Living with (flood)water

By Björn Carstens
Storms, flooding, heavy rainfalls: Climate change increasingly swamps coastal regions and metropolitan areas. Intercontinental teamwork is necessary to protect several hundred million people against rising water levels. Together with flood protection expert Dr. Helge Bormann “tomorrow” looks at possible solutions – from classic dikes to artificial reefs to sponge cities.
© shayes17/iStock, Rendering: Ivo Christov/Speedpool
Surfing Citys

Floating homes using innovative green technologies such as water recycling and solar power-stations can already be found all over the world – and their numbers are increasing. In the Maldives, which at just about one meter (3 feet) above sea level is the lowest country in the world and under acute threat of sinking, the world’s largest floating city is planned to be built. In “Maldives Floating City,” starting in 2027, up to 20,000 people are supposed to be living in the middle of a 200-hectare (500-acre) lagoon. Japanese architects are thinking in even larger dimensions. They’re planning their “Dogen City” project for 40,000 people, albeit 30,000 of them being tourists. The floating city is supposed to be built within a four-kilometer (2.5-mile) wave protection ring accommodating gardens, apartments, shops, and supply lines on several levels. On the inner part, the model shows several buildings for schools, sports facilities, and key industries such as food production. The developers are planning the servers for “Dogen City’s” data center to be placed in torpedo-shaped underwater containers. That’s supposed to facilitate cooling and save electricity. However, at this juncture, “Dogen City” is still a utopia because there aren’t any concrete plans yet to implement the project.

Living with (flood)water
This is what the planned swimming city "Dogen City" will look like

That’s what the expert says: “Cities floating on the ocean require a massive technical and financial investment. Unfortunately, solutions like these won’t be able to save everyone affected. Worldwide, some 300 million people are living in areas threatened by flooding. In the examples mentioned above, a few ten thousand people could live ­permanently. For me, that raises the question: for how many people would an investment in the construction of floating cities be ­realistic? However, as stand-alone solutions, even unconventional concepts such as ‘Dogen City’ do make sense and can presumably be realized.

The expert
Living with (flood)water© Privat

Teamwork in flooding protection plays an important role for Dr. Helge Bormann. The Head of Research Management at Jade University of Applied Sciences in Oldenburg, Germany, emphasizes international networking. “Together with many actors in Germany we keep looking at projects in the Netherlands and in Denmark,” he says. Bormann’s focal research areas include projects for climate adjustment and flood risk management.

Mega dams: mega solutions or mega problems?

People living on the coast have always been threatened by flooding. In North-Western Europe, people were building ring dikes as early as in the 12th century to protect settlements against flooding. At the beginning of the 16th century, coastal residents would ram wooden stakes vertically into the ground and stabilize them by an earth wall behind them. However, because these dikes called “Stackdeich” with a height of around two meters (6 feet) were often destroyed by breakers people started erecting elongated dikes with stretched profiles on which the waves during storm tides could peter out. The earth banks kept getting higher. In the middle of the 18th century, they had a height of about five meters (16 feet) and today’s ocean levees have reached heights of around nine meters (29 feet). Due to embankment slopes of at least 1:6, the earth walls are about 100 meters (330 feet) wide at the foot.

However, green dikes don’t always provide sufficient protection. The Netherlands, where more than half of the country’s 17 million inhabitants live below sea level, are a case in point: Ever since the national storm surge trauma in 1953 when nearly 90 dikes broke and more than 1,800 people lost their lives, the Dutch have massively invested in flooding protection. Kilometer-long storm surge dams were built along the coast. The Oosterschelde dam, for instance, was built through a nine-kilometer (5.6-mile) inlet. In case of an emergency, it’s closed. However, even this mammoth structure, like many other costly dams, could soon reach its limits due to climatic warming. Depending on how much CO₂ continues to be emitted, the sea level will rise between 30 centimeters and one meter (1 to 3 feet) by 2100, according to the Intergovernmental Panel on ­Climate Change (IPCC). Gloomier forecasts don’t even exclude a rise by two to five meters (6 to 16 feet) in roughly the next 130 years.

That’s why Dutch hydraulic engineers are untiringly working on projects to protect their country. One of their proposals calls for a three-kilometer-wide (1.9-mile-wide) sand wall in front of the coast, where the sea is about 20 meters (65 feet) deep. The “Haak-Seedeich” would be as much as 20 meters (65 feet) above today’s sea level. Lakes would then be created between the current and the new coast. Thanks to locks, ships should continue to be able to access the Rhine River from the sea. Fish ladders and ecological compensation sites are planned as well. Experts are currently clarifying the feasibility of this centennial project.

