Floating cities are emerging as a possible solution for residents of many of the world's sinking megacities, if they can be built and operated sustainably.
Many of the world's largest coastal cities are sinking at alarming rates due to factors such as groundwater extraction and the weight of built infrastructure.
A potential solution is moving their residents to floating cities, if they can be built without creating a damaging ecological footprint in the ocean.
The issue of land subsidence or 'sinking' is not a new one. Vertical subsidence of up to 9 metres has been observed in coastal areas during the 20th century.
The issue has been gaining increasing global attention because the areas most prone to sinking are megacities that house almost 20 per cent of the global urban population and these areas are also the most heavily impacted by sea level rise.
Given that almost half of the 48 largest coastal cities are sinking at a rate faster than 10 millimetres per year there is an urgent need for viable solutions.
Current approaches have tended to focus on trying to hold back the sea and building up coastal defences through hard engineering or nature-based solutions, but when subsidence is combined with sea level rise even the lowest projections will prove challenging for engineering works.
So-called sponge cities that can absorb and reuse water have been touted as one potential solution, but they address intermittent flooding rather than creeping sea level rise and eventual permanent inundation.
With more than 40.5 million people already displaced as a result of the climate crisis, the need to relocate people, businesses and even cities to safer regions is an increasing reality.
Where an inland movement is impossible then floating cities may be a way forward in addressing overdevelopment, sea level rise and land subsidence.
The idea that cities could be built to float is not a new concept and apart from existing floating villages in places like Thailand, the Philippines and Vietnam there has been an ongoing architectural debate since the 1950s that has recently gained significant traction with the United Nations calling for research into floating cities.
Technically, there are two types of structure that could support a floating city — pontoon structures that float on the water's surface and semi-submersible structures like oil rigs that have submerged pontoons connected to an operating deck by structural columns.
While there are still no floating cities in the ocean, the closest proposal is Oceanix which is based on a pontoon structure and so requires shallower waters and some protection from wave energy.
A prototype — Oceanix Busan — is due to begin construction in 2023 in South Korea and will start with three connected, floating islands including a lodging platform, research platform and living platform, with the potential to expand to 20 platforms over time.
The development of entire cities is creating new opportunities to ensure that they are designed, built and operated with sustainability at the forefront.
Oceanix Busan aims to create zero waste, have closed loop water systems, operate efficiently on clean energy, and support food production on a neighbourhood system.
Habitat regeneration is also an aim of the project that proposes using biorock to encourage marine growth.
Engaging early with marine ecologists and restoration practitioners would help support this goal and ensure that natural tidal cycles are considered for communities such as mangroves and saltmarsh that rely on them.
But there are other considerations to ensure that the rush to create new ocean cities doesn't create new problems for the underwater neighbours sharing this space.
Floating cities may have less impact on seafloor communities than artificial islands because they don't require extensive seafloor scaffolding. Those proposed for coastal environments, like Oceanix Busan, are an extension of the land and wouldn't require a connection to the seabed.
To move further offshore then anchoring to the seafloor might be necessary and mooring blocks or anchors and chains can cause significant scour, impacting on soft sediment communities.
Floating cities would also create physical structures where none existed previously and this type of construction can change water flow and may impact on movements and migration by marine life.
The creation of floating cities will also create significant shading of the waters and seafloor beneath.
This could impact on any organisms that rely on sunlight to create food through photosynthesis and includes sensitive communities like seagrass and corals that have already been decimated worldwide through other types of coastal development and climate impacts.
Designing built structures that reduce shading either through the incorporation of 'skylights' or using light transmissive materials whenever possible could help to address impacts from shading.
Australia now has guidelines for managing the ecological impacts of light pollution and these or similar guidelines should be considered when planning lighting for a floating city.
Research done in parallel with prototype and early-stage floating cities will help inform whether they are a viable solution for sinking cities without extending the impact of our urban footprint further offshore.
Katherine Dafforn is an Associate Professor in the School of Natural Sciences at Macquarie University. Her research investigates the impacts of coastal cities on marine life and provides eco-friendly solutions that can benefit humans and nature.
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