Floodplain compensation: level for level volume calculations
Compensatory storage has to be replaced within the same elevation band it was lost from, not just in the same total volume. The calculation method, the siting rules and the drawings that get it approved.
A floodplain compensation calculation is not a single volume figure. It is a table of volumes, band by band, showing that the storage removed between each pair of levels has been replaced between the same pair of levels. Replacing 400 m³ of storage lost between 12.0 m and 12.5 m with 400 m³ created between 13.0 m and 13.5 m satisfies volume for volume and fails level for level, and the Environment Agency will say so.
The reason is hydraulic. Floodplain storage fills progressively as a flood rises, and storage that only becomes available near the peak does nothing for the earlier stages, so the flood level upstream and the timing downstream both change.
Why compensation is required at all
National policy requires that development does not increase flood risk elsewhere. Raising ground levels, placing a building, or filling a hollow inside the modelled floodplain removes volume the river currently uses, and that volume has to go somewhere. Usually it goes onto a neighbour.
Compensation only arises once the sequential and exception tests have been dealt with. It is a mitigation measure for a site that has already been justified, not a substitute for the justification. If the Sequential Test has not been passed, no amount of compensatory storage design helps, and the Exception Test still has to show the development will be safe for its lifetime.
Be clear about what compensation does not address. It replaces storage. It does nothing for conveyance, which is the ability of water to pass through the site along its flow route. A scheme that compensates perfectly for lost volume while placing a building across a flow path still fails.
Level for level and volume for volume
Two terms get used interchangeably and should not be.
Volume for volume means the total storage lost is matched by the total storage created, without regard to the elevation at which each occurs. It is the weaker test and on its own it is rarely accepted for fluvial sites.
Level for level means the storage lost within each elevation band is matched by storage created within the same band. The band interval is usually 100 mm, sometimes 250 mm on a large site with gentle gradients. Agree the interval with the Environment Agency and the lead local flood authority before producing the numbers, because redoing the calculation at a finer interval is a week of work.
The combined requirement, level for level and volume for volume, is what most authorities and the Environment Agency ask for, and the accepted shorthand in correspondence is simply "level for level".
Calculating lost storage by band
The method is a cut and fill exercise between two surfaces, bounded by a third.
- Survey the site to Ordnance Datum Newlyn, at a density that captures the hollows and the banks rather than a grid that steps over them. Include the land beyond the site boundary where compensation is proposed.
- Build a digital terrain model of existing ground and a second model of proposed ground, including the building footprints as solid.
- Take the modelled flood surface for the design event from the hydraulic model, not a single flat level. A river water surface slopes along its length, and using the highest value across the whole site overstates the storage lost at the downstream end and understates it upstream.
- Slice the volume between existing ground and the flood surface into bands at the agreed interval, and do the same for proposed ground.
- Subtract, band by band, to get the loss in each band.
- Repeat for the compensation area to get the gain in each band.
The design event is normally the 1 per cent annual exceedance probability fluvial flood with a climate change allowance applied. The Environment Agency publishes the allowances by river basin district, allowance category and epoch, and they are revised, so take them from the current guidance at the time of the assessment and state the version used in the Flood Risk Assessment.
Present the result as a table with one row per band: lower level, upper level, volume lost, volume provided, net. A positive net in every band is the outcome. A single negative band invites a holding objection even where the total is positive.
Where compensation must be sited
Four requirements apply, and all four have to be satisfied together.
The compensation land must be hydraulically connected to the same floodplain, so that it fills and empties at the same time as the land being filled. A depression separated from the river by a bund or an embankment provides no storage at all until something overtops.
It must be within the applicant's control, and the control must survive the permission. Land in third-party ownership with a verbal agreement is worthless, and a condition requiring compensation on land you cannot access is undischargeable.
It must be in the same floodplain cell and, as a strong preference, adjacent or close to the development. Compensation two kilometres downstream does not help the properties immediately upstream of your fill.
