Reservoir inundation mapping and emergency planning

If your site sits downstream of a large raised reservoir, it will appear inside a mapped reservoir inundation extent, and that extent is not a flood zone. Reservoir inundation flood risk is a residual risk arising from the failure of a structure, mapped by consequence rather than by annual probability, and it does not move the site from Flood Zone 1 to Flood Zone 2 or 3. What it does do is bring safe access, egress and evacuation into the assessment, and on schemes with vulnerable occupiers it can change the layout.

Developers often discover this late, when a lead local flood authority consultation response asks for something the flood risk assessment does not contain.

What reservoir inundation mapping shows

The Environment Agency publishes mapping of the area that would be flooded if a large raised reservoir released its contents in an uncontrolled way. The modelling assumes a breach of the impounding structure and routes the released volume downstream, usually in two scenarios: a dry day, where the receiving watercourse is at normal flow, and a wet day, where the watercourse is already in flood.

The output is a maximum extent, sometimes accompanied by depth, velocity and hazard information. It is deliberately conservative. The mapping exists so that emergency planners can prepare, and the extent is drawn to capture the worst credible case rather than a likely one.

You can see the extent for a site through the Environment Agency's long term flood risk service, which reports risk of flooding from reservoirs alongside river, sea and surface water risk. For the flood zones themselves you need the Flood Map for Planning, and the two services answer different questions.

Why it is not a flood zone

Flood Zones 1, 2, 3a and 3b are probability classifications for flooding from rivers and the sea, assessed ignoring the presence of defences. Each zone corresponds to an annual exceedance probability, which is what makes the sequential and exception tests workable.

Reservoir inundation has no equivalent probability attached. The mapping tells you what would happen if a structure failed, and the chance of that failure is managed by a statutory inspection and supervision regime rather than expressed as a return period. Applying a probability-based policy test to a consequence-based map produces nonsense, which is why national flood risk policy treats reservoir inundation as a residual risk to be described and managed in the flood risk assessment rather than as a trigger for the sequential test or the exception test.

A site can of course be in both. A valley floor site downstream of a reservoir may be in Flood Zone 3 for fluvial reasons and inside the inundation extent as well, and the two then have to be assessed separately with different mitigation.

Which schemes need to assess it

Applications inside the extent do not all need a detailed treatment. Proportionality is the governing principle, and the factors that push a scheme towards a fuller assessment are the vulnerability of the use, the number of people, and whether they could get out.

• Highly vulnerable and more vulnerable uses, including care homes, hospitals, schools, nurseries, hostels, caravan and camping sites and basement dwellings
• Major development, where the number of people at risk is material
• Sites where the only access route lies within the extent, so occupants would have to travel through the hazard to escape
• Schemes proposing sleeping accommodation at ground floor level within the deeper parts of the extent
• Emergency services infrastructure and other uses that need to function during an incident

A single infill dwelling at the edge of the extent, on a plot with a dry route to higher ground, will usually need no more than a paragraph in the flood risk assessment confirming the position and the escape route.

The engineering regime behind the mapping

The reason the probability is not quantified is that it is actively managed. The Reservoirs Act 1975, as amended by the Flood and Water Management Act 2010, regulates large raised reservoirs in England, with the Environment Agency as enforcement authority.

A large raised reservoir is one designed to hold, or capable of holding, more than 25,000 cubic metres of water above the natural level of the surrounding land. The 2010 Act introduced a risk-based approach allowing smaller structures to be brought into the regime where a breach could endanger human life, and the commencement and designation position should be checked with the Environment Agency rather than assumed.

Undertakers must appoint panel engineers from statutory panels. A construction engineer oversees construction and alteration. An inspecting engineer carries out periodic inspections, in the normal case at intervals of no more than ten years. A supervising engineer keeps the structure under continuous supervision and reports annually. High-risk reservoirs are required to have flood plans prepared by the undertaker.

Set that out in the assessment. An officer reading that the structure upstream is inspected by a panel engineer and supervised continuously is being given the reason why the residual risk is tolerable, and that paragraph does more work than two pages of mapping.

Emergency planning, evacuation and what the authority can rely on

Emergency planning for reservoir incidents sits with the local resilience forum under the Civil Contingencies Act 2004, working from the Environment Agency mapping and the undertaker's flood plan. The planning system does not organise the evacuation and cannot promise that it will work.

National flood risk guidance draws a line here that is worth understanding. A flood warning and evacuation plan can be required and conditioned, and the authority's emergency planning officers should be consulted on it, but the existence of a plan is not a justification for allowing development that would otherwise be unacceptable. Safe access and egress has to be achievable in the first place.

Reservoir inundation also behaves differently from fluvial flooding in a way that affects the response. Onset can be rapid and depths can be high, so strategies that work for slow-onset river flooding, such as sheltering in place on an upper floor and waiting for the water to recede, are harder to justify. Where evacuation is the only realistic response, the route and the warning time have to be credible.

Speak to the emergency planning team early. A pre-application conversation that establishes whether they have a view on the site is worth more than a plan written in isolation and submitted at validation.

What the assessment should contain

A reservoir inundation section inside a flood risk assessment should answer a short list of questions clearly.

1. Which reservoir or reservoirs affect the site, who the undertaker is, and whether the structure is designated.
2. The mapped extent at the site, in both the dry day and wet day scenarios, with depth, velocity and hazard where published.
3. The statutory regime that applies to the structure and the inspection and supervision arrangements.
4. Proposed finished floor levels relative to the mapped levels, where levels are published or can be derived.
5. Access and egress, showing a route to land outside the extent and stating the distance and the gradient.
6. The flood warning arrangements available at the site and how occupants would receive a warning.
7. Residual risk after mitigation, stated plainly, with the matters that remain outside the applicant's control identified.

Do not present modelled depths as precise. The mapping is strategic, and if the scheme depends on a 200 millimetre difference in level, say that site-specific work would be needed to establish it. Choosing and citing the right datasets matters here in the same way it does for drainage design, where the dataset behind a calculation has to be identified, as set out in the note on which rainfall dataset to cite.

Where the risk changes the layout

On most sites the answer is modest. Put the sleeping accommodation outside the deeper parts of the extent, or raise it. Keep basements out of the extent altogether, because a basement in an inundation zone is the hardest case to defend. Place the vehicular and pedestrian access so that the route out runs away from the hazard rather than along the valley.

On larger sites it can shape the masterplan. Open space, parking, attenuation and sports pitches can occupy the extent without creating risk to life, and the built form can sit outside it. That is a better answer than engineering a defence, because a defence against a dam breach is not a proposition most authorities will entertain.

Where the whole site lies within the extent and the use is vulnerable, the honest conclusion is sometimes that the scheme should change. That is a difficult conversation to have with a client at planning stage and a much worse one to have at appeal.

Enter your postcode on PlanScope to see which flood and drainage documents your site is likely to need.

This is general guidance, not legal advice. Check the position with your local planning authority, the lead local flood authority, the Environment Agency 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.