SuDS Hierarchy Explained: A Developer's Guide

Sustainable Drainage Systems — SuDS — are no longer optional for development in the UK. The NPPF, the Planning Practice Guidance, and the requirements of Lead Local Flood Authorities all mandate that major development must incorporate SuDS, and the expectation is increasingly extending to minor development as well.

At the heart of the SuDS approach is the SuDS hierarchy — a priority-ordered list of options for managing surface water runoff. The hierarchy determines where surface water should go, and in what order of preference. Getting the hierarchy right is fundamental to a drainage strategy that will satisfy the LLFA and secure planning approval.

This guide explains each level of the SuDS hierarchy, how to demonstrate compliance, and the common pitfalls that cause drainage strategies to fail.

What Is the SuDS Hierarchy?

The SuDS hierarchy ranks the discharge destinations for surface water in order of environmental preference:

1. Infiltration to ground — disposing of surface water by soaking it into the ground
2. Discharge to a watercourse — releasing surface water to a river, stream, ditch, or other surface water body
3. Discharge to a surface water sewer — releasing surface water to the public surface water sewer network
4. Discharge to a combined sewer — releasing surface water to a public sewer that carries both surface water and foul water

This hierarchy is set out in the Planning Practice Guidance (Paragraph 080, Reference ID: 7-080-20150323) and is replicated in most LLFA guidance documents.

The principle is simple: surface water should be managed as close to source as possible and as high up the hierarchy as possible. Discharge to a combined sewer is the least preferred option because combined sewers are already overloaded in many areas, and additional surface water loading increases the risk of combined sewer overflows (CSOs) that discharge untreated sewage to watercourses.

Level 1: Infiltration to Ground

What It Means

Infiltration means disposing of surface water by allowing it to soak into the ground. The water percolates through the soil and subsoil, recharging the groundwater. This is the most environmentally beneficial option because it:

• Replicates the natural water cycle
• Recharges groundwater, maintaining base flows in rivers and streams
• Does not require any offsite discharge
• Provides the highest level of water quality treatment (through natural filtration)

Common Infiltration SuDS

Soakaways. Excavated pits filled with granular material or geocellular crates, which receive surface water and allow it to infiltrate into the surrounding soil. Soakaways are one of the most common SuDS features for smaller developments.

Permeable paving. Paving surfaces with gaps or porous materials that allow water to pass through and infiltrate into the ground below. Permeable paving is particularly effective for car parks, driveways, and lightly trafficked areas.

Infiltration basins. Shallow landscaped depressions that receive surface water and allow it to infiltrate over a large area. They are dry most of the time and can be incorporated into open space or amenity areas.

Infiltration trenches. Narrow trenches filled with granular material that receive surface water from adjacent hard surfaces and allow it to infiltrate through the sides and base.

Rain gardens. Planted areas designed to receive surface water from adjacent impermeable surfaces. The planting and growing medium promote infiltration while providing an attractive landscape feature.

When Infiltration Works

Infiltration is feasible when:

• The soil has adequate infiltration capacity (typically a minimum infiltration rate of 1 x 10-6 m/s, though some LLFAs accept lower rates with larger SuDS features)
• Groundwater levels are sufficiently deep (at least 1m below the base of the infiltration feature)
• The site is not in a Source Protection Zone 1 (where infiltration of contaminated surface water could affect a drinking water source)
• The site is not contaminated (where infiltration could mobilise contaminants into groundwater)
• There is adequate space for the infiltration features

Demonstrating Infiltration Feasibility

The LLFA will require evidence to support the use (or non-use) of infiltration. This evidence typically includes:

BRE 365 soakaway tests. The BRE Digest 365 test is the standard method for determining the infiltration rate of the soil. It involves excavating a trial pit, filling it with water, and measuring the rate at which the water level drops. At least three tests should be carried out at different locations across the site, and the results should be representative of the areas where infiltration features are proposed.

Borehole logs. Borehole logs from a geotechnical investigation provide information about the soil and subsoil profile, including the depth to groundwater, the presence of impermeable layers (such as clay), and the general ground conditions.

