Compensatory flood storage and level for level design
Floodplain compensation has to be level for level, volume for volume and hydraulically connected. Here is how the accounting works and what the Environment Agency checks.
Compensatory flood storage is the volume you give back to the floodplain when a development fills part of it, and the Environment Agency will expect it to be provided level for level and volume for volume, hydraulically connected to the same source of flooding, and available for every flood event up to the design standard. Get the principle wrong and the objection is fatal. Get the accounting wrong and you spend three months exchanging spreadsheets with a consultee who has already decided you have not demonstrated it.
Why displaced volume has to go somewhere
A floodplain holds water. Raise ground levels, build a plinth, or put a building footprint where water currently spreads, and that water goes somewhere else, usually deeper and faster onto land upstream, downstream or opposite. National policy in the National Planning Policy Framework requires development not to increase flood risk elsewhere, and the Planning Practice Guidance on flood risk and coastal change is the place to check how the tests are applied.
Compensation is the mechanism that makes a scheme neutral. It is not mitigation for the site itself. Your finished floor levels, access route and emergency plan deal with risk to the occupants. Compensation deals with the effect on third parties, which is the test the Environment Agency applies when it decides whether to object.
Two points get lost. Compensation is required for built footprint and for made ground, so a landscaped bund or a raised car park counts. And in Flood Zone 3b, the functional floodplain, most development is unacceptable in principle whatever compensation you offer, so the conversation only starts once the Sequential Test has been properly worked through.
Level for level against volume for volume
The two phrases are used loosely and mean different things. Volume for volume means the total lost storage equals the total replacement storage. Level for level means the lost storage is replaced at the same elevations, so the volume taken out between 8.20 and 8.40 metres above Ordnance Datum is given back between 8.20 and 8.40 metres.
| Approach | What it delivers | Where it is accepted |
|---|---|---|
| Level for level and volume for volume | Storage replaced in matching elevation bands, so the floodplain behaves the same at every return period | The default expectation for fluvial floodplain compensation |
| Volume for volume only | Total volume matched but distributed at different levels | Occasionally accepted where modelling shows no detriment at any return period |
| Off-site or remote storage | Volume provided outside the affected hydraulic unit | Rarely accepted, because it does not function in the same flood event |
The reason the Environment Agency insists on level for level is that flooding happens at many return periods, not one. If you take storage out low in the profile and give it back high up, the frequent events get worse even though the 1 per cent annual probability event balances on paper. Your model has to demonstrate neutrality band by band, typically in 100 or 200 millimetre increments.
Where the compensation land has to sit
Compensation has to be in the same hydraulic unit and, in practice, immediately adjacent to the loss, on the same bank, between the same structures. Land on the other side of a railway embankment or a flood defence is a different cell and will not fill and drain in step with the loss.
The land also has to be outside the existing floodplain, which is the part applicants find hardest. Excavating ground that already floods gives back nothing, because that volume is already counted in the baseline. Workable compensation almost always means lowering land that currently sits just above the flood level, which means you need either a wider site or a neighbour's field.
Check three things on the site plan before you promise anything: the existing ground levels on a survey tied to Ordnance Datum, the modelled flood levels for each return period including the relevant climate change allowance, and the position of any defence, culvert or structure between the loss and the proposed replacement.
Hydraulic connectivity and dead storage
Storage only works if water can get into it and out of it freely during the event. A hollow behind a bund with a 150 millimetre pipe as its only connection is dead storage, and an assessor will discount it entirely.
Dead storage comes in several forms. Ground below the invert of the connection that fills but never drains. Volume behind a raised threshold that only engages above a level the event never reaches. Depressions that hold water permanently, which cannot count because they are already full when the flood arrives. Each of these has to be removed from the calculation, which is why the honest net figure is often 15 or 20 per cent lower than the raw cut volume.
Design the connection deliberately. A wide, shallow, graded link at the invert of the excavation, with a gradient that drains back to the watercourse, keeps the storage live. And say in the Flood Risk Assessment how you have treated dead storage, because a reviewer who cannot see that you have thought about it will assume you have not.
