Swale Calculator — SuDS Storage Volume & Excavation

A swale is a shallow, broad, vegetated channel designed to convey and treat stormwater runoff. As a SuDS component, swales slow water flow, allow infiltration and filter pollutants through grass and soil. They are commonly used alongside roads, paths and in open spaces. Other SuDS options include soakaways and rain gardens. The CIRIA SuDS Manual (C753) provides detailed UK design guidance.

Storage volume = Cross-section area x Length. For a trapezoidal swale, cross-section = ((Top Width + Base Width) / 2) x Depth. For a parabolic profile, cross-section = (2/3) x Top Width x Depth. The storage volume must equal or exceed the runoff volume for the design storm event.

CIRIA C753 (The SuDS Manual) recommends a longitudinal gradient between 0.5% and 6% (about 1 in 200 to 1 in 17), with 1% (1:100) a common treatment-swale figure. Steeper gradients increase flow velocity and reduce treatment effectiveness. On slopes steeper than approximately 3% (1 in 33), check dams should be incorporated to slow the flow and maintain residence time. Flow velocity should stay below 0.3 m/s in the 1-year 15-minute event for treatment swales.

Check dams are small barriers placed across the swale at regular intervals (typically 5-15m apart on slopes) to slow water flow and increase storage volume. They can be made from stone, timber or earth, typically 150-300mm high. Per CIRIA C753, check dams should be incorporated on gradients steeper than approximately 3% (1 in 33) to maintain the residence time needed for water treatment.

Typical UK swales are 1.5-3m wide at the top and 300-600mm deep with gently sloping sides (maximum 1:3 gradient for maintenance mowing). The base width is typically 30-50% of the top width for trapezoidal profiles. Larger catchment areas need wider, deeper swales. Side slopes must be gentle enough for safe mowing.