The Complete Guide to Highway and Car Park Drainage

Highway and car park drainage refers to the whole system of gullies, channels, pipes and underground storage used to convey and control runoff from roads and car parks, keeping them safe to use and compliant with the applicable sustainable drainage standards. Get it wrong, and you get standing water, cracked paving and, increasingly, a planning objection. Get it right and it’s out of sight.

Highway and car park drainage combines two distinct functions, moving water off the surface fast (channels and gullies) and storing it before controlled release (attenuation tanks) — and England currently governs the second function through Defra’s 2025 non-statutory National Standards for Sustainable Drainage Systems, not a single “UK law.” Car parks over 800m² typically need a pollutant interceptor before discharge. Highways managed by National Highways follow a separate standard, DMRB CG501, rather than the Defra standards.

What Counts as Highway and Car Park Drainage?

Highway and car park drainage covers everything that removes and manages surface water from roads and parking areas: the water bill charge that funds treatment, and the physical network of gullies, pipes, channels, and attenuation storage that actually does the work — this guide separates the two and points to the highway drainage regulations that actually govern each. “Highway drainage” means two different things depending on who says it, and mixing them up wastes a lot of time.

• On a water bill, highway drainage is a charge – a line item for treating road and pavement runoff that enters the public sewer.
• Highway drainage as an engineering system is the physical network of gullies, pipes, channels, and ( increasingly ) sustainable drainage elements that convey and manage road and pavement runoff.

This guide is about highway drainage the engineering system. But highway drainage on your bill matters for context: Ofwat confirms that water companies charge for treating runoff from roads and footpaths, even though they don’t own or design the physical systems that manage it.

Who Is Responsible for Highway Drainage?

• Local highway authorities manage adopted public road gullies and prevent flooding on public roads.
• National Highways manages the public motorway and major roads network, with a design process based on its own standards, not local highway authority standards.
• The public sewer system is owned by the local water company, which also takes on responsibility for the water and consequently sends the highway drainage charge to you.
• Owners of private roads and car parks inherit the responsibility as it would otherwise go unaddressed, and this is precisely why many existing car parks now exist in a state of no one really being responsible for them — the trap a retail park operator falls into after inheriting a 15-year-old surface with no drainage drawings on file.

📐 Engineering Note: Highway and car park drainage schemes discharging to a range of application sectors — from local access roads to multi-storey car parks, sit under different regulatory owners even when the physical hardware (gullies, pipes, tanks) is functionally identical. Confirm which authority applies before assuming a “highway drainage” standard covers your car park, or vice versa.

How Surface Water Moves Through a Highway or Car Park: The 5-Stop Water Journey

Rain that falls on a road or car park doesn’t go straight to a river. It moves through five distinct stages, and a drainage scheme that only solves one or two of them fails during the storm it was supposed to handle — an application where this shows up first is a multi-storey car park deck, where every missed stage is immediately visible as a puddle rather than hidden underground.

1. Capture – gullies, channel gratings, or roadside ditches collect surface water off the trafficked area.
2. Conveyance – pipes or channels carry captured water away from the trafficked surface.
3. Treatment – pollutants (oil, silt, heavy metals) are removed before the water go further, where required.
4. Attenuation – water is stored temporarily so it doesn’t overwhelm the downstream sewer or watercourse during peak flow.
5. Discharge – water is released at a controlled rate to a sewer, watercourse, or the ground itself.

Most drainage complaints trace back to a gap in one of these five stops, not a total system failure — that single missing stage, not a design failure at every stage, is the mistake worth checking for first, because it is the cheapest one to fix before construction rather than after a 2025-vintage scheme floods in its first winter. A retail car park that floods usually has working gullies (stop 1) and pipework (stop 2) but no attenuation (stop 4), so the moment inflow exceeds the downstream sewer’s capacity, water has nowhere to wait and backs up onto the surface.

