Figure 3.2. Flowchart for daily sea level change modeling using FVCOM Global and regional sea level rise as the other factor that causing sea level
change is taken from research institution. In this research, data of global sea level rise was acquired from CSIRO, regional sea level change was acquired
from University of Colorado, and local sea level change was acquired from Coastline
Bathymetry
Editing Coordinate Transformation
Geoprocessing Merge Extraction
Extraction
Input data UNIX Programing
Coordinate Transformation Coordinate Transformation
Format Conversion
Grid Generation -
Create boundary -
Define Resolution -
Build Mesh -
Mesh Quality Control -
Interpolate bathymetri based on mesh pattern
UNIX Programing
Realistic tides model
BMKG Semarang. BMKG’s data is not corrected yet because tide gauge is located in unstable soil which sinking linearly. To obtain real sea level,
BMKG’s data must be corrected with average of regional sea level change. The period of data from BMKG and regional sea level change from University of
Colorado must be the same to obtained the correct result. Real sea level can be calculated as given:
1 where
is real sea level, SL is recent sea level from BMKG and Sreg is average of regional sea level change.
3.4.2. Estimating Land Subsidence
This research estimating land subsidence level by calculating the deviation from sea level which was recorded using tide gauge by BMKG in Semarang
City and isostatic sea level. Isostatic sea level is obtained by add up yearly sea level value in first
recorded year with yearly change of regional sea level. By this calculation, the isostatic sea level in next year can be calculated. Land subsidence level can be
calculated then by calculating the average of deviation in last year of data recorded by BMKG and isostatic sea level in same year.
2 where
is yearly land subsidence level, SL
ly
is sea level in last year data recorded from BMKG and SL
r
is real sea level value in the same year with SL
ly
and n is time period year.
3.4.3. Digital Terrain Model
Digital terrain model in this research is not measure directly when field survey was conducted. To represent terrain pattern, the combination of land
subsidence level and data of terrestrial survey from Public Work Agency of Semarang city was used. When recent terrain model has been drowned, the
submergence area can be drawn easily based on height pattern of the research location. The general step of terrain model mapping can be seen in figure 3.3
Figure 3.3. Flowchart for terrain model of Semarang City coastal area
3.4.4. Landuse and Infrastructure Mapping
Landuse and infrastructure is the properties that related to human activity. It change along the time whether it getting better or worse. In Semarang City, it
seems to be worse because the fact nowadays some of infrastructure has been covered by water permanently and can’t be used anymore as it function. One
factors that causing its worse is flooding. Analyzing the change as the effect of flooding, landuse and infrastructure map is needed and it will be easily to
analyzed if the format of map is in digital format. To create digital landuse and infrastructure map, there are two sources that
can be used i.e. printed rupabumi map of Semarang City that has been released by BAKOSURTANAL in 2002 and digital satellite images that derived from
ALOS-AVNIR in 2009 with 10 m of spatial resolution. The format of both data source is different, one is digital and the other one is in analog format. To
convert analog map to be digital, it must be scanned, registered and digitized where the result of digitizing is used as the baseline for correcting satellite image
geometrically. Once it corrected, it can be used to updating digital map as digitizing result from 2002’s printed map. The steps of landuse and
infrastructure mapping in this research can be seen in Figure 3.4. 2008 Terrestrial Survey Result
Semarang City Public Work Office
Subsidence Point Survey
InterpolateGridding InterpolateGridding
Subsided area Digital Elevation Modeling
DEM
Semarang City Terain Model
Figure 3.4. Flowchart for landuse and infrastructure Mapping
3.4.5. Ground Truth
Ground truth or field survey in this research in intended to refers of a process in which form on a satellite image, result of data processing and
analysis is compared to what is in the reality at the present time in order to verify the contents of the form on the image and result of analysis. Field survey
also intends to collect some other information, which will use in this research, which cannot be derived from other sources and must be taken directly in the
research location. Data that collected from ground majority truth will be used as reference for
infrastructure updating. Ground truth also utilized to comparing the result of projection and previous occurrence of inundation. Preliminary result of data
processing was consulted to the expert as part of the research. All of that has been conducted so that the final result will not be so differ from it reality.
