Depth and velocity hazard and safe access routes
Depth alone says little about safety. The hazard rating combines depth, velocity and debris, and it is what decides whether an access route out of a development is acceptable.
A flood hazard rating turns depth and velocity into a single number, and that number is what decides whether an access route out of a development is safe. Depth alone tells you very little. Shallow water moving fast is more dangerous to a person on foot than deeper water that is barely moving, and the published hazard method exists precisely because designers and officers kept arguing about depth in isolation.
The method matters at application stage because national policy requires safe access and escape in flood risk areas. An assessment that states a depth and concludes that access is acceptable has not answered the question.
Why depth alone does not define safety
Water at 300 millimetres standing in a car park is an inconvenience. The same depth moving at 2 metres per second across a road will take an adult off their feet. Add floating debris and the hazard rises again.
The research behind the current approach, carried out for Defra and the Environment Agency, produced a hazard rating that combines depth, velocity and a debris factor into one figure, and a classification of that figure against the degree of danger to people. Planning Practice Guidance on flood risk and coastal change expects safe access and egress to be demonstrated, and the hazard rating is the usual way of demonstrating it.
So the sequence is: model or estimate depth and velocity along the route, calculate the hazard rating, classify it, then decide whether the route is acceptable for the vulnerability of the development proposed.
Hazard rating and the factors in it
The rating multiplies the depth by the velocity plus a constant, then adds a debris factor. In words: hazard equals depth times velocity plus half a metre per second, plus debris.
Each input needs stating:
- depth in metres, for the design event including the climate change allowance
- velocity in metres per second, from the model rather than assumed
- a debris factor reflecting the land use and the depth, lowest in open pasture and highest in urban areas with fences, bins and parked cars available to be mobilised
The constant exists because even still water of any depth carries risk, and the formula would otherwise produce a hazard of zero where velocity is zero.
The debris factor is the input most often left out, and it is the one that moves urban results across a classification boundary. An urban street in an extreme event is full of mobile objects, and the factor should reflect that rather than defaulting to the lowest value.
Classifying a route against the rating
The published classification bands run from very low hazard, through danger to some people, to danger to most people, and then to danger to all including the emergency services. Each band carries a short description of who is at risk.
| Band | Broad description |
|---|---|
| Lowest band | Very low hazard, caution advised |
| Second band | Dangerous for some, particularly children and less mobile people |
| Third band | Dangerous for most people |
| Highest band | Dangerous for all, including trained emergency responders |
Quote the band boundaries from the current published source rather than from memory, since values are sometimes cited inconsistently in older reports.
The classification is about people on foot. It is not a classification of whether a car can pass, and it does not account for the behaviour of people who have not been told what is happening.
Dry routes, low hazard routes and refuge
The hierarchy applied in practice runs in this order.
- A dry route to a place outside the flood extent, at a level above the design flood level including freeboard and climate change allowance. This is the preferred answer and the only one that works without warning or intervention.
- A route where the hazard rating stays within the lowest band throughout, for the design event, with the depth and velocity stated at the worst point.
- A route in a higher band, acceptable only exceptionally and usually only for less vulnerable uses, with a flood warning and evacuation plan agreed with the emergency planning team.
- Safe refuge within the building above the design flood level, where no route works, combined with warning, a means of communication and a plan. Refuge is a last resort rather than a design solution.
The vulnerability classification of the development drives which of these is acceptable. More vulnerable uses, including housing for older people and other residential institutions, attract a stricter position than a warehouse on the same site. Where the Exception Test applies, the safety of access and egress forms part of the case that development will be safe for its lifetime, as the Exception Test material explains, and the earlier location question belongs with the Sequential Test.
Vehicular against pedestrian access
Two different questions get merged here, and officers increasingly separate them.
Pedestrian access is assessed on the hazard rating. Vehicular access is a different matter, because a car loses traction and can be moved in depths that a person can wade, and because emergency vehicles have their own operational limits. A route passable on foot may be impassable for an ambulance.
Where the development relies on vehicular access for its occupants or for emergency response, say what depth occurs on the route and ask the local emergency planning team for its view rather than asserting acceptability. Fire and rescue services often have a position on access depths for appliances, and it is better in the assessment than discovered afterwards.
Construction and occupation phases differ too. A site accessed across a flood plain during construction carries risks for the contractor that the finished development does not.
Evidence and modelling required
Proportionality applies, but the hazard rating needs numbers and numbers need a model.
For a site within a modelled area where the Environment Agency holds detailed modelling, request the model outputs and use the depths and velocities from them. For a site where only broad mapping exists, the published flood maps show extents and in some cases depths, and a hydraulic model may be needed to produce velocities. The surface water maps do not generally give velocities at all.
State in the assessment:
- the source of the depth and velocity data, with the model name and date
- the event modelled, and the climate change allowance applied
- the route assessed, drawn on a plan with chainage or named points
- the hazard rating at the worst point on the route, with the inputs shown
- the resulting classification and the conclusion for the development's vulnerability class
And show the route on a drawing. A hazard rating with no plan of the route it applies to cannot be checked.
How authorities and emergency planners respond
The lead local flood authority comments on surface water, the Environment Agency on main river and sea flooding, and the local emergency planning team on evacuation and access. Their positions are not always identical, and the planning officer has to reconcile them.
Three responses recur. A request for the velocity data where only depths were given. A challenge to a conclusion of safe access where the route crosses a road that floods earlier than the site does. And a requirement for a flood warning and evacuation plan secured by condition, which is a reasonable outcome and should be anticipated rather than resisted.
Where residual risk remains after the route has been designed as well as it can be, resilience measures inside the building are the next step, covered in residual flood risk and property flood resilience measures. Where sleeping accommodation is proposed below ground, the position is stricter again and set out in basements, flood risk and sleeping accommodation.
All of this belongs in the Flood Risk Assessment as a section with its own heading, its own plan and its own conclusion, rather than as a sentence in the residual risk chapter.
This is general guidance, not legal advice. Hazard classification values, climate change allowances and emergency planning expectations are revised periodically, so check the current published guidance and consult your lead local flood authority, the Environment Agency and the local emergency planning team before relying on it.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
How is flood hazard rating calculated?
Depth in metres is multiplied by velocity in metres per second plus a constant of half a metre per second, then a debris factor is added. The constant exists because standing water carries risk even at zero velocity. All three inputs should be stated, with velocity taken from a model rather than assumed.
What is a safe access route in flood risk terms?
The preferred answer is a dry route to land above the design flood level including freeboard and the climate change allowance. Failing that, a route where the hazard rating stays within the lowest band throughout the design event. Higher bands are acceptable only exceptionally and usually only for less vulnerable uses.
Is safe refuge an acceptable alternative to a route?
Only as a last resort where no route works, and only with a flood warning and evacuation plan, a means of communication and refuge space above the design flood level. Refuge depends on people receiving warning and acting on it, which is why it is treated as a fallback rather than a design solution.
Does the hazard rating cover vehicles?
No. The classification bands describe danger to people on foot. A car loses traction and can be moved in depths an adult can wade, and emergency vehicles have their own operational limits. Where the development relies on vehicular access, state the depth on the route and ask the emergency planning team for its view.
Where do depth and velocity data come from?
From detailed hydraulic modelling where it exists, which the Environment Agency holds for many main river catchments and will share on request. Published flood maps give extents and sometimes depths but generally not velocities, and surface water mapping does not provide velocities at all, so a model may be needed.
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.