Sea levels are rising, and the rate of rise is accelerating. All over the world, many of today’s dikes, sea walls and flood barriers won’t be enough to hold back the water in the future. This will be particularly a problem in countries that lack the resources to maintain or fund extensive engineering projects to protect their citizens. But we can all learn from alternative, more affordable and flexible approaches that adapt to the rising water currently emerging all around the world. Rather than only battling to keep ever-rising seas out, these natural solutions aim to help rebuild land above sea level. Researchers from Utrecht University are testing which of these strategies will work for specific regions to help tame the tide. And they’re also thinking ahead: how can we minimise the damage and ensure people have somewhere safe to go when the water does come?

Rising above water

Estimated reading time: 14 minutes

Painting of a seascape

October 2021. Water poured from the sky, like it often does in the Netherlands. Jana Cox was about to give a presentation about the impact of climate change on rivers to the Dutch Directorate-General for Public Works and Water Management (Rijkswaterstaat), when the Royal Netherlands Meteorological Institute (KNMI) released a report that stopped her in her tracks. “All sea level rise scenarios scientists have been working on since 2014 were underestimated, the report said. The new worst-case scenario is double previous estimates”, recalls Cox, a Physical Geography PhD candidate at Utrecht University.

Thinking that higher and wider dikes alone will save us is such a dangerous mentality.

“I scrapped my whole presentation, pulled up the headline and said: everything we’ve done for flood protection in the Netherlands, including my own research, – building and reinforcing the dikes, constructing the Delta Works – is based on sea level projections that are now way too low to keep people safe in the future”, recounts Cox. And as sea levels rise, she cautions, it’s not only the looming excess water we should worry about, but the lack of it: “If sea levels rise by one metre (as it’s likely by 2100), the biggest source of drinking water to millions of people in the Netherlands may become contaminated with saltwater. This is an often overlooked, but equally severe threat.”

Why are sea levels rising?

“The main reasons sea levels are rising are associated with climate change. One is thermal expansion, caused by the warming of the oceans as they absorb almost 95 percent of the extra heat from increased greenhouse gas emissions. As seawater warms, it expands, and sea levels rise as a result. Another is the increased melting of land-based ice, which means more water enters the oceans. The ice loss from Greenland and Antarctica and from mountain glaciers together account for about half of current sea level rise. On top of that, as ocean and air temperatures increase, coastal storms will become more intense and frequent, creating large storm surges at short notice”, explains Roderik van de Wal, Professor of Sea level Change and Coastal Impacts.

As sea levels continue to rise, the centuries-old strategy of dikes, dams and storm surges that have protected the Netherlands so far is no longer future-proof, says Cox. “Thinking that higher and wider dikes alone will save us is such a dangerous mentality. You can’t take on an ocean and win.”

Among other things, Cox explains: hard engineering structures (made out of concrete) don’t absorb all river and tidal energy, but can also deflect it or suffer from the excess energy, causing damage to structures and sometimes shifting the problem to adjacent areas and neighbourhoods along the coast. Over time, the land behind the dikes starts to sink, as they interrupt the transport of sediment from the coast. And that’s worrying, according to Cox, because sediment – and its availability - will determine the region’s ability to respond to sea level rise.

If there's too little sediment, even Internet access could be at risk.

Sediment matters

“It may seem intuitive, but we should remind ourselves that sea level rise is always relative to the height of the land. Thus the sediment that is laid down from rivers and seas is crucial to maintain land elevation and compensate for sea level rise and land subsidence”, explains Cox. “That’s why it’s such a problem that sediment is dramatically declining in deltas worldwide from human intervention”, says Cox, who’s calculated the Dutch delta loses 2 billion kilos of sediment every year from excess dredging for navigation in the river mouth; an amount that, according to her research, will increase to twelve billion kilos by 2085. This sediment is also in many cases being eroded from rivers banks and beds further upstream, which are degrading very quickly.

“If there’s too little sediment, intertidal areas, which are very important in terms of flood storage, are lost. And in many river channels, flood infrastructures and pipes, tunnels and cables underwater can become exposed and get damaged. Even services we take for granted like Internet access could be at risk”, says Cox.

The good news? There are solutions that stimulate natural processes to help land rebuild above sea level. Colleagues from the Water, Climate and Future Deltas hub have created the first global dataset documenting the efficiency and sustainability of 21 strategies that exist in various deltas to facilitate sediment build-up. Cox: “We found that 84% of these sediment-enhancing strategies are capable of offsetting even the most extreme of sea-level rise scenarios. However, they take time to set-up before they become effective, so the time to act is now.”