2009 ALOS-AVNIR Images Printed Rupabumi 1:25.000
Geometric Correction Scanning
Registration On Screen Digitizing
Creating Color Composite Image
Landuse Infrastructure Mapping Field Survey
Updated Landuse Infrastructure Map
29
Figure 3.5. Research framework Spring Tide
Sea Level Rise from BMKG
Highest Sea Level Semarang City Terrain
Model Recent Landuse and
Infrastructure Map
Inundation
Model and Previous Occurrence comparison
Analysis
Expert Consultation
Final Result Submergence Area
I V . R E S U L T A N D D I S C U S S I O N
4.1. Sea Level Change
4.1.1. Tides Projection
Realistic tidal elevation forecasting for 90 day from December, 2011 to February, 2012 in ocean part of Semarang city has been generated using
FVCOM where six components of tides were included in the model S2, M2, N2, K1, P1, and O1. The type of tides in Semarang city is mixed mainly diurnal
tides this type was generated using Formzal number F= 1.46. This tides type shows the same type if it is compared with the result of tide gauge data
processing by Wirasatriya et al. 2006 and Prameswari 2007. The ebb tides and peak tide is occur 2 times a day with different value of height in different
times. The second ebb tide has the higher level that the firs and the first peak tide has the lower level than the second peak tide. The illustration of tides in a
day can be seen in figure 4.1
Figure 4.1. Sea level dynamic in Semarang city based on the day when spring tide is occur
The phenomenon of the spring tide highest peak tide like shown in Figure 4.1, with 0.4730 m of sea level value, has occur in the day 44
th
from total 90 days of model generating tides. This phenomenon has occurred in the full moon
on the second month from total 3 months of model generating tides. It possibly occurs because the first three moon of the year has been known as the periods
when the Earth is closest to the Sun Perihelion if it is compared to other months
http:www.science.nasa.gov
. In this research, the first moon of model generating tides is in December so the full moon occurs in day 15
th
. Revolution period of Moon to the Earth is 29.5 days, so the next full moon in January will
be occur in day 44
th
of total 90 days. In day 15-16 of each month moon calendar Sun-Earth-Moon also will be in one line and it can affect tides to reach
it peaks. NASA
http:www.science.nasa.gov
also states that perigee Moon closest to the Earth generally occur in the first three months of the year
whether it is in January, February, or March. The combination of both phenomenon perigee and perihelion will increase the level of peak tide and
reaches it maximum high in day 44
th
of total 90 days of model generating tides. In spring tide, the highest sea level has found in the coastal area of Semarang
city and Kendal where the height level is constantly decrease toward to the west. It can indicate sea surface in Semarang city was reach it maximum and starting
to recede to lower level. This indication also supported by surface current speed and direction. Speed of surface current is less than 1.5 ms, spreading to all
direction unequally with the majority direction of movement to the part where sea level is lower than the other. Seawater also propagates to land area during
that time following water channelriver and it gives big impact to the land, because it inundate the land area that lower than level of sea level at springpeak
tide. This phenomenon nowadays occurs in Semarang City coastal region, seawater inundates land area as tidal flood rob and affecting people’s life. The
variation of sea surface level at the times when spring tides occur can be seen in figure 4.2.
Figure 4.2. Sea level variation at spring tide In contrary with peak tide, neap tide can’t reach it lowest value at that month
January. Neap tides occur in day 7
th
from total 90 days of model generated tides or in the first quarter revolution of Moon to the Earth with height of sea
level at that time was -0.2310 m or 0.2310 m below mean sea level MSL. By looking at figure 4.3, there are some part in northward research area that has
lower height of sea level than in coastal area of Semarang city so that water in the coastal area of Semarang City move northward.
