The residual risk section of a flood risk assessment

The residual risk section is the part of a flood risk assessment that assumes your mitigation fails. Floor levels are set, the defence is in place, the attenuation is designed, and the question becomes what happens when the river exceeds the design event, the defence breaches or the pump stops.

It is also the section most often written as three paragraphs of reassurance. Lead local flood authorities and emergency planners read it closely, and a weak version is a common reason an otherwise sound Flood Risk Assessment comes back with questions.

What residual risk means in an FRA

Residual risk is the risk left after everything you propose has been built and is working as intended, plus the risk of it not working.

On a typical site it covers five things. An event larger than the design event, including the climate change allowance. Failure or overtopping of a defence. Blockage of a culvert, trash screen or watercourse. Exceedance or failure of the site drainage, including pumped systems. And the consequences of any of those for the people inside the buildings.

National guidance frames the test as whether the development will be safe for its lifetime, taken as 100 years for residential and a shorter period for most commercial uses, without increasing flood risk elsewhere. Safe for its lifetime is a people question, not a property question, which is why this section is about occupants rather than about finish levels.

Say plainly which of the five apply to your site and which do not. A site in Flood Zone 1 with a surface water flow path across it has residual risk, and pretending otherwise invites a request for more information.

Defence failure and breach scenarios

Where a site sits behind a raised defence, the Environment Agency will usually want a breach assessment rather than a statement that the defence has a design standard.

A breach assessment models a defect in the defence at a chosen location, during a chosen event, and maps the depth, velocity and rate of onset across the site. The outputs that matter are how deep the water gets, how fast it arrives, and how quickly a person's route out is cut off.

Hazard is normally expressed by combining depth and velocity, with an allowance for debris, using the published research on flood risks to people. The result grades areas from low hazard through to danger for all. Plot it, do not describe it.

Agree the breach location, the event and the assumptions with the Agency before the modelling runs. A breach assessment carried out on assumptions the Agency did not accept is a wasted month, and this is one of the areas where its position varies by region and by defence.

Flood warning and the lead time you actually get

Flood warning is often relied on and rarely examined. Check three things.

Whether the site is in an area covered by the Environment Agency's flood warning service at all, since many surface water and small watercourse locations are not. What the realistic lead time is in that location, which in a rapid response catchment may be under two hours. And what happens at 3am, when the occupants are asleep and the warning arrives by text.

Surface water flooding generally comes with no warning. A residual risk section that relies on warning and evacuation for a surface water hazard is relying on something that does not exist.

Where the lead time is short, the answer is usually design rather than procedure: raised floor levels, a refuge above the hazard, resilient construction below it, and a Drainage Strategy with an exceedance route that takes water away from doors rather than towards them.

Refuge or evacuation

This is the decision the section turns on, and it should be made explicitly.

Evacuation means people leave before the water arrives, which needs warning, a route that stays passable, somewhere to go and someone to trigger it. Refuge means people stay, above the hazard level, until the water recedes or they are rescued, which needs a suitable floor at a suitable level with access to it from every dwelling.

Safe refuge normally means floor space at or above the design flood level with freeboard, reached internally, with an escape route to a place of safety if conditions allow. A first floor bedroom in a house with an internal stair can serve, while a single-storey flat cannot, and that is why single-storey accommodation in higher hazard areas is difficult.

Set out the level of the refuge, the number of people it serves, and the duration it has to work for. A site that floods for six hours and one that floods for three days are different problems.

Who signs off the evacuation plan

A point that catches applicants out. The Environment Agency does not approve flood warning and evacuation plans, and it says so consistently. Its remit is the flood risk assessment and its own consenting functions.

Approval of an evacuation plan is a matter for the local planning authority, advised by the council's emergency planning team and, where relevant, the local resilience forum. So send the draft plan to the emergency planners early, not with the application.

Emergency planners ask a narrow set of questions. Who decides to evacuate. How are occupants told. Where do they go and who has agreed to receive them. How are people with reduced mobility, children and pets dealt with. Who keeps the plan current when the management company changes.

Where the application is for a use in a higher risk zone, the Exception Test requires the development to be safe for its lifetime, and the evacuation and refuge arrangements are part of demonstrating that.

Management after occupation

A plan that nobody owns after practical completion is a plan that fails.

Set out who holds the plan: the management company for flats, the employer for a workplace, the operator for a care or leisure use. Commit to registration with the flood warning service where it is available, to an annual review, and to issuing the plan to every new occupier.

Secure it properly. A condition requiring the plan to be implemented is common, and on larger schemes a section 106 obligation requiring maintenance of the arrangements is stronger. Either way, name the mechanism in the assessment rather than leaving it to the case officer.

Insurance is worth a sentence. The Flood Re scheme does not cover homes built after 1 January 2009, so buyers of new homes in flood risk areas rely on the open market, and the quality of the residual risk case affects what they are offered.

Wording that gets the section agreed

Four habits separate a section that is agreed from one that is queried.

Give numbers. Depth, velocity, hazard rating, floor level, refuge level, freeboard, lead time and duration, each with its source and its return period.

Show the plan. A hazard plan and an access and egress plan with the route marked answer more questions than any amount of text.

Name the responsibilities by role and never by individual, and state what happens when that role is vacant.

And state the honest limitation. Where the site cannot be evacuated in the time available, say so and design for refuge, rather than describing an evacuation that would not happen. Officers and emergency planners recognise the difference, and a candid assessment is treated better than a confident one that does not survive a question.

PlanScope drafts a flood risk assessment structured to national policy, cites the sources it uses and marks the site facts it does not hold with a flag rather than inventing them. Enter the postcode to see which reports a site is likely to need.

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.