Painting of a seascape

Double dikes with transitional polders

One promising example is double dikes, which use the natural sedimentation processes enabled by salt marshes to raise land between two dikes, while offering extra protection in the case of flooding. The concept emerged as Utrecht scientists looked at historic dike breaches during the Christmas flood of 1717 and the North Sea storm of 1953 in the Netherlands. “We showed that salt marshes had a double function in flood safety: dikes located behind salt marshes had fewer breaches, and when they did breach, salt marshes limited the maximum depth of such breach. Calculations showed that this translates in more time to evacuate and fewer deaths if a dike is breached”, explains Tjeerd Bouma, professor of bio-geomorphological ecology of estuaries, deltas and coasts at Utrecht University, and one of the authors of the study.

On that basis, Bouma and colleagues devised the concept of double dikes with in between transitional polders that can grow with the rising sea level. “A temporary opening (tidal inlet) is made in the most seaward dike to allow seawater to flood the polder during high tide. This enables large quantities of sediment from the sea to deposit in the polder so that the land gradually silts up”, explains Bouma. “The new land created in the polder can be used for saltwater aquaculture/agriculture, nature and/or recreation”, says Bouma. “And when the ground has risen enough, the inlet can be closed, and the transitional polder can be put back into agricultural use. As the land will be much higher, food production will suffer less from saltwater infiltration. And the landscape as a whole will become safer!”

Transitional polders are a no-regret investment for coastal protection.

“Moving towards transitional polders is a conceptual change, in that we move away from a single barrier towards a flood-safety landscape. If you account for all costs and benefits, double-dikes with transitional polders offer a more affordable alternative to the conventional dike reinforcement. And this advantage will increase over time”, says Bouma, after comparing the costs and benefits of implementing double dikes in the Southwest Delta with the traditional ‘raising the dikes’ approach. “If the outer, most seaward dike was to breach during storms, the high accreted area in the transitional polder in front of the lower inner dike, would reduce the risk of flooding”.

Bouma and colleagues are now focusing on optimising the design of this concept, to enable the rising of the land to proceed as fast as possible. But from a flood safety perspective, Bouma is sure: “Transitional polders are a no-regret investment for coastal protection, and the sooner we start, the bigger the long-term gains.”

Dunes for the future

Another strategy consists of restoring coastal sand dunes. For decades across Northwest Europe, most dunes have been artificially reinforced to act as a first line of defence against rising seas and heavy storms. But that is unintentionally reducing our safety in the future, says Utrecht University Professor of Wave-Dominated Coastal Morphodynamics, Gerben Ruessink.

“We’ve planted marram grass to help stabilise the dunes, allowing them to grow upwards. By doing so, we’ve created high and narrow dunes that work as sand dikes: they hold back the sea and capture beach sand blown by the wind”, explains Ruessink. “One downside of that is that the area behind the foredunes can’t accumulate sand, which is crucial to sustain their biodiversity and ecological functions, but also to outpace sea level rise.”

For the past ten years, Ruessink and his team have studied how dunes evolve without marram grass. They’ve assessed openings in the dunes comparable to natural blowouts created by dune management authorities to let the sand blown from the beach gust through them onto the landward dunes. “These openings raise the area inland, which is out of reach from erosion by present-day storms to become a buffer for future storms and rising seas.” By allowing the sand to build up, the landscape also becomes more dynamic and diverse, and thus adaptive to climate change.

Ruessink and colleagues are investigating whether restoring dunes this way will be enough to cope with rising seas. That depends, in part, on how much sand is transported by the wind, how fast, and in which direction. “We have created a numerical model that can predict dune recovery. That is important for resilience. We’re also applying our research directly to a large-scale restoration project in Bloemendaal to investigate how these gaps work, when they work and how dune management authorities can implement them.” Results and lessons from this research can be used for high wave-energy coasts with high dunes, such as those in France, the United Kingdom, Germany or New Zealand.

An experiment like this isn’t suitable for all coastal dune regions, Ruessink admits. “Along beaches where dunes are only a few metres high, such as in the Gulf of Mexico, the last thing you probably want to do is to create trenches in them”, he says, adding that protecting some deltas may still require a combination of nature-based solutions and hard infrastructure.

Low-cost solutions

While conventional dikes may still be necessary for coastal protection in some places, they are ineffective or simply too expensive for others. That’s the case for soft, muddy coasts, like that along the north of Central Java in Indonesia, where Annisa Triyanti, a Postdoctoral researcher at Utrecht University, has extensively studied flood risk reduction strategies. “In some areas, communities experience inundation almost every day as a result of heavy coastal erosion. Dikes would only aggravate the problem by interrupting sediment flows”, explains Triyanti. “The government has also prioritised investments to protect the country’s big economic hubs like Jakarta or Semarang. Rural areas are dependent on nature to protect themselves.”

