Flood Hazard Rating Calculator: Depth, Velocity and Debris Factor
This flood hazard rating calculator works out the hazard to people on each stretch of an access or evacuation route. It takes the depth and velocity of floodwater on each stretch in the design flood. It uses the formula the Environment Agency (EA) states in its hazard mapping methodology, HR = d × (v + 0.5) + DF, and reads each result against the four classes from Very low hazard to Danger for all. Enter one row per stretch, and the calculator reports the worst one and gives a plain-text table that can go into a flood risk assessment (FRA) appendix.
Need the full evacuation plan? Send us the site and the council's comments through our fee proposal form. The service, with what a plan contains, is on our flood evacuation plan page.
What a flood hazard rating is, and when the council asks for one
A flood hazard rating expresses how dangerous floodwater is to a person standing or walking in it. Depth and speed both matter: water 0.3 m deep and still can be waded with care, while the same depth moving fast can knock an adult over. The rating combines the two with an allowance for debris carried in the water.
Councils and Lead Local Flood Authorities (LLFAs) ask for hazard ratings when an application relies on people walking out through floodwater. They also ask when they need to know whether a route can be used at all. That comes up in three places:
The FRA, where the development has to be safe for its lifetime, including access and escape. The Planning Practice Guidance on flood risk and coastal change says "Access routes should allow occupants to safely access and exit their dwellings in design flood conditions". It adds that "The acceptable flood depth for safe access will vary depending on flood velocities" (paragraph 047, Reference ID 7-047-20220825).
The flood warning and evacuation plan, which sets out who leaves, when, and by which route. Our guide to what the council wants in a flood emergency plan covers the rest of the plan.
The Exception Test, where the development has to be shown to be safe for its lifetime. Our page on the Sequential and Exception Tests explains when it applies.
The EA's flood risk assessment standing advice puts it simply in its advice for vulnerable developments, which covers more vulnerable and less vulnerable development in Flood Zone 2, including changes of use. Under "Access and escape" it says: "You need to provide details of your emergency access and escape plans if any part of your development is below the estimated flood level."
The formula, the classes and the source
The rating is calculated as:
HR = d × (v + 0.5) + DF
where d is the depth of flooding in metres, v is the velocity of the floodwater in metres per second, and DF is a debris factor of 0, 0.5 or 1.
The method comes from two Defra and EA research reports. FD2320 is the flood risk assessment guidance for new development, and FD2321 is Flood Risks to People, Phase 2. The two reports classify the higher ratings differently. In 2008 HR Wallingford and the EA published a Supplementary Note on flood hazard ratings and thresholds for development planning and control purpose to settle which to use.
The EA states the same calculation in the hazard mapping methodology it sends with Product 4 data, citing that note. Its classification table, which the calculator uses word for word, is:
Figure 1: The hazard rating and the four hazard-to-people classes, as the EA states them in its Product 4 hazard mapping methodology.
Two details matter at the class boundaries. A rating of exactly 0.75 is Danger for some, because the lowest class is "less than 0.75". A rating of exactly 2.0 is Danger for most, because the top class is "more than 2.0". FD2321 used 2.5 as the threshold for Danger for all. The 2008 note selects 2.0, as does the EA's Product 4 wording, and the calculator follows them.
The debris factor
The debris factor allows for the extra danger from objects carried in floodwater, such as branches, bins and parked cars. It is added to the result, not multiplied, so it moves shallow water up the scale more than deep water. The EA's Product 4 methodology gives it only as "0, 0.5, 1 depending on probability that debris will lead to a hazard".
AFC uses the debris factors of FD2320 Table 13.1: 0.5 where the water is 0.25 m deep or less, and 1.0 where it is deeper. The 2008 Supplementary Note extends Table 13.1 as its Table 4 and recommends it for development planning and control. The calculator's "Auto" setting applies these factors (Figure 2).
Other published tables vary the factor with land use, with lower values for pasture and woodland. If your LLFA or model uses one, enter the factor by hand: each row of the calculator can be set to 0, 0.5 or 1.
Figure 2: The AFC default debris factors (FD2320 Table 13.1), the calculator's "Auto" setting.
How to rate a route with the flood hazard rating calculator
Draw the route from the front door to a place that stays dry in the design flood and connects to the dry road network. A destination that is dry but surrounded by water (a dry island) is not a safe destination, however short the walk.
Split it into stretches wherever the depth or velocity changes: a dip at a junction, a railway underpass, a stretch beside a watercourse. On a 2026 Flood Zone 3a householder job in Datchet, AFC took the depth along the route at 10 m stations, so that no low point was missed.
Take the depth and velocity for each stretch from the design flood. The design flood is the 1% annual exceedance probability event with the climate change allowance, or the 0.5% event for tidal flooding. The sources, in order of preference, are a site-specific model, the EA's modelled outputs (Product 4 levels, Product 6 grids for velocity) and the council's Strategic Flood Risk Assessment mapping. Without a velocity, 0 gives a lower bound only.
Rate every stretch and report the worst. The route is only as safe as its most hazardous stretch. One rating for the whole route, or an average, hides the low point.
Test the alternatives. If the obvious route is Danger for some or worse, rate a shorter route, a drier one, or a route to a different destination, and show the comparison.
Figure 3: A route rated stretch by stretch (illustrative numbers from the calculator's example route, not a real site). The worst stretch decides the route.