Contamination data. If the site is brownfield, a Phase 1 Desk Study or Phase 2 Site Investigation may be needed to determine whether the soil is contaminated and whether infiltration is appropriate.

When Infiltration Does Not Work

Infiltration may not be feasible where:

• Clay soils — heavy clay soils have very low infiltration rates (often less than 1 x 10-7 m/s), making soakaways and other infiltration features impractical
• High groundwater — where groundwater is within 1m of the proposed base of the infiltration feature, infiltration is generally not acceptable
• Contaminated land — where infiltration could mobilise contaminants into groundwater
• Source Protection Zones — where infiltration could affect drinking water quality
• Made ground — where the soil profile has been disturbed by previous development, infiltration rates may be unpredictable

If infiltration is not feasible, the drainage strategy must explain why, supported by evidence, and move to the next level of the hierarchy.

Level 2: Discharge to a Watercourse

What It Means

If infiltration is not feasible (or not fully feasible), the next preferred option is to discharge surface water to a watercourse — a river, stream, ditch, or other surface water body.

Discharge to a watercourse is preferred over discharge to a sewer because:

• It maintains the natural drainage pattern
• It does not add loading to the sewer network
• Watercourses have more capacity to absorb additional flow than sewers (in most cases)

Requirements

Greenfield runoff rate. The discharge rate from the development to the watercourse must not exceed the greenfield runoff rate for the site. This is typically calculated as the QBAR (mean annual flood) rate using the IH124 method or the FEH statistical method, for the equivalent greenfield catchment area. Many LLFAs also require that the discharge rate is limited to the greenfield QBAR rate or 2 l/s per hectare, whichever is greater, with some requiring even lower rates.

Attenuation. Because the development generates more runoff than the equivalent greenfield area (due to impermeable surfaces), but the discharge rate is limited to greenfield rates, attenuation storage is required to hold the excess water temporarily. The volume of attenuation is calculated using computer modelling for a range of storm events up to the 1% annual probability event (1 in 100 year) with climate change allowance.

Water quality. Surface water must be treated before discharge to a watercourse. The level of treatment depends on the pollution risk — car parks and roads require more treatment than roofs. The CIRIA SuDS Manual provides guidance on the treatment levels required using the Simple Index Approach.

Consent or ordinary watercourse consent. Discharging to a watercourse may require consent:

• For main rivers (maintained by the Environment Agency), a Flood Risk Activity Permit may be required under the Environmental Permitting Regulations
• For ordinary watercourses (maintained by the LLFA or riparian owners), ordinary watercourse consent may be required under the Land Drainage Act 1991

Common SuDS for Watercourse Discharge

• Attenuation basins and ponds — large open features that store surface water and release it slowly to the watercourse via a flow control device
• Swales — vegetated channels that convey surface water to the discharge point while providing attenuation and treatment
• Wetlands — shallow water features with aquatic planting that provide attenuation, treatment, and biodiversity benefits
• Detention basins — dry basins that fill during storm events and release water slowly after the event

Level 3: Discharge to a Surface Water Sewer

What It Means

If neither infiltration nor discharge to a watercourse is feasible, the next option is to discharge surface water to the public surface water sewer network.

Surface water sewers are dedicated pipes that carry surface water only (not foul water). Discharge to a surface water sewer is preferred over discharge to a combined sewer because surface water sewers discharge to watercourses without treatment, whereas combined sewers discharge to treatment works, and overloading combined sewers causes untreated sewage discharges.

Requirements

Discharge rate. The water company will typically specify a maximum discharge rate, which may be the greenfield runoff rate or a rate agreed through pre-development enquiry. Some water companies are stricter than others.

Attenuation. As with watercourse discharge, attenuation storage is required to manage the difference between the development’s runoff rate and the permitted discharge rate.

Connection agreement. Discharge to a public surface water sewer requires a connection agreement with the water company under Section 106 of the Water Industry Act 1991 (for new connections) or Section 104 (for new sewers that will be adopted by the water company).

