While sea level rise seems increasingly inevitable, a separate but more solvable problem faces many of the world’s coastal cities: how to stop their gradual sinking into the ground.
Subsidence, or the gradual sinking of landscapes under their own weight, has long been associated with agricultural areas.
Years of pumping groundwater for irrigation caused parts of California’s San Joachin Valley to sink by nine metres in the space of 50 years during the twentieth century, according to the United States Geological Survey (USGS).
“The main cause of subsidence, in general, in the world, not even in cities, is lowering the groundwater table,” says Herbert Einstein, a professor of rock mechanics at the Massachusetts Institute of Technology (MIT).
However, a separate type of subsidence gave residents of San Francisco’s Millennium Tower a very different kind of sinking feeling, after the northwestern corner of the luxury high rise sank by more than 40 centimetres over the course of a decade.
According to Tom Parsons, a research geophysicist at the USGS, buildings like the Millennium Tower may be causing the entire city to sink, exacerbating the impacts of climate change-induced sea-level rise.
Parsons found general ranges of subsidence to be between five and 80 millimetres in the San Francisco Bay Area, with the most subsidence recorded around San Francisco International Airport, the region’s heaviest building.
He attributed this to the rapid increase in the mass of these buildings and its effect on the earth underneath.
“There’s the additional weight and then there’s the ability of the water underneath that is allowed to flow away,” says Jaap Nienhuis, a geomorphologist at Utrecht University who studies coastal subsidence. “If the water is allowed to flow away, then you suddenly have excess space, which can cause the land to sink.”
Parsons warns this problem will likely be worse in the developing world as more people migrate to coastal cities.
The Flanders Marine Institute, a non-profit organisation, estimates that 60 percent of the world’s population already lives in coastal areas, with an additional 65 percent of the world’s most populated cities also located on the coast. The
United Nations expects these figures to rise, along with sea levels.
“In a lot of countries it is considered a very big issue,” says Nienhuis. “It is important and needs to be highlighted more, definitely in comparison to climate change-driven sea-level rise.”
Perhaps the most prominent example of where urbanization-driven subsidence is exacerbating the impacts of sea-level rise is Jakarta, the capital of Indonesia.
Jakarta is sinking faster than any other major city in the world, mostly as a result of groundwater being pumped out faster than it can be replenished. An estimated 40 percent of the city is already below sea level.
Nienhuis believes that cities such as Jakarta exemplify why increased awareness should be given to subsidence.
“Subsidence is easier to manage [than sea level rise] because it is a local problem and in many cases is also much bigger,” he says. “In cities like Jakarta, you are going to have tens of centimetres of subsidence each year, whereas
sea level rises only a couple of millimetres per year, so that's a factor difference of 10 or more.”
In instances like this, one of the solutions to subsidence can be replenishing the groundwater. In the twentieth century, Tokyo sank by more than four metres as a result of pumping groundwater. However, the subsidence was largely arrested after the local
government banned the practice and began importing water.
“Replenishing groundwater is certainly a way to reverse subsidence,” says Einstein. “But depending on the type of soil that is certainly a very long-lasting process.”
“Pumping out is a much faster process than infiltration from rain into green space or reverse wells,” he adds. “The increase in groundwater level works but is a slow process. Prevention is what you should do.”
As a result, urban planners should design cities in ways that avoid subsidence. This includes plenty of green places, such as parks, to allow natural groundwater replenishment from rain.
It also involves constructing high rises and skyscrapers appropriately. For example, New York City is not at risk of subsidence because its largest buildings are constructed on top of a layer of bedrock.
“If your foundation reaches the bedrock, then the soil around you might sink, but your building is going to be fine,” says Nienhuis. “Any additional weight on soft soils is going to increase the subsidence.”
“In general, it is good to have a deep foundation of your building so that the weight is not compressing the soil and allows for enough groundwater to infiltrate,” he adds. “This keeps spaces open so that rainwater can fill in any void
and reduce subsidence in that way.”
Einstein agrees that building high rises and skyscrapers on bedrock or hard soils helps prevent subsidence. He adds smaller buildings that do not need such a deep foundation will not subside as long as the mass of earth moved from underneath them is equal
to its mass.
“MIT is a classic example of this,” he says. “We have some high rises using that system where you excavate the weight of the building and there's basically zero effect. Or you have high rises, which are on pile foundations on the rock.
Einstein adds that the old main building of MIT has subsided because the pile on foundations do not all reach the underlying bedrock.
This is the same problem faced by the tenants of San Francisco’s Millennium Tower, who reached a deal with the company that owns the building and the neighbouring construction project to fix the subsidence problem after years of litigation.
Their solution: spend USD 100 million drilling down to the bedrock to install a perimeter pile upgrade and reinforce the structure.
“If you're intent on urbanising a place, [subsidence] is not really something you can do much about,” Parson says. “It’s just gravity pushing down, you can’t really fight that.”