Exceedance flow routing in a drainage strategy

Exceedance flow routing is the part of a drainage strategy that answers what happens when the system is full. Every piped and attenuated design has a capacity, and rainfall above that capacity has to go somewhere. Non-statutory technical standards for sustainable drainage require the site to be designed so that flows from rainfall in excess of the 1 in 100 year event are managed along routes that minimise risk to people and property, and a lead local flood authority will ask for a drawing that shows those routes.

Consultants often treat it as an afterthought, a paragraph added at the end of the calculations. Reviewers treat it as a test of whether the layout was designed with drainage in mind at all, because by the time the levels are fixed the options have usually gone.

What exceedance covers

Three separate scenarios, and a strategy should be explicit about which of them it has considered.

Rainfall greater than the design event. The system performs as intended and is simply overwhelmed, which is the standard case.

Failure or blockage. A gully blinds over with leaves, an outfall is obstructed, a pumping station loses power, a flow control is silted. This can produce exceedance in a modest storm, and it is the scenario that causes most real-world flooding of new development.

Loss of the receiving system. A surcharged sewer or a watercourse in flood means the site cannot discharge at all, so attenuation fills and then overtops. Where the site is also at risk from an external source, that interaction belongs in the Flood Risk Assessment rather than being handled twice.

Finding the flow path before designing the layout

Water goes downhill, and the site's existing topography tells you where. Start from survey levels and a digital terrain model, look at the catchment upslope of the site as well as the site itself, and find the low points and the natural valleys.

Then check what arrives from outside. An overland flow route crossing the site from adjoining fields is a pre-existing condition, and blocking it with a terrace of houses transfers a problem onto someone else's land, which is not something a council will accept. Critical drainage areas and surface water flood maps are the first check, and a site sitting in one should be treated as a drainage-led layout from the start.

On a greenfield site, sketching the exceedance strategy at concept stage costs an hour and usually improves the layout. The open space ends up in the low corner, where it was going to be wet anyway.

Routing rules that survive review

The principles are simple to state and routinely broken in practice.

• Route flows over adopted roads, parking courts and hard landscaping, not through private gardens or between buildings.
• Keep flow paths away from thresholds, and set finished floor levels above the level of any adjacent flow route with a stated margin.
• Never route towards a basement, a lightwell, a sunken terrace or an undercroft. These are the features that turn 200mm of surface water into an insurance claim.
• Give the route a defined destination: a shallow basin, an area of open space, a playing field, an over-designed swale, or a controlled outfall to the watercourse. A route that ends at the site boundary is not a route.
• Keep depths and velocities within hazard criteria along any route people will use, and check the two together rather than depth alone.
• Do not export more water off site than the pre-development condition, at any return period.

The last point is the one that gets strategies rejected. A neat exceedance route down the access road and out onto the public highway has solved the applicant's problem by giving it to the highway authority.

Kerbs, falls and temporary storage depth

The detail is where a good exceedance plan is made.

A standard kerb upstand gives a usable conveyance channel along a road, and a road built with a crossfall and a longitudinal gradient will carry a surprising flow within the carriageway alone. Calculate it rather than assuming: channel capacity depends on the upstand, the crossfall and the gradient, and a flat road with a 25mm upstand at a dropped crossing carries almost nothing.

Dropped kerbs are the weak point. Every vehicle crossing and every pedestrian crossing point is a place where the channel loses its containment, and the flow leaves the road towards whatever is beyond. Mark them on the plan and check what is downhill of each one.

For temporary storage, state the depth and the duration. Parking courts and service yards are the usual places, and depths in the region of 100mm to 200mm over a car park for a short period are commonly accepted where signage and levels make it safe. Playing fields and amenity grass tolerate more. What is not acceptable is unquantified ponding depth against a building, or storage that drains only by evaporation.

Fences deserve their own line. A close-boarded fence across an exceedance route is a dam, and it will either hold water against a house or fail and release it suddenly.

Exceedance on a phased site

The completed scheme is not the risky condition. The risky condition is phase two with the attenuation basin for phase three not yet built, the permanent outfall not yet connected, and haul roads changing the levels across half the site.

Set out the interim arrangements for each phase, with temporary storage, temporary outfalls and the levels that will exist at the end of each phase. Where a phase relies on infrastructure in a later parcel, say what happens if that parcel is delayed, which is a normal occurrence on a multi-outlet site.

Also state who is responsible during construction. A Drainage Strategy that hands the interim position to an unnamed contractor is the version that leads to a flooded neighbour and a complaint to the council during the build.

What the drawing has to show

One drawing, at a legible scale, with:

1. Existing and proposed contours or spot levels, including finished floor levels.
2. Arrows showing each exceedance route, from where flow leaves the system to where it ends up.
3. Depth of flow and, where relevant, velocity along each route.
4. Temporary storage areas annotated with area, depth and volume.
5. Kerb upstands, dropped crossings and any check structures.
6. External flows entering the site, and where they are conveyed to.
7. The design event assumed and the blockage scenario tested.

Illustrative example: a 60-dwelling site falling 4m from north to south, with attenuation in the south-east corner discharging to a ditch. Exceedance leaves the network at three manholes, runs along the spine road channel behind a 125mm upstand, and is held in the southern parking court to a maximum depth of 150mm for under 90 minutes before draining back to the basin. Finished floor levels sit 300mm above the adjacent channel level, and the two plots downhill of dropped crossings have their thresholds raised a further 100mm. A blockage test on the basin inlet routes flow around the basin edge to the ditch rather than back into the estate.

What the reviewer checks first

In roughly this order.

Whether the exceedance plan and the levels drawing agree. They often do not, and that alone generates a request for more information.

Whether every route ends somewhere defined, inside the site or at a controlled outfall.

Whether any flow path passes a threshold, a basement or a lightwell.

Whether flows leaving the site at any return period exceed the pre-development position, and whether third party land is affected.

Whether the blockage scenario has been tested at all, because an untested strategy assumes every gully works in a storm that fills every gully with debris.

Then the maintenance schedule, because exceedance routes silt up, get fenced across, get planted and get built over. Say who inspects them and how often, and put the routes into the management company's plan with a drawing they can understand.

Browse the full list of report types to see how the drainage documents sit alongside the rest of the evidence base, or enter a postcode on PlanScope to start a draft.

This is general guidance, not legal advice; check the position with your lead local flood 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.