Culverts, de-culverting and the riparian owner's duties

If a watercourse runs through or along your land, you are a riparian owner, and that comes with duties you did not sign up for. You must let water flow through without obstruction, maintain the bed and banks, clear debris that could cause a blockage, and accept flood flows from upstream. Where the watercourse has been culverted, those duties extend to the culvert and to any trash screen on it. De-culverting a watercourse for planning purposes is usually proposed as a benefit of a development, and it often is one, but the starting point is that the pipe is your responsibility whether or not you ever build anything.

Two consent regimes sit over any work. For an ordinary watercourse, consent under section 23 of the Land Drainage Act 1991 is needed from the lead local flood authority, or from the internal drainage board where one has jurisdiction, before you alter or obstruct the flow. For a main river, an environmental permit for a flood risk activity is required from the Environment Agency. Planning permission authorises neither.

What riparian ownership means and what it obliges

Ownership is presumed to run to the centre line of the watercourse where it forms a boundary, and to the whole channel where it runs within your land. The presumption can be displaced by the title deeds, so check them rather than assuming.

The duties that follow are the ones that catch rural owners out:

• Pass on the flow without obstruction, pollution or diversion.
• Maintain the bed and banks, and the trees and shrubs on them, so the channel stays clear.
• Clear debris, including debris that did not originate on your land.
• Keep culverts and trash screens free of blockage.
• Accept flood flows from upstream, even where they cause damage.
• Obtain consent before altering the channel in any way.

There is no duty to improve a watercourse, and no duty to prevent flooding of your neighbours by natural flows. But there is liability if your obstruction causes flooding elsewhere, and that liability is where culverts bite.

Why authorities resist new culverts

The policy position across national guidance and Environment Agency practice is a presumption against culverting an open watercourse, with new culverting permitted only where there is no reasonable alternative, and then only for the shortest practicable length.

The reasons are concrete rather than aesthetic. A culvert has a fixed capacity that an open channel does not, and when it surcharges the water goes somewhere unplanned. Culverts block, and the blockage is invisible until it floods. They cut the channel off from its floodplain and from daylight, so the ecological and water quality functions stop. And they are expensive to inspect, maintain and eventually replace, with the cost falling on whoever owns the land above them in fifty years' time.

An applicant proposing a culvert for convenience, usually to gain a few extra plots or a simpler access, should expect the lead local flood authority to object and should price the alternative before submitting.

Opening up a culverted watercourse as betterment

De-culverting runs the other way and authorities welcome it. Where a development can open a culverted length and restore a channel with a natural profile, that is a benefit to weigh in the planning balance, and it often unblocks other objections.

It is also harder than it looks. The design has to deal with the existing culvert's invert levels, which are usually lower than a natural channel would be, so the restored channel either needs a long transition or a drop structure. Land take is substantial once you allow for a channel, two banks at a stable gradient and a maintenance strip. Services frequently cross the culverted line. And the hydraulic performance of the new channel has to be at least as good as the pipe it replaces across the full range of events, which has to be modelled rather than asserted.

Outfall and connection details are a design discipline of their own, and outfall design, bank stability and scour protection covers the points reviewers test.

Assessing the capacity of an existing culvert

Where the culvert stays, its capacity has to be established rather than estimated from the headwall diameter.

A proper assessment needs five things. A survey of the full length, usually by camera, recording diameter, material, condition and any change in section. Invert levels at both ends, measured rather than scaled. The upstream catchment area. A hydraulic calculation of capacity including entry and exit losses. And an assessment of blockage risk at the inlet.

The blockage case is the one reviewers care about. A culvert that passes the design flow when clear and surcharges with a 50 per cent blocked inlet is a flood risk, and the assessment should model that scenario and show where the water goes. On a site where development is proposed over or near the culvert, the exceedance route matters more than the clear-flow capacity.

Old culverts also fail in ways that are not about capacity. Collapsed sections, root intrusion, silt accumulation to half the diameter, and brick barrel deterioration are all common in Victorian culverts, and a camera survey finds them in an afternoon.

Blockage, maintenance and liability

The riparian owner is responsible for keeping a culvert clear, and that duty does not transfer to the council because the culvert runs under a road, unless the highway authority built and owns it.

Trash screens are a particular problem. A screen keeps debris out of the culvert, which is good, and concentrates blockage risk at a single point, which is not. Screens need a maintenance regime with a stated frequency and a response arrangement during storms, and they need safe access, because people have been injured clearing screens in flood conditions. Where a development relies on a screen, name who maintains it and how often in the drainage strategy.

The lead local flood authority and the Environment Agency both have powers to require works to be carried out, and to carry them out and recover the cost. That is the backstop and it is used.

An illustrative example

Illustrative only: a farm diversification scheme converting two barns, with an ordinary watercourse culverted in a 450 mm clay pipe for 60 metres under the yard. A camera survey finds two displaced joints and silt to a third of the diameter. The capacity calculation shows the clear pipe passes the 1 in 30 year flow and surcharges in the 1 in 100 year event, with overland flow heading towards the proposed conversion. The scheme de-culverts 35 metres across the open yard area, retains a 25 metre length under the access track in a new oversized pipe, raises the converted threshold levels by 350 mm, and secures section 23 consent for both elements. One watercourse, two treatments, one consent application.

Easements, access and third party land

De-culverting rarely stops at the red line. The upstream and downstream connections are usually on someone else's land, and a restored channel that ends at a boundary creates a problem for the neighbour.

Resolve the land position before the drainage strategy is finalised. That means identifying every owner along the affected length, establishing whether existing easements cover the culvert, and agreeing access for construction and for future maintenance. Where third party agreement cannot be obtained, the design has to work entirely within your own land, which often means retaining more culvert than you wanted.

Construction itself brings its own consent questions, and the consents needed for dewatering catch out contractors working in or near a channel.

What the drainage strategy should propose

Put the whole picture in one document. The Drainage Strategy should set out the watercourse status, whether it is a main river or an ordinary watercourse and who regulates it, the riparian ownership position with the extent shown on a plan, the condition survey findings, the capacity assessment with the blockage scenario, the proposed works with levels, the consent route for each element, and the maintenance regime with a named responsible body.

Where flood risk to the development is in issue, the Flood Risk Assessment should use the same levels and the same modelling outputs, and the two documents should agree on the exceedance route. On sloping rural sites the overland flow question is usually as important as the channel itself, and managing run-off on a steeply sloping site deals with that separately.

Councils normally expect a suitably qualified author for hydraulic work, and professional review before submission is strongly advised.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.