Flood hazard rating and the depth velocity product

Flood hazard rating has one specific meaning in English flood risk work: a single number that combines how deep the water is with how fast it is moving, plus an allowance for debris, so that a point on a flood map can be described as dangerous to some people, to most people or to everyone. The formula is hazard rating = depth × (velocity + 0.5) + debris factor, with depth in metres and velocity in metres per second. It comes from the Defra and Environment Agency research on flood risks to people published in 2006, and it is the basis on which a flood risk assessment argues that an access route is usable in a flood.

Four numbers, four bands, one awkward step change in the middle. That step change is where most arguments start.

What the rating combines

Depth does the obvious work. Velocity is added to 0.5 before multiplication, which means still water still produces a rating proportional to depth, and moving water produces a disproportionate one. A depth of 0.3 m with no flow gives 0.15. The same depth moving at 1 m/s gives 0.45.

The debris factor is the part that catches people out, because it is a step rather than a curve. It takes the value 0, 0.5 or 1 depending on the depth, the velocity and the dominant land use, with built-up areas attracting the higher factor because there is more loose material to be carried. Below shallow depths the factor is zero everywhere. Above a threshold in an urban setting it becomes 1, and adding 1 to a rating that was 0.35 moves the result across two band boundaries at once.

That is not a modelling error. It reflects the fact that being knocked off your feet by a wheelie bin in 300 mm of water is a different proposition from wading through 300 mm of clear water in a field.

The bands and what each describes

The wording of the bands matters as much as the numbers. "Danger for some" is not a statement that the route is impassable. It identifies a group for whom it is unsafe, which is why the band that applies has to be read against the occupants of the building. A route rated "danger for some" may be acceptable for a commercial unit with an evacuation plan and unacceptable for a care home.

Authorities and the Environment Agency generally look for access and egress at very low hazard where it can be achieved, and will consider a route in the "danger for some" band with a managed evacuation and a flood warning service in place. "Danger for most" on the only route out is usually fatal to the application.

Use in assessing an access route

The sequence for testing a route is mechanical once the modelling exists.

1. Identify the design event, which for a fluvial site is usually the 1 per cent annual probability flood with the relevant climate change allowance applied.
2. Extract depth and velocity grids for that event along the full length of the route, from the building door to a point outside the flood extent.
3. Calculate the hazard rating at intervals and record the worst value, not the average.
4. Check the timing. A route that is dry at the start of the event and 400 mm deep at the peak needs the warning time to be long enough to use it.
5. Check it again for the 0.1 per cent annual probability event, because that is where residual risk is assessed.

Point 3 is where assessments get marked down. A route with a single pinch point at the low spot of a lane is as safe as that pinch point, and averaging along the route hides it. Point 4 is where they get marked down twice, because safe access is a function of time as well as depth, and a site with no flood warning coverage cannot rely on an evacuation plan.

Where the route crosses third party land or a highway the authority does not control, say so. The Environment Agency's published standing advice sets out what it expects on access and egress for different proposals, and the note on standing advice from planning consultees explains when that advice applies instead of a bespoke response.

Application to designed exceedance routes

The same rating is used in surface water design, for a different purpose. A drainage scheme is designed to a return period, and flows above that have to go somewhere. The route they take is the designed exceedance route, and it is usually across car parks, footways and landscaped areas rather than through buildings.

Checking an exceedance route with the hazard rating answers a practical question: if 600 mm of water runs down this access road once in a lifetime, can a person walk out of it. The answer shapes kerb heights, falls, the position of door thresholds and whether a basement car park ramp needs a flood gate or a raised crest.

A Drainage Strategy that shows exceedance flow arrows without depths and velocities has not answered the question. Lead local flood authorities increasingly ask for the hazard rating along the exceedance route in the same way the Environment Agency asks for it on the flood access route.

Limits of the rating as a design tool

Three limits are worth stating in the assessment rather than leaving the reviewer to find.

The rating is a steady state measure taken from a model output at a moment in time. It says nothing about the rate of rise, and rate of rise is what determines whether people get out. A site where water reaches 400 mm over six hours is a different place from one where it does so in twelve minutes, and both can produce the same hazard rating.

Still deep water is understated. With velocity at zero, 1.4 m of standing water gives a rating of 0.7, which falls in the very low hazard band. Nobody sensible regards 1.4 m of water as a low hazard for a child. The rating was derived for flowing flood water and it should not be the only test applied to ponded areas.

And the output is only as good as the model. Mesh resolution, roughness coefficients, the treatment of buildings as blocks or porous, and the position of the model boundary all move depths and velocities by enough to cross a band boundary. State the model, the resolution and the assumptions, and expect them to be checked.

An illustrative calculation across a car park

The following is illustrative only and not taken from a real site. A surface car park sits beside a building in Flood Zone 3, and 2D modelling of the design event with the climate change allowance gives two conditions along the pedestrian route out.

At the mid-point of the car park, depth 0.15 m and velocity 0.3 m/s. The depth is below the threshold at which a debris factor applies in this setting, so the rating is 0.15 × (0.3 + 0.5), which is 0.12. Very low hazard.

At the low spot by the exit, depth 0.35 m and velocity 0.5 m/s. The depth now takes the urban debris factor of 1, so the rating is 0.35 × (0.5 + 0.5) + 1, which is 1.35. Danger for most.

The route is therefore rated danger for most, because the worst point governs. The design response is to raise the exit threshold or regrade the car park so that the low spot moves away from the only pedestrian route, not to average the two values. That is a 200 mm level change solving a problem that would otherwise need an evacuation plan the authority may not accept.

What a flood risk assessment should report

A Flood Risk Assessment dealing with hazard should set out, for each event modelled, the depth, the velocity, the debris factor applied and the resulting rating, with the location of the worst point identified on a plan. Report the events separately rather than presenting a single worst case, because the Exception Test and the residual risk discussion need different events.

Alongside that, the assessment should state the finished floor level and the freeboard above the modelled level, the proposed access and egress arrangements with the hazard rating for each, the availability of flood warning for the site, and the residual risk if a defence fails or is overtopped. Where the sequential and exception tests are in play, the hazard information feeds the safety limb of the Exception Test directly.

One honest uncertainty: the thresholds at which authorities accept a managed route rather than a dry one vary, and the published guidance leaves judgement to the decision maker. Ask the lead local flood authority and the Environment Agency at pre-application stage what they will accept on your route, because the answer changes the layout rather than the paperwork.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. To see which flood and drainage documents a site is likely to need, enter the postcode on PlanScope.

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