Swale & Bioretention Calculator — SuDS Design Storm Volume

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 core component of Sustainable Drainage Systems (SuDS). In England, SuDS are required through the planning system (National Planning Policy Framework and the non-statutory technical standards for SuDS); the statutory SuDS Approving Body regime under Schedule 3 of the Flood and Water Management Act 2010 is in force in Wales but has not been commenced in England (as of June 2026).

Bioretention (also called a rain garden) is a shallow planted area with an engineered filter medium that treats and infiltrates stormwater. Water ponds as open water on the surface (a maximum temporary storage depth of 150-300mm per CIRIA C753), then percolates through the filter medium (normally 750-1000mm of sand/compost mix, with 400mm an absolute minimum) and is collected by a perforated underdrain pipe. It removes pollutants very effectively.

For SuDS design in the UK, size the conveyance for the 1-in-30-year event (it should be conveyed before overland flow is permitted, per CIRIA C753 ch.24.11.1) and manage the 1-in-100-year event plus a climate-change allowance for exceedance (use the current gov.uk peak-rainfall allowances — commonly +40% for the upper-end allowance). A typical 1-in-30-year, 1-hour storm in England deposits roughly 30mm of rainfall, but you must use site-specific depths from the Flood Estimation Handbook (FEH) rainfall data for your location.

The filter medium is normally 750-1000mm deep (with 400mm an absolute minimum for very small catchments) and is typically sand-based with some organic matter for plant growth, per CIRIA C753 ch.18.9.2 (Box 18.1). A transition layer of at least 100mm (or a geotextile) prevents fines washing into the drainage layer, which gives at least 100mm cover to a perforated underdrain. The medium supports planting and filters pollutants from the runoff.

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. Use our Storm Water Attenuation Calculator for sizing overflow storage.