The exceedance flow section of a drainage strategy

Exceedance is what happens to surface water when the storm is bigger than the drainage system was designed for, and the exceedance section of a drainage strategy is where you show the lead local flood authority that those flows leave the site along a route you chose rather than through somebody's front door. Every drainage system has an exceedance condition. The only question is whether it was designed or discovered.

This is the section that most often triggers a holding objection from a lead local flood authority on a major application, because it is the section developers treat as a formality and drainage engineers treat as the whole point.

What exceedance means in drainage terms

Design events are conventional. Pipes and below-ground storage on most residential sites are designed against a range of return periods, with the critical check being the 1 in 100 year event including an allowance for climate change, and the requirement that no flooding occurs within any part of a building during that event.

Exceedance covers two separate situations. Storms rarer than the design event, and blockage or failure of the system during an ordinary storm. A gully blocked with leaves in October produces an exceedance flow on a day nobody would call extreme, and the second case is more common than the first.

The distinction matters for what you have to show. For rare storms you model. For blockage you reason: which single point of failure, what volume then goes where, and what is downhill of it.

Designing an overland flow route

The route is a designed feature, not an absence of one. It runs from the point where water leaves the system, across the site along falls you have set, to a place where it can stand or discharge without causing harm.

Good routes share the same characteristics. They follow roads, footpaths, car parks and open space rather than private gardens. They have positive falls the whole way with no low spots that are not intended. They keep water away from building entrances and from any below-ground access such as a basement ramp or a lift pit. And they finish somewhere with capacity, which usually means an area of open space, an attenuation basin with freeboard, or a watercourse.

Where the site falls towards a neighbouring property, the route has to turn the water before the boundary. That is a levels problem, and it is solved in the earthworks design rather than in the drainage layout, which is why the exceedance route has to be set while the site levels are still movable.

Protecting property during exceedance

The convention that protects buildings is freeboard: finished floor levels set above the modelled exceedance level, commonly 150 mm above adjacent ground or above the modelled flood level, with the exact figure set by the authority and by any Flood Risk Assessment for the site.

Thresholds and openings need the same attention. A level threshold required for accessibility conflicts directly with a raised finished floor level, and the resolution is usually a shallow ramped approach with a drainage channel at the door, detailed properly rather than assumed.

Do not forget what is below ground. Basement light wells, car park ramps, bin stores and substations are the assets that fill first, and a substation flooded by an exceedance flow takes the whole scheme off supply.

Levels, kerbs and safe conveyance

Conveyance capacity comes from the road cross-section. A standard carriageway between kerbs carries a surprising volume at shallow depth, and raising the upstand at a critical point, or setting a footway crossfall to contain flow, is often all the containment a route needs.

Safety is judged on depth and velocity together rather than on either alone. Shallow fast-moving water across a footway is a hazard to a child in a way that the same depth standing in a car park is not. Where a route passes a place people walk, show the depth and velocity at that point and compare them against the hazard criteria in the guidance the authority uses.

Temporary works deserve the same thought, because a half-built site drains nothing like the finished one, and the phasing set out in the construction phase sections of a management plan should say where water goes between the first dig and the final adoption.

Two details that get missed. Vehicle safety in car parks, where a flow depth that is harmless to a person will float a parked car. And the point of discharge, where concentrated flow leaving the site at one spot can erode a bank or overwhelm a ditch that the undeveloped site never loaded.

Showing the route on the drainage drawing

Lead local flood authorities want to see it drawn, not described. The exceedance drawing should show:

1. Arrows following the route from each exceedance point to the receiving area, with the contributing area for each
2. Spot levels and finished floor levels along the route, and the freeboard achieved at the nearest buildings
3. Depths and velocities at the sensitive points, with the hazard assessment where people are present
4. The volume the receiving area can hold and the level it reaches when it holds it
5. The condition assumed, whether that is a rare storm or a blockage, and where the blockage was assumed to occur

Cross-reference the drawing from the text of the Drainage Strategy and repeat the key figures in the text. Officers read the text and check the drawing, and a strategy where the two disagree gets sent back.

Maintenance implications

An exceedance route only works if it stays clear. That means the maintenance plan has to name it: no new fences across it, no raised planters, no informal parking in the conveyance corridor, and gullies and outfalls inspected before autumn rather than after.

The awkward part is future occupiers. A route that runs between two houses is a route that will have a fence built across it within five years unless the land is in common ownership or the deeds restrict it. Design routes along adoptable or commonly managed land wherever the layout allows, and where it does not, secure the corridor legally and say how in the strategy.

Where lead local flood authorities object

The objections repeat. No exceedance drawing at all. A route shown leaving the site boundary onto third-party land with no evidence of a right to discharge there. Finished floor levels given as a note rather than as levels. A receiving area whose volume is asserted rather than calculated. And a strategy that models the rare storm but never considers blockage.

The simplest fix for all of them is to draw the route early, while the engineer, the architect and the landscape designer are still able to change levels together. Once the housetype levels are fixed and the podium is priced, the exceedance route is whatever is left over, and what is left over is usually the gardens.

PlanScope drafts drainage strategies structured to UK planning policy, with site-specific facts flagged where they need confirming, and drainage design should be reviewed by a qualified engineer before submission. Enter your postcode to see which reports your 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.