SuDS Design for Small Sites: CIRIA C753, Attenuation Calculations, Infiltration Testing and Approval Process

Summary

Sustainable drainage replaces the traditional approach of routing all surface water to soakaways or combined sewers as quickly as possible. Instead, SuDS manage rainfall close to where it falls, using a hierarchy: prevent (permeable surfaces), store (swales, ponds, tanks), release slowly, and convey as a last resort. The aim is to mimic pre-development runoff rates — meaning the development should not increase flooding risk downstream.

For small residential and commercial sites, SuDS design is increasingly a planning condition and sometimes a statutory requirement. The planning engineer or drainage engineer on the project needs to produce a drainage strategy that demonstrates runoff rates and volumes have been managed. This document accompanies the planning application and, in Wales, goes to the SuDS Approving Body (SAB) for approval before construction can start.

The technical complexity ranges from simple permeable paving calculations for a single house extension, to multi-element attenuation systems with overflow routes for larger developments. CIRIA C753 (SuDS Manual) is the definitive technical reference, supplemented by local planning authority SUDs guidance which varies significantly by region.

Key Facts

• CIRIA C753 — SuDS Manual (2015); the primary technical guidance document; produced by CIRIA; covers all aspects of SuDS design
• Greenfield runoff rate — The rate at which rainfall leaves a site in its undeveloped natural state; measured in litres per second per hectare (l/s/ha)
• Brownfield sites — Must achieve greenfield runoff rates in most English planning policies; may use pre-development brownfield rate where lower
• Minimum standard (England) — Runoff should be limited to greenfield rate for all storm events up to and including 1:100 year event plus 40% climate change allowance
• Climate change allowance — For SuDS in England: apply 40% increase to rainfall intensity per DEFRA guidance (from 2016, applies to designs with >50 year design life)
• Greenfield runoff calculation — BRE Digest 365 method: Qbar = 1.56 × SAAR^0.88 × AREA^0.89 (simplified); or FEH HOST method preferred
• Infiltration testing — BRE Digest 365: soak test method; Ksat (hydraulic conductivity) must exceed a threshold for soakaway/infiltration to be viable
• SAB approval (Wales) — Mandatory for all new construction disturbing more than 100m² of surface; approval by local authority SuDS Approving Body
• Lead local flood authority (LLFA) — Responsible for strategic local flood risk management in England; consulted on SuDS in planning applications
• NPPF 2023 — National Planning Policy Framework: local planning authorities should ensure development avoids increased runoff; SuDS preferred
• Exceedance route — Required in all SuDS designs: a designed route for flows exceeding system capacity; must not flood buildings
• Water quality — SuDS treat water quality; treatment train approach: source control → conveyance → site-level attenuation

Quick Reference Table

Detailed Guidance

Design Hierarchy: The SuDS Triangle

The SuDS design hierarchy (CIRIA C753) prioritises:

1. Source control (highest priority): Prevent runoff or deal with it where it falls — permeable paving, green roofs, rainwater harvesting, filter strips
2. Local conveyance: Move water slowly across the site — swales, filter drains, bio-retention features
3. Site-level attenuation: Store water on site and release slowly — detention basins, ponds, underground tanks
4. Regional systems: Only when site-level management is genuinely insufficient

For small residential sites (single house, small housing scheme), source control and simple soakaways usually suffice. For larger developments, a treatment train combining multiple elements is expected.

Infiltration Testing: BRE Digest 365 Method

Before designing any infiltration-based SuDS, the soil's percolation rate must be tested. The BRE Digest 365 method:

Field test procedure:

1. Excavate test hole to the proposed soakaway depth (or to base of any proposed infiltration feature)
2. Fill test hole with water and allow to drain — repeat twice to saturate soil (takes 24 hours typically)
3. Refill to full depth; measure time for water level to fall a specific distance (the h measurement depends on hole size — use BRE 365 guidance)
4. Calculate Ksat in m/s

Minimum Ksat for infiltration viability:

• BRE 365: if Ksat > 1 × 10⁻⁶ m/s, infiltration is viable
• If Ksat < 1 × 10⁻⁷ m/s, infiltration is not viable and attenuation required
• Between: may be viable; detailed design needed

Ground conditions that prevent infiltration:

• Clay soils (low Ksat; almost always fail BRE 365)
• Made ground / contaminated ground
• High groundwater table (within 1m of soakaway base)
• Proximity to slope (risk of groundwater emerging on slope face)
• Within protection zone of water abstraction point

Minimum separation from soakaway to services:

• 5m from any foundation
• 3m from any service pipe (water, gas, electricity)
• 2m from site boundary
• 10m from any building if in contaminated ground

Greenfield Runoff Rate Calculation

The design must demonstrate that post-development runoff rates do not exceed greenfield rates.

Method 1 — Institute of Hydrology (FEH) simplified approach:

Qbar_greenfield (l/s) = 1.56 × AREA^0.89 × SAAR^0.88 / 1000

Where:

• AREA = site area in hectares
• SAAR = Standard Annual Average Rainfall in mm (from FEH data for the site)
• Result in m³/s × 1000 = l/s

This gives the mean annual flood rate — used as a starting point for more detailed analysis.

For small sites: Use the simple 2 l/s/ha rule of thumb as a conservative target where detailed calculation is not required by the planning authority. Always check local guidance — some LPAs specify a lower rate.

Attenuation Volume Calculation

Where infiltration is not viable, surface water must be attenuated and discharged at the greenfield rate.

