Exceedance flow routing when the drainage system fills
Above the 1 in 100 year event, water runs overland. Designing where it goes is a layout and levels exercise, and lead local flood authorities expect to see it drawn rather than described.
Exceedance flow routing is the design of the above-ground route water takes once the drainage system is full. The Defra non-statutory technical standards for sustainable drainage systems set the sequence: no surface flooding anywhere on site in a 1 in 30 year event, no flooding of any building or utility plant in a 1 in 100 year event including the climate change allowance, and flows above the 1 in 100 year event managed along exceedance routes that keep risk to people and property low. The third of those is the one that gets left out of drainage strategies, and it is the one a lead local flood authority asks about at the point you thought the scheme was agreed.
What happens above the design event
Every piped and attenuated system has a capacity. Above it, water does what gravity tells it to, which on most developed sites means running along the road, pooling against the lowest kerb line and finding the lowest threshold.
Three failure modes generate exceedance, and the design has to cope with all three rather than only the rainfall case.
Rainfall beyond the design return period is the obvious one. Blockage is the more common one in practice: a gully grating covered in leaves, a trash screen choked, a hydrobrake orifice silted. Mechanical or operational failure of a pumped system is the third, and it matters disproportionately because pumped systems fail at the worst moment, during a storm with a power cut.
The useful discipline is to treat exceedance as a layout question rather than a hydraulics question. You are not sizing a pipe. You are deciding where a sheet of water 50 to 150 millimetres deep will go across a site you have already drawn, and arranging the levels so that the answer is somewhere harmless.
Mapping overland flow paths across a layout
Start with the topographic survey, not the drainage layout. Contours at 250 millimetre intervals across the site and at least 20 metres beyond the boundary will show you the flow paths that existed before anyone proposed anything.
Then overlay three things: the Environment Agency surface water flood risk mapping, the proposed finished levels, and the position of every threshold. Reading the surface water map takes some care, because its extents are indicative and modelled at a coarse resolution.
What you are looking for is the set of low points. On a site with a consistent fall, exceedance is straightforward and the route is the road. On a site with a local hollow, which is normal where a layout has been fitted around retained trees or an existing access level, exceedance ponds. Ponding is acceptable where it is designed, bounded and shallow. It is not acceptable where it is an accident of the levels drawing.
A site that falls towards its own access is the hardest case, because the route out is also the route in for vehicles and pedestrians.
Kerbs, swales and designed depressions
The tools available are mundane and effective.
A standard 125 millimetre kerb upstand will convey a surprising volume along a carriageway channel before anything overtops. Raising a short run of kerb, or dropping it deliberately at a chosen point, is the cheapest exceedance control there is.
A swale running alongside a road gives conveyance and storage together and keeps water visible, which matters for maintenance. Swale invert levels need to sit below the road channel at the point where water is meant to transfer, and that transfer point needs a flush kerb or a kerb outlet rather than a 125 millimetre upstand.
Designed depressions in public open space, shallow and grassed, take the volume that conveyance cannot. A 300 millimetre deep depression over 200 square metres holds 60 cubic metres, and that calculation belongs in the strategy with the depth, area and volume stated.
Car parking courts work as temporary storage where the fall is away from buildings and the depth stays low. Children's play areas do not, and neither does anywhere a vehicle might be parked overnight.
Depth and velocity together decide whether a route is safe. The flood hazard rating used in flood risk work combines depth and velocity with a debris factor, and the bands run from a low hazard, through danger to some people, to danger to all. Keep designed routes in the lowest band, which in practice means shallow and slow.
Protecting thresholds and basement entries
Thresholds are where exceedance becomes a claim.
The conventional figure is a finished floor level at least 150 millimetres above adjacent external ground, with a greater freeboard where a flood level has been modelled. Where the site sits in Flood Zone 2 or 3 the Environment Agency will usually look for freeboard above the modelled 1 in 100 year plus climate change level, and the figure to use comes from the current climate change allowances for the relevant river basin district rather than from an older report. Those allowances are revised periodically.
Level thresholds required for accessibility pull against all of this. The resolution is normally a drainage channel across the door line draining to the system, a cross-fall away from the threshold over the first 1.5 metres, and a route that takes exceedance past the door rather than towards it.
Basement entries are a different problem, because gravity drainage is unavailable and the consequence of failure is a flooded habitable space. Stepped access down to a basement door is a sump unless it is drained and the drainage is independent of the main system. Basements, flood risk and sleeping accommodation covers the policy position where bedrooms are proposed below ground. Where a basement stays in the scheme, pair the drainage design with property flood resilience measures and say which ones, because residual flood risk and property flood resilience measures is the section a reviewer reads next.
