Tidal flood risk, breach modelling and coastal change

A tidal flood risk assessment differs from a fluvial one in a way that catches out teams who have only done rivers. The published flood zones ignore defences, so a site sitting behind a sea wall with a good standard of protection still shows as Flood Zone 3. The assessment then has to answer a question the mapping cannot: what happens when that wall fails.

That question is answered by breach modelling, and it is the part the Environment Agency will look at first.

How tidal flood zones are derived

Tidal Flood Zone 3 is land with a 1 in 200 or greater annual probability of flooding from the sea. Zone 2 covers land between that and the 1 in 1,000 annual probability. The zones are derived from extreme sea level statistics combined with ground levels, and they are drawn as though no defences existed.

That convention exists for a reason. Defences fail, defences are overtopped, and defences reach the end of their design life, so planning policy starts from the undefended position and then allows the assessment to show what protection actually exists. It is not an error in the mapping and arguing that it is will not advance an application.

The consequence is that most estuary and coastal town sites are in Zone 3 regardless of how good the wall is, and the sequential test and, for more vulnerable development, the exception test both engage.

Defended and undefended scenarios

A tidal assessment normally models three states of the world, and the drawings should present each separately.

The undefended case, which shows what the flood zone mapping is based on. The defended case, showing the extent and depth of flooding with the defence performing as designed, which is what happens in most events. And the breach case, showing what happens if the defence fails during a design tide.

Each answers a different question. The defended case tells you the likelihood of any flooding at all. The breach case tells you whether the development is safe, which is the test the exception test actually applies.

What a breach model has to include

Agree the parameters with the Environment Agency before running anything, because a model built on assumptions the regulator does not accept has to be rerun.

The parameters to settle are the breach location, the breach width, the time at which the breach occurs relative to peak tide, the duration before it is repaired, and the number of tidal cycles modelled. Breach locations are chosen where failure would be most consequential for the site, and more than one location is usually required. Timing at or just before peak tide produces the worst case, and modelling over several tidal cycles matters because water that enters on one tide may not drain before the next.

The output is not a single line on a plan. It is depth, velocity and the timing of arrival across the site, because a slow rise to 200mm is a different proposition from a fast-moving front at the same depth.

Hazard rating and what it is for

Depth and velocity are combined into a flood hazard rating using the method published for assessing flood risks to people, which multiplies depth by velocity plus a constant and adds a debris factor. The result falls into bands running from low hazard through to danger for all.

That rating is what the safe access and egress argument is built on. A route out of the site that sits in the danger band during the breach event is not a safe route, whatever it looks like on a dry day, and an assessment that presents a depth figure without a hazard rating has not addressed the question.

Use it honestly. Where the site floods to a hazard level that rules out evacuation, the answer is usually safe refuge above the flood level within the building, with a flood plan and a warning arrangement, rather than an evacuation route that only works in theory.

Wave overtopping

On open coast and exposed estuary frontages, overtopping is often a bigger practical issue than breach. Waves running up the face of a defence throw water over the crest, and the resulting discharge is expressed as litres per second per metre of defence.

Tolerable discharge rates differ sharply by what is behind the wall: a promenade with pedestrians, a road with vehicles, a building, or open ground all have different thresholds, and published European guidance on overtopping sets them out. Where a scheme puts habitable accommodation close behind a defence, the overtopping assessment may drive the layout more than the breach model does.

Wave conditions need deriving properly from fetch, bathymetry and offshore wave data. This is specialist coastal engineering work.

Sea level allowances and the defence itself

Apply the sea level rise allowances published by the Environment Agency for the relevant region and time period, over the lifetime of the development, which for residential is normally taken as 100 years. Allowances for extreme sea levels and for changes in wave height are set out alongside them and are revised, so take the figures from the current tables.

Then look at the defence as an asset. Who owns it, who maintains it, what standard of protection it provides now, what its residual life is, and whether it is in the Environment Agency's asset records with a condition grade. A private frontage wall of unknown construction, maintained by nobody, cannot be relied on in the way an agency-maintained flood embankment can.

Where the defence's residual life is shorter than the development's lifetime, say so and explain what happens at that point. That is the question officers ask and the one most assessments avoid.

Shoreline management plans

Shoreline management plans set non-statutory coastal policy for each stretch of coast over three time periods, using four policy options: hold the line, advance the line, managed realignment and no active intervention.

The policy for your frontage, for each epoch, is evidence you should put in the flood risk assessment rather than leaving the officer to find it. A frontage with a hold the line policy in all three epochs supports a case for long-term development. A frontage moving to managed realignment or no active intervention in the second epoch does not, and proposing a 100-year residential lifetime behind it invites refusal.

Where the local plan defines a coastal change management area, a different set of policy tests applies, and the note on coastal change management areas sets out what remains permissible in one.

What stays acceptable behind a defence

Development is not ruled out by tidal risk, but the design changes.

Set finished floor levels above the breach flood level where the levels allow it. Avoid ground floor sleeping accommodation and single-storey more vulnerable development in tidal Zone 3a. Provide safe refuge above the design flood level in every building, with a route to it that does not pass through floodwater. Design for rapid onset, because unlike a slow fluvial rise a breach gives limited warning even where the tide itself was forecast days ahead.

And write a flood warning and evacuation plan that the emergency planning officer can actually work with: who receives the warning, what the trigger is, what happens at night, and who is responsible for vulnerable occupiers.

This is general guidance, not legal advice. Breach parameters, overtopping criteria and sea level allowances are matters for the Environment Agency and a coastal engineer, so agree them before modelling.

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