FLOATING CITIES MAY HELP SOLVE FUTURE CHALLENGES OF GLOBAL CITIES

11.02.2020 / Ulf Kjellerup and Tina Vejrum

For centuries, the myth of Atlantis has captivated the imagination of man. But where the fictional island sank to the bottom of the sea, never to be seen again, modern floating cities, or specially designed artificial islands, are beginning to emerge as real solutions to some of the biggest challenges facing cities around the world: overpopulation and climate change. In Copenhagen, two such projects will also help the city get rid of massive amounts of surplus soil from its many construction projects.

Today, some 55 per cent of the world’s population lives in urban areas and this share is expected to rise to 68 per cent by 2050. Urbanisation coupled with a growing global population could see an additional 2.5 billion people in urban areas by 2050. At the same time, urban areas, most often located in coastal areas, are facing massive threats resulting from climate change, not least rising sea levels and flooding.

New sustainable solutions are needed to solve these challenges. One solution, once considered pure utopia or too far-fetched, is now frequently touted as a real solution to the problems of overpopulation and lack of housing coupled with the threat of rising sea levels: floating cities.

Artificial islands established through land reclamation on the outskirts of cities are by no means new inventions, but in the future these islands may be designed to serve as climate adaptation as well as habitation.

Floating structures anchored to the seabed

Floating cities rest on large floating structures anchored to the seabed or the shore. The structures can be pontoon structures, which are suitable for waters close to shore, or a semi-submersible structure known from the oil industry for deeper waters.

The concept of floating cities is still in the development phase, though the technology being applied is not new and it may not be many years before we will see the first prototypes being tested.

The technology behind floating cities is already known from floating bridges such as the Bjørnafjord Bridge in Norway, which will become the world’s largest floating bridge. A number of floating bridges are already constructed around the world, including Nordhordland Bridge and Bergsøysund Bridge in Norway, and the Okanagan Lake Bridge in Canada.

COWI was one of the working groups looking at possible bridge solutions to cross the five kilometres wide Bjørnafjord river. A number of different floating bridge concepts were looked at: multi-span suspension bridges based on tension leg platform (TLP) technology, a side-anchored straight bridge where mooring lines are taken down to the sea bottom at depths of up to 550 metres, and an end-anchored curved bridge.

The COWI working group carried out numerous analyses, drawing on top-level expertise from several in-house departments: marine operations, hydrodynamics, wind dynamics, structural engineering, risk analysis and load response to impact from ships.

However, before floating cities can be a reality, numerous tests in wave tanks will be required, for example tests to prove the resilience of the structures, their safety and reliability; risk analysis; and analysis of anchoring methods and hydrodynamics, that is the dynamic effects on structures reacting to waves.

Floating cities – are they suitable for people?

Comfort is an area that will have to be investigated thoroughly. How will it affect people to live on a floating structure? Will movements and vibrations affect people, for example when sleeping? It is quite possible that traditional engineering work will need to be combined with physiological and psychological studies to determine how people react to living on floating cities.

Even if the concept of floating cities is new, many of the technologies being applied are far from new. It is a new way of using existing technology and a new way to think. Expertise and knowhow will be gathered from many areas, such as floating bridges, marine structures and offshore installations like oil platforms. But whereas an oil platform is designed to last roughly 25 years, floating cities will have a life span of at least 100 years.

Designed to survive mega storms

The first floating cities are likely to be in a smaller scale. The Netherlands, for example, with its low-lying coastal areas and high population density, is an ideal place for floating cities, as are some Asian mega cities such as Singapore and Hong Kong.

Among the more interesting projects currently receiving a lot of media attention is Oceanix City, designed by Danish architect Bjarke Ingels and part of the UN-Habitat’s New Urban Agenda. Oceanix City aims to be the world’s first resilient and sustainable floating community, but is also flood-proof and designed to survive mega storms. In the event of a long-term major shift in weather patterns, the floating city can be unmoored and towed to another location.

