Jesse
Kearse, Te
Herenga Waka — Victoria University of
Wellington
Many cities around the
world are sinking because too much water is extracted from
the groundwater reservoirs beneath them.
This
sinking makes these coastal cities more exposed to rising
seas as the climate warms.
But as our new
research shows, this process can work in reverse; when
groundwater is replenished, cities can rise.
Our
findings also reveal an unexpected clue: faultlines play a
key role in shaping where the fastest groundwater recovery
and land uplift occur.
The global problem of
sinking cities
Land subsidence, the gradual
sinking of the ground surface, affects many of the world’s
major cities.
Parts of Jakarta, the capital of
Indonesia, are sinking more than ten centimetres each year,
prompting the government to plan a
relocation of the city.
The coastal city of
Tianjin, China, is home to 15 million people. It, too, is a
subsidence
hotspot. If sinking there continues unabated, 15% of the
city’s population will be underwater by
2120.
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Similar stories are playing out from San
Diego to Iran’s
major cities, wherever groundwater has been pumped
faster than it can refill.
In Aotearoa New
Zealand, recent studies show around 80%
of the urban coastline is sinking, and that groundwater
may be a contributing factor in some areas, including
Christchurch and Wellington.
How urban
groundwater leads to sinking
Many cities are built
above natural groundwater reservoirs (aquifers), and are
literally held up, in part, by the water beneath them.
Think of a gigantic water balloon beneath a city.
The balloon represents an aquifer full of water. When
groundwater is pumped out, the “balloon” deflates, and
the ground above sinks.
This matters most at the
coast. If the land is sinking while the sea is rising, the
two effects combine, meaning coastal cities experience
relatively higher sea levels, and the impacts of climate
change arrive sooner.
This effect of groundwater
pumping on land subsidence is well understood, and improved
groundwater management is often put forward as the key
strategy for slowing or halting the sinking.
What
is not clear is how the ground beneath a city responds when
long-term water aquifer recovery happens. Does the ground
simply rise back up, uniformly, like a balloon reinflating?
Or is the response more complicated?
To answer
this, we need two key things: long-term records of where the
water is accumulating in the underground aquifers, and
precise millimetre-scale measurements of elevation of the
ground surface above.
Osaka, a natural
laboratory
Osaka provided the perfect setting to
explore this. Between the 1920s and 1960s, heavy groundwater
pumping lowered water levels beneath the city by up to 30
metres, and the land sank by more than two metres in places.
This led the Japanese government to introduce
strict groundwater regulations in the early 1960s. In the
decades since, groundwater levels have been recovering
steadily.
To track the groundwater levels, we
looked at data from 44 monitoring wells, some reaching as
deep as 500 metres below the surface, and stretching back in
time to 1985.
To measure the tiny changes in
ground elevation, we used a technique known as
interferometric synthetic aperture radar (InSAR).
This involves the repeat acquisition of satellite radar
images of the Earth’s surface, tied to very accurate
global navigation satellite system measurements of ground
stations.
The results are striking. Across greater
Osaka, home to about 15 million people, the ground is
rising, and fast.
The speed of uplift averaged
about four millimetres a year, and was as high as 12
millimetres a year in some places. Groundwater levels have
also been steadily rising, at rates of up to a metre a year.
Where water levels climbed fastest, so did the
land.
But the map of uplift was also very patchy.
One city block could be uplifting fast, while less
than 100 metres down the road it could be much slower. These
abrupt changes aren’t random. They form narrow but
continuous corridors that ran across the city.
We
soon realised something unexpected: these corridors mapped
almost perfectly onto known tectonic fault
lines.
Faults act like dams
Faults are
fractures in the Earth’s crust, and are usually thought of
in terms of earthquakes. But we found they play another role
here.
Picture a fault as a curtain hanging
vertically underground, cutting through the aquifer. Instead
of holding back light, it holds back water, blocking its
sideways flow and acting like an underground
dam.
On one side of Osaka’s Uemachi Fault,
groundwater levels have been rising three times faster than
on the other side.
We found the same pattern
repeats at other faults across the city. Water dams on the
“upstream” side of a fault, where it can’t easily flow
through, driving faster groundwater recovery and faster
uplift there, while the “downstream” side lags
behind.
Our research highlights a potentially
important opportunity for coastal cities adapting to
sea-level rise.
Where groundwater depletion has
contributed to land subsidence, reducing extraction and
allowing aquifers to recover could reduce – and in some
places potentially reverse – the downward movement of the
land.
As seas continue to rise, keeping the ground
beneath coastal cities from sinking could be an increasingly
important part of adapting to a warming world.
Jesse
Kearse, Research Fellow in Earth Science, Te
Herenga Waka — Victoria University of
Wellington
This article is
republished from The
Conversation under a Creative Commons license. Read the
original
article.