Flood hazard rating and what the numbers mean

Flood hazard rating is a number that describes how dangerous moving water is to a person standing in it. It combines depth and velocity, with an allowance for debris, and it answers a different question from the flood zone on the map. Flood zones describe how likely flooding is. The hazard rating describes what the water will do to someone trying to walk out through it.

The published method expresses it as the depth in metres multiplied by the sum of the velocity in metres per second and 0.5, with a debris factor added. That arithmetic is why a shallow fast flow can be more dangerous than deeper still water, and why access and escape routes are assessed on hazard rather than on depth alone.

How the rating combines depth and velocity

Three inputs go in. Depth comes from the hydraulic model for the design event, including the climate change allowance. Velocity comes from the same model, at the same point and the same time step. The debris factor is read from a published table according to the depth, the velocity and the dominant land use around the route, because a wooded or built up catchment carries more material than open pasture.

The structure of the formula matters more than the figures. Because velocity is added to 0.5 before being multiplied by depth, any velocity above zero amplifies the effect of depth. Still water of a given depth produces the lowest rating available at that depth. Add movement and the rating climbs quickly.

Use modelled values at the location being assessed rather than a single figure for the site. A route crossing a road that becomes a flow path will have velocities far higher than the garden it starts in, and averaging across the site hides exactly the point the assessment exists to find.

The hazard categories and their labels

Results are grouped into bands with plain language descriptions.

The labels are doing real work. A route rated as danger for some is not acceptable for a care home or a scheme aimed at older residents, even though the same route might be defensible for a block of flats occupied by working age adults. A route rated danger for all is not acceptable for anything, because it is unsafe for the people who would be sent to assist.

Why velocity matters more than depth

An illustrative comparison makes the point, using the published formula with invented values.

Take 0.3 metres of water flowing at 2.0 metres per second along a road. The depth and velocity term gives 0.3 multiplied by 2.5, which is 0.75, before any debris factor. Now take 1.0 metre of effectively still water in a field, at 0.1 metres per second. That gives 1.0 multiplied by 0.6, which is 0.6. The shallow fast flow produces the higher hazard, even though it is barely above the ankle, and adding a debris factor can push it into the danger for some band while the deeper water stays below it.

That is counterintuitive to most applicants, who picture flooding as a question of how deep it gets. It is also why a Flood Risk Assessment that reports depth contours and no velocity data cannot support a safe access conclusion.

Application to access and escape routes

The assessment is about the route, not the building. Plot the route from the dwelling or the site access to a point outside the modelled flood extent, then report the hazard rating along it at intervals, for the design event with climate change and usually for a lower probability event as well.

Four things the authority and the Environment Agency look for.

• The route drawn on a plan with levels, not described in words.
• Hazard ratings at the worst points along it, with the depth and velocity behind each figure.
• The timing of flooding, because a route that becomes hazardous within twenty minutes of a warning is different from one that takes six hours.
• Whether the route remains available for the full duration, including on the falling limb when debris is at its worst.

Dry access, where the route stays above the flood level throughout, is what everyone would prefer and what many authorities expect for more vulnerable uses. Where dry access is not achievable, the hazard rating is how the alternative is judged.

Limits on development by category

The categories constrain the use rather than forbidding development outright. Sleeping accommodation is treated more strictly than commercial floorspace, and a scheme relying on vertical escape to an upper floor instead of a safe route out faces a harder argument, because emergency planners dislike rescuing people from windows.

Where the site is in the functional floodplain, the sequential approach comes first and the hazard rating is secondary. Both the Sequential Test and, where it applies, the Exception Test have to be satisfied before a safe access argument is reached at all. An assessment that leads with hazard ratings and never addresses the sequential approach has started in the wrong place.

Flood warning and evacuation plans support the case but do not replace it. A plan that depends on residents acting correctly at three in the morning is weaker evidence than a route that is physically safe.

An illustrative route assessment

Consider a single plot with its access along a lane that crosses a shallow valley. Illustrative modelled values for the design event with climate change give 0.15 metres at 0.4 metres per second at the plot entrance, rising to 0.45 metres at 1.6 metres per second where the lane crosses the low point, and falling again beyond it.

At the entrance, the depth and velocity term is 0.15 multiplied by 0.9, which is 0.135, a very low hazard. At the low point it is 0.45 multiplied by 2.1, which is 0.945, and with a debris factor the route moves into the danger for some band at that one location. The assessment then has to address that specific 20 metres of lane: whether the levels can be raised, whether an alternative route exists, or whether the proposal is appropriate given who will live there. Those figures are illustrative, but the pattern of one pinch point governing the whole route is typical.

What an assessment must present

A hazard assessment that an officer and the lead local flood authority can rely on contains the model provenance, including the software, the event modelled, the climate change allowance applied and its source, the depth and velocity outputs at each assessment point, the debris factor used and the basis for choosing it, the hazard ratings with their category labels, a plan of the route, the timing and duration of flooding, and a conclusion that engages with the vulnerability of the proposed use.

Two failures recur. Reporting hazard as a single site-wide figure, which hides the pinch point. And reporting it for the present day event only, which understates the position for the lifetime of the development.

Consultee responses on these points often arrive through published standing advice rather than a bespoke letter, so read a short response carefully rather than treating it as agreement, as we explain in standing advice from planning consultees.

For plant and site specific hydraulic modelling, use a competent engineer: the modelling assumptions decide the answer, and professional review is strongly advised. Enter the postcode on PlanScope to see which flood and drainage reports the site is likely to need.

This is general guidance, not legal advice; check the position with your local planning authority, the Environment Agency or a qualified professional 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.