Lidsey Drainage Impact Assessment: What Arun Requires and Why

A Lidsey Drainage Impact Assessment is a statement Arun District Council requires with every planning application in the catchment of Lidsey Wastewater Treatment Works. It explains how foul water and surface water will leave the site, and how the development avoids adding to a sewer network that floods when groundwater and rainwater get into it. Leave it out and an application in Barnham, Yapton or Walberton is incomplete.

The requirement comes from Policy W DM1 of the Arun Local Plan 2011–2031. Scale makes no difference. A single new dwelling or a change of use triggers it just as a large housing estate does, and the short title hides how much it asks for: in practice the assessment can only be written once the foul and surface water drainage for the site have been designed.

Arun's drainage engineers are consulted on every planning application in the Lidsey catchment, however small.

What is a Lidsey Drainage Impact Assessment?

It is the part of a planning submission that shows how a development in the Lidsey catchment will dispose of its foul water and its surface water, and what it does to protect the local sewer network. It has to deal with the scheme's own impact and with its share of the cumulative impact of all development in the catchment.

Policy W DM1 names four things the assessment must take account of:

• Foul water disposal. Where the sewage goes, whether the network can take it, and how the new pipework stays watertight.
• Surface water drainage. How rainwater is managed on site without any of it reaching the foul sewer.
• Flood storage capacity. Whether the development removes space that floodwater currently occupies.
• Flood flows. Whether runoff from the site adds to flooding downstream in the catchment.

That list is a whole drainage design. The assessment cannot be written without a foul drainage strategy for planning and a surface water drainage strategy behind it, because it summarises and justifies both. That is why it usually travels in the same report as those strategies, or inside a flood risk assessment, rather than as a separate document.

Which sites fall inside the Lidsey catchment?

The catchment is the area whose sewers drain to Lidsey Wastewater Treatment Works, south of Woodgate. Arun's catchment page lists Barnham, Eastergate, Flansham, Fontwell, part of Middleton-on-Sea, Norton, Aldingbourne, Slindon, Walberton, Westergate, Woodgate and Yapton.

The boundary follows the sewer network, not parish or ward lines, so a site near the edge of one of those villages may sit either side of it. The council publishes a catchment boundary map and shows the area on its planning validation interactive map. Check it before committing to a site. Most of Middleton-on-Sea and Felpham drain to Ford instead, which is why only part of Middleton is caught.

Householders, small builders and change-of-use applicants are all caught, and each needs a statement that deals with foul water as well as surface water.

Why does the Lidsey catchment have its own drainage rule?

Because the foul sewers around Lidsey fill with water they were never built to carry. In a wet winter groundwater seeps into pipes and manholes through failed joints and cracks, while rainwater pours in through manhole covers and gullies. The network then overloads, sewer flooding hits roads and gardens, and the treatment works sends the excess to storm.

The Local Plan records that communities in the catchment have suffered foul flooding of roads and property, with pollution of watercourses, and attributes it to groundwater infiltration and surface water inundation. The Lidsey Surface Water Management Plan, prepared in 2014 by West Sussex County Council with Southern Water, Arun District Council and the Environment Agency, set out the evidence.

Southern Water's infiltration reduction plan for Barnham explains the mechanism: once groundwater rises above the sewers it enters through leaking joints in public and private pipes. At a public meeting in April 2026, the company said groundwater was filling the network and causing storm overflow releases "even in dry weather", and set out a £50 million programme to more than double Lidsey's treatment capacity and continue sealing sewers. A spill in dry weather is a capacity signal in its own right, as the article on dry weather flow and dry day spills explains.

Every new connection adds flow to a network that already overloads in wet weather, which is why Arun weighs the cumulative effect of small schemes as well as large ones.

What do Policy W DM1 and Arun's validation page require?

Part 3 of Policy W DM1 sets the requirement and Arun's catchment page makes it a local validation requirement. The policy draws one distinction: major development needs a full Drainage Impact Assessment covering surface water and foul water, while every other application in the catchment needs one that addresses both its own impact and its share of the cumulative impact.

The council's catchment page, last updated in November 2025, sets out what the assessment must contain:

• Both drainage routes. How foul and surface water will drain from the site, and the measures that reduce the impact on the catchment.
• Water quality. Evidence that water bodies the proposal could affect, directly or indirectly, will not be harmed.
• The Surface Water Management Plan. An explicit reference to the Lidsey surface water management plan and its actions.
• National standards. A surface water design that meets the National Standards for Sustainable Drainage Systems.
• Local Flood Risk Zone detail. Extra content where the site sits in one of the SWMP's zones (covered below).

Under Arun's local validation list, a missing assessment means the application is not registered. The wider Arun flood risk and drainage requirements apply on top, including winter groundwater monitoring and infiltration testing wherever a design relies on soakage.

Surface water: why none of it can reach the foul sewer

Surface water from a development in the catchment must stay out of the foul system completely. The council's surface water design checklists list "no connection of surface water to the public foul sewer" as a critical item, and the council states that missing a critical item results in an objection. The drainage design has to find another destination for every square metre of roof and paving.

The order of preference is the SuDS hierarchy: rainwater reuse, then infiltration to ground, then restricted discharge to a watercourse, then a surface water sewer. Requirement H3 of the Building Regulations sets the same priority for rainwater: soakaway first, then watercourse, then sewer. The difference between foul, surface water and combined sewers matters here, because a pipe that looks like a storm drain can turn out to be foul.