The direction of surface current in spring and ebb tide near land area 0-10 km shows the different direction. In spring tides, the irregular surface current
mostly toward land area because possibly there are some land area which has lower height than sea level so that water moving to those land area and inundate
land. In ebb tide, the surface current direction also spreading to all direction irregularly but the major direction is to west part of the sea. This area has lower
height of sea level. It means that water moving to that part, not to land area as it is happen in spring tide.
Figure 4.3. Sea level pattern at ebb tide When surface current pattern near land area between spring tide and neap tide
is different, the surface current in open sea shows similar direction, moving from west to east. In December-February, timescale of model generating tides, the
periods has been known as west monsoon periods with it northwest wind propagation. Wind blowing from northwest passing Java Sea to the east,
causing current moves following wind direction. Wind propagation affecting surface current so that it direction was almost similar in open sea. This result
also supported by Sofian 2007 and Widyastuti 2010.
4.1.2. Regional Sea Level Change
As technology advances, the use of technology to record sea level data had been increasing. In some area where tide gauges data relatively rare andor
difficult to obtain, data record from satellite measurement can be a solution. Regional sea level data derived from TOPEXPoseidon has shown the variation
of sea level change during 2004-2010 Figure 4.4.
Figure 4.4. Trend of regional mean sea level around Indonesia data from University of Colorado
The result shows that in 2004, mean sea level was 22.986 mm above zero and in 2005 has reach 36.713 mm above zero. In 2006, sea level has increase around
1.7 times from it level in 2005 and in 2007 sea level was reach 64.569 mm above zero. In 2008, sea level has been increase significantly from it values in 2007 and
reach 137.356 mm above zero while in 2009 and 2010 mean sea level reach 105.164 mm and 124.159 mm above zero. The result also shows that average of
sea level rise from 2004 to 2010 was 2.535 cmyear. The complete data of yearly regional sea level change can be seen in table 4.1.
Table 4.1. Result of regional sea level change data processing
Year Mean Sea Level mm
Rising Level
2004 22.98621622
2005 36.71316216
13.72694595 2006
63.38036111 26.66719895
2007 64.56962162
1.189260511 2008
137.3559722 72.7863506
2009 105.5638649
-31.79210736 2010
124.1595676 18.5957027
4.1.3. Global Sea Level Change
Based on result of tide gauge data processing data from CSIRO, eustatic sea level change shows the increasing of sea level linearly since 1880-2009 follows
equation Y=1.54X – 3060.07 with 0.97 of coefficient determinates R
2
. This equation is not constant as it is and might be changed along the time because
input of sea level height is not linearly constant either it decreasing or increasing. Figure 4.5 shows the pattern of global sea level change during 1880 to 2009.
Assumption from those CSIRO data, time when level of seawater firstly equal to the land has been occurred on December 1982 indicate in figure 4.5 by red
color of dot. Before that time, sea level constantly rose from -181.4 mm in January 1980 to -0.5 mm in November 1982 or rising rate is 0.1465 mmyear.
After 1982, sea level still raised with rising rate around 0.2108 mmyear or 1.5 times it rising before seawater equal to the land. Figure 4.5 below shows the
relation of sea level value and year where the average of sea level increasing from 1880-2009 is 22.2859 mmmonth complete data is in Appendix 2.
Figure 4.5. Monthly average of global mean sea level pattern based on tide gauge recorded andor reconstructed data data from CSIRO
As technology advances, the use of technology to record sea level data had been increasing and global sea level data as data comparison of tide gauge data
has been recorded. High quality measurements of global sea level have been made since late 1992 by satellite altimeters. In particular, TOPEXPoseidon launched
August, 1992, Jason-1 launched December, 2001 and Jason-2 launched June, 2008 had been recording sea level data globally. Data sets as a combination of
data from TOPEXPoseidon, Jason-1 and Jason-2OSTM has shown a steady increase in global mean sea level. The illustration of sea level increasing in 1993-
2010 can be seen in Figure 4.6.
