Stormwater management - Southwest Washington

Biofiltration swale

Above: a well maintainted bioinfiltration swale. Below: a biofiltration swale showing a sediment trap in the foreground.

Bioinfiltration swale with sediment trap




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. 

Signs that maintenance is needed and a checklist for maintaining the facility including how to fix problems.

Additional elements to swales

Field inletField inlet
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.

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.