Water as Leverage- Setting the scene for a call for action

Water as Leverage for Resilient Cities Asia Setting the Scene for a Call for Action Content
3.2.1 Sinking City
Semarang is a sinking city, subsiding eight
centimeters each year in the most severely affected
areas. By comparison, sea level rise due to climate
change amounts to about two millimeters each year.
The weak soil also causes buildings to sink into the
clay. Pumping drinking water from the deep aquifers
is the main cause of land subsidence in Semarang.
A direct consequence of land subsidence is
damage to housing and infrastructure. In the future,
parts of the city will lie below sea level, increasing
vulnerability to flooding during high tide. Even if the
causes of land subsidence are tackled now, it will
still take many years for land subsidence to come to
a standstill. Even in the best scenario, the land will
still sink by 1.6 meters.
villages, land owners are elevating their
land and public space. This results in the
phenomenon of ‘sinking houses’, buildings
where the ground floor slowly disappears
below the surface, forcing people to move to
the first floor and, if they can afford it, build
another floor on top.
The waterways are not only used for waste
disposal; they provide vital resources as well.
Semarang’s water network is mostly supplied
by the Jatibarang reservoir and the Semarang
river. Nevertheless, most industrial, residential
and business complexes tend to pump up their
own water from the aquifers. As mentioned
before, this has contributed to land subsidence
and salt water intrusion issues in central and
coastal areas.
Natural compression
Sinking houses
Road
elevation
Load of buildings
and infrastructure
Groundwater extraction
The area around the city consists of a stretch
of alluvial plains formed by hundreds of small
rivers. This low-lying land consists of young
alluvial sediments. The natural mangrove
ecosystem had to make place for urbanization,
fish farming and other forms of agriculture,
leaving only small patches of the mangrove
intact. With the loss of the mangroves,
Semarang lost its natural coastal protection.
However, this is not the only issue that affects
the city. Land subsidence has by far the
largest impact on floods; some parts of central
Semarang sink up to eight centimeters per
year. In comparison, sea level rise is a few
millimeters per year.
Land subsidence is caused by groundwater
extraction, natural consolidation of alluvium
soil and by the pressure of the weight of
constructions. Besides increased flood risk,
land subsidence is related to issues of water
logging, salinization and impaired drainage.
What makes this process more threatening is
that the process is almost invisible, resulting
in a lack of a sense of urgency among
inhabitants.
Large restoration projects of the mangroves
aspire to protect the city from floods but,
although these developments have a positive
impact on ecology and the mitigation of
coastal erosion, they do not reduce the flood
risk related to land subsidence. Moreover, the
issue of land subsidence is not tackled at the
main source: people prefer to use water from
deep wells for drinking and cooking, rather
than the surface-water-based tap water that
is available in over half of the city, because it
has a bad taste. As a result, the construction of
deep wells is still a standard procedure when
new apartment buildings, hotels and shopping
malls are erected. Stopping or slowing down
this process is very hard; even if groundwater
extraction would stop today, subsidence would
continue for years, resulting in approximately
a further 1.6 meters of subsidence in some
areas, leaving them permanently below sea
level.
Restoring subsided areas is basically
impossible. Nevertheless, attempts to elevate
the land are common in the harbor areas,
where there is direct economic benefit from
flood mitigation. Also, at neighboring marine
Semarang is subiding;
the weak soil causes
buildings to sink into
the clay, forcing home
owners to give up their
ground floor.
Urban developments in
flood prone areas.
250 SEMARANG
Water Issues
PART 2/Background Information on the Partner Cities
251