Engineering sea defences: the problem of high seas and low lands

One effect of rising world temperatures is that water levels are also rising, with potentially disastrous effects for some low-lying regions. That’s leading many countries to consider whether it’s feasible to protect their seabords with walls or barriers. It’s becoming an urgent concern, even alongside the current political will around the globe to slow the warming by cutting carbon emissions. Even with a radical change in human behaviour, the Intergovernmental Panel on Climate Change (IPCC), which includes more than 1,300 scientists from around the world, forecasts a temperature rise of 1.4 to 5.6°C over the next century.

The consequences of this are likely to include more volatile weather systems – more droughts and heatwaves, more precipitation and hurricanes – although there is still uncertainty about future weather behaviour. The polar ice caps will reduce and, in the case of the Arctic, may disappear, resulting in loss of habitat and extinction for polar bears and other species.

What will also happen is that the sea level will rise, partly through meltwater from the ice caps and partly from expansion of water at increased temperature. According to IPCC, global sea level has risen by about 20cm since 1880 (when reliable records began) and is projected to rise another 30 to 120cm by 2100.

Dr Ivan Haigh is associate professor in coastal oceanography at the National Oceanography Centre, University of Southampton. He explains that even this broad range is ‘best-guess’. “The IPCC range isn’t the full range. They present the 95 per cent confidence range. It could be as much as 2.5m but it’s very unlikely to be higher than that.” At the extreme end of the scale, if all the ice melted, the sea level would rise 65m.  

‘It’s absolutely undeniable that sea levels have risen by about 20cm over the last 150 years. There is a very high certainty now that a large proportion of that has been driven by anthropogenic warming.’

These IPCC figures would represent a mean sea level rise, but it would not be equal everywhere. In some places the oceans would heat up and expand more than in others, and also there would be the change in weather patterns and ocean currents. If the Gulf Stream was strengthened then it could affect the USA’s eastern coast. One of the biggest reasons is to do with ice melt, which counter-intuitively raises the sea level more the further away it is from the polar regions. This is due to the ice caps being so massive they have their own gravitational pull.

Another effect of the ice is that the weight of it on land leads to a readjustment once it is gone. In the last Ice Age, which ended about 11,700 years ago, the northern half of the UK was covered in ice that was so heavy it compressed the ground beneath. Since it has melted there has been a gradual land rise of about 1cm a year in Scotland, while southern England compensates by sinking slightly.

Putting these factors together with the storm surges we can expect in more extreme weather, this could have a significant difference on coastal low-lying areas.

The ultimate example is the Maldives, a nation of only 430,000 people scattered over 26 atolls in the Indian Ocean. It spans the Equator and so could not be further away from the melting ice. Its average elevation above sea level is only 150cm and its highest point is 240cm. Even as it is, big waves can be a problem. Sea level rises above one metre would be catastrophic – a lost nation.

Solutions in the Maldives tend to be low-tech. Sand is shipped to beaches where erosion has taken it away, mangroves are being planted to reduce erosion and deaden the impact of waves, and protection is provided by fake coral reefs and sea walls that in some cases completely surround the islands. However, if the sea level continues to rise at its present rate then such protection will become inadequate and the vast majority of the Maldives could sink below the surface, probably in about 80 to 100 years. It is no surprise that the Maldives has fast-tracked its environmental agenda and plans to be carbon-neutral by 2020.

The Maldives are at least above sea level, unlike 40 per cent of the Netherlands – a country that has been largely reclaimed from the sea. It has invested heavily in protection.

The jewel in its sea defence crown is the Port of Rotterdam, which is situated at the mouth of the River Rhine, 25km inland from the North Sea coast. As Europe’s busiest sea port it plays an important role in the Dutch infrastructure. Protecting it is the Maeslant storm surge barrier – one of the Earth’s largest moving objects. It has two arms or doors, each the size of the Eiffel Tower and with a width of 210m, which together can span the Nieuwe Mass, one of the many channels that make up the Rhine delta.

The barrier is automatically deployed if a storm surge is expected that is more than 3m higher than typical high tide and will protect Rotterdam and surrounding countryside from a surge of up to 5m. Its 22m-high doors rotate around 10m diameter ball-and-socket joints that each weigh 680 tonnes. Once in position, the doors fill with water and sink to the bottom, the whole operation taking a couple of hours.

In early January 2018 as storm Eleanor blew in from the west, the Maeslant storm surge barrier was closed for the first time since 2007, along with the other four main storm surge barriers in the Dutch armoury. It was the first time all five had been deployed at the same time.