Showing exceedance on a drawing
A paragraph of text will not satisfy a lead local flood authority. An exceedance plan will, and it needs six things.
- Proposed finished levels across the whole site, including the external areas nobody has detailed yet.
- Arrows showing the direction of overland flow for each catchment within the site, including flow arriving from outside the boundary.
- Ponding areas hatched, with depth and volume labelled for each.
- Threshold levels for every building, annotated against the adjacent external level.
- The blockage scenario assessed, stated explicitly, with a 50 per cent blockage of the flow control a common assumption to check with the authority.
- The exit point or points, with the level at which water leaves and where it goes.
Add a long section through the critical route. One section, showing the road channel, the kerb, the swale invert and the nearest threshold, answers more questions than three pages of commentary.
The exceedance plan sits inside the drainage strategy rather than inside the Flood Risk Assessment, although the two have to agree. Where they disagree, and they often do because they are written by different people at different times, the reviewer will assume the worse of the two numbers.
Where the route leaves the site
This is the part that generates objections, because the exit is almost always someone else's land.
There is no general right to discharge surface water onto neighbouring land, and an exceedance route that terminates at a boundary fence is not a design. The options are a discharge to the highway with the highway authority's agreement, a discharge to an ordinary watercourse, which needs land drainage consent under section 23 of the Land Drainage Act 1991 from the lead local flood authority, a discharge to a main river with an environmental permit from the Environment Agency, or retention on site in designed storage.
Land drainage consent is routinely forgotten until after permission, and the process is described in land drainage consent for works to an ordinary watercourse.
National policy requires development not to increase flood risk elsewhere. An exceedance route that sheds a greater volume at a greater rate onto a downstream neighbour than the undeveloped site did will fail that test even where the 1 in 100 year design case passes. Model the existing overland regime as well as the proposed one, and show the comparison.
Conditions that secure the arrangement
Expect three conditions and draft them with the applicant's programme in mind.
A pre-commencement condition requiring full drainage details, which should be read to include the exceedance plan and the long sections. A levels condition fixing finished floor and external levels, which is what actually protects the thresholds, because a drainage condition alone does not stop a contractor raising a driveway by 100 millimetres. And a management and maintenance condition covering the lifetime of the development, naming who maintains the swale, the depression and the flow control.
Schedule 3 to the Flood and Water Management Act 2010, which would introduce SuDS approving bodies and a separate approval route, has still not been brought into force in England as at October 2026, so planning conditions and section 106 obligations remain the mechanism. Check the current position before you rely on that.
Enter your postcode on PlanScope to see which flood and drainage reports a site is likely to need.
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.
Frequently asked questions
What design events does a surface water system have to cope with?
The Defra non-statutory technical standards set three. No surface flooding anywhere on site in a 1 in 30 year event, no flooding of a building or utility plant in a 1 in 100 year event including the climate change allowance, and flows above that managed along exceedance routes that keep risk to people and property low. The third is a layout requirement rather than a pipe sizing one.
Does exceedance only happen when rainfall exceeds the design storm?
No. Blockage is the more frequent trigger, whether a gully grating, a trash screen or a silted flow control orifice. Pumped systems add a third mode, because they fail during storms when power is lost. A reviewer will usually ask what blockage scenario was assessed, and a 50 per cent blockage of the flow control is a common assumption to agree with the authority.
How much freeboard should a finished floor level have?
At least 150 millimetres above adjacent external ground is the conventional minimum on a site with no modelled flood level. In Flood Zone 2 or 3 the Environment Agency normally looks for freeboard above the modelled 1 in 100 year level including the climate change allowance. Take the allowance from the current published figures for the river basin district rather than from an older report.
Can an exceedance route discharge onto neighbouring land?
Not as a matter of right. A route that stops at a boundary fence is not a design. The workable exits are a highway discharge agreed with the highway authority, an ordinary watercourse under a land drainage consent from the lead local flood authority, a main river under an environmental permit, or retention on site in designed storage with the volume stated.
Which drawing shows exceedance, the flood risk assessment or the drainage strategy?
The exceedance plan belongs in the drainage strategy, with the flood risk assessment relying on it and agreeing with it. The two are often written at different times by different people, and where their levels or volumes conflict a reviewer will take the less favourable figure. Reconcile them before submission and say in both documents that they match.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.