Compensatory Flood Storage Explained: Level for Level Design for Planning

In simple terms, if development takes space away from the floodplain, that space must be given back in a way that works with how flooding actually occurs. This process is an example of compensatory flood storage, and is known as level for level compensation.

The requirement most commonly arises on sites within Flood Zones 2 and 3. A well-prepared flood risk assessment for planning applications should identify any loss of floodplain storage and demonstrate how it will be replaced.

Why flood storage matters when building in the floodplain

During a flood, water spreads across land and is temporarily stored within the floodplain. This natural flood storage area plays an important part in managing flood levels both upstream and downstream of a site.

When development raises ground levels or introduces buildings into that space, some of that storage is lost. The displaced water does not disappear. It is pushed elsewhere, which can increase flood levels or alter flow paths on neighbouring land.

Planning policy is therefore concerned not only with whether a site may flood, but whether development could make flooding worse elsewhere. This principle is set out in flood risk and coastal change planning guidance.

The starting point for understanding site risk is typically the Flood Map for Planning, supported by an understanding of flood zones in England.

What does “level for level” actually mean?

It does not mean simply replacing the same total volume of flood storage somewhere else. It means replacing lost storage at the same flood levels.

Flooding happens progressively. As water levels rise, different parts of a site become occupied. If development removes storage at multiple levels, that storage must be replaced at those same stages, not concentrated at one lower level.

In practice this is demonstrated through a cut and fill exercise. The floodplain is divided into level bands, often at 100mm or 200mm increments, and the volume of fill in each band is matched by an equal volume of cut.

A simple way to understand it: Archimedes’ “Eureka” moment

The idea behind compensatory flood storage is closely related to a principle first described by Archimedes.

When he stepped into a bath, the water level rose because his body displaced a volume of water equal to the space it occupied. That observation led to the well-known “Eureka” moment.

Floodplain development works in a similar way. When buildings or raised ground take up space in the floodplain, they displace water that would otherwise sit there during a flood.

Level for level compensatory storage is the response to that displacement. If space is taken away at certain levels, it must be replaced at those same levels so the floodplain continues to function properly.

Why total volume alone is not enough

Replacing the same total volume of storage is not usually sufficient.

If replacement storage is provided at lower levels than the storage that has been lost, it may fill too early in a flood event. As water levels rise further, the site may still have reduced capacity where it matters most.

This can result in higher flood levels elsewhere. The question is therefore not just how much storage is replaced, but where within the flood range it is replaced.

Direct and indirect compensatory flood storage

Flood compensation schemes fall into two categories.

Direct compensation is level for level compensation: ground levels elsewhere on or adjacent to the site are lowered so that the same storage volume is available at every stage of a flood. This is what the Environment Agency expects in almost all cases.

Indirect compensation relies on displaced floodwater entering a storage area that fills or empties in a different way, for example at a controlled rate. Because it does not replicate the natural behaviour of the floodplain, it is normally only considered where direct compensation has been shown to be impractical, and it needs detailed justification, usually supported by hydraulic modelling.

If a layout cannot deliver direct level for level compensation, that is usually a prompt to revisit the layout rather than to assume an indirect scheme will be accepted.

Can underground tanks provide compensatory flood storage?

We are regularly asked whether an underground or subsurface compensatory storage system, such as a buried tank or crate system, can replace lost floodplain storage.

In almost all cases the answer is no. A buried tank does not fill and drain naturally with the floodplain and cannot demonstrate level for level performance, so it is treated as indirect compensation at best. Long-term maintenance and the risk of blockage count against it too.

Floodable voids beneath buildings are a separate concept. A building raised on stilts or over an open undercroft can allow floodwater to continue occupying the space beneath it, which retains floodplain storage rather than replacing it. We cover this in our guide to floodable voids, stilts and undercrofts.

When is compensatory flood storage likely to be needed?

It is typically required where development reduces existing floodplain storage, most commonly within Flood Zone 3 and increasingly within the climate change extents of Flood Zone 2.

Whether it is needed depends on the site layout, proposed levels, and how the development interacts with flood risk, which is why the assessment must be site-specific. The quality of that assessment is often decisive, and it is one of the reasons flood risk assessment expertise matters.

How compensatory flood storage is worked out

The process compares existing and proposed ground levels against the design flood level. For most sites in England this is the 1 in 100 year fluvial flood level plus an allowance for climate change.

That comparison identifies where storage is lost across the flood range. Replacement storage is then designed so that equivalent space is available at the same levels, with the results presented in a cut and fill table.

This relies on accurate technical inputs, including topographical survey data and Environment Agency flood data products.

For more complex sites, flood modelling to support compensatory storage design is often required to demonstrate that flood behaviour is not adversely affected.

Why hydraulic connectivity matters

Replacement storage must function as part of the floodplain.

That means floodwater must be able to enter and leave the flood compensation area naturally during a flood. If it is disconnected from the flood mechanism, it may not provide effective compensation, however large it is.

What the Environment Agency will usually look for

The Environment Agency will assess whether compensatory storage has been properly identified and designed. This typically includes:

• the amount of storage lost;
• the levels at which it is lost;
• how that storage is replaced;
• whether the replacement storage is hydraulically connected; and
• whether flood risk is unchanged elsewhere.

Understanding the Environment Agency’s role in flood risk planning helps ensure proposals align with expectations.

What to submit with a planning application

Where level for level compensation is proposed, the supporting flood risk assessment will normally need to include:

• a topographical survey of the site;
• the design flood level, usually the 1 in 100 year plus climate change level;
• a plan showing where floodwater is displaced by the development;
• a plan showing the location and levels of the flood compensation area; and
• a cut and fill table demonstrating that compensation is achieved on a level for level basis.

Common mistakes that cause problems

Issues often arise where compensatory storage is misunderstood or left too late.

Typical problems include focusing only on total volume, using incomplete level data, assuming excavated space automatically counts as storage, proposing buried tanks in place of open storage, or failing to consider how the compensation area connects to the floodplain.

Why this should be considered early in site design

Compensatory storage can influence layout, levels, access and developable area. Addressing it early allows the design to respond properly, rather than forcing changes later in the process.

Getting the strategy right from the start

Compensatory flood storage is a technical requirement, but the principle is straightforward: if development removes flood storage, it must be replaced in a way that works at the same levels and within the same flood system.

Handled early, it can be addressed clearly within the planning process. If overlooked, it can lead to delay, redesign and objection.