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FAQ4 Impermeable Liners in SuDS: Choosing the Right Materials for Your Project
When you design or build sustainable drainage systems, the impermeable liner is often the quiet hero. Get it right and your pond holds water, your tank stays watertight, and your pollution risk drops. Get it wrong and you face leaks, ground contamination, and costly rework. This guide explains what impermeable liners do in SuDS, compares common materials, and shows you how to select the right option for durability, compatibility, and environmental safety, with practical UK insights you can apply on site.
What impermeable liners do in urban drainage
Impermeable liners create a watertight barrier that prevents uncontrolled infiltration or exfiltration. In SuDS, they are used to:
Contain water where storage is required, such as in attenuation ponds, detention basins, wetlands, and tanks.
Protect groundwater in contaminated land scenarios by isolating stored runoff from the soil.
Maintain hydraulic performance where infiltration is not permitted, for example above sensitive basements or close to building foundations.
Separate clean and dirty water pathways to improve water quality outcomes and simplify maintenance.
Typical applications include:
Attenuation tanks around geocellular storage where watertightness is essential until controlled discharge via a flow device.
Ponds and wetlands where you need a reliable, long term water level for ecology and amenity.
Cutoff barriers under permeable surfaces where subgrade infiltration is undesirable due to high groundwater, low permeability soils, or contamination.
If you are assessing options for tanks or ponds, you will often specify a geomembrane with protective geotextiles, sealed at seams and penetrations to deliver the required leakage rate.
Common liner materials compared
Selecting a liner starts with understanding the properties of the main material families you will see in UK SuDS projects.
WASP HDPE geomembrane High chemical resistance and very low permeability.Excellent durability under correct protection and with welded seams. Stiffer than alternatives, so detail design for corners and penetrations needs care.
WASP LLDPE geomembrane More flexible than HDPE, helpful around complex profiles and penetrations.Good chemical resistance and robust welded seams. Slightly lower stiffness can ease installation in cool weather.
WASP Polypropylene (PP) geomembrane Good chemical resistance (including many hydrocarbons) and no plasticiser-migration risk; hot-weldable and durable. More flexible than HDPE and LLDPE, making it ideal for detailing.
Whichever you choose, pair the liner with protective geotextiles above and below to control puncture risk. For UK commercial SuDS, HDPE and LLDPE are common where long design life and weldable seams are priorities.
How to select the right liner for your SuDS project
Focus on fitness for purpose, not just material data sheets. A simple selection checklist helps:
Function and hydraulic regime
Are you storing water temporarily or permanently, and what is the allowable leakage rate?
Will the system sit full for long periods, such as an attenuation pond, or cycle frequently like a tanked system?
Ground conditions and risks
Assess subgrade stiffness, angularity, and settlement potential.
Check for contaminants that could attack certain polymers.
Review groundwater levels and buoyancy.
Structural and installation constraints
Consider site access, lifting limits, weather windows, and welding methods.
Detail around inlets, outlets, sumps, and flow controls.
Durability and design life
Target a 50 to 100 year design life for buried liners, aligned with project requirements.
Specify UV protection if any sections will be exposed, even temporarily.
Use protective geotextiles and sacrificial layers in high traffic zones.
Compliance and QA
Require factory and site welding QA, spark or vacuum testing, and trial seams.
Document acceptance criteria and hold points in the ITP.
Confirm installer competence and certification.
Whole life performance
Consider maintenance access and the impact of silt, debris, and cleaning regimes.
Plan for repairs, with patching procedures and records.
Factors that affect liner durability and performance
Longevity is a combination of material selection, detailing, and site practice. Pay attention to:
Subgrade preparation Remove sharp inclusions and compact to a smooth, firm surface. Place a cushioning geotextile before the liner, and another above it under ballast or backfill.
Mechanical protection Use boards or geotextile pathways for foot traffic during installation. Avoid point loads, such as scaffold feet or sharp tools.
Welding and seams Use qualified welders and maintain welding logs. Non destructive testing, such as air channel testing for dual track welds, is vital before cover.
Penetrations and terminations Detail pipe boots, puddle flanges, and upstand anchorage properly. Provide movement joints where differential settlement is expected.
Chemical and biological exposure Check compatibility against hydrocarbons, leachates, and cleaning chemicals. Protect against root intrusion where landscaping interfaces occur.
UV and temperature Store rolls out of direct sunlight and limit exposure time on site. Consider cold weather welding procedures for LLDPE and HDPE.
Inspection and records Photograph subgrade, protection layers, seams, and penetrations. Keep as built records for future maintenance and liability clarity.
Where liners fit in the SuDS toolkit
Impermeable liners support a range of UK SuDS features:
Tanked storage with an attenuation tank connected to a calibrated outlet, often a vortex device or orifice plate in a chamber.
Wet features such as an attenuation pond or balancing pond that require stable water levels for ecology.
Retrofit sites where infiltration is not viable and surface water must be captured and released slowly.
When lining tanks or ponds, integrate controls and pre treatment. Silt capture upstream reduces maintenance and protects the liner from abrasion. Controlled discharge, sized by hydraulic analysis, ensures downstream assets are not overloaded.
Practical tips for a smooth installation
Order prefabricated panels where possible to reduce site welding and speed installation.
Specify geotextiles by mass per unit area and puncture resistance, not just weight targets.
Protect edges early with anchor trenches and temporary ballast.
Coordinate liner penetrations with MEP and civils to avoid late changes.
Include inspection chambers near controls for easy access and testing.
Bringing it together
Impermeable liners are central to reliable SuDS performance. Choose the material that suits your hydraulic aims, ground risks, and buildability constraints. Protect it with the right geotextiles, detail penetrations carefully, and insist on robust QA from delivery through to cover. If you would like support with material selection, detailing, and hydraulic checks, our team can help you specify a suitable geomembrane and integrate it with flow control and pre treatment so your system delivers from day one.
For more on materials and typical applications, see our page on geomembranes. You can also explore how storage products interface with liners on our attenuation tanks page. Finally, if you are planning lined surface features, read more about attenuation ponds.
