Predicting Land Use Affected by Land Subsidence in Semarang Based on Topographic Map of Scale 1:5.000 and Leveling Data Heri Sutanta, Arif Rachman R, Sumaryo, Diyono Department of Geodetic Engineering, Universitas Gadjah Mada Jl. Grafika, no. 2, Yogykarta, Indonesia Email: sutanta@alumni.itc.nl Abstract Semarang is a coastal city located in the coastal lowland of Central Java, Indonesia. The city is facing an unseen, but yet devastating, natural hazard caused by land subsidence. It is a natural phenomenon, but in Semarang is suspected to be accelerated by human factors. The obvious impact of it is the increase of sea water flooding. This work tries to predict physical impact of land subsidence by calculating land uses to be subsided, since it will to have economic and social consequences to the city. Large scale topographic map of scale 1:5.000, produced in 2000, have been used to construct digital terrain model (DTM) and detail land use map. Seven land use classes were defined in the work: building, road network, railway, fish pond, river, agricultural field, and bare land. To predict of the rate of land subsidence, a series of leveling survey data conducted between 1996 and 1996 were used. The intermediate results were land subsidence maps for the year of 2005 & 2010, which then used to predict DTM of 2005 and 2010. The final results indicate that that in 2005 and 2010, approximately 9.740,61 Ha and 9.787,83 Ha land to be subsided, respectively. The calculation also shows the area to be subsided for each land use classes. Complete field verification still need to be done in order to check the modeling results. However, the results have already given better information about the extent of land to be affected by land subsidence. Background Semarang located in 6 o 56 08 7 o 06 57 south latitude and 110 o 16 17 110 o 30 31 east longitude. It is the capital of Central Java Province, covers approximately 373,7 sq km of land area. The city consists of 16 sub-district (kecamatan) and 177 villages (kelurahan). The population in 2003 is about 1.389.000. During the last decade, Semarang is suffering from increasingly inundation of sea water. Local people called the phenomenon as rob, which means sea water flooding. Actually there are two types of flooding in Semarang, the second is coming from some rivers that passes through it. River flooding occurs during rainy seasons, from October to April. Sea water flooding reach its peak in the dry seasons. People living near port of Semarang, Tanjung Emas, told that they start to suffer from sea water flooding in the year of 1992 or 1993. Since that time the magnitude of the flooding is increase landward. The effect was continuous inundation in some parts of the lowland area of Semarang. Furthermore, some people were forced to flee from their homes, and many government buildings and private companies factories were abandoned (Figure 1). Suara Merdeka (13 July 2005), a local newspaper, reported that the extent of sea water flooding reach part of the airport of Semarang, Ahmad Yani airport. Although there is clear impact to the physical and social environment, but only recently publication in the newspaper related the flooding with land subsidence.  Figure 1. Impact of land subsidence on the housing and office building. Previous works have been carried out to study the land subsidence, its cause and impact in Semarang. Haryono (1995) write about impact of land use changes in the hilly part of Semarang which cause the reduction in the recharge of groundwater in lowland Semarang. The reduction, together with extensive groundwater extraction, was thought to be the cause of land subsidence. British Geological Survey (2000) studied the fall of piezometric head in Semarang which in part lead to the subsidence of the land. Tobing, et.al., (1999) reported the leveling measurement in Semarang and the relation with other data and geological properties of the city. Sutanta (2002) calculated predicted land uses to be subsided based on subsidence rate obtained from leveling data and medium scale map. The map was derived from classification results of the band 5, 4, 2 of the Landsat-7 ETM+ fused with its panchromatic band. The current work still incorporated the same leveling data, but with the addition of large scale topographic map of scale 1:5.000. The purpose of this research is to refine the previous work in order to have better estimate of the predicted impacts to the land use. Land Subsidence, their measurement and prediction Land subsidence occurs when the elevation of the land is lowered from its previous position, with respect to a fix height reference system. It might be caused by natural phenomena, human activities, or the combination of both factors. Those factors include normal consolidation of clayey sediment, drying out of soils due to a fall of water table, building loads, extensive groundwater pumping. Land subsidence mostly occurs in the coastal lowland, in which the soil was formed by alluvial deposit. Some mega cities in the Asia, like Jakarta and Bangkok experience land subsidence. Semarang city is not the exception, since it is located in relatively young alluvial deposit. British Geological Survey (1999) estimated that the consolidation of clay layer will reach 5.5 m. The rate of it will follow a logarithmic trend, with 50% of the consolidation will occurs in about 12 years, and 90% in about 50 years. Terboyo industrial estate which located in the north-eastern part of the city was estimated to have land subsidence up to 5 m. This estimation was confirmed by leveling measurement that shows the rate of subsidence in an area located 3 km on the west was very high, 21 cm in 3 years (Basuki, 2000). Leveling technique is the most establish method of measuring height difference and change. This technique was used by the Directorate of Environmental Geology (DoEG) and Office of Mine and Energy of the Central Java Province to monitor subsidence rate. The measurements were done annually, but sometimes for different part of the city in the successive years. This research was making use of leveling data as reported by Tobing et.al., (1999), and Basuki (2000). Some of the leveling data are presented in table 1. 
