Residual flood risk: writing about breach and blockage

The residual flood risk assessment section is where most flood risk assessments go thin, and it is the section a consultee reads hardest. Residual risk is what remains once the defences, the levels and the drainage design have done their work: the embankment that breaches, the culvert that blocks, the pump that stops, the event larger than the one you designed for. Writing it well means naming each mechanism, saying what happens to people if it occurs, and saying who does what about it. Writing it badly means a paragraph asserting that residual risk is low.

What residual risk means after mitigation

Mitigation reduces probability or consequence. It does not remove either, and the assessment has to say what is left.

Four families of residual risk turn up on English sites. Failure or exceedance of a defence, whether by breach or by overtopping in an event beyond its standard. Blockage of a conveyance structure, usually a culvert, a screen or a bridge soffit. Failure of an active system, meaning pumps, penstocks, non-return valves and the power that runs them. And the exceedance event itself, the flood larger than the design event including the climate change allowance.

Two others appear on particular sites: reservoir or canal breach above the site, and surcharge of the public sewer network.

Deal with each one that applies, and say explicitly which do not apply and why. A consultee who can see that you considered and dismissed breach on a site with no defences will trust the rest of the document.

Defence breach and overtopping scenarios

Breach and overtopping are different failures with different consequences and should not be merged.

Overtopping happens when the water level exceeds the defence crest in an event beyond its standard of protection. The result is usually a rising, relatively slow inundation from a broad front, and the depth is bounded by the level of the flood.

Breach happens when the structure fails, often below crest level, and produces a fast, deep, locally violent flow. The hazard to people comes from velocity as much as depth, and the onset can be sudden enough that a warning is useless.

Where breach modelling is required, the scope is a matter for agreement rather than assumption. Settle four things with the Environment Agency or the lead local flood authority before anyone runs a model. The breach locations, chosen as worst case for the site rather than at a convenient point. The breach geometry, meaning width and invert. The timing of the breach relative to the peak, since a breach on the rising limb behaves differently from one at the peak. And the events to be tested, including the design event with the appropriate climate change allowance.

Present the results as depth, velocity and a hazard rating at receptors rather than as an extent on a plan. The hazard rating combining depth and velocity, developed in the Defra and Environment Agency research on flood risks to people, gives bands running from caution, through dangerous for some and dangerous for most, to dangerous for all. Those bands are what an emergency planner needs, because they answer whether a person can walk out.

Blockage of culverts and trash screens

Blockage is the most common residual mechanism on small urban and semi-rural sites, and the one most often skipped.

Assess it in scenarios rather than as a single case. A partial blockage, conventionally taken at half the cross-sectional area, and a total blockage are both worth running, because the consequences are not proportionate. Where the blockage backs water up onto a road, a total blockage can redirect flow into a whole street that stays dry in the partial case.

Consider what actually blocks the structure. Debris from upstream vegetation, fly-tipping at an accessible screen, sediment in a low-gradient culvert, and ice in a cold snap all behave differently. The screen design guidance published by CIRIA sets out how screens should be configured, raked and accessed, and a scheme that relies on a screen nobody can reach with a rake has not mitigated anything.

Then say who maintains it. Riparian owners carry duties for watercourses passing through their land, and works to an ordinary watercourse need consent from the lead local flood authority, as our note on land drainage consent for works to an ordinary watercourse explains. Where the structure is on third-party land, name the owner and say what agreement exists.

Failure of pumps and control devices

Active systems fail predictably and the assessment should treat them as failing.

State the failure modes: power loss, mechanical failure, control fault, blocked intake, and the coincidence of power loss with the flood that makes the pump necessary. Then state the design response for each: duty and standby units each sized for the full flow, alternating operation, a high-level alarm with telemetry to a monitored address, emergency storage sized for a stated outage period, and standby generation or an uninterruptible supply where people sleep below the flood level.

Non-return valves, penstocks and anti-flooding devices need the same treatment. They fail silently, which is worse than failing loudly, so the mitigation is inspection frequency and a maintenance record rather than a redundant unit.

Put the assumed outage period in writing. Four hours and twenty-four hours produce very different storage volumes, and an assessment that does not state which was assumed cannot be checked.

Expressing residual risk without modelling everything

Proportionality is allowed and is often the right answer. A three-house infill behind a defence does not need breach modelling if the strategic assessment already contains rapid inundation mapping for that defence.

Work through the less costly sources first. Level 2 strategic flood risk assessments frequently contain breach and hazard mapping for defended areas. The Environment Agency holds breach modelling for many defences and will release it on request. Reservoir inundation mapping is published. Where any of those cover your site, cite them, apply your surveyed levels, and explain the result.

Where nothing exists, a qualitative assessment with a sensitivity test can be enough on a small scheme: state the defence crest level, the ground levels, the volume that could plausibly arrive, and the depth that implies across the site, then design the levels and the escape route with margin. Say plainly that the approach is qualitative and why it is proportionate.

Do not invent a number to fill the section. An assessment that says "breach depth is estimated at 0.4 metres" with no basis is worse than one that says the depth has not been modelled and explains the precautionary response.

Management measures and who carries them out

Mitigation that depends on a person doing something needs that person named.

• Flood warning registration, with the service that covers the site identified and a note of whether the site is within a warning area at all.
• A flood warning and evacuation plan setting out trigger points, actions, the safe refuge location and level, and the route out, prepared in consultation with the authority's emergency planning officer.
• Safe refuge above the design flood level where dry egress cannot be achieved, with the floor level stated.
• Inspection and maintenance of screens, pumps, valves and attenuation features, with frequency, responsibility and record keeping specified.
• Property flood resilience measures where they are appropriate, covered in our note on residual flood risk and property flood resilience measures.
• The mechanism that binds all of it: a management company, a condition requiring a plan to be implemented and maintained, or an obligation in a section 106 agreement.

An assessment that recommends a maintenance regime without saying who holds the duty after the developer leaves is incomplete, and consultees have become good at spotting it.

Wording that satisfies a consultee

Structure each residual mechanism the same way and the section writes itself: the mechanism, the scenario tested and its source, the predicted consequence in depth, velocity and hazard terms at named receptors, the mitigation proposed, the position after mitigation, and the party responsible.

One worked sentence is worth more than a page of policy. "In the modelled breach at chainage 1,250, depths at the site reach 0.6 metres with velocities below 0.5 metres per second, giving a hazard rating in the dangerous for some band. Finished floor levels at 8.9 metres above Ordnance Datum remain dry and the pedestrian route to Mill Lane stays passable throughout, so the evacuation plan relies on dry egress rather than refuge." That figure set is illustrative, and the shape is what matters.

Keep the levels consistent with the Drainage Strategy and the rest of the Flood Risk Assessment. Most consultee objections to residual risk sections are not about the analysis at all. They are about a document that contradicts itself.

To see which flood documents a site is likely to need, enter your postcode on PlanScope and run the constraint check.

This is general guidance, not legal advice. Flood risk work should be prepared by a suitably qualified author and reviewed before submission. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.