Presenting residual flood risk after mitigation

The residual flood risk assessment section is the part of a flood risk assessment that gets read first and written last. An Environment Agency officer or a lead local flood authority engineer opens the document, scrolls past the site description and the zone classification, and looks for what happens when the mitigation does not work. If that section says "residual risk is considered to be low", the response is predictable, and it is a holding objection.

Residual risk is the risk that remains after everything proposed has been built and is working as designed, plus the risk that arises when it is not. Those are two different questions and most assessments answer neither.

What residual risk means and why officers read it first

National flood risk guidance expects an assessment to consider the risks that remain once mitigation is in place, and to show that the development will be safe for its lifetime taking climate change into account. The lifetime figures in that guidance are long, 100 years for residential development and a shorter period for most commercial uses, so the question is not whether the scheme copes with today's design event.

Officers read the section first because it is diagnostic. A residual risk chapter with numbers in it, depths in millimetres, velocities in metres per second, warning times in hours, tells the reader that the modelling has actually been interrogated. A chapter of adjectives tells them it has not.

There is also a procedural reason. Where the exception test applies, the second limb asks whether the development will be safe for its lifetime without increasing flood risk elsewhere, and the residual risk analysis is the evidence for that limb. The sequencing is set out in more detail in the Exception Test page, and a weak residual case fails the test regardless of how strong the wider sustainability argument is.

Defence overtopping, breach and blockage scenarios

For a site behind a raised defence, three failure modes need separate treatment.

Overtopping happens when the water level exceeds the crest. It is a gradual process, the volumes are limited by the duration of the exceedance, and it is usually modellable from the same hydraulic model that produced the design levels. Report the depth on site and the time to that depth.

Breach is a structural failure of the defence, and it behaves completely differently. The inflow is rapid, the velocities near the breach are high, and the hazard to people is driven by velocity as much as by depth. A breach assessment needs an assumed breach location, which should be the one that produces the worst case for the site rather than the most likely one, an assumed breach width and an assumed time to failure, all stated and justified. Run it at the design event level and, where the defence standard is lower than the design event, at the defence crest level.

Blockage affects culverts, trash screens, bridges and small watercourses. A culvert running under the site, or immediately upstream of it, should be tested with a blockage scenario, and the convention of assuming partial blockage is explained in modelling a partially blocked flow control. Trash screens on urban watercourses block routinely, and an assessment that assumes a clear screen for the lifetime of the development is not credible.

Present the three as a table of depth, velocity, hazard and onset time at identified points across the site, not as a paragraph.

Failure of pumps and the loss of power case

Any scheme that relies on pumping has a residual risk the moment the power goes off, and flood events and power interruptions are correlated.

The cases to assess are pump failure with power available, where duty and standby arrangements determine the outcome, and loss of power, where nothing runs. Set out the pumped volume, the storage available upstream of the pump, the time to fill that storage at the design inflow, and what happens after it fills.

Basement car parks are the recurring problem. A ramped entrance at or below external ground level, a pumped sump at the bottom and a reliance on mains power produces a structure that fills in a predictable sequence. The mitigations that work are a threshold level raised above the modelled flood level at the ramp head, a gravity overflow where topography allows one, a second power source, level alarms linked to a management response, and a design that keeps no sleeping accommodation and no essential plant below the flood level.

Where pumped foul drainage serves the site, deal with surcharge separately. A pumping station that cannot discharge during a flood backs up into the lowest connections, and that is a residual risk inside the building rather than on the site.

Exceedance of the design event and where water goes

Drainage is designed to a return period. Rainfall does not respect it.

Surface water systems are normally designed for the 1 in 30 event with the 1 in 100 plus climate change event attenuated on site, and above that the system is in exceedance. The question the assessment has to answer is where the water goes when it is, and the answer must be a route rather than a reassurance.

A proper exceedance analysis identifies the overland flow path from each point of the drainage network to a safe destination, shows the depth and width of flow in that route, confirms that the route does not pass a door threshold or a basement light well, and demonstrates that flows leaving the site do not increase risk on third-party land. Level design does the work: dished carriageways, raised thresholds, kerb upstands, a car park used as temporary storage, a swale aligned on the flow path.

Rainfall choice matters here, because exceedance depths scale directly with the storm used. State the dataset and the version, for the reasons set out in which rainfall dataset to cite in a drainage calculation, and say where the climate change uplift came from. Those allowances are revised periodically, so cite the current published allowances and check them at the time of writing rather than copying a figure from an earlier report.

Quantifying rather than describing the residual case

The difference between an assessment that is accepted and one that is queried is almost entirely about numbers. For each residual scenario, give:

1. Maximum depth on site, in metres, and the finished floor level in metres above Ordnance Datum alongside it, so the freeboard is visible.
2. Maximum velocity, in metres per second, at the locations where people would be.
3. A hazard rating derived from depth, velocity and a debris factor using the published flood hazard methodology, with the resulting hazard category stated.
4. Onset time and rate of rise, because a scheme with six hours of warning is a different proposition from one with twenty minutes.
5. Duration of inundation, which drives whether people can be evacuated or must shelter in place.
6. Number of occupants affected, split between mobile and less mobile where the use involves either.

Hazard rating is the one that persuades. Depth alone understates danger in fast-flowing breach water and overstates it in slow-filling ponding, and the published methodology exists precisely to combine the two. State the formula you used and the inputs.

Put the results in the Flood Risk Assessment as a table keyed to a plan showing the assessment points. Councils expect a suitably qualified author for hydraulic work of this kind and professional review of any draft is strongly advised.

Management measures that reduce residual risk

Management is not a substitute for level design, and it is the right answer for the part of the risk that cannot be designed out.

A flood warning and evacuation plan is the standard document, and a useful one states the warning source and the registration arrangements, names the person responsible, sets out trigger levels and the actions at each, identifies the evacuation route and shows it is passable at the depths the assessment predicts, identifies a refuge above the design flood level where evacuation is not possible, and explains how occupants are told about any of it. Plans that fail are the ones whose evacuation route floods before the trigger level is reached.

Flood resilient and resistant construction has a place for the parts of a building below the design level: solid floors, closed-cell insulation, raised electrical services, water-resistant finishes and demountable barriers where there is time and someone to deploy them. Barriers that require manual deployment should not be relied on where the warning time is short.

Secure the parts that matter by condition or legal agreement. A warning and evacuation plan that is not required to be implemented and maintained is a document, not a mitigation.

Honest treatment and the credibility it buys

The strongest residual risk chapters are the ones that concede something. A statement that the breach scenario produces 0.4 metres of water at the northern boundary with a hazard category of danger for some, that the proposed finished floor level provides 0.6 metres of freeboard above it, and that the evacuation route to the north-east remains passable throughout, is a case an officer can accept.

A statement that residual risk is low, with no scenario tested, invites every one of those questions to be asked at consultation instead, and the answers then arrive piecemeal over two months.

Write the chapter early. Running the breach and exceedance cases before the levels are fixed is what lets you raise a threshold by 150 millimetres at design stage rather than argue about it later. And set out plainly what you have not modelled and why, because the one thing that destroys a flood case is the discovery that a scenario was omitted rather than addressed.

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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