Artificial islands built on excess soil

Land reclamation from surrounding waters to extend a city’s area is not a new idea and has been used for years, for example in Singapore. In Denmark, land reclamation is known from Copenhagen suburbs Amager and Nordhavn. The latter has been extended by 50 per cent using excess soil from Copenhagen’s many construction projects.

Both floating cities and artificial islands require huge investments, but in Copenhagen, the Nordhavn extension as well as two new projects on the drawing board have been conceived with the prime purpose of getting rid of excess soil and reusing the city’s natural resources. COWI is involved in both projects, namely Lynetteholmen in the Port of Copenhagen, and the Avedøre Islets in the southern part of Greater Copenhagen.

Every year, at least three million tonnes of soil are excavated from the Copenhagen underground. Lynetteholmen and the Avedøre Islets will require a total of 90 million tonnes of soil over a period that spans two to three decades and will thus help the city get rid of its excess soil while providing new land for homes and businesses.

Urbanisation is expected to gain speed in coming decades, and it is vital that politicians and developers thoroughly address how urban development affects the environment and identify the optimal use of resources such as excess soil from building projects. There are undoubtedly vast areas around the country where homes and offices can be built, creating whole new urban areas, but it is more sustainable to concentrate people in existing urban areas that already have well-functioning infrastructure in place. This will also help solve the puzzle of how to reuse excess soil rather than having to dispose of it, which would create a whole new challenge.

When reclaiming land, a perimeter is established at sea and a frame of some sort, sheet piles for example, are driven into the seabed. The area is then emptied of water and the emptied volume is filled with soil extending above the water line, to a level high enough to resist the rising sea levels of the future.

Lynetteholmen will be located at water depths of up to 13 metres whereas the nine islets at Avedøre will at water depths of between 3.2-5.5 metres, making it an altogether easier project to handle. Instead of sheet piles being driven into the seabed, large sea-stones will be placed into the waters at Avedøre to create a frame, which can then be filled with soil.

Major challenge: soil transport

Lynetteholmen will cover 282 hectares and house 35,000 people when it is completed in 2070. It will be located at the northern part of the Port of Copenhagen, and every day for around 40 years, hundreds of lorries carrying soil will have to access the site, passing through central Copenhagen, potentially causing disturbances to residents and workplaces.

With a total area of three million square metres filled with soil, the Avedøre Islets will see the creation of a green industrial area for clean-tech and high-tech companies. Thanks to its location in the southern part of Greater Copenhagen and close to the motorway, soil transport will be much easier and pose little hazard to the citizens of Copenhagen.

An alternative being considered for Lynetteholmen is transportation by ship, but this is very expensive and will have a more negative impact on the environment. Soil would still have to be transported by lorry to Nordhavn, then transhipped by barge for 350 metres to the Lynetteholmen site.

Alternatively, soil can be obtained in Southern Sweden, for example in Gothenburg, which is currently home to the largest urban development project in the Nordic region.

This bottleneck is one of the primary challenges requiring a solution, which will hopefully be resolved within 18 months. It will be part of the environmental impact analysis (EIA), which has to be carried out before politicians can make a decision on the Lynetteholmen project.

Islands as bulwark during flooding

Lynetteholmen and the Avedøre Islets will be located at opposite ends of the Port of Copenhagen, and they are intended to contribute to the future flood protection of Copenhagen. They will act as initial bulwark during situations with high sea levels, and the plan is to design an intelligent solution: A flood control device that can come into use to prevent flooding. The device can be a curtain or gate on the seabed that inflates during flooding to prevent water from entering the inner harbour.

Lynetteholmen will act as flood protection during southbound incidents, that is when water is pushed from the North Sea into the straits and into the Baltic Sea. Similarly, the Avedøre Islets will act as flood protection during northbound incidents, that is when the wind turns, and water is pushed back to the straits and back to the North Sea.

As outlined above, floating cities, or artificial islands, may become a more frequent answer to some of the major challenges facing coastal cities.

The technical solutions are already available. What needs to be further addressed is potential challenges that may have an adverse impact on the environment and how to eliminate these. Also, how will it affect people to live on a floating structure? This should be further investigated through physiological and psychological studies.

We expect an exciting journey ahead.