Wave overtopping on a tidal frontage and freeboard

On a tidal frontage the design flood level is not the still water level. It is the still water level plus whatever the waves do on top of it, and freeboard is what you add after both have been quantified. Miss the wave component and a finished floor level that looks 600mm clear of the extreme tide can be wet twice in a winter.

The assessment has two halves. Establish the extreme still water level for the design event, then calculate mean overtopping discharge at the defence crest for the wave condition that coincides with it. Both belong in the Flood Risk Assessment, and on any site of scale the Environment Agency will read that section first.

Still water level against the overtopping component

Extreme still water level combines astronomical tide with surge. For tidal flood risk, the boundary between Flood Zone 3 and Flood Zone 2 sits at a 0.5 per cent annual probability, so the 0.5 per cent event is the usual starting point for the design still water level. Add the sea level rise allowance for the region and the development lifetime from the Environment Agency's climate change allowances, which are published by river basin district and by epoch and are revised periodically.

Wave action sits above that. A wave breaking on a vertical wall at high tide can throw water several metres beyond the crest even when the still water level is below it, and the volume that lands behind the defence is the number that matters for people and property. So the deliverable is a discharge, expressed as litres per second per metre of defence, rather than a level.

Two mistakes recur. Using a mean sea level rise figure from an older allowance table because it was in the previous project file. And applying the fluvial 1 per cent event out of habit on a site whose dominant source is tidal.

Discharge rates and the tolerable limits

Mean overtopping discharge is calculated using the empirical formulae in the EurOtop manual, which is the reference UK and European practice uses for wave overtopping at coastal structures. Tolerable limits differ by receptor and they differ by more than an order of magnitude between them. A pedestrian standing on a promenade, a vehicle on an access road, a building behind a defence and the structural integrity of the defence itself each have their own threshold.

Take those limits from the current edition of the manual rather than from a spreadsheet inherited from a previous job. They have changed between editions, and an objection that your limits are out of date is one you cannot answer at committee.

Peak individual wave volumes deserve separate attention. A frontage can pass the mean discharge test and still be unsafe, because a single large volume arriving in two seconds is what knocks a person over. Where a public route or an escape route sits behind the defence, report both mean discharge and maximum individual volume.

Fetch, wind and the design condition

Offshore wave conditions come from hindcast data or a nearshore transformation model. What the assessment needs is the inshore condition at the toe of the defence, which means taking the offshore significant wave height and period and transforming them across the local bathymetry, accounting for shoaling, refraction and depth-limited breaking.

Fetch matters most on estuarial and sheltered frontages, where the design wave is locally generated by wind rather than swell arriving from deep water. On a narrow estuary the critical direction is often not the prevailing one, because the longest fetch may lie across the channel. Check the fetch in every direction, not just the windward one.

The design condition is a joint probability question. The extreme tide and the extreme wave do not necessarily occur together, so combining the 0.5 per cent tide with the 0.5 per cent wave overstates the event. Use a joint probability approach and state the dependence assumption explicitly, because that assumption is the single most contestable input in the whole calculation.

Crest levels and set-back as alternatives

Raising the crest is the obvious answer and often the worst one. A higher wall reduces overtopping and blocks the sea view that made the site viable, and on a promenade frontage it usually attracts a landscape and townscape objection of its own.

The alternatives are worth modelling before the wall goes up:

1. Set the buildings back so the overtopping volume dissipates across a defined splash zone.
2. Change the defence profile, since a sloped or stepped revetment with a recurve at the crest can cut discharge substantially compared with a plain vertical face.
3. Add roughness, through rock armour or a stepped apron, to dissipate energy before the wave reaches the crest.
4. Accept a higher discharge and design the receiving surface for it, with falls, drainage and no habitable use at ground level.

Set-back is usually the cheapest and the one planning officers prefer, because it reads as an urban design move rather than a flood defence. It also avoids committing a future authority to maintaining a structure it did not ask for.

Sea level rise over the development lifetime

The allowance you apply depends on the lifetime you claim. Residential development is normally assessed over 100 years, commercial over a shorter period, and the lifetime has to be stated and justified rather than assumed.

This is where a tidal site differs sharply from a fluvial one. Fluvial climate change allowances increase peak flows; sea level rise raises the baseline the waves sit on, which increases overtopping non-linearly. A 300mm rise in still water level against a fixed crest can multiply mean discharge several times over, because the freeboard between crest and water is the dominant term in the overtopping formulae. Model the end-of-lifetime case, not just the present day.

Illustrative example. A 40-unit scheme behind a vertical masonry sea wall passes the overtopping test at present-day levels with the wall crest 1.2m above the design still water level. Apply the sea level rise allowance for the 2120 epoch and the effective freeboard falls, which pushes mean discharge above the tolerable limit for the pedestrian route immediately behind the wall. The scheme was consented with a set-back and a raised terrace rather than a raised wall, and with a condition tying the terrace levels to the modelled case. The figures here are illustrative and are not a substitute for site-specific modelling.

Mitigation at the building rather than the boundary

Where the frontage cannot be changed, the mitigation moves to the buildings. Finished floor levels above the design flood level, with freeboard on top, is the first move. Practice commonly uses 300mm of freeboard for fluvial sites, but on a wave-affected frontage the Environment Agency may ask for more, and the figure should be agreed at pre-application rather than assumed.

Beyond levels, the usual package is no habitable accommodation at ground floor on the seaward side, flood-resistant construction to the exposed elevation, safe access above the design level, and a written flood warning and evacuation plan. Note that safe access is the requirement that most often defeats an otherwise well-designed tidal scheme, because a dry building reached across a flooded road is not a safe development.

Residual risk has to be stated rather than argued away. If the defence fails or is overtopped beyond the design case, say what happens, how deep it gets and how long it lasts.

How the assessment is presented for review

Structure the wave section so a reviewer can reproduce it. State the source of the extreme still water levels, the offshore wave data set and its period of record, the transformation method, the joint probability assumption, the defence geometry with a survey reference, the formulae used and the edition of the manual, and the tolerable limits with their source.

Include the sensitivity runs. A reviewer who can see that your result holds under a more pessimistic dependence assumption will accept the central case. One who is shown a single run will ask for the others anyway, three weeks into determination.

Where the site also needs the Exception Test, the overtopping work carries directly into the safety limb, and the drafting is easier if the two are written together rather than bolted on. The two limbs and the evidence each needs are covered in exception test evidence, and the sequential approach that comes before it in the Sequential Test.

Wave overtopping modelling is specialist coastal engineering work and needs a suitably qualified author. Enter your postcode on PlanScope to see which flood and drainage reports your site is likely to need before you commission it.

This is general guidance rather than legal advice. Check the position with your local planning authority, the Environment Agency or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.