On the coastal plain infiltration often fails on groundwater alone: winter levels sit too close to the surface for a soakaway to keep the 1 metre of unsaturated ground Arun requires beneath it.

Infiltration only counts once winter groundwater monitoring and BRE 365 infiltration testing prove it, and where they rule it out the design falls back to attenuation with a restricted discharge to a watercourse, which on a flat coastal-plain site can mean a long pipe run or an outfall across someone else's land. Arun's checklists also treat a gravity-based design with no pumps as a critical item. In practice that makes a pump the last resort, and a site where levels force a pumped outfall needs that case made in the strategy itself.

Foul water: proving capacity and keeping groundwater out

The foul section of the assessment has two jobs: show that the flow from the development can be accommodated, and show that the new or retained pipework will not let groundwater or surface water in. The first is arithmetic. The second is a matter of pipe materials, chamber construction and detailing, most of it taken directly from the Surface Water Management Plan's guidance for new development, which was written for exactly this catchment and its groundwater.

Planning practice guidance sets the foul drainage hierarchy: connection to the public sewer first, then a package treatment plant, then a septic tank. Policy W DM1 adds a general presumption against non-mains foul drainage in sewered areas. Most Lidsey sites therefore connect to Southern Water's network under the section 106 right to connect, and the assessment has to address what that connection adds.

A change of use: showing foul flows fall

Conversions are different. Where an existing building changes use, the strongest argument is often that foul flows go down. One recent Lidsey assessment for a former care home converted to 25 dwellings compared the two uses using the British Water Flows and Loads 4 allowances. The care home produced an estimated 16,650 to 18,600 litres a day. The 25 dwellings, at 150 litres per person per day for about 62 residents, came to roughly 9,300 litres a day: close to half the care home's figure.

The existing foul connection was kept, but the assessment still recommended inspecting manholes, chambers and the pumping station for ingress routes, and upgrading anything replaced to the standards below. A fall in flow does not excuse leaking pipework.

New dwellings: building a sealed foul system

New homes add flow, so the assessment has to show the connection is acceptable and the new system is sealed. A Southern Water pre-planning enquiry gives early guidance on capacity. The Surface Water Management Plan's practical guidance for new development then sets out how to keep clear water out:

• No surface water connections. Roof and paved runoff goes to the surface water system only, in line with Part H of the Building Regulations.
• Fusion-jointed MDPE pipes. These resist groundwater infiltration and need little maintenance.
• Mass concrete surrounds. Inspection chambers and manholes set in mass concrete keep groundwater out of the chamber walls.
• Keep foul assets off flow routes. Foul pipes, manhole covers and sink waste gullies avoid overland flow paths and ponding areas, or are sealed where they cannot.
• Bolt-down sealed covers. Used on chambers and manholes wherever surface water could pond.
• Protect the building from surcharge. A non-return valve rather than an external gully where surface water flooding is a risk, and traditional external vent pipes rather than air admittance valves, which can cause bubbling WCs and blown traps when the sewer surcharges.

Listed generically, those six measures read as boilerplate. Tied to the layout, the levels and the existing drainage survey, they show the foul system was designed for this catchment.

What extra does a site in a Local Flood Risk Zone need?

The Surface Water Management Plan divides the catchment into 27 Local Flood Risk Zones, each with its own modelled flood extents, property impacts and preferred interventions. Arun's catchment page requires a site in one of those zones to go further than the standard assessment. Applications outside the catchment boundary but inside a zone are caught too.

1. Name the zone. Identify which Local Flood Risk Zone the site falls within.
2. State the predicted impact. Give the SWMP's predicted flood impact for that zone.
3. Include the summary tables. Reproduce the zone's summary of flood risk tables from the plan.
4. Commit to interventions. Say which of the zone's preferred interventions the development will deliver.

The last item can cost money. It asks the development to help fix the catchment as well as doing no harm. The zone tables sit in chapter 7 of the SWMP, alongside the integrated sewer and surface water model built for the plan.

What happens if the assessment is missing or thin?

A missing assessment stops the application at validation, and a thin one can sink it at determination. The consequences run in sequence: an invalidated planning application loses time before anyone has looked at it, and a weak assessment gives the council a reason for refusal it can rely on at appeal.

In 2022 Arun refused five homes at Woodgate, and one of the reasons was that no drainage impact assessment had been submitted taking account of the individual and cumulative impact on the catchment. Design, trees, housing mix, access and the absence of a draft section 106 agreement made up the other five.

A refusal on drainage grounds means a redesign and a resubmission, and where the missing evidence is winter groundwater data, the scheme has to wait for the next winter monitoring season.

Permission does not close the matter. Policy W DM1 allows conditions on implementation dates and technical detail, and requires a statement from a suitably qualified and experienced engineer confirming the measures were properly built before those conditions are discharged. At the discharge of drainage conditions an engineer has to certify what was built, so gaps left in a vague assessment come back as problems on site. Arun's published checklists show how much construction detail the council already expects before determination.

Frequently asked questions

Applications in the catchment move fastest when the drainage evidence is planned before the layout is fixed. A drainage strategy for a planning application that covers both systems, with the Lidsey assessment built in, answers the validation requirement and the engineers' questions in one document. Where the site also needs a flood risk assessment for planning, the two can be prepared together. Call 01293 214444 or email enquiries@unda.co.uk to talk through a site in the Lidsey catchment.