In the rural area of Demak, communities have been working with researchers to build permeable dams that, unlike closed dikes, enable sediment to build and mangroves to grow. “These permeable structures made of fairly cheap and local materials like bamboo or twigs are placed along the shore. As they attenuate erosive waves, trap sediments and increase bed level, they create the optimal conditions for mangroves to settle and grow, becoming a firm, first barrier against coastal inundation again”, explains Triyanti. “As importantly, these living structures enable other species to come in, providing opportunities for farmers to develop their livelihoods around aquaculture systems that can co-exist with the restored mangroves.”

These nature-based solutions buy us time to adapt and keep people safe.

Adaptive and low-cost: this is the type of innovation needed to respond to the urgent needs of communities who are already living on borrowed time in the island of Java, Kiribati or Bangladesh, says Triyanti. “These ecosystem-based approaches are still small-scale, so we can't rely on one type of measure only”, notes Triyanti. “But if we start building one upon another, they buy us time to adapt and keep people safe for the next 5, 20 or 100 years.”

Here too, the whole is greater than the sum of its parts. “Integration between coastal adaptation measures and coordination among all stakeholders is most important to realise integrated coastal management”, stresses Triyanti, “so that whatever is applied in one part of the coastal area will not have a negative impact on another. Instead, they'll complement each other.”

Utrecht researchers are now developing climate adaptation pathways, which could help scientists and policymakers assess the applicability of any of these measures in their respective deltas and where it would be best to place them. But whether any of these solutions will be able to counter rising seas in the future depends very much on how high and how fast the oceans will rise. That brings us back to sea level rise projections, a field in which Utrecht University researchers are working hard to reduce uncertainty.

Decoding Antarctica

One of them is polar meteorologist Michiel van den Broeke, who studies a major source of sea-level rise uncertainty: the melting of the Antarctic and Greenland ice sheets. Van den Broeke has shown consistently over the past few years that the Antarctic ice sheet is melting faster than ever before. “As much as three times faster over the last two decades”, condenses Van den Broeke.

“If the Antarctic ice sheet melts faster, you can also expect the sea level to rise faster, reducing our ability to adapt.” And that’s only the tip of the iceberg: “If it were to melt entirely, the Antarctica ice sheet holds enough water to raise global mean sea level rise by more than 58 metres.”

While the potential is huge and scary, the exact impact of the Antarctic ice sheet on sea level rise remains difficult to predict, as many of the processes driving the melting are hard to measure, and even harder to model, says Van den Broeke. “The melting of the Antarctic ice sheet is taking place from below, due to warming oceans. To study it, you’d need to go underneath these hundreds of metres thick floating glaciers. We only have limited tools for that. That’s also one of the reasons why it’s more difficult to predict what’s going to happen with the Antarctic ice sheet.”

We hope these projections will enable policy makers to take better action.

One approach Van den Broeke and colleagues in Utrecht are taking is to look at the past to understand the variability of the system and better predict the future. Advanced satellite technology also gives ice sheet researchers in Utrecht real-time information that wasn’t available 30 years ago when van den Broeke first set foot on the Greenland ice sheet as a Master's student. “With satellite observations and weather stations, we can keep an eye on every individual glacier these days and see how it behaves on a daily basis”, he explains. “We can use all that information to improve our climate models, which allows us to predict ever-more precisely how these ice sheets will behave in the future.”

For now, Van den Broeke at IMAU has been setting up the interdisciplinary Earth System Modelling Group, in which glaciologists, oceanographers and meteorologists will work together to simulate all those interactions between the land, the ocean and the atmosphere that can give them hints. “We hope these projections will enable policy makers to take better action.”

For all the uncertainty, however, one thing is certain: “We absolutely need to reduce emissions to limit global warming to a maximum of 1.5°C”, says Van den Broeke. “If we continue with business as usual, and the Earth warms 3 degrees above pre-industrial levels, we’ll be easily talking about more than a metre rise by 2100 and a further several metres by 2200.” (For comparison, the Dutch delta works are prepared to withstand a 50 cm to 1 metre rise).

Painting of a seascape

Away from risk

While reducing emissions in line with the Paris Agreement goals can avert the worst consequences of sea level rise, about one billion people living in low-lying, coastal cities will still be at risk from sea level rise by 2050, according to the last IPCC report. “We need to think more critically about the location of new developments, and take sea level rise more into account in our decisions on where and how to build”, says Utrecht University Assistant Professor of Regional Water and Climate Governance, Dries Hegger. At some point in the future, we might even need to think about coordinated relocation of assets and people: managed retreat. “Moving people from their homes is a controversial and politically sensitive issue. But it would be better to have an open and transparent discussion about which risks we want to face as a society before it is too late”, Hegger says.

