SuDS Pond Liner Specification UK — CIRIA C753 Compliance Guide
SuDS Pond Liner Specification UK — CIRIA C753 Compliance Guide
Sustainable Drainage Systems (SuDS) are now a mandatory element of most new major developments in England, following the implementation of Schedule 3 of the Flood and Water Management Act 2010. Correctly specifying the pond liner for SuDS features — particularly attenuation ponds, detention basins, and constructed wetlands — is critical to achieving LLFA (Lead Local Flood Authority) approval and ensuring long-term performance.
This guide sets out the key specification requirements, liner selection criteria, and compliance framework for SuDS pond liners in the UK. For project-specific advice, contact our specialist team.
What is CIRIA C753?
CIRIA Report C753, The SuDS Manual (2015), is the primary technical reference for SuDS design, construction, and maintenance in the UK. It is referenced by the LLFA Approval Standards and is effectively mandatory for SuDS schemes requiring drainage approval. The manual covers:
- Design principles for all SuDS components
- Hydraulic design methods
- Water quality treatment trains
- Construction guidance and quality control
- Maintenance requirements and access
- Adoption and long-term management
Types of SuDS Feature Requiring Pond Liners
Attenuation Ponds
Attenuation ponds are designed to receive, store, and gradually release surface water runoff at a controlled rate, preventing downstream flooding. They are typically lined where:
- The subgrade has high permeability and uncontrolled infiltration is undesirable
- The pond is sited near sensitive receptors (drinking water abstractions, SSSSIs)
- Groundwater levels are high and liner prevents groundwater ingress affecting storage volume
- Contaminated runoff must be contained to prevent groundwater pollution
HDPE geomembrane (0.5mm–2.0mm) is the most commonly specified liner for attenuation ponds due to its durability, low maintenance, and compatibility with the long design lives (typically 60–100 years) required by adopting authorities.
Detention Basins
Detention basins are dry or wet basins that temporarily store runoff during storm events. Where a liner is required, EPDM or HDPE are commonly used depending on the base geometry and access requirements.
Constructed Wetlands
Constructed wetlands provide biological water quality treatment. A liner is typically required to maintain the water level necessary for plant establishment and to prevent contaminated runoff from reaching groundwater. EPDM is frequently specified for wetland applications due to its flexibility and compatibility with planting schemes.
Liner Selection for SuDS Applications
HDPE Geomembrane
HDPE is the most widely specified liner for large or formal SuDS ponds. Key properties:
- Thickness: typically 0.75mm–2.0mm depending on application
- Density: minimum 0.940 g/cm³ (GRI-GM13)
- Tensile strength: ≥29 kN/m (smooth, 1.5mm)
- Chemical resistance: excellent resistance to acids, alkalis, and most hydrocarbons
- Design life: 50–100 years when properly installed
- Seaming: hot-wedge or extrusion welded
EPDM Geomembrane
EPDM is preferred for irregular pond shapes, wildlife ponds, and smaller SuDS features. Key properties:
- Thickness: typically 0.75mm–1.02mm
- Elongation at break: 300–400% (excellent conformity to irregular shapes)
- UV resistance: excellent — no protective cover required
- Fish safe: certified EPDM-45 grade
- Seaming: tape or liquid adhesive
CIRIA C753 Liner Specification Requirements
CIRIA C753 does not prescribe specific liner types or thicknesses, but specifies performance criteria that the liner specification must meet:
- Hydraulic performance — the liner must achieve the design permeability requirements (typically ≤1×10⁻⁹ m/s for attenuation ponds near sensitive receptors)
- Structural integrity — the liner must withstand installation stresses, operational loads, and any settlement or differential movement
- Durability — the liner design life must match the adopted infrastructure design life (typically 60 years minimum)
- Constructability — seaming and jointing must be achievable with the specified method and quality control regime
- Maintenance access — the liner system must allow for inspection and any required maintenance activities
Environment Agency Permit Requirements
SuDS attenuation ponds that discharge to a watercourse may require either:
- An Environmental Permit (water discharge activity) under the Environmental Permitting Regulations 2016
- Registration as an Exempt Activity — where the discharge meets the criteria set out in the Water Discharge Activities Exemptions
The EA's position on lined ponds is that they should not cause deterioration of the water environment. Pre-application discussion with the EA is strongly recommended for any SuDS pond that discharges to a main river or that is in a sensitive catchment.
Installation Quality Assurance for SuDS Liners
For ponds being adopted by a local authority or highway authority, a Construction Quality Assurance (CQA) plan is typically required. This should include:
- Subgrade preparation and acceptance criteria
- Liner deployment sequence and panel layout drawings
- Seam testing regime (destructive and non-destructive)
- Defect repair protocols
- As-built record drawings
- Material certification (manufacturer's test certificates per GRI-GM13)
Our installation support service can assist with CQA documentation for adoptable SuDS schemes. Contact us for project support →
SuDS Pond Liner FAQ
Do all SuDS ponds need to be lined?
No. SuDS ponds only require lining where uncontrolled infiltration is undesirable — for example near sensitive groundwater receptors, where the pond must maintain a design storage volume, or where contaminated runoff must be contained. Many wildlife ponds and infiltration basins are deliberately unlined to promote groundwater recharge. The decision should be based on a site-specific assessment and agreement with the Lead Local Flood Authority.
What liner thickness is required for an attenuation pond?
HDPE attenuation ponds typically use 0.75mm to 1.5mm liner, depending on the pond depth, berm geometry, and any anticipated loads. For ponds with vehicular access to the berm, 1.5mm or greater is commonly specified. Deep ponds (over 3m) or those with steep side slopes may require textured HDPE to improve interface friction and slope stability. A geotechnical assessment should inform the final specification.
