A swale is a broad, shallow, vegetated channel that forms a key component of sustainable drainage systems. Swales serve a dual function: they convey surface water along their length while simultaneously providing treatment through filtration, settlement, and biological uptake of pollutants.

There are three main types of swale:

Dry swales: Designed to drain down between storm events, with an underdrain (perforated pipe in a gravel trench) beneath the channel. Suitable for most soil types and the most commonly used in UK developments.
Wet swales: Designed to retain a permanent or semi-permanent water level, functioning similarly to a linear wetland. These provide excellent water quality treatment and biodiversity value but require sufficient baseflow or groundwater.
Infiltration swales: Constructed without an underdrain, allowing collected water to infiltrate directly into permeable soils. These combine the functions of a conveyance swale and an infiltration feature.

Design considerations for swales include:

Side slopes: Typically no steeper than 1 in 3 for maintenance access and safety
Longitudinal gradient: Between 1% and 4% for effective conveyance without erosion
Manning’s roughness: Reflecting the vegetated surface to slow flow velocities and promote treatment
Check dams: Low weirs placed across the swale to increase residence time and attenuation storage

Swales are particularly effective along roads, at the edges of car parks, and as boundaries between development parcels. They occupy linear space that often aligns with landscape buffers or open space requirements, making them a cost-effective SuDS solution.

Aegaea incorporates swales into drainage designs as part of the SuDS management train, as outlined in our SuDS design guide.