And it must not be land that already provides floodplain storage. Lowering ground that already floods in the design event creates nothing, because that volume was already in the model. This is the single commonest error in a first submission: an applicant proposes to excavate an adjacent field that turns out to be functional floodplain already.
When compensation is not accepted
Tidal floodplains are the main category. The volume of a tidal system is so large relative to a site that removing storage has no measurable effect on levels, so compensation is often considered neither necessary nor effective, and the assessment focuses instead on levels, safe access and the breach scenario. Confirm the position with the Environment Agency rather than assuming either way.
Compensation is also refused where delivering it would cause other harm: removing a hedgerow, excavating within the root protection area of veteran trees, damaging a designated habitat, or disturbing contaminated ground. Where the compensation area discharges back to the watercourse through a new or modified outlet, the scour and bank stability questions covered in outfall design, bank stability and scour protection come into play as well.
A further category is the site where no compensation is physically possible. Urban infill in Flood Zone 3a with no adjacent open land is the classic case. The honest answer there is a design that does not reduce storage: a building on piles or columns with a flood-permeable undercroft, void formers under slabs, or a layout that accepts flooding at ground level with habitable accommodation above. Those approaches need to be modelled, not asserted.
The drawings and the paperwork
A compensation case that gets approved first time usually contains the following.
An existing levels plan with the survey points shown and the datum stated. A proposed levels plan at the same scale with contours at the band interval. Sections through the fill area and the compensation area on a common vertical scale, with the design flood level and the finished floor level marked. A cut and fill plan with colour coding by depth. The volume table. And a long section of the modelled water surface through the site showing the levels used at each end.
Add a short note on how the compensation area will be maintained, because a storage void that silts up or is later landscaped out of existence stops working. Expect a condition securing the levels, and expect to submit an as-built level survey to discharge it.
And set the finished floor levels explicitly. A freeboard of 300 mm above the design flood level is commonly applied, but the figure is set by the authority and the Environment Agency rather than by convention, so ask.
Note: compensation volumes should be calculated by someone competent in hydraulic modelling and topographic survey interpretation, and the local planning authority will expect the flood risk assessment to be prepared by a suitably qualified author. An AI-assisted draft gives you the structure, the policy references and the section headings, and the modelling has to come from the engineer.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. For specialist technical reports, qualified professional input may be required. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. To see which flood and drainage documents a site needs, enter your postcode on PlanScope.
Frequently asked questions
What is the difference between level for level and volume for volume?
Volume for volume matches the total storage lost with the total created, regardless of elevation. Level for level matches the loss within each elevation band with a gain in the same band, usually at 100 mm intervals. Level for level is the test that matters on fluvial sites, because storage only available near the flood peak does nothing earlier in the event.
Can I use adjacent fields for compensatory storage?
Only if they are not already functional floodplain. Lowering ground that already floods in the design event creates no new storage, because that volume is already in the hydraulic model. The land must also be hydraulically connected to the same floodplain, within your control, and close enough to benefit the properties affected by your fill.
Which flood level should the calculation use?
The modelled water surface for the design event, usually the 1 per cent annual exceedance probability fluvial flood with a climate change allowance, taken from the hydraulic model rather than as a single flat level. A river surface slopes, so using one value across the whole site misstates the volumes at both ends.
Is compensation required in a tidal floodplain?
Often not. The volume of a tidal system is large enough that removing site-scale storage has no measurable effect on levels, so the assessment usually concentrates on finished floor levels, safe access and egress, and the breach scenario instead. Confirm the position with the Environment Agency rather than assuming either way.
What if no compensation is physically possible?
Design so that storage is not reduced. A building on piles or columns with a flood-permeable undercroft, void formers beneath slabs, or a layout that accepts flooding at ground level with habitable rooms above can all work. Each has to be modelled rather than asserted, and the residual risk section has to address it.
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.