What is a SuDS swale?

A swale is a shallow, wide, vegetated channel that conveys and treats surface water runoff. It slows flow, filters pollutants through vegetation and allows infiltration into the ground. Swales are a key SuDS (Sustainable Drainage Systems) component required under Schedule 3 of the Flood and Water Management Act.

What is bioretention?

Bioretention (also called a rain garden) is a shallow planted area with engineered filter medium that treats and infiltrates stormwater. Water ponds on the surface (typically 100-300mm depth), then percolates through 600mm of filter medium (sand/compost mix) and is collected by an underdrain pipe. It removes pollutants very effectively.

What design storm should I use?

For SuDS design in the UK, use the 1-in-30-year storm for conveyance and the 1-in-100-year storm plus 40% climate change allowance for exceedance management. A typical 1-in-30-year, 1-hour storm in England deposits about 30mm of rainfall. Check the EA Flood Estimation Handbook for your location.

What filter medium is used in bioretention?

The standard filter medium is 600mm deep and consists of approximately 60% sand, 20% topsoil and 20% compost by volume. It must have an infiltration rate of 100-300mm/hr. Below this, a 150mm transition layer of pea gravel covers a perforated underdrain pipe. The medium supports plant growth and filters pollutants.

Do I need an overflow for a swale?

Yes. All SuDS features must include an overflow for storms exceeding the design capacity. For swales, this is typically a weir or pipe at the maximum water level. For bioretention, an overflow standpipe connects to the underdrain. The overflow route must be planned to avoid flooding buildings.

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Swale & Bioretention Calculator — SuDS Design Storm Volume

Design sustainable drainage (SuDS) swales, bioretention areas and detention basins with storm volume, cross-section and filter medium calculations.

Catchment Area

Impermeable area draining to SuDS

Design Rainfall Depth

1-in-30-year storm depth

SuDS Type

Swale Length (optional)

Leave 0 for auto-sizing

Price Per m² (optional)

Installed cost per m² of SuDS feature

How We Calculate This

This calculator sizes SuDS features based on the design storm volume and catchment area.

The formula

Storm Volume = Catchment Area × Rainfall Depth / 1000

Swale Length = Storm Volume / Cross-Section Area

Bioretention Area = Storm Volume / (Ponding Depth × Porosity)

Typical dimensions

Swale: 0.6m deep, 3:1 side slopes, 0.6m base
Bioretention: 0.3m ponding, 0.6m filter medium
Detention basin: 1.0m max depth, 3:1 slopes

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.