Can EPDM be used for SuDS attenuation ponds?
Yes, EPDM is suitable for smaller SuDS attenuation ponds, particularly those with irregular shapes or where a wildlife-friendly edge finish is required. However, for larger formal ponds being adopted by a highway or local authority, HDPE with welded seams is generally preferred due to its higher tensile strength and more straightforward CQA regime. The choice should be made based on the specific pond geometry, design life requirements, and adopting authority preferences.
Specifying a SuDS pond liner? Our technical team can review your design and provide a compliant liner specification. Get free expert advice → | View all pond liners →
LLFA Approval Process — What Documentation Is Required
Since April 2024, all new major developments in England must have their SuDS drainage designs approved by the Lead Local Flood Authority (LLFA) before planning consent is granted. The LLFA approval process typically requires the following documentation for lined attenuation ponds:
Hydraulic Design Documentation
- Hydrological calculations demonstrating that the pond provides the required storage for the design storm events (typically 1:1 year, 1:30 year, 1:100 year, and 1:100 + 40% climate change)
- Hydraulic routing calculations for inflow, storage, and controlled outflow at each return period
- Evidence that the pre-development runoff rate is not exceeded for all return periods
- Flood mapping showing that the pond overflow route does not create unacceptable flood risk
Structural and Geotechnical Information
- Berm/embankment cross-section drawings with slope gradients and dimensions
- Slope stability analysis (if side slopes steeper than 1:3 H:V)
- Subgrade conditions and compaction specification
- Settlement assessment for ponds on made ground or compressible soils
Geomembrane Liner Specification
- Liner type, thickness, and compliance standard (e.g. "HDPE 1.0mm smooth, GRI-GM13 compliant")
- Manufacturer's GRI-GM13 product data sheet
- Seaming method and testing regime (air pressure test procedure, destructive test frequency)
- CQA plan — who will carry out CQA, documentation format, and handover package
- Liner design life evidence (typically required to match adopted asset life, minimum 60 years)
- Anchor trench specification and detail drawings
- Inlet/outlet pipe penetration detail
Maintenance and Management Plan
- Responsible organisation for long-term maintenance (usually Management Company, adopting authority, or riparian owner)
- Maintenance schedule: sediment removal, vegetation management, liner inspection
- Liner inspection protocol: frequency, method, reporting
- Trigger criteria for liner repair or replacement
- Funding mechanism (service charge, commuted sum, reserve fund)
Case Study: 2,000m² HDPE Attenuation Pond — Housing Development
Project context: 250-unit residential development, Midlands England, 2023–2024. Total impermeable area: 6.2 hectares. Design storm: 1:100 year + 40% climate change factor.
Pond specification: 2,000m² surface area, average depth 1.8m, maximum depth 2.5m at base. Controlled outflow at 2 l/s/hectare. 3:1 side slopes on three sides; 5:1 entry slope for emergency vehicle access.
Liner:
- Base and gentle slopes: 1.0mm smooth HDPE, GRI-GM13
- 3:1 side slopes: 1.0mm single-sided textured HDPE (textured face against subgrade)
- Total liner area: approximately 2,650m² (allowing for slopes and anchor trench overlap)
Seaming: Hot-wedge fusion welding throughout. Extrusion welding at 75mm HDPE pipe penetrations (3 inlet pipes, 1 controlled outflow, 1 emergency overflow).
CQA:
- 100% air pressure testing — all dual-track seams tested same day as welding
- Vacuum box testing — all pipe penetrations and extrusion welds
- Destructive testing — 14 tests over 2,100 linear metres of seam (one per 150m)
- All destructive tests: FTB achieved in peel, shear strength >100% parent
- One minor repair required: 40mm extrusion weld defect at outlet pipe penetration
LLFA approval: Approved at first submission with no technical queries. CQA handover package submitted within 14 days of pond completion.
Outcome: Pond successfully attenuated two significant rainfall events in autumn 2024 without reaching emergency spillway. LLFA annual inspection reported no defects.
Pipe Penetration Details for SuDS Liner
Pipe penetrations are the most common location for seam failures in HDPE-lined ponds. Each penetration requires:
- Boot seal: A factory-fabricated HDPE boot welded to the liner around the pipe. The boot is extruded-welded to the liner panel in the workshop and vacuum-tested before delivery.
- Stainless steel compression ring: Mechanically clamps the HDPE boot to the pipe, providing a secondary seal.
- Site welding: Extrusion weld around the full boot perimeter where it meets the liner panel on site.
- Testing: Vacuum box test over each boot after site welding; re-test after backfilling around pipe.
Never attempt to cut a hole in an installed HDPE liner and attach a boot on site without factory fabrication — this creates an unreliable joint that is not testable to the required standard.
Interaction with Other SuDS Components
Lined attenuation ponds do not operate in isolation — they form part of a SuDS treatment train with other components:
- Upstream filter drains/pervious paving: Provides first-stage pollutant removal before runoff enters the pond. Reduces sediment loading on the liner.
- Bioretention cells: Can be located upstream of the lined pond to provide water quality treatment. Combined design reduces the volume of pollutants reaching the pond liner.
- Green roofs: Reduce peak runoff volumes and first-flush pollution, reducing the hydraulic and pollutant loading on the lined pond.
- Downstream watercourse: The controlled outflow from the lined pond must meet the receiving watercourse's capacity. EA consultation is required if the outflow discharges to a main river.