44.464 cm, isostatic mean sea level in 2009 was reached 49.535 cm, and isostatic mean sea level in 2010 was reached 52.070 cm. The complete data about local
mean sea level from BMKG and real isostatic mean sea level as the result of
calculation can be seen in table 4.3. Table 4.3. Real sea level values in Semarang City
Year MSL
from BMKG cm
Calculated Isostatic
MSL increase
2.535 cmyear
2004 36.859
36.859 2005
47.764 39.394
2006 50.204
41.929 2007
59.292 44.464
2008 80.972
46.999 2009
89.610 49.535
2010 94.669
52.070
4.2. Land Subsidence
Land subsidence in Semarang city had been occurring in northern part of its city as the area that experiencing highest subsidence level. There are three factors
that causing land subsidence in Semarang City i.e.; geological structure of soil as natural factor, groundwater extraction, and buildingconstruction load that
accelerating land subsidence in Semarang City and become major factor of land
subsidence itself.
This research was not measured subsidence directly in the field or using active remote sensing technique but the value of subsidence can be estimate by
calculating the deviation between mean sea level from tide gauge data and real sea level value. Real sea level value can be calculated by adding rising value of
regional sea level to tide gauge data in the first year in this research, 2004 is the first year when data of MSL was recorded by tide gauge and this first year data
became the baseline to estimate real sea level value in next year. In short, MSL in 2004 as the result of tide gauge recording is considered as real mean sea level at
that time. The deviation of tide gauge data and calculated isostatic MSL can be seen in table 4.4
Table 4.4. Real sea level values in Semarang City and it deviation from tide gauge
data Year
MSL data from BMKG cm
Calculated Isostatic MSL
increase 2.535 cmyear
Deviation cm
2004 36.859
36.859 2005
47.764 39.394
8.370
2006 50.204
41.929 8.275
2007 59.292
44.464 14.827
2008 80.972
46.999 33.973
2009 89.610
49.535 40.075
2010 94.669
52.070 42.599
Based on the deviation of mean sea level data from BMKG and calculation, land subsidence value in the area where tide gauge is located can be estimate. The
value of land subsidence at that location is:
= 96.449 – 52.0702010-2004 = 42.5996
= 7.100 The level of subsidence, with decreasing level 7.1 cmyear, is not so different
with other result Abidin et.al. 2010 found the level of subsidence was 7.5 cmyear between October 2007 to December 2008.
4.3. Recent Impact of Sea Level Change To Land Area
There are some impacts of sea level rise to the land like flooding, expanding of submergence area, wetland and estuary ecosystem lost, coastline change and
many more. Semarang City especially its coastal area had been facing sea level change in daily tides and regional sea level rise as the respond of global sea level
rise. Sea level rise increases the risk of flooding in low lying areas.
This study only considered in expanding of submergence area as the effect of sea level rise. Van Bemmelen 1949 reported that the shoreline of Semarang
progresses relatively quick toward the sea, namely about 2 km in 2.5 centuries since 1660 or about 8 myear. Helmi 2010 also found that land area in
Semarang City coastal area from 1660-2007 has been increased ± 1.7 km. The result shows that land area is not increase anymore, not even stabile, but decrease
maximally around 0.8 km in last 97 years or about ± 8.2474 myear if we compare it with Van Bemmelen’s research result. That comparison can be made as fact,
referenced and basic theory that submergence area had been expanding in Semarang City coastal area.
4.3.1. Coastal Submergence by Sea Level Rise
Sea level has been changed globally and it’s affecting regional and local sea level including sea level in Semarang City coastal area. One of the effects of that
change, in form of sea level rise, is flooding in land area near coastline and with some conditions also covering area that has lower altitude than sea level value at
specific time. When values of sea level continuously increase, flooded area will permanently covered by water and loose it land it called submergence area.
The effect of sea level change in form of rises in Semarang City coastal area can be various, but the main sources of that effect comes from flooding. Estuary
ecosystem lost, human migration, water intrusion and many more are disaster that occur by flooding. The area that has lot of potencies in the past has been
transformed to submergence area with water as properties that covered it land. Submergence area in Semarang City at recent time 2011 can be measured
by satellite image classification and corresponded with result form field survey. Using ALOS-AVNIR image, 2009 of time acquisitioned, land area that was
covered by water can be drawn clearly so that the submergence area can be measured. Figure 4.7 shows the area that has been covered permanently by water.
Total area of submergence area at that time was 1,043.527 Ha, located along the coastline and illustrated by light blue color. The widest area that permanently
covered by water at that time was embankment 950.866 Ha, which was used by peoples for various purposes. 25.433 Ha area of building and 8.75 Ha area of