The table reveals that there is spatial variability of the subsidence rate. Different rate occurs for the same area in the different years. It creates difficulties in define the most possible scenario of the subsidence. A simplification was made to generate land subsidence prediction. The rate of subsidence is assumed to be linear. The results indicate that it still give conservative estimates, still lower that BGR estimate. The leveling data were than interpolated to create a subsidence map. Land Use Map and Digital Terrain Model Land use map is needed in the process of risk assessment. In 1999 there was an aerial photo mapping of the city, which results in the topographic map of scale 1:5000 released in 2000. This scale is the highest one available that cover the whole city. In total 64 map sheets cover the whole city, but only 33 of them were used. The research area covers the lowland part of the city in the north. Figure 2 illustrated research area, composed by a DTM based on topographic map of scale 1:25.000 and Landsat-7 ETM+ image overlaid on it. Figure 2 is a view from the east. Width of the coastal lowland area in the eastern part of the city is approximately 10 km, while in the western part approximately 5 km. central- and eastern-north of the city is extensively used as residential, business and industrial area. Fishpond is the main land use in the western-north.  Figure 2. DTM of Semarang with fused Landsat-7 ETM+ image (Sutanta, 2002) Southern part of the city is hilly, and therefore not subject to land subsidence. In this area, detailed land used map was generated with the following land use classes: house, road, agriculture field, fishpond, railway, river, and bare land. The land use map was then processed in GIS software to calculate width of each class. It was also calculated the number of feature in each class, e.g. the number of houses in the research area. Besides land use classes, the topographic map contain layers of contour and spot height. The interval contour was 2 meters, while the spots height was well distributed in the lowland part where contours are scarce. These data were sources to refine the contour, using digital terrain model (DTM) as an intermediate result. To DTM of the area, contours and spots height were combined. Inverse Distance Weighted interpolation method was used. The DTM presented in a grid format with 1 m pixel size, which later converted into vector map. Calculation of Land Use Classes in the Predicted Elevation Class Having the land use map and elevation map in the vector format, the calculation was performed to obtain information about land use in a specific elevation class. The land actually already been subsided in 2000, as the data listed in table 1 show. To predict the condition in 2005 and 2010, the estimation on land subsidence should be incorporated. There is also additional factor that may affect the extent of the increasing of sea water flooding, the global sea level rise. Some research in the global context provide estimate on the rise of sea level, which generally consist of three estimates: low, medium and high. Semarang is also subject to sea level rise, but very limited data available to verify the magnitude of it. Since the area is subsided, the relatively short period of tidal data may also be biased. A portion of sea level data in Semarang is as follow (BAPPEDA, 2000).
There is un-usual variation in the mean sea level (MSL), which is most likely coming from other cause(s) than global sea level rise. A study sponsored by ADB (1994) stated that the high estimate of the rise of sea level in Indonesia region will only about 15 cm in the year 2010. Due to very small influence and also regarding the interval contour of the topographic map, it was decided that the absolute sea level rise projection was excluded in this study. Actually sea level in Semarang is rise, given the fact that the land is fall. This phenomenon is called as relative sea level rise, which make the land become inundated by sea water. To predict the magnitude and extent of inundation, new DTM were derived from the existing one and the land subsidence map. Simplification was made by making prediction of land subsidence as linear trend. In this case, if a particular area has been subsided by 6 cm per year as revealed from leveling data, then that area is predicted to be subsided by 30 cm and 60 cm in 2005 and 2010, respectively. The process was done using the following equation: DTMt1 = DTMt0 (RoLS * (t1 t0)) where: DTMt1 = DTM at the year to be estimated DTMt0 = DTM at the initial condition RoLS = rate of land subsidence, which already being translated into map. The result will be the predicted DTM2005 and DTM2010. The DTM were then classified into elevation classes, in the form of contour. Height interval between classes was 0.5 meter. The objective of this step was to obtain a vector map to be overlaid with land use map. Simple operation of intersection was carried out. Two input maps were land use map and contour map. The result were land use in different elevation class, e.g. number of building located in area between 0 0.5 meter, etc. (Table 2). 
Number of building in area situated below 0 m is increase substantially from 2005 to 2010. The land use class building consists of house, factory, school, warehouse, and public facility. There area many residential areas located in the vicinity of coastline near port of Semarang. Therefore, many inhabitants will be or already affected by the impact of land subsidence. It is confirmed by field visit in 2005 that many houses in different location in the northern part of the city have been abandoned by the owner. The fishpond area covers the largest area at the lowest elevation. This is not surprisingly, because the fishpond located very close to the sea where they can obtain water. This situation means that they are very vulnerable to the impact of land subsidence. The likely impact will be further inundation of the fishpond area, making it useless and causing great economic loss. The increasing number of buildings or area of certain land use class in a particular elevation class was not linearly following the changes in other elevation class. It has to do with spatial distribution of the land use class. Conclusion and Recommendation There are clear evidence that land subsidence is occurs in Semarang, and affecting northern part of the city. The rate of subsidence varied in spatial and temporal aspect. With regard to the available data, a simplification was made in projecting future land subsidence by using linear trend. This projection was converted into vector map and used to derive predicted future DTM in the year 2005 and 2010. Intersection operation between predicted DTM and land use map resulted in a prediction of land use to be affected by continuous process of land subsidence. The number of building, as well as other land use, situated in area with elevation below 2.5 m is increase substantially. Considering that the population will grow, and many people do not afford to buy house in safer places, then more people are predicted to be affected by this coastal hazard. Acknowledgement Initial work leading to this paper started when the first author study at ITC, the Netherlands. Universitas Gadjah Mada, through its Research Institute, is thanked for provide funding in 2003 & 2004 to continue the research. We would like to extend our thanks to BAPPEDA Kota Semarang who provide the topographic map, and the Agency of Mine and Energy of the Central Java Province who provide their research report and data. References
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