Of even more mammoth proportions is a plan that the Royal Netherlands Institute for Sea Research proposed a few years ago: a roughly 480-kilometer (300-mile) reservoir dam between Scotland and Norway combined with a 160-kilometer (­100-mile) barrier between France and England. That action could all at once save 25 million Europeans from the rising sea levels. Costs are supposed to amount to something between 250 and 550 billion euros.

That’s what the expert says: “There’s no such thing as 100-percent flooding protection using steel and concrete. At some point, the structures are no longer adaptable; they can’t be raised to just any desired level. The question, instead, must be: how far must we humans adapt ourselves to live with the water? The objective must be to manage the residual risk of flooding in the best possible way (see page 74). As far as that goes, I’m hesitant when it comes to mammoth projects. Wouldn’t it make more sense to invest those huge sums of money into predictive coastal protection to mitigate the rise of the sea level instead of in costly structures that in a few decades will be too low again? Moreover, huge barriers only create new problems that are at least as big as flooding. Take Oosterschelde, for example, in the estuary mouth area, ecologically valuable tidal flats are being lost.”


concrete pillars weighing up to 18,000 metric tons (20,000 short tons) form the backbone of the Oosterschelde storm surge dam. It’s the most impressive structure of the so-called Delta Works in the Netherlands that are also called the “Eighth Wonder of the World.” The movable leaf gates with widths of up to 42 meters (138 feet) between the pillars can be closed to protect against imminent spring tides. The rest of the time the dam is open.

Living with (flood)water© RDPhotos/iStock
Venice’s life belt

Without the “Mose” flooding protection system that was put into operation in 2020 Venice, one of the world’s most beautiful cities, would presumably soon be doomed. “Mose” consists of 78 lowerable barrier modules erected at the three access areas to the lagoon. Each module weighs around 250 metric tons (276 short tons), is 20 meters (66 feet) wide, 30 meters (98 feet) high, up to five (16 feet) meters deep and kept mobile with bearings from Schaeffler. The barriers are normally filled with water and lying on the bottom of the sea. When activated, the water is pushed out by means of compressed air. One box after the other rises so that all of them together form a barrier.

Living with (flood)water
The flood protection system in Venice consists of 78 lowerable barrier modules
Making room for water

Effective coastal protection is combined with effective water management behind the dike. To the extent that water enters the interior it must be possible to get it out again. The principle behind that is to give nature more room and let it reconquer its natural flood plains, so-called polders. The Dutch are pioneers in that regard as well. Since 2012, they’ve been pursuing new pathways with a project called “Room for the River.” While rivers used to be straightened by dikes more than 30 individual actions on the Maas, Rhine, and Waal rivers have now reduced the risk of flooding.

Those actions include:

  • Widening and deepening of riverbeds so that they can absorb more water.
  • Providing relief to the main river by means of separate watercourses.
  • Rerouting dikes and creating flooding areas (polders) in front of them to give floods room and to protect the interior of the country against masses of water. Subsequently, the flooding polders can evolve into wetland habitats for an abundance of species and serving as brooding areas for birds.

That’s what the expert says: “It’s imperative that we give the rivers in the interior more room to get a better handle on the convergence of an event of interior flooding with a coastal storm surge. That’s another area in which the Dutch are pioneers having emphasized solutions in harmony with nature by identifying polders. The Netherlands have an additional problem. As a downstream country, they get all the water from Switzerland and Germany via the Rhine River. Everything that we don’t hold back flows toward them and they must deal with it. Another interesting aspect in that regard is that the Dutch establish different protection levels for their polders, depending on population density, value of the infrastructure, or intensity of agricultural use. As a result, financial resources can be concentrated more effectively and systematically by investing most of the money in areas enjoying the highest protection level. In Germany, for instance, we don’t discuss the question of different protection levels at all. It certainly makes sense to question whether the potential of technologies should be used systematically or be randomly distributed.”

600 mn.

euros. That’s roughly how much the creation of several polders along the Schelde estuary on the Dutch and Belgian side is supposed to cost. The project is scheduled for completion in 2030. By comparison, the flooding damage that would occur if the polders were not to be built would be much higher, i.e., up to one billion euros per year by 2100, according to estimates.

Living with (flood)water© ollo/iStock
Sand flushed ashore

Gaining protective land due to hydraulic sand fills. The sand is typically pumped ashore from the deeper seabed via pipelines or delivered by ships. In the estuarial area of the Mississippi, for instance, sand has been hauled into the delta via a more than 20-kilometer (12-mile) pipeline since 2013. That creates ecologically valuable sandbars on which large salt marshes can evolve across ­several square kilometers in the coming decades that will act as natural coastal protection. Specifically, cities south of New Orleans are supposed to be protected from flooding in that way. ­Another ­example is the western part of the popular ­German tourist island of Sylt, where since 1972 some 60 million cubic meters (2,1 billion cubic feet) of sand have been pumped ashore to preserve the beach.