“Highway drainage is just about getting the water off the carriageway as quickly as possible” is myth number one. Once the water is in the pipes it still needs to go somewhere, if the downstream system doesn’t have capacity, the highway drainage will be ineffective regardless of how fast it clears the surface.

Gullies themselves are also a narrower tool than most people assume. They’re designed for a 1-in-1-year or 1-in-2-year rainfall event, and they block with debris even when maintained on schedule – which is why swales and filter strips are increasingly specified alongside them on new schemes to slow water flow, and why a genuinely extreme rainfall event needs a separate overland flow route, not just a bigger gully.

This five-stop breakdown is this guide’s own synthesis rather than a direct quote from any single document, but it follows the same underlying order Defra sets out in the National Standards for Sustainable Drainage Systems: work out where the water should go before deciding how big to build the pipe.

Surface Drainage Channels vs Underground Attenuation Storage

Surface drainage channels and underground attenuation storage solve two different problems, and treating them as interchangeable – or assuming one substitutes for the other – is the single most common design mistake in this guide’s research. A drainage channel move water off the trafficked surface quickly; an attenuation tank stores water so peak flow doesn’t overwhelm the sewer or watercourse downstream.

⚠️ Common Misconception: It’s tempting to assume a channel system can simply feed an attenuation tank during an extreme rainfall event. It generally can’t. Gullies and channel gratings are sized for routine flow, not the volume a 1-in-100-year event pushes toward underground storage, the intake path into a tank needs its own dedicated sizing, not a repurposed surface channel.

A big 1,100m channel drainage car park could fully clear surface but overfill site if there’s no space for that drained water to park up during a storm – a real case on a big shopping centre car park.

If you’ve space down the sewer and only normal rainfall to deal with then just a well-specified channel system might be all that’s needed.

If down the sewer space is limited or there’s a requirement from the local authority for SuDS-compliant rates, or if it’s just that your peak rainfall is more than average, then you’ll need attenuation storage along with the surface capture, not in lieu of it, that’s the decision framework we’re using for the remainder of this guide.

• Channel drainage alone: normal rainfall, space down sewer, no SuDS rate required.
• Channel drainage + attenuation: space limited down sewer, SuDS rate required or big, high run-off site.

For trafficked areas storage please check out our D400 rated heavy duty soakaway crate range.

Sustainable Drainage Systems (SuDS): England’s Standard for These Schemes

Sustainable drainage systems, known as SuDS, form the current English system for dealing with surface water run-off at source — often by letting it soak away and recharge groundwater — rather than taking it straight to the foul sewer. The National Standards for Sustainable Drainage Systems were published by Defra in June 2025 and cover the whole of England.

• Standard 1, the hierarchy standard.
• Standards 2-7, the fixed standards.

Here’s the gap many guides get wrong, and the one that trips up developers working across a border: these standards aren’t legislation, they’re advisory to inform policy in the National Planning Policy Framework, and although they would be expected to be referenced, they don’t impose a legal requirement in the same way as building regulations do — a mistake a specifier moving from a Welsh scheme to an English one can easily make by assuming the same statutory footing applies, because Wales made SuDS approval mandatory in 2019 while England, as of 2025, still has not.

Nor do they apply to the National Highways trunk road network (that’s covered by DMRB next). Nor are they intended for retro-fits — in practice, a car park resurfacing project is judged against different criteria than a new-build application on a greenfield site.

England vs Wales: Two Different SuDS Regimes

This surprises most people planning cross-border projects. Under Schedule 3 of the Flood and Water Management Act 2010, Wales became the first country in the world to make SuDS legally mandatory for new developments in 2019, requiring approval from a statutory SuDS Approving Body before construction.

England’s government considered doing the same but decided against it in December 2014, relying on planning policy instead, Defra is still “considering implementation” with no confirmed date. The Welsh government has stated its approach “contributed to England also now preparing for very similar legislation.”

