Floodplain compensation storage and level for level

Floodplain compensation storage is the volume of flood water you give back to a river when a development takes some away. Put a house, a raised platform or an engineered embankment on land that currently floods, and the water that used to spread across that ground has to go somewhere, which means slightly higher levels or a wider extent somewhere else. National policy requires development not to increase flood risk elsewhere, and compensation is how that requirement is met where development in the floodplain is accepted in principle.

The design test is level for level, not simply volume for volume. That distinction is the single thing most schemes get wrong, and it is the reason an Environment Agency objection often arrives after a developer has already shown that the numbers balance.

Why volume for volume is not enough

Imagine a site where development raises ground from 10.0 m to 11.0 m above Ordnance Datum across 1,000 m2, in a floodplain whose 1 in 100 plus climate change level is 10.8 m. The lost storage is not a single figure. It is a loss of 1,000 m2 of storage at every elevation band between 10.0 m and 10.8 m.

Provide the replacement volume in a deep pit whose base sits at 8.5 m and whose top is at 9.5 m, and the arithmetic balances while the hydraulics do not. During the design event, that pit fills early and is already full by the time the river reaches 10.5 m, so it offers nothing at the level where the storage was taken. Flood levels upstream still rise.

Level for level compensation replaces the lost volume band by band, so that for each elevation slice the storage removed is matched by storage created at the same elevation. In practice that means a graded, shallow excavation rather than a pond, and it means the compensation area is usually larger in plan than the area being filled.

Where the compensation land has to sit

Three locational requirements, and all three are about hydraulics rather than convenience.

The land must be outside the existing floodplain, because lowering ground that already floods creates no new storage. It must be hydraulically connected to the same watercourse at the relevant levels, so that water can enter and leave as the river rises and falls. And it must be in the same part of the floodplain as the loss, which generally means adjacent and either opposite or upstream.

Compensation downstream of the loss does not work for the properties that matter. Water displaced from your site raises levels upstream and across the channel, and storage created a kilometre downstream fills too late to help. The Environment Agency will look at this before it looks at your volumes.

Dead storage and the connectivity check

Storage that cannot fill, or cannot empty, is dead storage and does not count.

Three failures account for most of it. A compensation area separated from the channel by a high point only fills once the river overtops that ridge. A connection channel with an invert above the lower elevation bands leaves the bottom third of the excavation dry in the design event. And an area with no positive outfall traps water, leaving a wet depression after every flood.

Design the connection explicitly. Show the invert levels of the link, the ground levels along the flow path, and a cross-section through the compensation area with the elevation bands marked. Then confirm the area drains back to the watercourse by gravity, or state the maintenance arrangement if it does not. The same care with flow paths and blockage that applies to a flow control, as set out in our note on modelling a partially blocked flow control, applies to the inlet of a compensation area.

Modelling the before and after case

The evidence is a hydraulic model, baseline and proposed, run for the design event with the current climate change allowance for the relevant river basin district and epoch.

Where the Environment Agency holds a model for the reach, request it and the associated detailed flood risk data rather than building from scratch. Using the Agency's own model, updated to include your proposal, removes an entire category of argument about model build, roughness coefficients and boundary conditions.

What the model has to show is no detriment at identified receptors: flood levels, extents, depths and velocities in the proposed case no worse than in the baseline, at the design event and usually at a sensitivity event too. Report the results as a table of levels at named nodes rather than as a statement that there is no increase. Expect the Agency to look at velocity and hazard rating as well as level, because a scheme can hold levels constant while making flows faster through a constriction.

Set the whole exercise out in the Flood Risk Assessment, with the compensation design referenced to drawings and the model outputs appended. The assessment also has to confirm that compensation does not substitute for the Sequential Test, which is answered separately and first.

Land ownership and securing the area

Compensation land is frequently outside the development's red line, and that is a legal problem rather than an engineering one.

Include the compensation area within the application site boundary where you can. Where it is in third party ownership, you need the owner's agreement, the correct ownership certificate and notice under the application procedure, and a mechanism that binds the land: a section 106 agreement including the owner, an easement, or a transfer. An option agreement that expires before the compensation is built is not security.

The agreement should also prevent the compensation area being filled, built on or landscaped up at some later date, and should carry a maintenance obligation covering vegetation management and silt removal so the volume survives. Flood storage lost to twenty years of accumulated silt and scrub is a real failure mode, and the Agency has seen it often enough to ask about it.

Permits, consents and the works themselves

Excavating within 8 metres of a main river, or 16 metres of a tidal main river, is a flood risk activity needing an environmental permit from the Environment Agency under the Environmental Permitting Regulations 2016, separate from planning permission and granted on different criteria. For an ordinary watercourse, land drainage consent comes from the lead local flood authority or internal drainage board instead.

Dewatering during construction brings its own consents, as our note on construction dewatering and the consents you actually need sets out, and where the works involve a new or modified outfall the bank stability and scour issues in our note on outfall design, bank stability and scour protection apply.

Build the permit programme into the development programme. Permit determination runs on its own timetable and is not accelerated by a planning committee date.

Conditions that secure it

Expect a condition requiring the compensation works to be constructed in accordance with approved drawings and completed before the development is occupied, or before the land raising takes place where that is sequenced first. Expect a verification condition requiring an as-built topographic survey demonstrating the designed levels and volumes have been achieved, signed off by the designer.

Also expect a condition or obligation preventing the raising of ground levels elsewhere on the site without consent, because incremental filling during construction is how balanced schemes become unbalanced. And expect a long-term management plan.

One honest caveat: compensation makes displacement acceptable, and nothing more. It does not make development appropriate in the functional floodplain, it does not answer the vulnerability classification, and it does not pass the Exception Test. Those are separate hurdles, and a well-designed compensation scheme attached to a proposal that fails them still fails.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Enter your postcode on PlanScope to see which reports your site is likely to need.

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