S3: Management of Extreme Rainfall and Flooding

S3

What S3 requires

Source: Defra Non-Statutory Technical Standards for Sustainable Drainage Systems (2015, refreshed 19 June 2025), Standard S3. Implemented through CIRIA C753 §3 (Designing for water quantity), the Sewerage Sector Guidance (SSG) and Environment Agency climate-change allowances (EA, May 2022).

S3 addresses peak discharge control for rainfall events larger than everyday rain, ensuring that development does not increase downstream flood risk. The standard uses rainfall events defined by their rarity: the 50% Annual Exceedance Probability (AEP) event (1-in-2 year) and the 1% AEP event (1-in-100 year). These are determined using rainfall data for the site and uplifted for climate change using Environment Agency allowances.

Baseline: Greenfield Runoff Rate

For any site, the greenfield runoff rate is calculated as if the entire site were undeveloped (all grass or permeable soil). This is computed using the IH124 QBAR method (Marshall & Bayliss, 1994; see codes of practice section). The resulting rate varies widely depending on rainfall, soil drainage class, and catchment characteristics.

Discharge Limits

• 50% AEP (1-in-2 year) event: Peak discharge must be limited to the greenfield runoff rate OR 3 litres/second/hectare (l/s/ha), whichever is greater. This provides flexibility: sites in low-rainfall areas or with very high greenfield rates will discharge at the greenfield rate; sites in low-permeability soil or high-rainfall areas may discharge at 3 l/s/ha.
• 1% AEP (1-in-100 year) event: The discharge limit depends on whether the runoff volume from the site would exceed the greenfield runoff volume. If it does not (i.e., the site is retaining water or has high infiltration), then the 50% AEP limit applies. If the runoff volume does exceed greenfield (which is common for urban sites), then the discharge is limited to 50% AEP greenfield rate OR 3 l/s/ha, i.e., the peak cannot increase beyond the 50% AEP rate even though the total 1% AEP volume is larger.

Climate Change Uplift

The Environment Agency publishes climate change allowances for peak rainfall intensity (upper percentile projections). These are applied to rainfall frequency curves to inflate the rainfall depth for a given return period. The 2025 National Standards require the Upper End Allowance (95th percentile) to be applied, providing a robust design safety margin.

Urban Creep Uplift

The standard assumes that 10% of the impermeable area of the site will be replaced with slightly permeable or low-permeability surfaces over time (e.g., as paved areas age or owners modify gardens). This "urban creep" is accounted for by increasing the modelled impermeable area by 10% for discharge calculations. Exception: residential flats/apartments with no external private permeable space are assumed not to suffer urban creep (0% uplift).

Greenfield Baseline NOT Climate-Changed

A common point of confusion: the greenfield baseline rate (used as the discharge limit) is calculated for the current climate, NOT uplifted for climate change. Climate change is applied to the developed site's projected rainfall, but the baseline remains current-climate. This ensures that discharge limits are set relative to what natural, undeveloped land would produce today, not a hypothetical future greenfield baseline.

Flooding Within the Development

All rainfall up to the 1% AEP event must be managed within the development without flooding property, roads, or access routes. Exceedance routes (paths for water to flow if storage is exceeded) must be identified and designed to lead water safely away from buildings to areas that can accommodate it.

Sizing Approach: Designers typically use hydrological modelling (SWMM, HEC-HMS, or UK-specific tools) to route rainfall through the development, calculate peak flows and storage volumes, and size SuDS features accordingly. The Attenuation Storage Calculator in the Catchment can help determine storage depth and orifice sizing.

→ Attenuation Storage Calculator