Beyond building sea defences higher, stronger and more creatively, the Dutch have also looked at other ways that can alleviate flooding. In Delft, Rotterdam and Amsterdam there are a number of floating buildings – still a novelty but an indication of one potential way forward. Rotterdam also has underground car parks and sunken public places that can effectively act as a mini flood plain and absorb flood water.

While storm Eleanor marked only the second operation for the Maeslant barrier, it caused the Thames barrier to notch up its 180th closure. With the original design life of the barrier only extending to 2030, what comes next?

“One option is to build a much bigger barrier downstream, but that would cost billions,” says Haigh. “Rather than rushing into that decision, they’ve taken an adaptive approach. They find that, if they make some small design changes, rather than the Barrier just coming upright, they could over-rotate it. If they add some flood storage zones downstream they think they can cope with 2m of sea-level rise. But if the sea level rises more than 2m, they have to build a new, bigger barrage. So I really like the London Thames Estuary 2100 approach. Rather than making a decision now they’re going to assess the situation every five years.”

This is just one implementation of the UK’s Shoreline Management Plan, which has divided the coastline into manageable chunks, each of which can define its own plan. These can broadly be split into four categories: hold the line (typically meaning building a sea wall); do nothing; managed retreat; and adaptation. The last option is essentially looking at the sort of solutions being introduced in Rotterdam – create urban flood areas and new housing types to embrace ‘living with water’ rather than keeping it away from us.

The problem with ‘holding the line’ is that it costs a lot of money. As it is, the Netherlands puts aside €1bn a year on flood defences, but even that is not enough to cover the requirements of the future. Haigh says: “Seawalls are very expensive. For small villages it just doesn’t make economic sense to put in a massive seawall.  Unfortunately, over time – maybe not now, but in 50 years’ time, when sea levels are much higher – the government will have to make some difficult decisions, and just say, ‘Look, we can’t defend this stretch of coastline.’”

‘Doing nothing’ applies when the money has run out for land or property that can’t be justifiably protected, but perhaps the most interesting approach is the ‘middle way’.

“In the UK we’ve got a number of so-called Managed Retreat Schemes,” says Haigh. “This is where we’ve actually breached a hole in the flood defence and let the land behind it flood. There is a very good example in Steart Peninsula [on the Bristol Channel].”  In some cases the government is buying up coastal farmland specifically for this purpose.

An Adaptive Management Plan could be part of such a scheme or even an alternative – the Thames Barrier being one prime example of this. In the case of Managed Retreat Schemes, it could involve building a sea wall behind both the existing coastline and the area that is being allowed to flood. Instead of building a very expensive sea wall that could cope with a sea level rise of 2m, even if that is the long-term forecast, the idea would be to build a defence, say 0.5m if that was the medium-term sea level rise prediction, that could be adapted – built on – in the future to accommodate greater rises.

Using farmland and coastal marshes to absorb flood waters is one of a number of techniques following the trend of moving away from ‘hard engineering’.  Haigh explains: “Lots of flood defences were built 50 years ago, but often in the long run, they do more damage than good.  Increasingly we’re going towards soft engineering – ‘working with nature’. Nature provides the best flood defences. Mangroves, salt marshes, even beaches, are all good. If you’ve got a very healthy beach with lots of sand, the waves crash on the beach, not on your road, or your seawall.

“So [it would be beneficial] if we can encourage salt marshes to grow up and be healthy again, if we can replant mangroves, if we can stop people destroying coral reefs, and even if we can put sand on beaches to build them back up, so that the energy dissipates on the beach, not directly on the coast.  In Holland, they use a ‘sand engine’, where they’ve dumped millions and millions of tonnes of sand on the beach to protect that stretch of coast, and that sand naturally moves along the beaches and builds up beaches elsewhere.”

Beyond the coastal protection there are two major challenges that, if met, could improve our resilience to flooding. Firstly there is the redesign of cities, so called ‘blue-green cities’, to cope with the increased likelihood of flooding. These include the sort of infrastructure design features being introduced in Rotterdam, while in in the USA there are floating bridges, like the Evergreen Point Floating Bridge in Seattle, opened in 2016, which is 2.35km long.

On a smaller scale houses could be designed with all of their electricity at least a metre above floor level, small flood barriers at the door, using stone floors rather than carpets and tiled walls instead of plaster.

Moreover, rather than making houses flood-resilient it would be better to build them in places that didn’t flood. Haigh says: “We really need new regulations that mean that we don’t build in flood-prone areas. Flood plains are good. Flooding is good. Certainly, river flooding is good, it brings sediment and nutrients on to the soil. Even the coast is supposed to flood, and the fact that we’ve built seawalls and built cities there isn’t good. So, I think the government really needs to bring in new regulations to say we don’t live in flood-plain areas. Unfortunately, in many cases, we’re too late for that.”