Parson's Brook Flooding & Flood Risk

Parson’s Brook is under four kilometres long and runs through farmland between Itchingfield and Coolham in West Sussex, passing within half a kilometre of Barns Green before it joins the River Adur West Branch. It is not a river anyone writes about. What makes it worth understanding is its legal status. The Environment Agency has not designated any part of Parson’s Brook a Main River, which puts the brook, its banks and anything built near it under a different regulator, a different consent regime and a different set of duties from the ones most people assume apply. Unda prepares flood risk assessments, drainage strategies and related reports for sites along the brook and across the Adur West Branch catchment. Reports for sites along the Parson's Brook come from Unda Consulting Limited, a CIWEM Business Partner. To talk it through, call 01293 214444 or email enquiries@unda.co.uk.

Parson’s Brook is an ordinary watercourse along its entire length. A query of the Environment Agency’s Statutory Main River Map on 9 July 2026 returned no Main River within 50 metres of the brook, and the nearest designated Main River is 514 metres away. Works in or near the channel therefore need land drainage consent from West Sussex County Council, not a Flood Risk Activity Permit from the Environment Agency.

• An ordinary watercourse, end to end. No reach of Parson’s Brook appears on the Environment Agency’s Statutory Main River Map. West Sussex County Council regulates it as Lead Local Flood Authority.
• Inside a flood alert area, outside any warning area. The brook falls within the Environment Agency’s River Adur West Branch flood alert area, which covers the West Branch and its tributaries from Coolham to Bines Green. No flood warning area covers the brook itself.
• No gauge on the brook. The nearest Environment Agency monitoring station, at Coolham, sits about half a kilometre downstream of the confluence, on the Adur West Branch rather than on Parson’s Brook.
• Development pressure at the headwaters. A live application for 68 dwellings at Chapel Road, Barns Green includes works to a culverted watercourse on site, and has drawn objections on surface water grounds.

Does Parson’s Brook flood, and who does it affect?

Nobody publishes a flood record for Parson’s Brook. The reason matters more than the gap. Small ordinary watercourses are not gauged, not modelled at national scale, and not investigated after a flood unless the flooding crosses a threshold. West Sussex County Council, as Lead Local Flood Authority, opens a full Section 19 flood investigation only where more than ten properties flood in a single incident. Parson’s Brook has never triggered one. That tells you nothing about whether the brook floods. It tells you that when it does, it floods fields, lanes and the odd property, and nobody writes it up.

What the Environment Agency does say is where the brook sits in its warning system. The whole of Parson’s Brook falls inside flood alert area 065WAF432, which the agency describes as covering the River Adur West Branch and its tributaries from Coolham to Bines Green. A flood alert means flooding of low-lying land and roads is possible somewhere in that area. It will not tell you a specific property is at risk, and it will not be issued for the brook alone.

No flood warning area covers Parson’s Brook. The nearest one, Coolham and Shipley on the River Adur, covers the West Branch downstream of the confluence. Be clear about what that gap means. A property beside Parson’s Brook can flood without any Environment Agency warning being issued, because the agency does not run a warning service for this watercourse. Flood warnings near this brook cover the river it drains into, not the brook. You can still check for flooding and sign up for alerts on the Adur West Branch.

Check flood risk along Parson’s Brook

The map below draws Parson’s Brook in coral from its source near Itchingfield to its confluence with the River Adur West Branch at Coolham. There are no Environment Agency gauges on the brook, so no live gauge pins appear on it. Enter a postcode near the brook to zoom in and screen that location against the Environment Agency’s flood data for rivers and sea, surface water, reservoirs and the flood zones used for planning. It is an indicative screening tool, not a flood risk assessment.

Where Parson’s Brook runs

Parson’s Brook rises in farmland near Mareland Farm, in the parish of Itchingfield, and runs roughly south-southwest for 3.9 kilometres to Slaughter Bridge Farm near Coolham, where it enters the River Adur West Branch. It is a single continuous channel with no significant tributaries. Its course is drawn on the map above in coral, taken from OpenStreetMap by way of WaterwayMap.org (© OpenStreetMap contributors) and checked against Ordnance Survey Open Rivers.

The brook does not run through a town. It passes the edge of Barns Green at about 420 metres, and its lower reach curves within a kilometre of Brooks Green and Coolham. Those distances are measured off the mapped centreline, and they explain why the brook rarely appears in flood reporting. There is very little in its path to flood.

Barns Green is the exception. The village sits close to the brook’s upper reach on ground that residents describe as flooding in heavy rain, and the drainage there runs through ditches and culverts rather than an open channel. Horsham District Council is the planning authority for all four settlements, within West Sussex.

Is Parson’s Brook a Main River or an ordinary watercourse?

It is an ordinary watercourse, along its whole length. That single fact changes more about a project here than anything else you could learn about the brook.

The distinction is legal, not a description of size. A Main River is a watercourse the Environment Agency has formally designated and drawn on its Statutory Main River Map. Everything else is an ordinary watercourse. We queried that map directly for the brook’s corridor on 9 July 2026 and it returned nothing. No Main River intersects Parson’s Brook, and the nearest designated Main River line lies 514 metres away, on the Adur West Branch it eventually joins.

So the regulator is not who most people expect. Consent for works in, over or under an ordinary watercourse comes from West Sussex County Council as Lead Local Flood Authority, under section 23 of the Land Drainage Act 1991. The Environment Agency has no consenting role here, and its eight-metre byelaw margin does not apply. Our guide to ordinary watercourse consent sets out how the application works.

There is also no Flood Risk Activity Permit for Parson’s Brook, because that permit exists for Main Rivers. Applicants who budget for one are budgeting for the wrong consent. Applicants who assume no consent is needed because no permit exists are wrong in the other direction, and that mistake is the more expensive of the two.

