, it often traps sediment and debris. Regular maintenance is important. Keep the opening clear of obstructions. A field inlet is usually cleaned by a truck with a  vacuum hose but sometimes can be cleaned with hand tools.

Biofiltration With Sediment Trap
Good: Biofiltration swale showing
sediment trap in foreground.

Sediment trap

A sediment trap is a concrete structure fitted with a slotted grate or multiple slotted grates (debris barriers). Storage area below the outlet pipe allows sediment and debris to settle out of the stormwater runoff. Regular maintenance is important to remove trash, vegetation and sediment buildup.

Problem Sediment Trap
Problem: This trap needs cleaning.