Sea levels are rising, and the rate of rise is accelerating. All over the world, many of today鈥檚 dikes, sea walls and flood barriers won鈥檛 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鈥檙e 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
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 that stopped her in her tracks. 鈥淎ll 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鈥, 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.
鈥淚 scrapped my whole presentation, pulled up the headline and said: everything we鈥檝e done for flood protection in the Netherlands, including my own research, 鈥 building and reinforcing the dikes, constructing the Delta Works 鈥 is based on sea level projections that are now way too low to keep people safe in the future鈥, recounts Cox. And as sea levels rise, she cautions, it鈥檚 not only the looming excess water we should worry about, but the lack of it: 鈥淚f sea levels rise by one metre (), 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.鈥
Why are sea levels rising?
鈥淭he 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鈥, 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. 鈥淭hinking that higher and wider dikes alone will save us is such a dangerous mentality. You can鈥檛 take on an ocean and win.鈥
Among other things, Cox explains: hard engineering structures (made out of concrete) don鈥檛 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鈥檚 worrying, according to Cox, because sediment 鈥 and its availability - will determine the region鈥檚 ability to respond to sea level rise.
If there's too little sediment, even Internet access could be at risk.
Sediment matters
鈥淚t 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鈥, explains Cox. 鈥淭hat鈥檚 why it鈥檚 such a problem that sediment is dramatically declining in deltas worldwide from human intervention鈥, says Cox, who鈥檚 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.
鈥淚f there鈥檚 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鈥, 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 that exist in various deltas to facilitate sediment build-up. Cox: 鈥淲e 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.鈥
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. 鈥淲e 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鈥, explains Tjeerd Bouma, professor of bio-geomorphological ecology of estuaries, deltas and coasts at Utrecht University, and one of the authors of the .
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. 鈥淎 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鈥, explains Bouma. 鈥淭he new land created in the polder can be used for saltwater aquaculture/agriculture, nature and/or recreation鈥, says Bouma. 鈥淎nd 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!鈥
Transitional polders are a no-regret investment for coastal protection.
鈥淢oving 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鈥, says Bouma, after comparing in the Southwest Delta with the traditional 鈥榬aising the dikes鈥 approach. 鈥淚f 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鈥.
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: 鈥淭ransitional polders are a no-regret investment for coastal protection, and the sooner we start, the bigger the long-term gains.鈥
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.
鈥淲e鈥檝e planted marram grass to help stabilise the dunes, allowing them to grow upwards. By doing so, we鈥檝e created high and narrow dunes that work as sand dikes: they hold back the sea and capture beach sand blown by the wind鈥, explains Ruessink. 鈥淥ne downside of that is that the area behind the foredunes can鈥檛 accumulate sand, which is crucial to sustain their biodiversity and ecological functions, but also to outpace sea level rise.鈥
For the past ten years, Ruessink and his team have studied how dunes evolve without marram grass. They鈥檝e assessed comparable to natural blowouts created by dune management authorities to let the sand blown from the beach gust through them onto the landward dunes. 鈥淭hese 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.鈥 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. 鈥淲e have created a numerical model that can predict dune recovery. That is important for resilience. We鈥檙e also applying our research directly to a to investigate how these gaps work, when they work and how dune management authorities can implement them.鈥 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鈥檛 suitable for all coastal dune regions, Ruessink admits. 鈥淎long 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鈥, 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鈥檚 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. 鈥淚n 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鈥, explains Triyanti. 鈥淭he government has also prioritised investments to protect the country鈥檚 big economic hubs like Jakarta or Semarang. Rural areas are dependent on nature to protect themselves.鈥
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. 鈥淭hese 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鈥, explains Triyanti. 鈥淎s 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.鈥
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. 鈥淭hese ecosystem-based approaches are still small-scale, so we can't rely on one type of measure only鈥, notes Triyanti. 鈥淏ut 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.鈥
Here too, the whole is greater than the sum of its parts. 鈥淚ntegration between coastal adaptation measures and coordination among all stakeholders is most important to realise integrated coastal management鈥, stresses Triyanti, 鈥渟o 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.鈥
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. 鈥淎s much as three times faster over the last two decades鈥, condenses Van den Broeke.
