How Sea Level Rise Changes Seabed Stress and Offshore Wind

Rising sea levels might seem certain to increase stress on the seabed because there is more water above it. However, the underlying physics indicates the reverse.

A new study has examined what happens beneath the surface. Its findings for north-west Europe matter for every wind turbine, cable and organism positioned on the seafloor.

A little-studied boundary

Climate research commonly focuses on warming at the ocean surface or receding shorelines. Far less attention is paid to the makeup of the seabed and the animals that inhabit it or live on top of it.

Dr Julia Rulent, an oceanographer at the National Oceanography Centre (NOC), headed a team seeking to address this gap.

The researchers brought together ocean and wave models with climate projections extending to 2093.

Their case-study region was the North Western European Shelf, although the physics involved apply to shelf seas around the world. In particular, the team investigated how sea-level rise and more intense storms could alter stress on the ocean floor.

Sea-level rise and storms: opposing forces

The research separates two key influences. By moving the sea surface further away from the bed, sea-level rise reduces the effect that waves and tidal currents have at the bottom.

This makes the seabed calmer, more regular and more stable. Storms, by comparison, have the opposite effect.

A warmer atmosphere is projected to bring fewer, yet stronger, winter storms to north-west Europe. Every storm sends brief but powerful bursts of energy down to the seabed.

Before this research, nobody had measured how these two influences work together across a whole shelf sea: when and where they occur, and how strongly the seafloor experiences them.

The subdued influence of sea level

Using UK climate projections, the researchers modelled two levels of sea-level rise: approximately 28 centimetres (11 inches) by the middle of the century and 71 centimetres (28 inches) by 2100. They applied the same weather conditions to each scenario.

Greater water depth weakens almost every effect. Bottom tidal currents become slower, while wave energy finds it harder to reach the seafloor.

Amphidromes - stationary locations where tidal range is zero - shift by as much as 3.9 kilometres (2.4 miles).

The mean decline in seabed stress across the shelf is modest but persistent. It is most pronounced in shallow waters and high-tide estuaries such as Morecambe Bay.

There is, however, an exception. As waves cease dissipating over Dogger Bank, more wave energy reaches the coast, potentially creating larger nearshore waves in the German Bight.

Storm-driven seabed stress

Storm patterns present a contrasting picture. Warmer seas are associated with more powerful low-pressure systems.

Recent forecasts indicate that UK storm severity may increase by 30% by 2080, chiefly because storms will span larger areas.

In Rulent’s modelling, an intense winter storm at the end of the century may impose up to 15 newtons per square metre of stress on the seabed.

That exceeds twice the force generated by the strongest spring tides today. In certain areas, storm stress could rise by a full order of magnitude - ten times the level in calm conditions.

Larger sediment grains begin to shift

The size of material a current can carry is determined by the force it applies. Under present-day calm summer conditions, only fine sand particles below 0.1 millimetres (0.004 inches) are lifted.

Major storms can move grains larger than 11 millimetres (0.4 inches), including small pebbles. On Atlantic-facing shorelines, peak storm conditions can already roll stones exceeding 25 millimetres (1 inch) - approximately the size of a quarter.

At present, the transition between summer and winter alters the type of sediment the sea can transport across more than 500,000 square kilometres (193,000 square miles) of shelf.

By the end of the century, that threshold is expected to extend beyond 640,000 square kilometres (247,000 square miles) during winter.

A growing seasonal divide

Together, these changes create an unusual new cycle. Summers are expected to become calmer as wave conditions ease and higher sea levels increase water depth.

Winters, meanwhile, are forecast to become more severe as stronger storms arrive. Benthic habitats - including the worms, clams, crabs and other organisms that rely on a stable seabed - developed under the current cycle.

“Increased storminess may create more and bigger disturbance regimes for benthic communities,” Rulent and her colleagues wrote.

Species that require quiet periods to recolonise disrupted areas of seabed may therefore have increasingly limited opportunities to do so.

Offshore wind infrastructure at risk

This shelf is among the world’s most heavily industrialised marine areas. Offshore wind capacity in the EU and UK reached 36 gigawatts in 2023, and 110 gigawatts are planned by 2030.

Turbines, steel foundations, scour-protection rocks and seabed cables are all installed on a floor whose behaviour is changing. Rock armour designed for present-day currents may consequently prove insufficient during the storms expected by 2080.

A related study, led by several of the same researchers, found that one turbine foundation can more than double the force a current applies to the seabed behind it. Climate-driven forces could then be added to this effect.

What happens next

For the first time, projected changes in seabed stress and sediment movement have been mapped across a complete shelf sea. The difference between summer and winter conditions is expected to grow throughout the century.

Sea-level rise will reduce stress at the bed in a gradual and predictable way. Storms, in contrast, will disrupt it more forcefully and more frequently, producing an ever-wider seasonal divide.

These changes affect offshore wind engineering, marine protected area planning and fisheries management. The same physical processes apply throughout shelf seas worldwide.