Presenting the volume accounting
This is where good submissions separate themselves. The assessor wants a table they can check in five minutes, not a narrative.
Give a single table with a row for each elevation band, showing the volume lost, the volume provided and the balance, with a total at the foot. Add the levels source, the model run reference and the date. State the datum. State the return period and the climate change allowance used, and cite the Environment Agency allowances you applied rather than describing them loosely, because those figures are updated and a reviewer will check the vintage.
An illustrative example. A scheme in Flood Zone 3a raises a 0.4 hectare platform by an average of 0.6 metres, losing about 2,400 m³ of storage between 8.10 and 8.70 metres above Ordnance Datum. The compensation area, an adjoining paddock currently 0.3 metres above the 1 per cent flood level, is lowered in three graded terraces to give 2,650 m³ across the same bands, of which 180 m³ is discounted as dead storage below the connection invert. The net position is a surplus of about 70 m³. These figures are illustrative and would need a site specific model.
Land you do not own
Compensation land in third party ownership is the most common reason a promising scheme stalls. A promise to deliver storage on a neighbour's field is worth nothing to a decision maker unless there is a mechanism.
Three routes are used. Buying or optioning the land before the application, which is cleanest. A planning obligation under section 106 where the third party is a party to the agreement, which requires their solicitor to engage. Or a Grampian style condition preventing occupation until the storage is constructed, which only works where there is a realistic prospect of delivery, and a council should not impose one where there is not.
Whichever route you take, deal with the long term. Storage that is filled in, fenced off or planted up by a future owner has failed. Maintenance and a restriction on raising levels belong in the agreement, alongside the drainage arrangements set out in your Drainage Strategy.
What the Environment Agency will ask about the model
Expect questions on model provenance, boundary conditions and sensitivity. Where you have used the authority's own model, say which version and whether you have altered the mesh or the roughness values. Where you have built a new model, explain calibration against recorded events or against the published flood outlines.
The comments that recur are about extent rather than volume. Does the post-development run show any increase in flood level off site, anywhere, at any return period, including the frequent events. Have you tested the blockage scenarios on structures. Have you shown the difference plots as well as the tables. What the lead local flood authority checks in a drainage strategy overlaps here, and the two consultees will read each other's comments.
One honest uncertainty. Practice on compensation for small infill sites in urban floodplain is inconsistent, and some authorities accept a proportionate argument where the volume involved is trivial while others do not. Ask at pre-application stage and get the answer in writing.
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.
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Frequently asked questions
What is the difference between level for level and volume for volume compensation?
Volume for volume means total lost storage is matched by total replacement storage. Level for level means it is replaced in the same elevation bands, so the floodplain behaves identically at every return period. The Environment Agency normally expects both, because matching only the total can worsen more frequent flood events.
Can compensatory storage be provided off site?
Rarely. Storage has to be in the same hydraulic unit and fill and drain in step with the loss, so land separated by an embankment, a defence or a structure will not be accepted. Adjacent land on the same bank between the same structures is the workable position.
What counts as dead storage?
Volume that cannot fill and empty freely during the flood event. That includes ground below the invert of the connection, volume behind a threshold the event never reaches, and permanent standing water. It has to be discounted from the calculation, which often reduces the usable figure noticeably below the raw excavation volume.
Do I need compensation for a raised garden or car park?
Yes, if it sits in the floodplain. Compensation is required for any loss of flood storage volume, whether from a building footprint, a bund, made ground or a raised parking area. The test is the volume displaced between the relevant flood levels, not whether the works are habitable.
How do I secure compensation land I do not own?
Either acquire or option the land before applying, bring the owner into a section 106 agreement as a party, or accept a Grampian style condition preventing occupation until the storage is built. A condition is only appropriate where delivery is a realistic prospect, so most councils prefer a land interest or an agreement.
Does compensation help in Flood Zone 3b?
Usually not. The functional floodplain is where water has to flow or be stored in times of flood, and most development is unacceptable there in principle. Compensation does not overcome that policy position, so the sequential approach to site selection has to be worked through first.
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.