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DOSSIER
LAND SUBSIDENCE
The subsidence of soft soils such as peat is a global problem.
California, Italy and Indonesia have developed their own
ways of dealing with the consequences of land subsidence.
‘Sound science is the only way to convince people.’
BY PJOTR VAN LENTEREN
ITALY, AMERICA AND
INDONESIA STRUGGLE
WITH SOFT SOILS
PHOTO: THINKSTOCK
15
DELTARES, FEBRUARY 2015
F
or anybody who works with agriculture and
homes in peat areas, there is no escaping
the challenges: land subsidence, flood risks
and greenhouse gases. The Italian Po Valley,
Sacramento Delta in California and the coastal
zones of the Indonesian islands are all facing the
same difficulties, but they have adopted different
approaches. How can local scientists get the topic
onto the agenda, how are they trying to counteract
the effects of land subsidence, and what can other
countries learn from them?
Po valley: a fine balance
Everyone knows that Venice can just keep its head
above water, but the real problems are found where
tourists don’t go, in the Marghera, the agricultural and
industrial area around the city. Here, the peat ground
is located a number of meters below sea level. Until
the 1970s, this was caused by deep groundwater
being pumped to the surface. After an aqueduct was
installed, land subsidence continued, but now as a
result of the drainage of agricultural land. ‘This type of
subsidence is not as fast, but it is much more difficult
to stop’, argues Pietro Teatini of the University of
Padua. ‘If we pump too much, the land subsides; if we
pump too little, there are floods.’ Water authorities in
the Po valley have to meet the challenge of striking
a fine balance in both natural and political terms. ‘It
has been agreed with farmers to plough 30 cm rather
than 70 cm. That is already an enormous help.’
California: convincing farmers and the
public
In Venice, the problems are easy for all to see; in the
USA, there are areas where it is much more difficult
to convince the government and business about
land subsidence. ‘America is affected by all types of
land subsidence’, explains Devin Galloway of the US
Geological Survey, ‘but the most frequent solution is
to move the problem. There are plenty of uninhabited
areas.’ In the Sacramento Delta near San Francisco,
farms are located on peat islands between busy river
arms. ‘If you stand at the bottom of the bowl, you
look up at sea-going vessels as they sail by.’ Even
here, it hasn’t proven easy to raise awareness of
the problem among local people and farmers: the
farmers themselves live safely on higher land. ‘On top
of that, the economic stakes are considerable: one
third of the USA’s table vegetables are produced here.’
Forward-looking areas are trying to buy out farmers
and submerge the land in question. Sedimentation
creates new land. ‘That sounds good, but the process
is not fast enough and we are struggling with the
salinisation of the groundwater. We do hope it will buy
us time so that we can find better solutions.’
Indonesia: palm oil and paper pulp
Not many people associate the tropical coastal
zones of Indonesia and Malaysia with peat. But that
is precisely what you find below the jungles, three
quarters of which have already been cut down: 25
million hectares, roughly the surface area of Great
Britain. Until the 1990s, the main industry here
was forestry, with the selective felling of sustainable
timber such as meranti. ‘Since they started producing
palm oil and pulp for paper, things have taken a turn
for the worse’, explains Aljosja Hooijer of Deltares.
‘Drainage leads to the loss of peatland and these
areas are now facing more and more floods, with the
country’s emissions of greenhouse gases almost
matching the USA and China.’ Due to the lack of
proper maps showing peat and land levels, it is easy
to question whether the problems are real. At the
moment, environmental organisations and business
are the main actors pushing for more sustainable
approaches, financing research into the loss of
peat and land subsidence, and drawing up accurate
maps using aeroplanes and laser technology. The
data make it possible to predict how fast flooding
will increase, and what can still be done by reducing
drainage. ‘But it continues to be a difficult discussion:
should a company or a government agency pass on
profits that can certainly be made this year for the
sake of more sustainable, but lower, profits in ten
years from now? The only way to convince people is
by doing good science, and producing sound data.’
Land subsidence in areas with soft soil, like those in and around Venice, can, in
combination with poor foundations, result in the subsidence of historic buildings,
as here in the case of the Santa Maria Gloriosa del Frari.