Exceedance routing above the design event

Exceedance flow routing is the part of a drainage design that deals with the rain the system was never sized for. Pipes and attenuation are designed to a stated return period. Above that, water still arrives, it flows over the surface, and the question the lead local flood authority will ask is where it goes and what it does on the way. An exceedance plan answers that question on a drawing.

Designs fail this test more often than they fail the hydraulic calculations. The attenuation volume is right, the discharge rate is agreed, and nobody has drawn the overland route from the car park to the front doors.

Why design events are not the whole answer

A surface water system is normally designed so that the pipe network does not surcharge in the smaller event and so that flooding does not reach buildings in the larger one, with a climate change allowance applied to rainfall. The Defra non-statutory technical standards for sustainable drainage set the expectations most authorities work to, and the Environment Agency publishes the climate change allowances to apply.

Anything above the design event is exceedance. It is not a design failure, it is a designed condition, and the national standards expect flood routes and flows to be managed so that the risk to people and property is minimised.

So the design question is not how to prevent exceedance. It is how to choose where the water goes when it happens.

Identifying overland flow routes across a site

Start with the topography, not the drainage layout. Take the proposed finished levels, not the existing ground, because the layout changes the catchment. Then find every low point and every flow path a surface would create.

Work through this sequence:

1. Plot proposed contours or a digital terrain model of the finished site.
2. Identify all low points, including those created by the design: the base of a ramp, a sunken courtyard, a dished junction, the inside of a kerb radius.
3. Trace the flow path from each catchment area to its lowest point and onwards.
4. Identify what lies at the end of each path, and whether it is a threshold, a basement light well, a substation or an open area.
5. Check what arrives from off site, including overland flow from higher ground beyond the boundary.
6. Check what leaves the site, and whether sending it downstream creates a problem for a third party.

That last point matters legally as well as technically. Concentrating flow onto neighbouring land where it did not previously go invites a civil claim as well as an objection.

Off-site flow is routinely missed. A site at the bottom of a hill receives water regardless of how its own drainage performs, and the surface water flood risk mapping gives a first indication, with the caveats set out in reading the surface water flood risk map without overstating it.

Designing roads and landscape to convey water safely

The usual answer is to use the highway as a channel. A road with upstand kerbs, falling in the direction you want the water to go, carries a surprising volume at shallow depth and takes it to somewhere that can hold it.

Three design moves do most of the work. Set the road network so it falls towards an open space, a basin or a watercourse rather than towards the buildings. Keep a continuous route, with no high point interrupting it, because a kerb line that rises for 20 metres turns a channel into a pond. And use the landscape at the end of the route, with a swale, a basin or a playing field designed to flood in extreme events and drain afterwards.

Depth and velocity both matter where people are present. A road carrying water at a shallow depth is passable. The same volume squeezed through a narrow gap between buildings becomes fast and dangerous, and the hazard that results is assessed on depth and velocity in the same way as river flooding, which is why the flood risk assessment and the drainage design have to agree on the figures.

Avoid routing exceedance through private rear gardens. It works hydraulically and fails in practice, because fences, sheds and raised beds appear within two years of occupation and the route disappears.

Protecting thresholds and vulnerable areas

Freeboard at thresholds is the simplest protection available. Setting finished floor levels above the adjacent external ground, with the external levels falling away from the building, keeps shallow overland flow out of the house. Many authorities expect a stated minimum difference between the threshold and the surrounding level, so check the local requirement.

Level access requirements pull in the opposite direction, and that tension is real. The resolution is usually to keep the threshold level at the door and to fall the external surface away quickly, with a drainage channel at the door line connected to the system, plus a secondary overflow path that does not point at the opening.

Basements, light wells and sunken terraces deserve particular attention because they collect everything. Where a basement contains sleeping accommodation the issue is more serious still, and the position is set out in basements, flood risk and sleeping accommodation.

Electrical substations, lift pits, plant rooms and refuse stores should sit off the flow routes. A substation flooded in an extreme event takes the pumps with it, which is how a manageable event becomes a serious one.

Modelling or mapping exceedance

Proportionality applies. For a small site on a slope, a levels drawing with arrows, a note of the receiving feature and a check on thresholds is enough, and asking for more is unreasonable.

For a larger or flatter site, or one where exceedance has to pass between buildings, a two-dimensional surface flow model gives depths and velocities across the layout and is worth the fee. State the rainfall event modelled, the climate change allowance applied, whether the piped system was assumed blocked or full, and the model's limitations.

The blockage assumption is where reviewers concentrate. A model assuming a fully functioning network above the design event is modelling the wrong thing. Many authorities expect a blockage scenario, typically on a critical inlet or outfall, to be tested separately.

Conditions and the exceedance plan

Exceedance is usually secured through the drainage condition rather than in a policy of its own. The submitted Drainage Strategy should contain the exceedance plan as a numbered drawing rather than a paragraph, showing flow arrows, low points, threshold levels, the designed flood storage areas and the receiving feature.

Expect the condition to require the strategy to be implemented as approved, and expect the lead local flood authority to check the as built levels against it. Where the contractor regrades a verge during construction and reverses a fall, the route stops working and nobody notices until the first serious storm.

Keep the plan consistent with the landscape drawings. A basin shown as flood storage on the drainage layout and as a formal lawn with raised edging on the landscape plan is a contradiction officers find quickly.

Common failures in flat urban layouts

Flat sites are the hard case, because there is no gradient to work with and no obvious receiving feature. The failures follow a pattern.

A single low point in a shared courtyard, with the only exit a ramp down to a basement car park. Podium decks where the designed overflow discharges onto a route used for level access. Attenuation tanks with no surface overflow at all, so once the tank is full the water finds its own way. A perimeter wall with no gap, turning the whole site into a tank. And flow concentrated into a narrow alley between blocks, producing high velocity exactly where pedestrians walk.

On flat sites the honest answer is often to design a shallow, broad area of deliberate flooding over hard landscape, with levels set so the depth stays low and the water drains within hours. That requires the landscape architect, the engineer and the client to agree in advance that an area is designed to flood, which is a conversation worth having before planning rather than after.

This is general guidance, not legal advice. Design standards, freeboard requirements and modelling expectations vary between lead local flood authorities, so check the local requirements and the current climate change allowances with your authority or a qualified professional before relying on it.

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