Residual risk and finished floor levels in an FRA
The finished floor levels section has to state the design flood level, the freeboard and what happens in the event beyond the design case. Most assessments skip the third, and consultees keep asking.
Residual risk is the flood risk that remains after every mitigation measure in the scheme has been designed in, and finished floor levels are the main tool for managing it. In a flood risk assessment the finished floor levels section has to do three things: state the design flood level with the climate change allowance applied, state the proposed floor level and the freeboard between them, and explain what happens in an event larger than the design case. Most assessments do the first two and skip the third, which is why consultees keep asking the same question.
What residual risk means once mitigation is designed in
Residual risk is not the risk the site started with. It is what is left over.
For a site behind a raised defence, the residual risk is breach and overtopping, and the depth and speed of flooding that follows a breach can be worse than an undefended floodplain because the water arrives quickly and has nowhere to go. For a site drained by a culvert, the residual risk is blockage. For a site with a pumped surface water system, it is pump failure and power loss. For every site, it is the event larger than the one the scheme was designed against.
Naming those mechanisms specifically is what separates a useful assessment from a template. "Residual risk is low" is not an assessment. "The site lies behind a defence with a standard of protection to the 1 per cent annual probability event, so the residual mechanisms are overtopping in a rarer event and breach at any time, modelled below" is.
Setting floor levels above the design flood level
The design flood level is normally the 1 per cent annual exceedance probability fluvial level, or the 0.5 per cent tidal level, with the appropriate climate change allowance added for the development's lifetime. Take the allowance from the Environment Agency's published allowances for the relevant river basin district and epoch, and state which epoch you used and why. A residential scheme is conventionally assessed over a 100-year lifetime, which pushes you into the later epoch.
Freeboard is then added on top. The convention that most lead local flood authorities and the Environment Agency apply is 300 mm above the design flood level, and it exists to absorb model uncertainty rather than to provide extra protection. Some authorities ask for more where the modelling is coarse or where the site is close to a defence.
Set out the arithmetic explicitly, in a small table, with levels to two decimal places and stated to a datum:
| Item | Level (m AOD) | Source |
|---|---|---|
| Existing ground level, lowest point | Survey | Topographic survey, dated |
| 1 per cent annual probability level, present day | Modelled | Environment Agency product 4 or site-specific model |
| Climate change allowance applied | Uplift | Published allowance, epoch stated |
| Design flood level with climate change | Calculated | As above |
| Freeboard | Typically 0.3 m | Local requirement, stated |
| Proposed finished floor level | Calculated | Design |
An assessment that gives the flood level as a depth above ground rather than as a level to datum will be sent back. Levels have to be comparable with the survey and with the drainage design.
Illustrative example
An illustrative example, for method only: a design flood level of 12.40 m AOD with the climate change allowance already applied, plus 300 mm of freeboard, gives a finished floor level of 12.70 m AOD. The existing ground is 12.15 m AOD, so the building platform rises 550 mm. That single figure then drives the threshold detail, the ramp gradient to the entrance, the level of the drive, the drainage outfall invert and whether a pumped connection is needed. Work it out at concept stage, because discovering it at technical design usually costs a redesign.
Where raising levels conflicts with access and heritage
Raising floor levels is easy on a greenfield plot and difficult everywhere else.
On a tight urban site, a 550 mm rise means either a ramp at 1 in 20 running seven metres from the entrance, or steps and a platform lift, and Approved Document M sets the standards the entrance has to meet. The ramp has to sit inside the plot, and on a terraced frontage there is no room for it.
In a conservation area or on a listed building, raising the floor level changes the relationship between the cill line, the plinth and the pavement, which is exactly the relationship a conservation officer is protecting. Retrofitting flood resistance into a historic building is usually a resilience exercise rather than a level-raising one.
Raising levels also displaces flood water. On a site within the floodplain, any imported fill has to be compensated for on a level-for-level basis, and the compensation storage has to be hydraulically connected to the source of flooding. Councils reject volume-for-volume calculations that ignore which level the storage sits at.
And where new levels are built up near retained trees, the fill sits over the root protection area and can kill the tree it was meant to keep. The categorisation of trees under BS 5837 categories A to U should be settled before the levels strategy is fixed, not after.
Resistance and resilience compared
Resistance keeps water out. Resilience accepts that water may enter and limits the damage and the recovery time.
