Exceedance routing: designing for the storm beyond design

Exceedance flow routing for a SuDS scheme answers a question the attenuation calculation does not: where does the water go when the system is full. Design events have a return period, rainfall does not respect it, and every drainage system has a failure mode. The national technical standards for sustainable drainage require that flooding from the system does not enter any building during events up to the 1 in 100 year rainfall, and that flows above the design event are conveyed and stored safely. That is a separate drawing and a separate piece of analysis from the storage sizing.

Most refusals and most lead local flood authority objections on drainage come from the absence of that drawing.

Why exceedance is assessed separately

Attenuation design answers how much volume the system holds and at what rate it discharges. Exceedance design answers what happens when the inflow exceeds the design capacity, when a gully blocks, when a trash screen chokes, or when an outfall is drowned by a high river level.

Those are different failure modes with different consequences, and the standards treat them separately for a reason. A scheme can be perfectly sized for the 1 in 100 year event with a climate change allowance and still flood three houses in a 1 in 200 year storm, because nobody looked at where the surface water would run once the pipes surcharged.

The analysis is also required where no attenuation is proposed at all. A site discharging entirely to infiltration still generates overland flow when the features are saturated, and that flow has to go somewhere.

Identifying overland flow paths across a site

Start with the pre-development situation and read the ground. Contour survey at 0.25 m intervals, the surface water flood risk mapping, and a walkover after heavy rain tell you where water already goes. Look for the low point in each boundary, the historic ditch line, the culverted stream nobody mentions, and the point where flow currently leaves the site.

Three things matter more than modelling at this stage. Where flow enters the site from upstream, because you are obliged not to make things worse for anyone downstream and you cannot block an existing flow route. Where it leaves. And what the ground does in between, particularly any hollow that already ponds.

Then compare that with the proposed levels. Regrading a site changes flow paths, and a road built across a natural flow line becomes either a conveyance route or a dam. The Flood Risk Assessment should carry the pre-development flow route plan, and the drainage design should carry the post-development one, with any change explained.

Designing safe routes and shallow ponding areas

A safe exceedance route conveys water at a depth and velocity that does not endanger people or property, and delivers it to a place where temporary ponding does no harm.

The practical design toolkit is short. Roads act as conveyance channels, using kerb upstands and crossfalls to hold flow in the carriageway. Car parks, play areas, verges and amenity grass act as shallow storage. Landscaped depressions and basins take the overflow deliberately, with a freeboard above the design water level. Footpaths and shared surfaces can convey flow if levels are controlled. And boundary walls can be detailed with gaps or grilles to let flow through rather than pond against them.

Depth and velocity both need stating, because the hazard is a product of the two. Shallow flow moving fast across a footway is a hazard to a child. Still water 300 mm deep in a car park overnight is an inconvenience. Say which you are designing for at each point on the route.

Where ponding is proposed on land in other ownership, or where the route crosses a boundary, the arrangement needs legal certainty rather than a hopeful arrow on a drawing.

Protecting dwellings and basements

Finished floor levels are the primary defence. A floor level set above the adjacent ground with the ground falling away from the building means an exceedance flow passes rather than enters, and that costs nothing at design stage.

Four details do most of the work. A minimum step from external ground to threshold, with the ground graded away from the building. Driveways that fall to the highway rather than to the garage door. Level thresholds, which are required for accessibility, protected by a channel drain and a local high point beyond it. And no exceedance route directed towards a building line, however convenient the fall.

Basements and lower ground floors need explicit treatment, because a route that passes a house at ground level enters a basement at the lightwell. Set lightwell surrounds above the exceedance level, drain them independently with a pumped sump and a non-return valve, and show the flow route passing clear of them. The interaction between basements and flood risk is a recurring objection in its own right, dealt with in more depth in the note on basement development and the flood risk objection.

An illustrative example. An estate road falls 1 in 80 towards a cul-de-sac head with eight houses around it. Gullies at the low point are sized for the design event. In exceedance, flow arrives at the cul-de-sac with nowhere to go, and the houses sit 150 mm above the road. Moving the low point to a landscaped swale beyond the turning head, raising the road crown at the two most exposed frontages and taking the plot thresholds to 300 mm removes the risk. The figures are illustrative and every layout needs its own analysis.

Kerb levels, road profiles and the drainage plan

The exceedance drawing should be a levels drawing, not a schematic. What a reviewer wants is a plan showing spot levels along every flow route, kerb upstand heights, the direction of flow with arrows, the location and extent of each ponding area with its volume and maximum depth, the exceedance outfall or discharge point, and the finished floor levels of every building adjacent to a route.

Two labels save a round of questions. Mark the maximum water level in each ponding area and the freeboard to the nearest threshold. And mark the point where flow leaves the site, with a note on where it goes and whether the receiving system or land can accept it.

Coordination with the highway authority matters here, because using a carriageway as a conveyance route needs the adopting authority's agreement, and some resist kerb upstands above their standard. Raise it early rather than at technical approval.

Modelling assumptions the authority questions

Where 2D overland flow modelling is used, the assumptions are what get challenged. Six recur.

1. Blockage scenarios, typically a partial or complete blockage of trash screens, inlets or gullies, applied to the most sensitive element rather than an average one.
2. The urban creep allowance for future paving over of gardens and yards, which the standards expect to be included.
3. The climate change allowance applied, by river basin district and epoch, with the source and version stated.
4. Peak rainfall intensity allowances for the drainage system, as distinct from river flow allowances.
5. Whether the receiving watercourse level is assumed to be free-flowing or in flood at the same time, since a joint probability assumption changes the answer.
6. Gully and inlet capacity in practice rather than in theory, including grate efficiency at the design flow.

State each assumption with its source in a short table in the Drainage Strategy. A reviewer who can see the assumptions will argue with one of them. A reviewer who cannot will send the whole document back.

Attenuation features that also serve another function, such as rainwater harvesting tanks, need care in exceedance because their available volume depends on the antecedent condition. The interaction with attenuation credit for rainwater harvesting is worth setting out explicitly rather than assuming an empty tank.

Securing the arrangement by condition

An exceedance route only works if it survives occupation, and residents fill in swales, raise kerbs, pave gardens and block gaps in walls.

The mechanisms that hold it are the approved levels drawing referenced in the condition, a management and maintenance plan naming the party responsible for each element, restrictive covenants or estate rules preventing alteration of ponding areas and flow routes, adoption where the highway authority will take the conveyance route, and the verification report at completion confirming levels were built as approved.

Ask for the condition to reference the exceedance drawing by number. A condition requiring drainage in accordance with the approved details is much harder to enforce when the exceedance plan was never listed.

One honest uncertainty. There is no single national standard for the acceptable depth and velocity of an exceedance flow across a development, and lead local flood authorities apply different thresholds, some borrowed from flood hazard rating work. Ask which criteria your authority uses before you design to a figure of your own choosing.

Browse all 66 report types on PlanScope to see which drainage and flood documents your scheme is likely to need.

This is general guidance, not legal advice. Check the position with your local planning authority, the lead local flood authority or a qualified professional before relying on it.

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