Residual risk and safe access in a flood risk assessment

The residual risk safe access flood risk assessment section is the part officers read last and object to first. It deals with what is left after the mitigation has been applied: the defence that breaches, the culvert that blocks, the event larger than the design event, and above all how people get to and from the building when the water is up. A finished floor level protects property. Safe access protects people, and it is assessed separately.

Two sites with identical flood zones can reach opposite conclusions on it.

What residual risk covers after mitigation

Residual risk is the risk that remains once the proposed measures are in place. On most sites it has four components worth separating out.

Exceedance of the design event, where a storm larger than the modelled 1 in 100 year plus climate change allowance occurs. Failure of an asset, which covers defence breach and overtopping, culvert and trash screen blockage, and the failure of any pumped system the drainage relies on. Change beyond the allowance applied, where the epoch chosen for the development lifetime turns out to be optimistic. And failure of management, where a barrier is not deployed, a pump is not serviced or a drainage feature has silted up.

Each needs a sentence saying what happens, not a sentence saying it is unlikely. An assessment that lists residual risks and then dismisses each one as low probability has not done the work. The question the officer has is what the consequence looks like, and whether anyone is trapped.

Dry access against safe access

These are different standards and the distinction carries most of the argument.

Dry access means a route from the building to land outside the flood extent with no water on it during the design event. It is the preferred outcome, it is easy to demonstrate on a plan, and on sites in Flood Zone 3 it is frequently impossible.

Safe access means a route where water may be present but the depth and velocity are low enough that people can use it without unacceptable hazard. That is a judgement, informed by published hazard criteria, and it is where the disagreement happens. The Environment Agency's standing advice and most local guidance set out what each authority will accept, and some specify a maximum depth on the route in millimetres. Check the local position before designing the access.

A third category exists and is worth naming honestly: no safe access, with occupants remaining in the building. That can be acceptable for some uses with the right design, and it is never acceptable for others. Saying it plainly in the assessment is better than describing a route that anyone with a section drawing can see would be a metre deep.

The depth and velocity judgement

The published approach derives from Defra and Environment Agency research on flood risks to people, which combines depth, velocity and a debris factor into a hazard rating. The rating bands describe the population endangered, running from a caution level through danger for some people, danger for most people, and danger for all including the emergency services.

Two practical points follow. Modelled velocities on floodplain edges are often low, so a route that fails on depth alone can sometimes be made acceptable by raising it locally rather than redesigning the scheme. And the route has to be assessed along its whole length to the point where it leaves the flood extent, including the section on the public highway, which is the part applicants most often omit.

Evacuation against staying put

Evacuation before the flood arrives is the safest outcome where there is warning and a route. Where warning is short or the route is unsafe, staying in place above the flood level can be the better strategy, provided the building supports it.

Staying put requires a refuge at a level above the design flood level with climate change, enough space for the occupants, and a means of communication. For residential development it also raises questions about vulnerable occupants and about how long the water stands, because a first floor refuge with no power, heating or water for two days is a different proposition from one for four hours.

The Environment Agency's position is that it does not approve flood warning and evacuation plans. That is a matter for the local planning authority and its emergency planners. Applicants sometimes submit a plan expecting an Environment Agency sign-off that will never come, and the application stalls while everyone waits for the wrong consultee.

What flood warning services can and cannot do

Flood warnings are issued for many areas at risk from rivers and the sea, and occupants can register for them. That coverage is not universal. Rapid response catchments can flood faster than a warning can be issued and acted on. Surface water flooding is not warned at all. Groundwater flooding develops over days but is not covered by the same service.

So the assessment has to state whether the site is within a flood warning area, what lead time is realistic, and what the strategy is if there is none. A strategy that depends on a warning service that does not cover the mechanism causing the flooding is the commonest defect in this section.

Illustrative example: a five unit scheme beside a small urban watercourse in a rapid response catchment. Floor levels are set 300mm above the design level, so the buildings are protected. The access lane floods to 400mm within twenty minutes of the peak rainfall, and there is no warning service for the catchment. The accepted strategy is a first floor refuge in each unit, a raised pedestrian route to higher ground on the northern boundary, and a management plan requiring occupants to be informed at letting. Evacuation by vehicle is not part of it.

Emergency planning consultation

The authority's emergency planning officer, and in some areas the Local Resilience Forum, will be consulted on the evacuation strategy for anything beyond a single dwelling. Their concerns are operational: can an appliance reach the site, where would people be taken, who holds the plan, and who tells the occupants.

Engaging with that team before submission costs nothing and shortens determination. It also surfaces local knowledge that no model contains, such as the road that always closes first or the primary school used as a rest centre. Where the wider case turns on the two limbs of the flood risk exception test, this section is part of the safety limb, and the Exception Test argument has to rest on the same facts.

Writing the section so it can be conditioned

The officer needs something they can condition. Give them these:

1. A plan showing the access route, its levels, the flood extent for the design event and the point at which the route leaves the extent.
2. A short table of depth and velocity along the route, with the hazard rating and the criterion applied.
3. The refuge level and location where staying put forms part of the strategy.
4. A named flood warning and evacuation plan document, listed as a document for approval rather than described in outline.
5. The management arrangements, including who holds the plan, who updates it and how occupants are informed at each change of occupancy.

Write the plan as a document that can be handed to a management company, not as a chapter of the assessment. Conditions requiring a plan to be submitted and approved before occupation are standard, and a plan that already exists in the right form discharges the condition at the first attempt. Everything in this section should agree with the rest of the Flood Risk Assessment, and where the two disagree on a level or an extent, the objection writes itself.

PlanScope drafts a Flood Risk Assessment structured to UK planning policy from your site data and guided answers, and flags modelled depths, velocities and route levels as still to be confirmed rather than inventing them. Hydraulic modelling and hazard assessment need a suitably qualified author, and for specialist technical reports, qualified professional input may be required. This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

If you are scoping the evidence base for a site in Flood Zone 2 or 3, enter your postcode on PlanScope to see which reports the constraints are likely to trigger.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.