Maintenance falls to the landowner. Riparian responsibility for the bed, banks and flow of an ordinary watercourse sits with whoever owns the land it crosses. Nobody is coming to dredge Parson’s Brook.

What that means for a site near the brook

Culverts are where this bites. A live application to Horsham District Council, reference DC/25/2057, proposes 68 dwellings on land north of Little Slaughterford, Chapel Road, Barns Green, and its description expressly includes works to an existing culverted watercourse on site. CPRE Sussex objected in February 2026, arguing that the proposed drainage was not assured to prevent surface water flooding off site, and that homes on the east side of Chapel Road, some of them Grade II listed, are at risk of flooding. Whether the culverted watercourse named in that application is Parson’s Brook itself or a ditch feeding it is not something we have established. The site lies in the brook’s headwater reach. Settling that question is where an assessment starts.

1. Identify the watercourse. Establish which watercourse crosses or bounds the site, whether it is culverted, and whether it is a Main River or an ordinary watercourse.
2. Establish ownership and duties. Work out who owns the channel and what riparian duties come with the boundary.
3. Check the consent route. Confirm whether the works need land drainage consent from the county council, and apply before the design is fixed.

A site that misses a culverted ordinary watercourse early tends to discover it at the drainage condition stage, when the design is fixed and the options have gone. Early preliminary drainage design and a topographical survey usually pay for themselves here.

The flood risk assessment is a separate question from the consent. Whether one is needed turns on the Environment Agency’s flood map for planning and on the size of the site, and for a brook of this scale the national mapping may show little or nothing along it. A site can sit outside every flood zone and still carry real surface water risk and a culvert that could block. National mapping has limits, and it is worth understanding how accurate flood maps really are.

Flood warning and live levels near Parson’s Brook

There is no gauge on Parson’s Brook. The closest Environment Agency monitoring station is at Coolham, roughly half a kilometre downstream of the confluence, and it measures the Adur West Branch. It tells you the state of the river the brook drains into, which is useful context and not the same thing as the brook’s own level.

• Live river levels. Track the nearest gauged reaches on our live river levels map, or on the Environment Agency’s Coolham station record.
• Screen a location. Try our free flood risk map, the drainage calculators, the BRE 365 infiltration calculator and the geology and soakaway map.
• Flood alerts. Sign up for Environment Agency flood alerts covering the River Adur West Branch, and expect that they will not name the brook. Floodline is 0345 988 1188.
• In a flood emergency. Call 999 if life is in danger, and 105 for power cuts.

Treat a rising Coolham reading as a signal about the receiving watercourse rather than a forecast for your boundary ditch.

Who manages flood risk here

Responsibility is split, and knowing the split saves a lot of wasted correspondence.

• West Sussex County Council is the Lead Local Flood Authority. It consents works to ordinary watercourses, manages surface water and groundwater flood risk, and decides whether to investigate a flood. Its threshold for a full Section 19 investigation is flooding to more than ten properties in one incident, as set out in its flood reports and policies.
• Horsham District Council is the local planning authority. It determines applications along the brook and holds the strategic flood risk assessment for the district. See what local planning authorities expect on flood risk and SuDS.
• The Environment Agency regulates the Adur West Branch as a Main River downstream, publishes the flood map for planning and runs the flood alert covering the brook’s catchment. It does not regulate the brook.
• Riparian owners maintain the channel where it crosses their land.

The county council’s revised local flood risk management strategy pushes a catchment based approach, capturing and controlling water closer to source in upper catchments so that it arrives more slowly further down. Parson’s Brook is an upper catchment watercourse in exactly that sense. What gets built on and around it changes what reaches Coolham and Shipley, and a site in a critical drainage area will face tighter limits still.

Flood risk and drainage services for sites near Parson’s Brook

A development near Parson’s Brook can call for several reports, and they answer different questions. A flood risk assessment, a sequential or exception test and a drainage strategy are three distinct pieces of work. They often travel together in a planning submission, but they address whether the site is safe from flooding, whether the development belongs here at all, and where the surface water goes.

• Flood risk assessments for planning. Site specific assessments addressing the flood zones, the surface water risk and the culverted watercourses national mapping tends to miss. See our flood risk assessment for planning service.
• Assessments for property purchase and insurance. Independent reports for buyers, owners and lenders, through our flood risk survey for property purchase and flood risk assessment for insurance.
• Drainage strategies and SuDS. Surface water designs meeting the discharge limits the Lead Local Flood Authority sets, which is the constraint that decides most schemes on a small ordinary watercourse. See our sustainable drainage strategy service and the four pillars of SuDS.
• Sequential and exception tests. Locational justification where policy requires it, through our sequential and exception test reports.
• BREEAM and groundwater. A BREEAM flood risk assessment for the Pol 03 credit, and a groundwater flood risk assessment where a high winter water table is in play.

Where national mapping is not enough, hydraulic flood modelling can derive flood levels and extents for an ungauged watercourse, and infiltration testing establishes whether the ground can take the runoff at all. All of it sits within the NPPF and its planning practice guidance.

How Unda helps with flood risk on Parson’s Brook

Unda understand how a small, ungauged, partly culverted ordinary watercourse behaves, and why it causes more trouble in the planning system than its size suggests. The brook has no gauge, no flood warning service, no Section 19 report and, in places, no open channel at all. Missing evidence is not the same as low risk, and building the evidence base is the job. A planning authority and a lead local flood authority both have to accept it.

We work for developers, planning consultants, architects, agents and homeowners with sites along the brook and across the wider Adur catchment. If you have a site near Parson’s Brook, talk to one of our experienced flood risk consultants about what your application will need.

Parson’s Brook flood risk: your questions answered