The practical dividing line is depth. Holding water out against depths beyond roughly 600 mm risks structural damage, because hydrostatic pressure on a domestic wall becomes substantial and the wall may be pushed rather than wetted. Above that, the honest answer is to allow water in and design for recovery.
Resistance measures include flood doors and barriers, non-return valves on drainage, sealed service entries, and waterproof render or membranes to a stated height. Every one of them depends on somebody being there to deploy it, so a barrier that needs manual installation is only as good as the flood warning and the occupier.
Resilience measures include solid floors with closed-cell insulation, raised electrical circuits and sockets, moisture-tolerant wall construction below the design level, kitchens with removable plinths, and stairs and internal doors specified to dry out rather than delaminate. On residential schemes, the measure that matters most to the occupier is having a habitable floor above the design flood level to retreat to.
State which strategy each element of the scheme follows, and to what level. A specification that mixes both without saying where the boundary is cannot be checked.
Safe access, egress and dry routes
Planning guidance expects safe access to and from the development in a flood event. The preference is a dry pedestrian route to land outside the floodplain, above the design flood level including climate change.
Where a fully dry route is impossible, the assessment has to describe the route, the maximum depth and velocity along it, and the resulting hazard, using the depth and velocity hazard rating from the Environment Agency and Defra research on flood risk to people. Say who is expected to use the route and when. A route that is safe for an adult in daylight may not be safe for a resident with limited mobility at night.
Vehicular access is a separate question and a weaker one. Emergency services can reach a site by other means, so a wet vehicle route is more often accepted than a wet pedestrian one, provided the pedestrian route works.
Where the site is dry but the surrounding roads flood, that is an island site, and the assessment should say so plainly and address whether occupants can safely remain in place. Councils are increasingly unwilling to accept sheltering in place for ground floor residential accommodation.
Evacuation plans and who comments on them
The Environment Agency does not comment on the adequacy of flood warning and evacuation plans. That is a matter for the local planning authority and its emergency planners, and the agency's standing advice says so.
So a flood risk assessment that ends with "an evacuation plan will be agreed with the Environment Agency" has misdirected the reader. What the assessment should do is confirm whether the site is in a flood warning area, describe the warning lead time, and set out the trigger points, the responsibilities and the destination for occupants. Then send it to the council's emergency planning officer.
Sleeping accommodation is where this bites. Bedrooms on a ground floor in Flood Zone 3, and any residential use in the functional floodplain, attract objections that levels alone do not resolve.
A flood risk assessment is a report type where councils expect a suitably qualified author, and professional review is strongly advised. For specialist technical reports, qualified professional input may be required. Scoping the wider evidence base against the full list of report types early avoids the common sequence where a drainage strategy contradicts the levels in the flood risk assessment.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it, and take flood levels and climate change allowances from the current published sources.
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Frequently asked questions
How much freeboard should a finished floor level have?
The convention most lead local flood authorities and the Environment Agency apply is 300 mm above the design flood level, including the climate change allowance. It exists to absorb uncertainty in the modelling rather than to add protection. Some authorities ask for more where the model is coarse or the site sits close to a defence, so confirm the local requirement.
What counts as residual risk in a flood risk assessment?
The risk that remains after mitigation is designed in. Behind a defence it is breach and overtopping. On a culverted watercourse it is blockage. On a pumped drainage system it is pump or power failure. On every site it includes the event larger than the one the scheme was designed against. Name the mechanisms rather than describing residual risk as low.
When should a scheme use resilience instead of resistance?
Broadly where flood depths exceed about 600 mm, because holding water out beyond that risks structural damage from hydrostatic pressure on domestic walls. Below that, resistance measures such as flood doors, non-return valves and sealed service entries are workable, provided somebody is present to deploy anything manual when a warning is issued.
Does the Environment Agency approve evacuation plans?
No. The agency's standing advice is that it does not comment on the adequacy of flood warning and evacuation plans. Those are for the local planning authority and its emergency planners. An assessment should confirm whether the site is in a flood warning area, describe lead times and triggers, and then be sent to the council's emergency planning officer.
What is a dry pedestrian route?
A route from the development to land outside the floodplain that stays above the design flood level including the climate change allowance. Where one is not achievable, the assessment must give depths and velocities along the route and assess the hazard, and say who is expected to use it and when. Vehicular access is judged less strictly than pedestrian access.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.