Demonstrating That Higher Options Are Not Feasible

To justify discharge to a surface water sewer, the drainage strategy must demonstrate that:

• Infiltration is not feasible (with evidence from BRE 365 testing)
• Discharge to a watercourse is not feasible (because there is no watercourse within a reasonable distance, or because the route to the watercourse crosses third-party land with no available easement)

Level 4: Discharge to a Combined Sewer

What It Means

Discharge to a combined sewer is the least preferred option. Combined sewers carry both surface water and foul water, and they are already overloaded in many urban areas. Additional surface water loading increases the frequency of combined sewer overflows, which discharge untreated sewage to rivers and the sea.

When It Is Acceptable

Discharge to a combined sewer is only acceptable when all higher-priority options have been demonstrated to be infeasible:

• Infiltration is not feasible (evidence required)
• Discharge to a watercourse is not feasible (evidence required)
• There is no surface water sewer available (evidence required)

Even where discharge to a combined sewer is the only option, the drainage strategy must:

• Limit the discharge rate to the lowest practicable level
• Provide maximum attenuation to reduce peak flows
• Achieve a significant reduction in discharge rate compared to the existing situation (for brownfield sites)

Water Company Requirements

Water companies are increasingly reluctant to accept surface water connections to combined sewers. Many water companies have published guidance stating that they will only accept connections to combined sewers as a last resort, and they may require significant attenuation and flow limitation.

Beyond the Hierarchy: The Four Pillars of SuDS

The SuDS hierarchy addresses the discharge destination, but good SuDS design goes further. The CIRIA SuDS Manual identifies four “pillars” that SuDS should address:

1. Water Quantity

Managing the volume and rate of surface water runoff to prevent flooding. This is the primary focus of the SuDS hierarchy.

2. Water Quality

Treating surface water to remove pollutants before it is discharged. Different SuDS features provide different levels of treatment — a treatment train approach, using multiple SuDS features in sequence, provides the most effective treatment.

3. Amenity

Designing SuDS features to create attractive, usable spaces. SuDS can be designed as parks, gardens, play areas, and other amenity features that enhance the development and add value.

4. Biodiversity

Designing SuDS features to create and enhance habitats. Ponds, wetlands, swales, and rain gardens can all support a wide range of plant and animal species, contributing to biodiversity net gain.

A drainage strategy that addresses all four pillars is more likely to be approved by the LLFA and to add value to the development than one that treats drainage purely as a technical problem.

Common Pitfalls

”Infiltration does not work on clay”

This is a common assertion, but it is not always correct. While heavy London Clay may have infiltration rates too low for conventional soakaways, some clay soils have sufficient infiltration capacity for larger SuDS features such as infiltration basins. The only way to know is to test. BRE 365 testing should be carried out before concluding that infiltration is not feasible.

Skipping Levels of the Hierarchy

Some drainage strategies jump straight to discharge to a sewer without properly investigating infiltration and watercourse discharge. The LLFA will require evidence that each higher-priority option has been genuinely investigated before accepting a lower-priority solution.

Undersizing Attenuation

Using outdated rainfall data, incorrect climate change allowances, or inappropriate modelling assumptions can result in undersized attenuation storage. This leads to increased flood risk and will be identified by the LLFA during their review.

Ignoring Maintenance

SuDS features require ongoing maintenance, and the drainage strategy must include a credible maintenance plan. A strategy that proposes complex SuDS features without explaining how they will be maintained is incomplete.

Treating SuDS as an Afterthought

SuDS should be designed as an integral part of the site layout, not bolted on after the layout has been finalised. Early integration of SuDS can deliver better outcomes, reduce costs, and create more attractive developments.

How Aegaea Can Help

Aegaea’s SuDS and drainage engineers design SuDS-compliant drainage strategies for developments of all types and sizes. We work from the earliest design stages to integrate SuDS into the site layout, ensuring compliance with the SuDS hierarchy and the LLFA’s requirements.

Our services include:

• Drainage strategy reports for planning applications
• BRE 365 infiltration testing and ground investigation
• Computer modelling (MicroDrainage/InfoDrainage)
• SuDS design and specification
• SAB applications (Wales)
• Detailed drainage design for construction
• S104 adoption applications

If you need a drainage strategy or SuDS design, contact our team for a no-obligation discussion.