Volume calculation steps:

1. Determine inflow volume: Using rainfall depth for the design storm event (1:100 year return period + 40% climate change) × impermeable area

   • Rainfall depth: obtain from FEH Web Service or Wallingford HydraSoft (commercial tools)
   • For simple calculations, use a rainfall depth value from the local planning authority's guidance

2. Calculate outflow volume: Greenfield rate (l/s) × storm duration (seconds)

3. Required storage volume: Inflow volume − Outflow volume = Attenuation storage needed (m³)

Simplified worked example:

• Site: 0.5 ha development; 0.4 ha impermeable after development
• Greenfield rate: assume 5 l/s for 0.5 ha site (at 2 l/s/ha greenfield)
• Design storm: 1:100 + 40%CC; duration 60 minutes; depth 40mm (hypothetical; use actual FEH data for site)
• Inflow volume: 0.4 ha × 40mm = 4,000 × 0.4 × 0.04m = 16m³ (note: 0.4 ha = 4,000m²) Actually: 4,000 m² × 0.04m = 160m³ inflow
• Outflow during storm: 5 l/s × 3,600s = 18,000 litres = 18m³
• Required attenuation: 160 − 18 = 142m³

This would require a significant underground attenuation tank (many small sites use 1m diameter corrugated polypropylene pipes in a ring main arrangement, each pipe providing approx. 0.785m³/m length).

Wales: SAB Approval Process

Schedule 3 of the Flood and Water Management Act 2010 is enacted in Wales (not yet in England). This means:

• Any new construction draining more than 100m² of surface requires SAB approval
• SAB is a function of the local authority
• SuDS must be built to adoption standards (Statutory Standards for Sustainable Drainage in Wales, November 2018)
• Pre-application discussions with SAB strongly recommended
• SAB approval is separate from planning permission; both required
• Developer constructs SuDS; SAB inspects; local authority may adopt for maintenance

SAB submission requirements:

• Drainage Strategy (at planning stage): demonstrates SuDS hierarchy and greenfield rate compliance
• Detailed design drawings
• Hydraulic calculations
• Construction specification
• Maintenance plan

Adoption: Not all SuDS are adopted; private maintenance agreement may be required for on-plot features.

England: Planning and LLFA

In England, SuDS are required through planning policy (NPPF 2023) rather than statute (except major developments which are subject to statutory requirements that LLFAs manage):

• Planning applications for major developments must include drainage strategy
• LLFA consulted by LPA; LLFA may recommend refusal if SuDS inadequate
• Non-major applications: check local planning guidance
• Schedule 3 (Wales-style mandatory SuDS) proposed for England but not enacted as of 2024 [verify current status]

Practical implication for engineers:

• Pre-application discussions with LLFA can save abortive design costs
• Many LPAs publish SuDS SPDs (Supplementary Planning Documents) with specific local requirements — always check
• Drainage condition typically imposed as pre-commencement condition: drainage scheme must be approved before groundworks begin

Frequently Asked Questions

Does SuDS apply to a single house extension or driveway replacement?

For a house extension: if it increases impermeable area significantly, it should ideally include source-control SuDS (permeable paving, rainwater harvesting), but there is no strict planning requirement for single domestic extensions in most cases. Replacing a driveway: permitted development for solid impermeable paving front gardens was removed in 2008 — permeable paving or a drainage provision is required for driveway replacements over 5m².

Can an underground attenuation tank replace a traditional soakaway?

Yes, where infiltration is not viable (failed BRE 365 test), an attenuation tank connected to a controlled discharge to a watercourse or surface water sewer is the standard alternative. The tank stores water during peak events and releases at the greenfield rate through a flow control device. This is more expensive than a soakaway but provides reliable performance regardless of soil conditions.

Who maintains SuDS features on a residential development?

This varies. On adoption roads, local authorities may adopt the roads but not the drainage features. Management companies, residents' associations, and individual homeowners may all be responsible for different elements. The maintenance plan submitted with planning approval defines this. Maintenance agreements must be secured (typically via planning condition or Section 106 agreement) before discharge of conditions.

What is an exceedance route and why is it required?

An exceedance route is a designed path for water to follow if the SuDS system is overwhelmed (e.g., a 1:1000 year event). Without a designed exceedance route, water finds its own path — usually into buildings. The exceedance route should direct overflow away from buildings, across the site, to a watercourse or public space. It must be shown on drainage drawings and considered in landscaping.

Regulations & Standards

• CIRIA C753 — The SuDS Manual (2015): comprehensive technical guidance for all aspects of SuDS design

• Flood and Water Management Act 2010 — Schedule 3 (enacted in Wales); framework for SuDS statutory approval

• NPPF 2023 — National Planning Policy Framework: England; SuDS policy requirements

• Statutory Standards for Sustainable Drainage Systems (Wales, 2018) — Technical standards for SAB-approved SuDS

• Building Regulations Approved Document H — Drainage and waste disposal; soakaway design (BRE Digest 365 reference)

• BRE Digest 365 — Soakaway design: percolation testing methodology

• BS 7533-12 — Pavements constructed with clay, natural stone or concrete pavers: code of practice for permeable pavements

• CIRIA SuDS Manual C753 — Free download from CIRIA website

• Environment Agency: SuDS in England — Gov.uk guidance on SuDS policy in England

• Welsh Government SAB Guidance — SAB process and statutory standards for Wales

• septic tanks — Off-mains drainage options where surface water SuDS interact with foul water management

• part g sanitation — Building Regulations sanitation requirements that interact with drainage design