Reservoir inundation areas and what they mean for a site

Reservoir inundation is a residual risk, and it does not change the flood zone your site sits in. A site in Flood Zone 1 that also sits inside the mapped inundation extent of a large raised reservoir is still in Flood Zone 1 for the sequential test. What changes is what the flood risk assessment has to say, and on some sites what the emergency planning officer will want to see. Reservoir flood risk planning questions usually arrive late, when a search or a consultee response mentions an inundation area nobody had looked at.

The mapping exists because reservoirs are regulated, not because failure is expected. England has thousands of raised reservoirs, many of them Victorian water supply structures on the edge of towns that grew up beneath them, and the extents drawn from those structures reach a long way downstream. Plenty of ordinary suburban streets sit inside one.

What the reservoir flood maps show

The Environment Agency publishes reservoir flood risk information through the long-term flood risk service, showing the extent that could be flooded if a large raised reservoir released the water it holds. Some datasets carry depth and velocity, and the modelling assumes the reservoir fails without warning at a time of dry weather or in combination with a river flood.

Two things follow from how that modelling is built. The extent is deliberately conservative, because a model designed to support emergency planning errs on the side of a wider footprint. And there is no return period attached to it. Fluvial and tidal flood zones carry an annual probability. Reservoir inundation does not, because the probability of failure is managed by the safety regime rather than estimated from a hydrological record.

How the Reservoirs Act 1975 regulates the risk

The Reservoirs Act 1975 applies to large raised reservoirs, those holding more than the statutory volume above the natural level of the surrounding land. The Flood and Water Management Act 2010 changed how the threshold and risk designation operate in England, so check the current position with the Environment Agency rather than relying on an older summary.

Under the regime, reservoirs are registered, inspected by panel engineers on a cycle, supervised between inspections, and those designated as high risk require flood plans. The Environment Agency is the enforcement authority in England. That framework is the mitigation, and a flood risk assessment should say so plainly rather than treating the inundation extent as an unmanaged hazard.

Why it is residual rather than a zone

The sequential test operates on the flood zones shown on the Flood Map for Planning, which cover flooding from rivers and the sea. Surface water, groundwater, sewers and reservoirs are other sources that the assessment has to consider, and national guidance expects an assessment to address all sources relevant to the site.

So the structure of the argument runs like this:

1. Establish the flood zone and apply the sequential test on that basis.
2. Identify every other source of flooding that affects the site, including reservoir inundation.
3. Assess the risk from each and set out mitigation.
4. Deal with reservoir inundation as residual risk, alongside matters such as breach of a raised defence.

Getting that sequence wrong is the common error. An assessment that runs the sequential test against the reservoir extent will be asked to redo it, and one that ignores the extent entirely will be asked to add it.

What the flood risk assessment should say

A proportionate treatment for a site inside a mapped extent covers seven points:

• The reservoir, its undertaker and its distance and elevation relative to the site.
• The mapped extent, with depth and velocity where published, and the date of the dataset.
• Confirmation that the reservoir falls within the statutory regime and is inspected and supervised under it.
• Whether the site would be affected in the dry day scenario, the wet day scenario, or both.
• Safe access and egress, and whether the escape route leaves the extent.
• Flood warning arrangements and how future occupiers would be informed.
• The conclusion on residual risk, expressed as a judgement rather than a probability.

Most of that is desk work. A Flood Risk Assessment for a minor development will cover it in a page, and for a major scheme it belongs in the residual risk chapter next to the defence breach analysis.

Where the proposal includes rooms below ground, the analysis has to go further, because a basement inside an inundation extent raises the same questions as any other basement flood risk case and the escape route is the part officers focus on.

Emergency planning and evacuation

Reservoir inundation is primarily an emergency planning matter. Local Resilience Forums hold multi-agency flood plans, and reservoir undertakers of high risk reservoirs prepare on-site flood plans that feed into them.

For most development, the planning response is limited to flood warning registration and an occupier information pack. For care homes, schools, hospitals and other uses where occupants cannot evacuate themselves, expect a harder look. The council's emergency planning officer may be consulted, and the answer they want is a route out of the extent that does not depend on the residents being mobile.

An illustrative example. A single storey extension to a bungalow inside a reservoir extent in Flood Zone 1 needs a sentence in the covering statement and nothing more. A 60 bed care home on the same plot needs an evacuation strategy agreed with the emergency planning team before the application is submitted. Both sites carry the same mapped risk. The difference is who occupies the building.

Do not overstate it either. A flood risk assessment that devotes four pages to a reservoir five kilometres away and half a page to the surface water risk on the site itself has the balance wrong, and reviewers notice. Match the length of the treatment to the significance of the source.

Consultation and who says what

The Environment Agency is a statutory consultee on flood risk in the flood zones and for major development, and it will comment on reservoir matters where they arise. The lead local flood authority comments on surface water and ordinary watercourses, and its drainage team will see the same information when reviewing what the drainage strategy proposes. Where a canal, a water company asset or a raised ordinary watercourse forms part of the picture, the relevant undertaker may also respond.

One point on searches. A conveyancing search may report reservoir flood risk in terms that sound alarming to a buyer or a lender, and applicants sometimes arrive expecting the planning system to resolve it. It cannot. The planning assessment records the risk and the mitigation, and the insurance and lending questions run on their own track.

Ask the question early rather than answering it late. A pre-application enquiry that names the reservoir and asks what the authority expects will usually produce a one paragraph answer that settles the scope.

Check the constraints before commissioning anything, then work out the document list. Enter your postcode on PlanScope to see which reports the site is likely to need.

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