the economic loss of each land area loss where in 2010 the economic losses will reach 0.03x10
6
, in 2050 economic losses will reach 0.14x10
6
, and in 2100 economic loss will reach 0.69x10
6
.
2.6 Land Subsidence
Land subsidence is a gradual settling or sudden sinking of the Earth’s surface owing to subsurface movement of earth Materials. Land subsidence is merely the
surface symptom of a variety of subsurface displacement mechanisms. Not all of these mechanisms are well understood. Subsidence processes are hiding below
ground, their development to the point of surface deformation may involve long periods of time, and for at least some mechanisms, significant evidence may lie
outside the area directly beneath the surface subsidence. Furthermore, at some sites more than one condition favorable to subsidence occurrence may be present
and require consideration in analyzing causal mechanisms and devising remedial procedures USGS, 2000.
Subsidence is a familiar accompaniment of a variety of natural events that comprise the geologic history of many areas. For practical reasons geologic
processes that are accompanied by subsidence have been examined for evidence that the range in their rates of progress extends into a time frame that may produce
damaging effects in terms of man’s time scale. The processes investigated are those that remove or withdrawal subsurface materials to produce void space or
significant volume reduction-solution, underground erosion, lateral flow, and compaction-or, in the case of tectonic activity, deep-seated downward
displacement. For all of these naturally occurring geologic processes, examples of related surface subsidence have been found, though some are rare Allen, 1970.
The incidence of subsidence is greater where some of these geologic processes are set in motion or accelerated by man’s engineering activities that involve
excavation, loading, or changes in the ground-water regime. The term subsidence is used because it representing the sinking in a broad
sense to include both slows downwarping and the collapse of discrete segments of the ground surface. Displacement is principally downward, although the
associated small horizontal components have significant damaging effects. The
term is not restricted on the basis of size of area affected, rate of displacement, or causal mechanism.
Land subsidence in Semarang city had been occurring with northern part of its city as the main region of subsided. There are three major factors that causing land
subsidence in Semarang City i.e.; geological structure of soil, groundwater
withdrawal, and buildingconstruction load.
Geologically, Semarang has three main lithologies, namely, volcanic rock, sedimentary rock, and alluvial deposits. According to Sukhyar 2003, the
basement of Semarang consists of Tertiary Claystone of the Kalibiuk Formation. Overlying this Formation is the Notopuro Formation, which consists of
Quaternary volcanic material. Those two formations crop out in the southern part of the Semarang area. The northern part of the Semarang area is covered by Kali
Garang deltaic alluvium up to a depth of 80 to 100 m in the coastal area. Aquifers are found at depths ranging from 30 to 80 m in this alluvium. Van Bemmelen
1949 reported that the shoreline of Semarang progresses relatively quick toward the sea, namely about 2 km in 2.5 centuries or about 8 myear. That “new” land
area is located in northern part of Semarang City deltaic area is composed by very young alluvium with high compressibility. Alluvium, as the upset soil layer
in coastal area of Semarang City is not stabile and its density can be easily changed. The instability of soil layers in deltaic area will easily change the pattern
of soil layer and without human factor it can causing land subsidence.
As development started to continue, there are some impacts that occur in Semarang City, one of them is land conversion. Swamp forest, agriculture area
and human non-infrastructure use had been converting into houses, building and infrastructure. In 1660, the first area that was converted into human uses building
and infrastructure is the area that nowadays known as Tanjung Mas harbor area. Around 600 years ago, Tanjung Mas harbor area is not land area but it known as
the area that covered by seawater completely. Deltaic sedimentation, river deposit, and sediment cover from tides process formed this land area so that the soil layer
in that area is still young. To supported Tanjung Mas harbor area as the central area of development, the buffer area of Tanjung Mas also converted and becomes
houses, building and infrastructure. In 1741, the conversion reach around 44.69
