Swale is a shallow, broad vegetated channel designed to convey, store and filter surface water run-off at low velocity. Swales are one of the most widely used conveyance components in UK Sustainable Drainage Systems (SuDS), replacing or supplementing piped networks while providing water quality treatment, amenity and biodiversity benefits.
How swales work in a SuDS management train
A swale intercepts run-off from adjacent roads, car parks or roofs and conveys it slowly towards downstream storage or discharge points. The dense grass cover reduces flow velocity, allowing sediment to settle and pollutants to be filtered through vegetation and, where infiltration is feasible, through the underlying soil. In the SuDS management train, swales typically act as source control and conveyance between upstream components (permeable paving, filter strips) and downstream storage such as basins, ponds or underground attenuation tanks.
Types of swale
Three forms are recognised in UK practice:
- Standard conveyance swale — a simple grassed channel that conveys and treats run-off. Suited to road verges and site boundaries.
- Dry (enhanced) swale — incorporates a permeable soil or gravel underdrain layer so the channel drains down quickly between storms. Preferred where standing water is unacceptable.
- Wet swale — retains a permanent pool or marshy conditions where the water table is high, maximising treatment but requiring greater land take and maintenance.
Swale design to The SuDS Manual (C753)
CIRIA C753 (The SuDS Manual) sets the UK design framework. Typical parameters: side slopes no steeper than 1:3, longitudinal gradient between 0.5% and 6% (check dams on steeper ground), maximum flow velocity around 1 m/s in the design event to prevent erosion, and a maximum design flow depth of 100–300 mm below the vegetation height for treatment events. Swales should convey the 1-in-30-year event within the channel and be checked against the 1-in-100-year event plus climate change allowance for exceedance routing.
Swales vs French drains and filter drains
A swale is an open, vegetated surface feature, whereas a French drain or filter drain is a buried gravel trench with a perforated pipe. Swales provide better water quality treatment, visibility for maintenance and amenity value; filter drains suit constrained corridors where surface features are impractical. The two are often combined, with a swale overlying a filter trench in an enhanced dry swale arrangement.
Related ViaCon solutions
Swales are frequently combined with underground storage where infiltration alone cannot manage the design event. See our ViaCon Storm Solutions for corrugated steel attenuation tanks and our stormwater solutions range. Related glossary entries: SuDS, surface water drainage and French drain.
Frequently asked questions about swale
What is a swale?
A swale is a shallow, broad vegetated channel that conveys, stores and filters surface water run-off at low velocity. It is a core conveyance and treatment component of Sustainable Drainage Systems (SuDS) in the UK, designed to CIRIA C753.
What is the difference between a swale and a ditch?
A ditch is designed purely to convey water quickly, often with steep sides and no treatment function. A swale is intentionally shallow and wide with gentle side slopes (1:3 or flatter) and dense vegetation, so it slows flows, settles sediment and filters pollutants as well as conveying run-off.
How deep and wide should a swale be?
Typical UK swales have a flow depth of 100–300 mm in treatment events, a base width of 0.5–2 m and side slopes of 1:3 or flatter. The channel should convey the 1-in-30-year storm and be checked for exceedance in the 1-in-100-year event plus climate change allowance.
Are swales considered SuDS?
Yes. Swales are a source-control and conveyance component in the SuDS management train, delivering all four pillars of SuDS design: water quantity, water quality, amenity and biodiversity.