Breach and overtopping scenarios behind a defence

A site behind a flood defence is still a site at flood risk, and the flood risk assessment has to quantify what happens if the defence fails or is exceeded. That means two scenario families: overtopping, where water passes over the crest because the event is larger than the defence was designed for, and breach, where the structure fails and releases water through a gap. The Environment Agency's flood zone mapping ignores defences, so a defended site often sits in Flood Zone 3 on the map while the issue that decides the application is residual risk rather than the zone itself. A defence breach assessment for planning is about depth, velocity, warning time and escape, assessed for the lifetime of the development.

Developers often arrive expecting the defence to settle the matter. It rarely does.

Why a defended site still carries flood risk

Flood zones are defined ignoring the presence of defences, so the zone tells you the risk if the defence were not there. That is deliberate, because defences fail, are overtopped, are not maintained for ever, and because the consequences behind a defence can be worse than on an undefended floodplain.

The reason is depth. Land behind a raised defence is often lower than the design flood level, so when water does get in it fills a bowl, with no route out until it is pumped. On an undefended floodplain the water rises and falls with the river. Behind a defence it stays.

So the Flood Risk Assessment has to deal with the standard of protection, the residual risk, and the consequences for the people who will live or work there. The sequential and exception tests still apply on their own terms, and the Sequential Test is not avoided by the presence of a defence.

Standard of protection and what it means

The standard of protection is usually expressed as an annual exceedance probability, for example a 1 in 100 year standard for a fluvial defence or 1 in 200 for a tidal one.

Three things that figure does not tell you. It does not say the defence will perform to that standard in 80 years, because climate change raises the water level against a fixed crest and the effective standard falls over time. It does not say the structure is in the condition it was designed to be in, which is what the asset condition grade tells you. And it does not say the defence will be maintained indefinitely, because maintenance depends on funding and on the shoreline or catchment management policy for that reach.

Ask the Environment Agency for the defence's design standard, crest level, asset condition grade and the management policy for the reach. Those four facts shape the assessment, and they are information you request rather than derive.

Breach location, width and timing assumptions

Breach modelling is scenario modelling, and the scenarios have to be justified rather than chosen for convenience.

What has to be stated and agreed:

• the breach location or locations, usually the worst case for the site, which is typically the nearest point of the defence upstream
• the breach width, commonly taken as a defined length for an earth embankment and a shorter length for a hard structure
• the breach invert level, whether the defence fails to ground level or partially
• the timing, meaning whether the breach occurs at the peak of the event or on the rising limb, the latter often producing a worse result for the site because more water follows
• the duration before any intervention
• the event return periods assessed, including the design event and a more extreme event, each with the appropriate climate change allowance for the development's lifetime

Agree these with the Environment Agency before modelling. A breach scenario the regulator does not accept is a modelling exercise repeated at the applicant's cost.

Overtopping against breach consequences

The two scenarios behave differently and the design responses differ.

Overtopping tends to govern finished floor levels and property resilience. Breach tends to govern layout, the position of vulnerable uses and the escape strategy. A scheme that addresses only one of them has a gap a reviewer will find.

Depth, velocity and hazard behind the defence

The outputs that matter are mapped, not tabulated: depth, velocity and the hazard rating that combines them, across the site, at the time of maximum hazard.

Hazard is conventionally expressed by combining depth and velocity with a debris factor, giving categories from low hazard, where there is caution for some groups, up to levels described as danger for all. Those categories are what determine whether a route out of the site is usable, because a 300mm depth with 2 m/s velocity is more dangerous than 600mm of still water.

Then the time dimension. The assessment should give the time from breach to the arrival of water at the site, the time to reach a depth that prevents walking, the duration of inundation and the rate of rise. Those numbers drive the flood warning and evacuation plan, and a plan that assumes people will drive out needs the road to be passable at the time it is needed.

Where the site includes basements or any sleeping accommodation at or below ground level, the hazard assessment is decisive rather than informative, which is covered in basements, flood risk and sleeping accommodation.

Mitigation and layout responses

Layout first, then levels, then resilience, then management. That order reflects what the Environment Agency and most local plans expect.

Layout responses: put the most vulnerable uses in the lowest hazard part of the site, keep sleeping accommodation above the modelled breach level, place open space and parking in the highest hazard areas, and provide a dry escape route to land above the design flood level rather than relying on rescue.

Level responses: finished floor levels set above the modelled level for the relevant event including climate change, with freeboard, and safe access at a level that works for the warning time available.

Resilience responses: flood-resistant construction up to a stated level, services and electrical distribution raised, materials that dry and clean, and a property flood resilience specification, which is dealt with in residual flood risk and property flood resilience measures.

Management responses: registration with the flood warning service, a flood plan naming who does what, and a safe refuge where evacuation is not realistic. Management measures carry the least weight on their own, because they depend on people.

Evidence and agreement with the regulator

Pre-application engagement with the Environment Agency is the step that saves the most time on a defended site, and the questions to put in writing are specific.

Ask for the defence design standard, crest levels and asset condition grade. Ask whether an existing breach model exists for the reach, because many urban defended reaches have been modelled already and reusing that model is far quicker than building one. Ask which breach scenarios they expect to see. Ask which climate change allowance applies for the development lifetime and the river basin district. And ask whether they will accept a hazard-based safe access argument or whether they require dry access.

Submit the model build note, the scenario schedule, the depth and velocity mapping, the hazard mapping, the timing analysis and the flood plan as separate appendices. Reviewers read the scenario schedule first to check it matches what was agreed, and a mismatch there discredits everything after it. The groundwater element, if relevant, is covered in evidencing groundwater flooding in a flood risk assessment.

A flood risk assessment involving breach modelling is work councils and the Environment Agency expect a suitably qualified hydraulic modeller to carry out, and professional review is strongly advised. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.