Compensatory flood storage: level for level in practice

Compensatory flood storage replaces the floodplain volume a development takes out, band by band, so that the same amount of water can still be stored at the same level at the same moment in a flood event. Replacing the total volume anywhere on the site is not enough. Storage provided at a higher level fills later in the event, which means the water that used to sit on your site at a lower level now sits somewhere else, usually on someone else's land.

That is why the Environment Agency objects to volume for volume schemes that look balanced on a spreadsheet. The arithmetic of the total can be right while the behaviour of the floodplain is wrong.

Why displaced water has to be replaced

National policy on flood risk requires development to be safe for its lifetime without increasing flood risk elsewhere, and that second part is where compensation sits. Raising ground, building a plinth, filling a hollow or putting a solid-walled building into land that currently floods removes storage volume from the floodplain, and the water that volume held does not disappear.

In the functional floodplain, the land where water has to flow or be stored in times of flood, the policy starting point is that built development should not be there at all. Compensation does not cure that. It is the measure that makes an acceptable proposal neutral, after the Sequential Test and, where needed, the Exception Test have been passed on their own terms.

The functional floodplain extent is defined by the local planning authority with the Environment Agency, usually through the strategic flood risk assessment, and it is not simply the Flood Zone 3 outline on the published map. Ask for the authority's own definition early, because the answer changes the whole approach.

The level for level principle explained

Divide the flood volume into horizontal slices, typically at 100 millimetre or 250 millimetre intervals between the lowest ground level affected and the design flood level. For each slice, calculate the volume lost to the development and provide at least that volume of new storage within the same slice.

The last row catches more schemes than it should. Lowering ground that already floods, or excavating a pond in the floodplain, moves soil without adding storage. Compensation has to come from land that is currently above the flood level being compensated, which in practice means lowering dry land immediately adjacent to the floodplain.

The new storage also has to be hydraulically connected, so water can enter and leave freely at the right levels. A lowered area behind a bund, or one that drains only through a pipe, does not fill when the river does.

Where compensation must be located

Adjacent to the loss, within the same flood cell, and on land that floods from the same source. Compensation on the other side of an embankment, downstream of a structure, or in a separate washland is a different hydraulic system, and the modelling will show it.

Three practical tests before you commit to a layout. Is the compensation land within the same hydraulic unit as the loss? Can it be connected at the levels required without a control structure? Is it clear of the conveyance route, so that lowering it does not divert flow towards a neighbour?

Onsite compensation is always preferable. Offsite compensation is accepted where it is close, connected and secured, and it brings a land control problem that is usually harder than the engineering.

Modelling the before and after case

The assessment runs a baseline and a post-development case in the same model, with the same boundary conditions, and compares flood levels, extents and velocities at defined points. Expect to show the design event with the current climate change allowance for peak river flow, a sensitivity run at a higher allowance, and the extreme event for the safety of people as distinct from the economics of the scheme.

What the Environment Agency looks for is no increase in flood level off site, no increase in extent onto third party land, and no adverse change in flow routes or velocities. And small increases are not automatically acceptable. The usual answer to a modelled rise of a few millimetres on a neighbour's land is more compensation rather than an argument about model accuracy.

Where a flow control, culvert or trash screen sits in the system, the blockage scenarios matter as much as the design event, and the approach to that is set out in the note on modelling a partially blocked flow control. The modelling belongs in the Flood Risk Assessment, with the compensation drawings, the slice calculations and the connection levels as appendices rather than as a separate submission.

Land ownership and deliverability

Compensation land is often outside the red line and frequently outside the applicant's ownership, which creates three separate problems: a legal interest sufficient to carry out the works, a mechanism to keep the land in that condition for the lifetime of the development, and the correct ownership certificate and notices with the application.

An option agreement is not enough on its own if the land must stay as storage in perpetuity. Authorities will want a planning obligation binding the land, a restriction on raising levels, and a maintenance regime that survives a change of ownership. Where the land is in agricultural use, the obligation has to be compatible with farming it.

An illustrative example. A site of about 1.2 hectares on the edge of a market town sits partly in Flood Zone 3 with a dry paddock to the north. Raising the developable part removes a modest volume below the design level, and the paddock can be lowered to replace it slice by slice, connected by a shallow swale at the right invert. The engineering is simple. The twelve months spent agreeing a deed with the paddock owner is the real programme item.

Securing the works by condition

The usual package is a condition requiring the compensation works to be constructed and certified before any part of the development is occupied, or before any ground raising takes place, together with a condition fixing finished floor levels and a restriction preventing future infilling or level changes. On larger sites the compensation is phased to match the raising, which needs the phasing plan to be a condition document rather than an illustrative drawing.

Construction sequencing is where this goes wrong on site. Lowering the compensation area usually means excavation below the water table at some point, and the consents for that are a separate matter covered in construction dewatering and the consents you actually need. Works in, over or within eight metres of a main river also need a flood risk activity permit from the Environment Agency, which is not a planning matter and runs on its own timetable.

Where compensation cannot be achieved

Sometimes there is no adjacent dry land, no willing owner, or no way to connect new storage at the levels required. At that point the options narrow to reducing the footprint in the floodplain, designing flood-permeable construction such as an undercroft or a void beneath a raised floor so that storage is retained within the building line, relocating the built form to the part of the site in Flood Zone 1, or accepting that the scheme does not work.

Be straight with the client at that point. An application that goes in with unresolved compensation attracts a holding objection from the Environment Agency, and an authority that refuses on that basis is on strong ground at appeal. Testing the compensation case before the layout is fixed costs a fraction of testing it afterwards.

This is general guidance, not legal advice. Flood modelling and compensation design should be prepared by a suitably qualified engineer, climate change allowances are updated periodically, so check the current published allowances, and the local definition of the functional floodplain should come from the authority's own strategic flood risk assessment.

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