Worked example: a hazard rating the LLFA asked for in Bristol
On application 25/14248/F at 2 Stillhouse Lane, Bristol (three dwellings in Flood Zone 2), AFC wrote the revised FRA and the flood warning and evacuation plan submitted in January 2026. Bristol City Council's Flood Risk Manager, the LLFA, commented on them on 23 March 2026. The comment found AFC's route lacking on exactly this point:
"An access and egress route has been provided but this is over a fairly long distance and links back through wet areas. The flood hazard ratings have not been provided for this route and there are potentially dry routes or routes with lesser depths of flooding over shorter distances that have not been considered."
The plan had a route but no hazard rating for it, and no comparison with other routes. The lesson applies to any evacuation plan: a route on a map is not evidence that it is safe. The council needs the depth, velocity and hazard rating for each stretch, the worst stretch identified, and the drier or shorter options tested and compared.
The application is undecided and the flood point is still open.
The traps
Setting DF to 0 for still water. This is the most common error. With DF = 0 and v = 0, the rating is half the depth, so still water rates as Very low hazard at any depth under 1.5 m. With the AFC default debris factor (FD2320 Table 13.1), any still water deeper than 0.25 m is at least Danger for some. From 0.5 m deep it is Danger for most (Figure 4). The calculator shows both results when one row is set to 0 and an identical row is left on Auto.
Figure 4: The still-water trap. With DF = 0, still water under 1.5 m deep reads Very low hazard; with the AFC default (FD2320 Table 13.1), 1.0 m of still water is Danger for most.
One rating for the whole route. An average depth, or the depth at the front door, says nothing about the low point 150 m away. Rate every stretch.
Mixing up events. A depth from a residual event (for example the 0.1% annual exceedance probability flood, or a breach scenario) may be reported as if it were the design flood, or the other way round. Either makes the whole table unreliable. Label every depth with the event and the model it comes from.
A dry island as the destination. A route that reaches a dry spot surrounded by flooded roads leaves people stranded. The destination must be outside the flood extent and stay connected to the wider dry network.
A velocity of 0 because none is mapped. Water on a road is rarely still in a real flood. Where no velocity is available, say so and treat the result as a lower bound.
What the result means for the application
Very low hazard throughout, or dry: the route can usually be put forward as safe access and escape in the design flood, subject to the levels being checked against a survey.
Danger for some or worse at any point in the design flood: the route should not be relied on while water is on it. The plan then needs a trigger to leave before water arrives, normally an EA Flood Warning or a gauge level. It also needs a safe refuge above the flood level for people who cannot leave in time, and evidence that a drier route was looked for.
For a vulnerable use, or where flooding can arrive quickly with little warning, the council may not accept the scheme at all. Raising the floor or the access is often the better answer, as covered in our guide to finished floor levels in flood zones.
What to submit:
a plan of the route split into stretches;
a table of depth, velocity, debris factor, hazard rating and class for each stretch, with the event and data source stated;
the worst stretch named, and the alternatives compared;
the warning trigger and the refuge arrangements.
Resilience measures for the building itself are covered on our page on flood resistance and resilience measures, and the flood zones on Flood Zones 1, 2 and 3 explained. For an example of a plan built around a rapid-onset flood, see the Skinningrove chapel conversion case study. Our floodplain compensation calculator covers the other volume check a Flood Zone 3 scheme often needs.
Frequently asked questions
How do you calculate flood hazard rating?
Multiply the depth of water in metres by the velocity in metres per second plus 0.5, then add the debris factor: HR = d × (v + 0.5) + DF. Read the result against the classes: less than 0.75 Very low hazard, 0.75 to 1.25 Danger for some, 1.25 to 2.0 Danger for most, and more than 2.0 Danger for all. The calculator above does it for each stretch of a route.
What is the debris factor in a flood hazard rating?
An allowance for objects carried in the water, added to the result. AFC uses the factors of FD2320 Table 13.1: 0.5 where the water is 0.25 m deep or less and 1.0 where it is deeper. The 2008 Supplementary Note extends that table and recommends it for development planning and control. Other published tables vary the factor with land use, with lower values for pasture and woodland; if your LLFA or model uses one, enter the factor by hand. Do not set it to 0 just because the water is still.
What does "Danger for some" mean on a flood evacuation route?
A hazard rating from 0.75 to 1.25: the water is dangerous for some people, including children, the elderly and the infirm. A route rated Danger for some in the design flood should not be relied on while water is on it. The plan should get people out before the water arrives, or keep them in a safe refuge.
Does the council need hazard ratings for my flood evacuation plan?
Where the route passes through floodwater in the design flood, usually yes. The LLFA on a 2026 Bristol application commented that "The flood hazard ratings have not been provided for this route". A route that stays dry needs no rating, but the levels should show that it does. Our flood evacuation plan service covers the whole plan.
Where do I get flood depth and velocity for my route?
From a site-specific model if there is one. Otherwise use the Environment Agency's modelled outputs (Product 4 for levels, Product 6 for depth and velocity grids where the model holds them) or the council's Strategic Flood Risk Assessment. Use the design flood with the climate change allowance, and say which event and model each figure comes from.
Is a route safe if the water is shallow but fast?
Not necessarily. Water 0.25 m deep flowing at 1.0 m/s rates 0.25 × 1.5 + 0.5 = 0.875 with the AFC default debris factor (FD2320 Table 13.1), which is Danger for some. The guidance says the acceptable depth for safe access varies with velocity, and the formula is how that is measured.
Ask us about your evacuation route
AFC prepares flood risk assessments and flood warning and evacuation plans with route hazard tables, for householder schemes, conversions and new dwellings. The tables are built from the EA's modelled data or the council's flood mapping. Send the site address, the drawings and any comments from the council or the LLFA through our fee proposal form, and we will set out what the plan needs.
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