How do we stop cities from sinking?
28.02.2019 / Jørgen S. Steenfelt
Over-exploitation of groundwater resources is causing cities around the world to sink below sea-level. The problem is set to accelerate due to rapid urbanisation and climate change. So, what can we do about it?
Roughly 83 million people is added to the world's population every year. By 2050, we are going to be 9.8 billion people in the world and two thirds will live in cities. With an additional 2.5 billion urban dwellers by 2050, no questions asked; our future is urban.
Just as cities can drive prosperity and human progress, the increasing pressure on infrastructure, housing and resources following urbanisation calls for smart solutions. And climate change does not make it any easier to plan and design our future cities.
However, there is another major, yet almost invisible, threat to life in cities; the land below us is subsiding. Cities all over the world are sinking below sea level damaging infrastructure and increasing flood risks. But why is that? And what can be done to make our cities more resilient to land subsidence?
Why cities sink
Land subsidence can have natural causes like tectonics, glacial isostatic adjustments and natural sediment compaction. However, the main reason behind sinking cities today is the anthropogenic changes to the bearing capacity of soil following heavy loading and, especially, excessive extraction of groundwater (or oil and gas).
You can think of it as a mattress filled with water. If you prick holes in the mattress (extract groundwater), no matter how big or small, the water will start to seep out, emptying the mattress and causing its surface to sink.
You can think of it as a mattress filled with water. If you prick holes in the mattress (extract groundwater), no matter how big or small, the water will start to seep out, emptying the mattress and causing its surface to sink.
If you place something heavy on top, like a city building, it speeds up the process. The higher the mattress (depth of groundwater extraction), the more pronounced the depression of the surface. And if the mattress is made of soft materials like clay (soil composition), it is especially compressible and vulnerable to subsidence.
That's the reality of many of the world's megacities, especially in coastal areas and urban centres that over-exploit groundwater resources. Sinking cities is a global phenomenon, although populous Asian cities top the list with Jakarta being the fastest sinking of them all.
Home to over 10 million people, 40 per cent of Jakarta is already below sea level. Large parts of the city have settled two to four metres since the 1970s, and at current rates, Jakarta is sinking by up to 20 cm a year. The northern areas of the city most prone to subsidence are predicted to sink by up to seven metres by 2100.
Impacts of land subsidence
That cities sink is problematic in itself. Adding climate change to the equation, extreme weather events and changing sea-levels only exacerbate the consequences. Impacts of land subsidence include damaged buildings and foundations, infrastructure, drains and sewage systems. And moreover, flood risks increase.
Cities all over the world struggle with these consequences. To mention a few, the Kansai Airport, situated on a sinking island outside Osaka, Japan, was covered in water during a typhoon earlier this year. In Mexico City it is nothing new to see cathedrals settle dramatically or drainage systems flow uphill. And Venice, as the city continues to sink, is experiencing more and more severe cases of rising waters, flooding the streets of the old, historic city.
Incoming water hits sinking cities hard, especially in coastal areas, affecting the livelihoods of millions of people. A total of 800 million people is estimated to live in cities where sea-levels could rise by more than half a metre by 2050 . Moreover, it's a costly affair. The World Bank and OECD estimate that global flood damage in large coastal cities, could cost USD $1 trillion a year, when adding in the risks from sea-level rise and sinking land.
What are the solutions?
Fortunately, there are ways to prevent our cities from sinking. To break it down, three areas of concern must be integrated in urban planning to make our cities more resistant to land subsidence:
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.
MEET THE EXPERT
"Medical doctors bury their failures, whereas geotechnical engineers bury their successes."
Despite this dire statement, I have always found geotechnical engineering intriguing and rewarding. When we are successful and provide society with feasible and economic solutions, nobody pays attention. We are only noticed when things go wrong – just look at the leaning tower of Pisa.
Through my career I have been blessed with the opportunity to work on a multitude of megaprojects (bridges, buildings, tunnels and marine works). These have presented significant opportunities and challenges for the geotechnical profession. They challenge the individual's ability to solve difficult soil-structure-water-environment problems and call for innovative and lateral thinking.
This is what motivates me. Sinking cities is such a humongous challenge calling out for the proper solutions.
Get in contact
Jørgen S. Steenfelt
Technical Director
Marine and Foundation Engineering, Denmark
Tel:
+45 56401388
jos@cowi.com