Designing Drainage for a Highway: Standards and Sizing Basics

Highway drainage design in England can be complicated by the divide between the system used on local roads (Defra National Standards above) and the separate system for the motorway and trunk road network managed by National Highways, covered in DMRB CG501: Design of Highway Drainage Systems, v.2.1.0.

• CG501 covers principles of drainage design to limit carriageway flooding on National Highways’ roads: calculation of storm flow, Flood Risk assessment, component choice.
• CG502 deals with certification of the drainage design itself – in other words, schemes for trunk roads require a formal sign-off, unlike the more typical treatment of local road schemes.

The practical advice: if you’re designing for drainage at a trunk road junction or motorway approach, cite CG501/CG502 rather than the Defra National Standards — citing the wrong document is a common mistake on mixed schemes where a site straddles a slip road and a local access road, because CG501 (version 2.1.0) and the 2025 Defra standards are separate documents with separate sizing tables, not two editions of the same one. If designing for a local access road or for a car park, use the relevant Defra standard (or Schedule 3 for schemes in Wales) instead — the same application, a retail car park resurfacing scheme, can sit under either regime depending on which side of the England-Wales border it falls.

Designing Drainage for a Car Park: Load Class, Layout, and Storage Sizing

A car park drainage design presents two separate sizing questions – the load class required to carry the surface, and the amount of storage volume the site needs. The same two questions apply to smaller parking areas and shared driveways, though a specialist drainage contractor will usually scale a full car park drainage system down rather than reuse the same spec unmodified.

How Do You Drain Water From a Parking Lot?

Design follows a pattern of surface capture — channel drains or point gullies rated for the site’s traffic — discharging to a piped network, with an interceptor ahead of the discharge point on larger or more sensitive sites. Where rainfall can’t be accepted quickly, that network feeds an attenuation tank sized to hold peak flow for controlled release, and the layout usually follows site topography (fall toward a collection point) rather than a fixed design standard.

📐 Engineering Note, Worked Storage Estimate: A rough first-pass storage estimate: for a 100-space car park with an average bay-plus-access footprint of roughly 25m² per space, the impermeable catchment area is approximately 2,500m². At a typical design storm depth of 20mm (a simplified planning-stage figure, not a substitute for a full hydraulic calculation), that catchment generates roughly 50m³ of runoff to attenuate before controlled discharge, the starting point a drainage engineer would then refine using site-specific rainfall data, permeability, and the applicable climate change allowance from Environment Agency guidance.

Storm Manage’s own container loading data shows one 40-foot high-cube container can carry up to 330m³ of installed geocellular storage volume with optimised stacking, meaning a single container shipment covers approximately 600% of this example car park’s estimated storage need, a useful sanity check when a supplier’s quote looks disproportionate to the site size.

You can run your own estimates using our attenuation storage volume calculator or its associated on-site detention (OSD) calculator for projects where OSD is the dominant design criterion.

Pollution Control: Interceptors, Silt Traps, and Petrol/Oil Risk in Car Park Runoff

Rainwater runoff from any road surface, or from a car park, is polluted in practice, not just in theory. It will pick up heavy metals, oil and other organic compounds from vehicles, and also the road surface itself — the application that makes this most visible is a fuel-retailer forecourt or a vehicle maintenance yard, where the same rain that would be harmless off a residential driveway becomes a genuine discharge risk — so even a well maintained gully pot offers very limited treatment on its own.

Current GOV.UK pollution prevention guidance sets a specific, checkable threshold: a separator is typically needed for car parks larger than 800m² or with 50 or more parking spaces, plus smaller car parks discharging to a sensitive environment, refuelling areas, and vehicle maintenance sites. Below that threshold, a well-maintained silt trap may be sufficient — but the risk sits right at the boundary, because a retail car park sized at 780m² today that adds 30 more spaces next year crosses the 800m² trigger without anyone necessarily re-checking the pollution control requirement.

