Flood defences and the residual breach scenario

A site behind a flood embankment is still in Flood Zone 3. The Flood Map for Planning ignores the presence of defences, and it does so deliberately, because the zones describe the probability of flooding from rivers and the sea in the absence of defences. A breach scenario flood risk assessment is what fills the gap between that map and the actual risk to people on the site.

Developers are often surprised by this. The land has not flooded in living memory, the defence is maintained, and the Environment Agency still objects. The objection is almost never about the frequency of flooding. It is about what happens on the day the defence fails.

Why a defended site is still in Flood Zone 3

Two consequences follow from the undefended basis of the zones.

The sequential test applies as it would on an undefended site, so the Sequential Test still has to be passed or the site has to be allocated in a plan that passed it. And for more vulnerable uses in Zone 3a, the exception test applies, with its requirement that the development be safe for its lifetime.

Safe, in a defended context, means safe in the residual scenario. That is the whole of the argument.

The zones also tell you nothing about surface water, groundwater or the possibility that the defence blocks drainage from the landward side. A site protected from the river can still flood from behind it, and a defended site with a pumped internal drainage system carries a dependency the assessment has to address.

Standard of protection and design life

Start by establishing what the defence actually does. Its crest level, its standard of protection expressed as an annual probability, the date it was built or last raised, its current condition grade, and who maintains it.

The Environment Agency holds asset data and condition information for the assets it manages, and that data is the evidence base for this section. Where the defence is privately owned, or maintained by an internal drainage board, or is in fact a raised road or railway embankment performing the function incidentally, say so. A defence nobody is obliged to maintain is a very different proposition from a maintained Environment Agency asset.

Then compare the standard of protection with the lifetime of the development. A defence built to a 1 in 100 standard in 1975 does not provide that standard today, because the flows it was designed for have been revised upward and climate change allowances have moved again since. Where the authority has a published plan to raise the defence, note it and note that it is a plan rather than a commitment.

Breach and overtopping are different failures

Overtopping happens when the water level exceeds the crest. It is progressive, it is predictable from the hydraulics, and it usually produces a slow rise landward of the defence.

A breach is a structural failure of the defence itself, from erosion, piping, animal burrowing, wave action or overtopping scour. It is sudden, it produces high velocity flow, and the resulting depth can exceed anything that would occur in an undefended situation because the water is delivered through a concentrated opening under head.

Model both. The overtopping case is the easier one and is often not the critical one. The breach case has to be defined by its location, width, duration and timing relative to the peak of the design event, and each of those is a judgement to agree rather than a value to assume.

Convention is to model the breach at the point of greatest consequence to the site, at or shortly before the peak, with a width appropriate to the construction of the defence. Test more than one location. A breach upstream of the site can produce a worse result than one directly opposite it.

Rapid inundation and hazard to people

Depth on its own understates the danger in a breach. The established hazard rating combines depth and velocity with a debris factor, and the resulting bands run from low hazard through moderate and significant to danger for all.

Map the hazard across the site at intervals through the event, not just at the maximum. The moment that matters for evacuation is when the route out first becomes impassable, which is frequently an hour or more before peak depth is reached.

Rate of rise is the other output to report. A site that goes from dry to a metre deep in twenty minutes cannot be evacuated in response to a warning, and a design that assumes it can will not be accepted.

Design responses inside a defended area

The responses that work in a breach scenario are different from those that work on an undefended floodplain.

Raising floor levels above the breach level is often impossible, because breach levels can be high. So the design shifts towards vertical refuge: habitable accommodation at first floor and above, safe internal access to that refuge from every unit, and ground floor uses that can be allowed to flood.

Layout matters more than construction. A dry route to land outside the hazard area, raised above the breach level, is the strongest single feature a defended scheme can have. Where no such route exists, the scheme becomes a shelter in place proposition, and that is a much harder argument that requires the building itself to be the refuge with services protected accordingly.

Flood resistant and resilient construction at ground level, raised electrical and heating plant, and drainage designed to work with the internal system all follow. Basement accommodation is effectively ruled out, for the reasons set out in the note on basement development and the flood risk objection.

Highly vulnerable uses, including basement dwellings and some forms of care accommodation, should not be proposed behind a defence without a very clear route through the policy tests.

Evidence the regulator expects

A defended site assessment that gets through consultation usually contains all of the following.

1. Defence details: ownership, crest levels surveyed, standard of protection, condition, maintenance regime and any funded improvement.
2. Breach scenarios agreed in advance with the Environment Agency, including locations, widths, timing and duration, with the reasoning recorded.
3. Modelled depth, velocity, hazard rating and rate of rise across the site, presented as maps at several time steps.
4. Overtopping assessed separately from breach.
5. Climate change allowances applied to the event and the development lifetime stated.
6. Access and egress assessed against the hazard mapping, with the time at which the route fails.
7. The design response, with floor levels, refuge provision and construction measures tied to the modelled outputs.

Agree the scenarios before modelling. A breach assessment run on assumptions the Environment Agency does not accept is a cost incurred twice, and its consultation response will say so in one line.

The whole package feeds the Flood Risk Assessment and, where a more vulnerable use is proposed in Zone 3a, the Exception Test. The second limb of that test is where the breach evidence does its work, as set out in the note on exception test evidence.

Conditions, warning and evacuation

Expect conditions requiring finished floor levels, flood resistant construction details, and a flood warning and evacuation plan submitted before occupation.

Be realistic about what a plan can deliver. Where the rate of rise exceeds the warning lead time, evacuation is not a mitigation and the plan should say so, setting out shelter in place arrangements instead. Emergency planning officers read these documents and they respond badly to plans that assume two hours of notice on a site that floods in thirty minutes.

Name who holds the plan, who acts on a warning, how residents are told, and how the arrangements survive a change of management company. A plan with no named holder is a document, not a measure.

Enter your postcode on PlanScope to see whether a flood risk assessment is likely to be triggered for a site. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.

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