Land Subsidence Sea Level Change and Its Impact To The Coastal Area of Semarang City, Central Java
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
Ha and it increased to 109.78 Ha 44.70 in 1880. In 1927, the building area expanded toward southern part of Semarang City with the increasing to348.14 Ha
and in 2009, building and infrastructure has covered all part of coastal area with total area reach 37,984.41 Ha more than 100 times than in 1927. Semarang City
especially coastal area, with very young unstable alluvium soil layer and can be decrease without human factor, since 1660 started experience land subsidence
which also caused by building load as the impact of development. The illustration of building and construction distribution increased over Semarang city, especially
coastal area, can be seen in Figure 2.5.
a b
Source: Interpretation of ALOS AVNIR-2 images and Helmi 2010
Figure 2.5. Building and construction distribution over Semarang city in a red in 1600, green is increasing until 1741, violet is increasing until 1880, blue is
increasing until 1927 and b yellow is increasing until 2009
Development with the expansion in number and area of building and infrastructure has been encouraged the increasing of population through
urbanization. Peoples started moved to the area near facilities office building, hospital, road, school, etc. where all of facilities were centralized built in deltaic
area had been knowing also as Semarang Bawah. As the effect of increasing population, the needs of water also increase. Groundwater is one of the water
source that can be easily access by peoples and cheaper than other sources. Peoples and industries started utilize ground water as their main water source.
When groundwater in the upper layer was not enough to supply human and industries need, deeper groundwater has been drilled and pumped as the
alternative. It can fulfill human and industries need of groundwater but in contrary the side effect of groundwater pumping in instable soil layer like Semarang City
especially Semarang Bawah costly the decreasing of ground water level and increasing the speed of subsided. Marsudi 2001 also found the similar things on
his research where tremendous amount of groundwater pumping in Semarang City that exceed it aquifer capacity has led the occurrence of land subsidence. Figure
2.6. below shows the increasing amount of groundwater pumping in Semarang city where it amount is increasing sharply since early 1990s, the era that has been
known as early industrial era in Semarang City Abidin et.al., 2011.
Source: Murdohardono in Abidin et.al. 2011
Figure 2.6. Ground water abstraction and registered drilled wells development in
Semarang City
There area several former land subsidence research in Semarang i.e.; I.
Marsudi 2001 conducted land subsidence prediction research in Semarang from geological point of view. Based on the 1-D consolidation model, the
result of land subsidence prediction from his research conclude that for the year of 2013 the ranges of subsidence will be between 87 to 190 cm from
annual condition. He also concluded that the amounts of land subsidence become larger toward north-northeast direction and increases toward coastal
area.
II. Kuehn et.al in 2009 conducted land subsidence research in Semarang using
Persistent Interferometery Scattered PSI technique. Persistent Scatterer Interferometry technique PSI has been applied to improve land subsidence
mapping because former maps show an overall distribution of land subsidence, but are inaccurate in detail. PSI measures ground motion using
SAR satellite images. For the PSI processing, 35 radar satellite images acquired between 2002 and 2006 have been selected. Derived land subsidence
rates vary from fractions of 1 mm per year to values of 10 cm per year and even beyond. Classification and visualization of the ground motion data
illustrate the boundary between mainly stable ground in the south of Semarang and increasingly land-subsidence-affected land towards the coast Kuehn et.al,
2009. III.
Abidin et.al in 2008 and 2009 conducted land subsidence investigation using GPS survey method. By studying the characteristics and rate of change in the
height components of the coordinates from survey to survey, the land subsidence characteristics can be derived. In order to achieve level of accuracy
the GPS static survey method based on dual-frequency carrier phase data processing should be implemented, with stringent measurement and data
processing strategies. GPS surveys for studying land subsidence in Semarang have been conducted on 7-13 July 2008 and 5-11 June 2009. The number of
observed points was 48 at first survey and 52 points at second survey. Results from GPS show that land subsidence in Semarang has spatial variations,
ranging from 0.8 to 13.5 cmyear with the mean of 5.9 cmyear. Northern region of Semarang city exhibits higher rates of subsidence compare to its
southern region Abidin et.al, 2010.
I I I . M A T E R I A L S A N D M E T H O D