Common Failure Points: Flooding, Standing Water, and Maintenance

As part of Defra’s National Standards 2025, continuing to maintain drainage isn’t simply a good idea, Standard 7 specifies the requirement for a management and maintenance plan, as well as a review of behaviour in the event of failure. In other words, if your new scheme fails to include a maintenance plan, that’s a compliance issue as well as an operational risk on your existing assets.

• Blocked gullies: many highway networks report more than half the gullies to be blocked at any one time, raising the risk of standing water on roads and lowering pedestrian safety as flow spreads across the carriageway. Increased water depth accelerates the formation of a pothole — starting as a small hole in the surface and widening with every heavy rain event — as a result of repeatedly saturating the road surface, sometimes prompting a temporary lane restriction while repairs are carried out — a pattern WRc’s Nick Orman has observed directly because gullies are designed for a 1-in-1-year event, not the debris load a badly-timed maintenance gap allows to build up.
• Underground or covered drainage flooding: this application — a basement or multi-storey car park deck — sees several examples of direct discharge to the pavement or basement, without a proper collection system. The result can be the flooding and damaging of parked cars once system capacity is reached, something a collection-stage design review will prevent.

How Underground Attenuation Tanks Fit Into a Highway or Car Park Drainage Design

After capture, conveyance and treatment have been designed, the final step of the car park attenuation design process involves adding a store of volume. For most highway schemes or car park sites where above ground space is a luxury that can’t be spared, a geocellular attenuation tank will be installed beneath the ground surface, made from modular crates.

Storm Manage supplies geocellular attenuation crate systems from an injection-moulding production base established in 2014, running 8 high-tonnage moulding machines (1,000-2,000 ton clamping force) across a 8,000m² Shenzhen facility with roughly 5,000m³ of monthly output and 60,000m³ annual capacity, with dedicated compression-performance verification, dimensional tolerance checks, and Virgin PP/Recycled PP material ratio control at batch level, the kind of QC detail that matters for a durable, reliable system once a tank is buried and effectively unreachable for a decade or more. Standard lead time runs 15-30 days after order confirmation, with a 12m (40-foot) high-cube container preferred for FOB Shenzhen/Yantian/Shekou shipping.

Please refer to our highway and car park attenuation tank systems page for a full worked example of highway drainage installation, from initial sizing through to a design a specifier can sign off. This gives a detailed view of the evidence required from the specifier’s perspective for this element of drainage.

Industry Outlook: What UK SuDS Policy Changes Mean for Highway and Car Park Schemes

The verdict for any 2026-2027 project: England’s SuDS standards are moving in one direction, towards being mandatory, not away from it, though no-one can put a date on it — the risk for a developer is treating today’s non-statutory guidance as optional and then facing a retrofit bill once Schedule 3 does commence. So if you’re specifying drainage now, the safer trade-off is to assume it’s a requirement and over-spec to current Defra standards, because a scheme built to yesterday’s minimum in 2024 is the one most likely to need expensive retrofitting once the mandate lands rather than a scheme already sized against the 2025 standard — in practice, that’s the difference between a car park resurfacing application that sails through a future SAB review and one that gets sent back for redesign.

• 2020, Jenkins Review recommends mandatory SuDS standards
• 2021, Government accepts the recommendation in principle
• 2023 – Defra publishes an implementation review of Schedule 3.
• 2025 – Non-statutory National Standards published as an interim step.

What Wales Shows Us

Wales offers a window into what that future regime for England might look like: mandatory SuDS Approving Body sign-off before building work starts, national standards for design and maintenance, and the end of an automatic right to connect to the public sewer. A 2023 review found the process successful enough to extend, not repeal — a developer working both sides of the border already sees two sign-off regimes in one portfolio.

Background context: UK flood cleanup costs have run as high as an estimated £566 million per year in recent reporting, with a majority of councils reporting overwhelmed sewer capacity during peak events, directional context for why regulatory attention on surface water management continues to increase, not a precise annual constant or a planning input on its own.

Frequently Asked Questions