Living with (flood)water© maodesign/iStock

That’s what the expert says: “Hydraulic sandfills created by means of pipelines have proven effective. But natural sedimentations accelerated by spur dikes and rows of piles, so-called breakwaters, can be used to cause buffer areas to grow as well. In addition, such areas can be very valuable ecologically in terms of salt marshes, for instance.”

21 mn. m3

(740 million cubic feet) of sand at the Dutch coast between Den Haag/Scheveningen and Hoek van Holland have been hydraulically filled to create a hook-shaped peninsula to solve the local erosion problem. Such an amount of sand would suffice to cover about 60 soccer fields 50 meters (164 feet) high. The artificial island acts as a natural sand depot that erodes due to waves, tidal currents, and the wind over several decades while permanently supplying fresh sediment to the 17-kilometer (10-mile) coastal section to compensate for the erosion.

Nature vs. nature

Why not draw on the forces of nature to protect coastal areas from the forces of nature? Experts refer to that as ecosystem-based coastal protection. In Vietnam, for instance, mangrove forests were restored to slow down erosion. In addition to providing a natural flood buffer, the forests serve as a nursery ground for many fish species. Other areas use oyster beds and coral reefs as natural wave breakers. Coral reefs can disperse 97 percent of the wave energy before the waves reach the coast. A relatively new measure in the portfolio of ecosystem-based coastal protection is the planting of seagrass meadows on the ocean floor. These maritime green-fields have proven to be effective and extremely resilient current breakers.

Dutch startup “Reefy” chose another anti-erosion method, having sunk 17 mammoth concrete Lego bricks, each weighing six metric tons (6.6 short tons) and hollow inside, to the bottom of the Maas River near Rotterdam. The artificial reef is supposed to be conquered by plants and animals to restore the diversity of the species in the sea. At the same time, it serves as a wave barrier counteracting the turbulences caused by tens of thousands of ships arriving at Europe’s biggest port every year.

Living with (flood)water
Seagrass meadows on the ocean floor© SnapsbyT/iStock

That’s what the expert says: “All these actions make sense but can never be the only solution. Ecosystem-based coastal protection leads to a reduction of erosion in the land in front of the dikes, compacting of the substratum, and prevention of sediment erosion. However, solutions like those aren’t suitable for every coast. Planting of mangroves is excluded on a coastal strip with dense construction or in front of ports. In cases like these, artificial reefs can be built or seagrass meadows created in front of dikes to ­protect the shore. However, in the case of great water depths, those actions cannot be taken either, so that ­rigid and classic coastal protection on the shore remains the only solution.”

> 250,000

metric tons (275,000 short tons) of seagrass per year are washed ashore on the Baltic Sea’s beaches. The maritime foliage typically rots or is collected and composted in landfills. That’s a shame because dried seagrass is a natural construction material with very good properties: the material regulates moisture, doesn’t mold, provides good insulation, and is non-flammable. On the Danish island of Læsø, the roofs of houses have been covered with dried seagrass instead of thatch for 400 years (pictured at left). As a result, seagrass can not only protect coasts but also nature in general.

Living with (flood)water© privat
Streets with suction power

Generally, the sponge city concept provides for less concrete and more vegetation in urban areas to make use of rainwater instead of fighting it. The idea is to create areas that can absorb large amounts of water in order to release it again in dry and hot conditions through condensation. The objective is to catch rainwater where it falls and to feed it into the natural hydrological cycle there as well. Water-permeable road and street surfaces, façade greening, and rooftop rainwater collection tanks are just some of the actions to be taken on the journey toward sponge cities. In addition, they include various types of seepage areas such as roadside depressions combined with underground rainwater tanks (trenching) or extensive green areas like parks or meadows.

Living with (flood)water© TH Treibhaus

That’s what the expert says: “The sponge city principle or the principle of water-sensitive cities is the right approach to managing heavy rainfalls or river flooding. Water-sensitive urban development promotes natural physiological processes. It’s more about seepage, more water storage, more blue-green structures, in ­other words more vegetation, more bodies of water in inner cities to reduce heat islands. Plus, it’s about multifunctionally usable areas to retain water in the event of heavy rain. I’m thinking about parking areas or generally about areas with low damage potential. In case of extreme weather, we should consider flooding as something that can and may happen without causing major damage.”


to 100 percent of precipitation can be held back by covering rooftops and walls of buildings with vegetation (“living walls”). In addition, covering buildings with vegetation provides effective insulation against heat, cold, and wind, and has a very positive effect on the urban microclimate as well.