1. Sustainable water management
Growing populations living better lives in rapidly expanding cities require great amounts of water for domestic and industrial use. A trend which is not set to stop; by 2050, global water demand is expected to increase by 55 per cent while, at the same time, water stress will affect half of the world's population.
To prevent water shortage, sustainable water management is crucial. Most importantly in terms of land subsidence, we must find new ways to supply our cities with water than continue the excessive extraction of groundwater. Purifying surface waters or desalinating seawater can be answers. However, that requires large amounts of renewable energy to be fully sustainable.
An example of visionary, urban water management is the town of Nye, which among other solutions collects all rainwater to supply toilet and washing machines with secondary water. Potentially housing 15.000 inhabitants, the solution saves drinking water resources of around 30 million litres annually.
2. Restoring water ecosystems in cities
While reducing groundwater extraction, what is just as important is to build up groundwater tables. To stay with the mattress metaphor: To keep it from sinking, the leaking mattress must get refilled with water to prevent compression.
But what characterizes most cities today is the presence of bricks and concrete rather than parks and rivers, challenging the natural recharge of groundwater basins. Solutions can be permeable asphalt and soakaways, which ensure that rainfall does not end up in a city's sewage system but instead infiltrates into the ground. Also, bringing back existing watercourses and green spaces can contribute to re-establishing water ecosystems in cities.
Artificial recharge, which happens by injecting treated wastewater into the underground, can also be a way to compensate for extracted water resources. Doing so requires in-depth knowledge of soil conditions, the geochemistry of aquifers and water pressure in order not to cause further instabilities in the ground.
In Chesapeake Bay, Virginia, USA, such a project is currently taking place. To slow down land subsidence, the goal is to inject a total of 120 million gallons of wastewater per day by 2030, making it the largest groundwater recharge effort in the USA .
3. 'Waterproof' urban planning
Finally, we must consider how to construct our cities to prevent them from sinking and cope with incoming waters. Protecting shorelines of coastal sinking cities is crucial to preventing the sea from taking over. Building on stilts is another way to prepare for changing water levels. And compensated foundations can reduce the stress on a city's surface, which otherwise is a consequence of heavy loading.
We can also plan and design our cities with infrastructure made for solving problems of incoming water. An example of such is tunnels with dual purposes; beside being an efficient traffic solution, they can, in times of flooding, capture enormous amounts of water for subsequent infiltration.
Recreational areas can be designed to have the same dual function. In Bangkok, the Centennial Park was established to protect the sinking city from flooding during heavy rainfalls. With artificial wetlands and a water tank below, the park can store up to 4,564 cubic metres of water .
Last but not least, some researchers believe that it is time to stop 'fighting' and instead 'cooperate' with the ocean. One way of doing so is to build floating cities fully adaptable to changing water levels.
In many ways, we are at a crossroads when it comes to sinking cities, as is the case for overall handling of climate change. It is last call if we want to turn the tide and do something about sinking cities to avoid unrest and turmoil following the laissez-faire attitude taken so far.
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