鈥淚f the Antarctic ice sheet melts faster, you can also expect the sea level to rise faster, reducing our ability to adapt.鈥 And that鈥檚 only the tip of the iceberg: 鈥淚f it were to melt entirely, the Antarctica ice sheet holds enough water to raise global mean sea level rise by more than 58 metres.鈥
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. 鈥淭he melting of the Antarctic ice sheet is taking place from below, due to warming oceans. To study it, you鈥檇 need to go underneath these hundreds of metres thick floating glaciers. We only have limited tools for that. That鈥檚 also one of the reasons why it鈥檚 more difficult to predict what鈥檚 going to happen with the Antarctic ice sheet.鈥
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鈥檛 available 30 years ago when van den Broeke first set foot on the Greenland ice sheet as a Master's student. 鈥淲ith 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鈥, he explains. 鈥淲e 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.鈥
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. 鈥淲e hope these projections will enable policy makers to take better action.鈥
For all the uncertainty, however, one thing is certain: 鈥淲e absolutely need to reduce emissions to limit global warming to a maximum of 1.5掳C鈥, says Van den Broeke. 鈥淚f we continue with business as usual, and the Earth warms 3 degrees above pre-industrial levels, we鈥檒l be easily talking about more than a metre rise by 2100 and a further several metres by 2200.鈥 (For comparison, the Dutch delta works are prepared to withstand a 50 cm to 1 metre rise).
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 . 鈥淲e 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鈥, 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. 鈥淢oving 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鈥, Hegger says.
Several societal actors, including scientists but also the Dutch Delta Commissioner, are raising attention for more water-conscious developments. 鈥淚t 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鈥, Hegger says.
If you're going to build in subsiding parts, one option could be to let houses float.
鈥淚f you鈥檙e going to build in subsiding parts, one option could be to let houses float鈥, suggests Hegger, who鈥檚 part of a think tank that is interrogating the idea of floating urbanisation. 鈥淭he 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.鈥
The biggest challenge, says Hegger, is making these climate-proof floating houses accessible to every resident, not just the well-off. 鈥淭hat is often what you see in all kinds of efforts to make cities green and more resilient: you don鈥檛 reach all sectors of the population.鈥 As several have shown, some of these green infrastructure projects 鈥 green roofs, rain harvesting gardens or greenways, which, among others, mitigate flooding and improve storm water management in cities, can harm socially vulnerable groups. 鈥淢any projects to 鈥榞reen鈥 the neighbourhood end up evicting people who can no longer afford them.鈥
Protecting the oceans
Adapting homes to rising seas and pluvial floods is a luxury receding small islands can鈥檛 afford. 鈥淪mall 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鈥, 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?
鈥淎t the moment, we don鈥檛 have a clear legal framework that offers protection to displaced persons because of environmental disasters or climate change consequences鈥, says Trevisanut. 鈥淪ea level rise can make the land disappear, or it can make it less productive. If people are leaving because they can鈥檛 work the land anymore, will we consider them displaced persons and will we grant them protection, or not?鈥, asks Trevisanut pointing to the already ailing cross-border migratory regime that neglects those fleeing a ruined livelihood from famines to hurricanes to wars.
鈥淲e need careful planning to ensure millions of people have somewhere safe to go. Because we鈥檙e not talking about a few thousand people from small island states. We can expect massive displacements across the US, India, or Europe鈥, says Trevisanut.
But there鈥檚 something we can do to turn the tide: 鈥淚f nature-based solutions offer the key to adapting to rising sea levels, our priority should be to protect nature鈥, says Trevisanut, who leads the Sustainable Ocean Research project. 鈥淭hink about it: oceans are the real lungs of our planet. By polluting them and increasing their acidity, we鈥檙e decreasing the capacity of the oceans to be a natural sink of CO2鈥, she explains. 鈥淭hat鈥檚 the one function of the ocean that we should safeguard in the fight against rising sea levels.鈥 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. 鈥淏y protecting the oceans, we鈥檙e protecting ourselves.鈥
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鈥檚 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 鈥 not only in the Netherlands, but in deltas worldwide.
Share
What can you do?
Meet the experts
Jana Cox
PhD candidate of Physical Geography at Utrecht University
Roderik van de Wal
Professor of Sea level change and coastal impacts at Utrecht University
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
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
Professor of International Law and Sustainability at Utrecht University