Several societal actors, including scientists but also the Dutch Delta Commissioner, are raising attention for more water-conscious developments. “It starts with halting or adjusting construction on vulnerable, low-lying coastal zones or floodplains of the river. This includes a recent plan to build 8,000 new houses in the Zuidplaspolder, between Gouda and Rotterdam, which sits at 6 or 7 metres below sea level”, Hegger says.

If you're going to build in subsiding parts, one option could be to let houses float.

“If you’re going to build in subsiding parts, one option could be to let houses float”, suggests Hegger, who’s part of a think tank that is interrogating the idea of floating urbanisation. “The technology to live on the water is already available, and developments in floating urbanisation are getting scale: from a floating farm in Rotterdam to entire floating neighbourhoods in Delft or Amsterdam.”

The biggest challenge, says Hegger, is making these climate-proof floating houses accessible to every resident, not just the well-off. “That is often what you see in all kinds of efforts to make cities green and more resilient: you don’t reach all sectors of the population.” As several studies have shown, some of these green infrastructure projects – green roofs, rain harvesting gardens or greenways, which, among others, mitigate flooding and improve storm water management in cities, can harm socially vulnerable groups. “Many projects to ‘green’ the neighbourhood end up evicting people who can no longer afford them.” 

Protecting the oceans

Adapting homes to rising seas and pluvial floods is a luxury receding small islands can’t afford. “Small islands such as the Maldives, Marshall Islands or Tuvalu are already losing large areas of territory. If we apply strictly the law that exists, a loss of territory means their maritime zones, including their exclusive rights to fishing and natural resources, shrink”, says Seline Trevisanut, chair of International Law and Sustainability at Utrecht University. As islands lose the means for their survival or even disappear underwater, the question is: where will their inhabitants go?

“At the moment, we don’t have a clear legal framework that offers protection to displaced persons because of environmental disasters or climate change consequences”, says Trevisanut. “Sea level rise can make the land disappear, or it can make it less productive. If people are leaving because they can’t work the land anymore, will we consider them displaced persons and will we grant them protection, or not?”, asks Trevisanut pointing to the already ailing cross-border migratory regime that neglects those fleeing a ruined livelihood from famines to hurricanes to wars.

“We need careful planning to ensure millions of people have somewhere safe to go. Because we’re not talking about a few thousand people from small island states. We can expect massive displacements across the US, India, or Europe”, says Trevisanut.

But there’s something we can do to turn the tide: “If nature-based solutions offer the key to adapting to rising sea levels, our priority should be to protect nature”, says Trevisanut, who leads the Sustainable Ocean Research project. “Think about it: oceans are the real lungs of our planet. By polluting them and increasing their acidity, we’re decreasing the capacity of the oceans to be a natural sink of CO2”, she explains. “That’s the one function of the ocean that we should safeguard in the fight against rising sea levels.” Trevisanut notes that tighter regulation of land-based sources of marine and coastal pollution (plastic, pesticides and other by-products of farming, or discharge from industrial processes) can lead the way. “By protecting the oceans, we’re protecting ourselves.”

The pace of sea level rise is forcing societies to change course - away from an overreliance on dikes, walls and barriers towards solutions that work with nature to adapt dynamically to the rising water. The main challenge with these so-called nature-based solutions is further research into their effectiveness is necessary so that they can be accepted as robust alternatives in coastal protection. That’s why researchers at Utrecht University are working hard to assess and validate solutions that can offer protection in the short-term, while building resilience in the long-term. Together, they can help us stem the tide – not only in the Netherlands, but in deltas worldwide.

  • Portrait of Marta Jimenez

What can you do?

Meet the experts

  • Jana Cox MSc

    Jana Cox

    PhD candidate of Physical Geography at Utrecht University
  • Roderik van de Wal

    Roderik van de Wal

    Professor of Sea level change and coastal impacts at Utrecht University
  • Tjeerd Bouma

    Tjeerd Bouma

    Professor of Bio-geomorphological Ecology of Estuaries, Deltas and Coasts at Utrecht University
  • Gerben Ruessink

    Professor of Wave-Dominated Coastal Morphodynamics at Utrecht University
  • Annisa Triyanti

    Annisa Triyanti

    Postdoctoral Researcher Water and Climate Governance in Deltas, Copernicus Institute of Sustainable Development at Utrecht University
  • Michiel van den Broeke

    Professor of Polar Meteorology at Utrecht University
  • Dries Hegger

    Assistant Professor of Regional Water and Climate Governance at Utrecht University
  • Seline Trevisanut

    Seline Trevisanut

    Professor of International Law and Sustainability at Utrecht University

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