What is a SuDS swale?

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. The CIRIA SuDS Manual (C753) provides detailed UK design guidance.

How do I calculate swale storage volume?

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.

What gradient should a swale have?

Swales typically have a longitudinal gradient of 1:100 (1%). Steeper gradients increase flow velocity and reduce treatment effectiveness. On slopes steeper than 1:50, check dams are needed to slow the flow and increase storage. Maximum velocity should be below 0.3 m/s for treatment swales.

What are check dams in a swale?

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. Check dams are essential on gradients steeper than 1:50 to maintain residence time for water treatment.

How wide and deep should a swale be?

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.

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Swale Calculator — SuDS Storage Volume & Excavation

Calculate swale storage volume, cross-section area, excavation, check dams and residence time for SuDS design.

Catchment Area

Impermeable area draining to swale

Rainfall Intensity

Design storm (50mm/hr = 1:100yr)

Swale Length

Top Width

Depth

Excavation Price

How We Calculate This

This calculator determines swale dimensions, storage volume, excavation quantities and check dam requirements for SuDS design.

Storage volume

Trapezoidal: Volume = ((Top + Base) / 2) x Depth x Length
Parabolic: Volume = (2/3) x Top Width x Depth x Length
Check dams at regular spacing on slopes steeper than 1:100.

Catchment runoff

Runoff volume = Catchment Area x Rainfall Intensity (mm/hr) / 1000 per hour. The swale storage must accommodate this volume for the design storm duration.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.