Figure 4.6. Monthly average of global mean sea level pattern based on data of tide
gauge and altimetry satellite
The average level of rising between over from 1993-2010 based of satellite- derived data was 0.28 mmyear. If this rising is compared with rising level from
tide gauge data in 1903-1920 and 1953-1970 100 and 50 years ago, the rising average nowadays is 4.8 times higher than the value in 100 years ago and 3.9
times higher than the value in 50 years ago. Even if we compared global mean sea level from tide gauge data in the same previous periods 100 and 50 years ago
with the tide gauge data in this period 1993-2010, the rising average nowadays is 5.2 times higher than the value in 100 years ago and 4.2 times higher than the
value in 50 years ago. The value of the change is not the same between tide gauge and satellite, but the pattern of chart Figure 4.6 represent the same meaning, the
linear increasing of sea level with uncertain value. The data of sea level rise in from tide gauge and satellite in specific time can be seen in table 4.2.
Table 4.2. Global rising average from tide gauge and satellite in specific time scale
Period Global
Rising Average from tide
gauge data
mmyear Period
Global Rising
Average from
Satellite data
mmyear 1903-1920
0.058796296 1993-2010
0.28226601 1953-1970
0.072222222 1993-2010
0.317241379
4.1.4. Eustatic And Isostatic Sea Level Change
As the effect of regional and global sea level rise, mean sea level in Java Sea also increases. Mean sea level data from BMKG Semarang as the measured of
tide gauge represent those rising. The data that used in is data from January 2004 to August 2010 even there is exist previous data from 1985 to 2005 from
PT. PELINDO Tanjung Mas, Semarang Appendix 5. Those data data from PT. PELINDO can not used in this research because there is abnormal trend of
sea level change from 1998 to 2005. Different with data from PT. PELINDO, data from BMKG BMKG’s tide
gauge located in the same area with PT. PELINDO’s tide gauge shows the increase of sea level change in Semarang City follows linear regression function
where Y=10.58X-21180.35 with R
2
=0.87. In 2005, sea level average 47.764 cm was increased around 10.904 cm from it level in 2004 and level sea in 2006
50.204 cm was respectively increased around 2.440 cm from it level in 2005. Monthly mean sea level from January 2004 to August 2010 also shows the
increasing with 65.64 cm of total increase level on those periods or with monthly average of rising was 0.94 cmmonth complete data from BMKG Semarang can
be seen in Appendix 4. As the increasing of regional sea level sea level in Indonesia area with rising
level 2.535 cmyear, isostatic mean sea level in 2005 was reached 39.394 cm. isostatic mean sea level in 2006 was reached 41.929 cm, isostatic mean sea level
in 2007 was reached 44.464 cm, isostatic mean sea level in 2008 was reached
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
settlement also becomes submergence area. Peoples whose stricken with submergence hazard had been giving high effort as their adaptation effort to this
situation.
Figure 4.7. Recent Submergence Area
4.3.2. Adaptation Strategy
Every peoples has their own strategy to face the problem of flood in Semarang City. Peoples, whose their homes was covered by water and also
sinking by subsidence, rebuilt their home if they have budget for that or even move to saver place. For peoples whose haven’t budgeted for that, they must
live in their flooded home with their own strategy. Industry also adapted this
situation with modified their building or working facility, because it not easier and cheaper to relocate their building to another place like settlement. When
industrial building permanently flooded, employee continuing their work in that area.
Transportation facilities still running it function, car passed the road even 149 km of road network in Semarang City especially coastal area was transformed
into submergence area. For industrial or government office that has more budgets, the building was moved to saver place and older building had been
utilizing by human for some purposes like fishing pond. All fact of those different strategies as adaptation strategy to facing submergence area can be seen
in figure 4.8 below.
a b
c d
Figure 4.8. Various adaptation strategies in submergence area a plastic bed cover in permanent flooded bedroom, b car try to passing road that was transformed to
be submergence area, c working activity in submergence area, d fishing activity in former warehouse of an industry
