Monitoring land subsidence in Semarang, Indonesia
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- Marfai, M.A. & King, L. Environ Geol (2007) 53: 651. doi:10.1007/s00254-007-0680-3
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Semarang is one of the biggest cities in Indonesia and nowadays suffering from extended land subsidence, which is due to groundwater withdrawal, to natural consolidation of alluvium soil and to the load of constructions. Land subsidence causes damages to infrastructure, buildings, and results in tides moving into low-lying areas. Up to the present, there has been no comprehensive information about the land subsidence and its monitoring in Semarang. This paper examines digital elevation model (DEM) and benchmark data in Geographic Information System (GIS) raster operation for the monitoring of the land subsidence in Semarang. This method will predict and quantify the extent of subsidence in future years. The future land subsidence prediction is generated from the expected future DEM in GIS environment using ILWIS package. The procedure is useful especially in areas with scarce data. The resulting maps designate the area of land subsidence that increases rapidly and it is predicted that in 2020, an area of 27.5 ha will be situated 1.5–2.0 m below sea level. This calculation is based on the assumption that the rate of land subsidence is linear and no action is taken to protect the area from subsidence.
KeywordsLand subsidenceMonitoringDigital elevation modelILWISSemarangIndonesia
Land subsidence, as a movement of a surface downwards relative to a datum such as sea level, is a major constraint to the development in many regions all over the world. It can damage existing buildings, roads, bridges, industrial estates, and result in loss of homes, even cause injury or loss of life. There are many factors causing land subsidence; it can occur naturally or by human activity. Natural subsidence may occur by collapses, for instance mine and limestone collapses (e.g. Hasan 1996). It also commonly occurs over man-made voids, such as tunnels, wells and covered quarries. Subsidence is also possible by faulting (e.g. Rathje et al. 2003; Gutiérrez 2004), in the foot wall of normal geological faults. Furthermore, subsidence may occur by thermal contraction of the lithosphere (e.g. Hamdani et al. 1994). It may also be caused by extraction of natural gas (e.g. Cassiani and Zoccatelli 2000). However, the most phenomenal causing of land subsidence is extreme groundwater withdrawal (e.g. Sun et al. 1999; Mousavi et al. 2001; Bhattacharya et al. 2004; Chai et al. 2004; Lamont-Black et al. 2005). Groundwater withdrawal results in fluid-pressure change in the layers, especially in sedimentary and clay materials. Excessive pumping of such aquifer systems, which contain rich clay materials, may cause permanent compaction that cannot be recovered after stress and leads to land subsidence. When large amounts of water are pumped out, the subsoil compacts, thus reduces the size and number of the open pore spaces in the soil that previously hold water. This can result in a permanent reduction in the total storage capacity of the aquifer system as well. This phenomenon is common on coastal regions and urban areas with dense housing, buildings and industrial estates all over the world.
Many urban areas in Indonesia, like Jakarta, Bandung, and Semarang suffer from land subsidence. Excessive groundwater withdrawal is probably the main factor causing land subsidence in urban areas of Indonesia. According to Abidin (2005), several areas of Jakarta have been subsiding at annual rates ranging from 20 to 200 cm over the years. The impact of subsidence in Jakarta appeared for the first time in 1978 in the form of cracking on permanent constructions, expansion of flooding areas, lowering of groundwater level and increased inland–seawater intrusion. Land subsidence in Bandung is caused by rapid sinking of water tables due to the increasing population and industry since the 1990s. Meanwhile, in Semarang land subsidence is mainly due to excessive groundwater withdrawal and natural consolidation of alluvium soil. Therefore, monitoring land subsidence in suspected cities is required for groundwater withdrawal regulation, effective control of floods and seawater intrusion, conservation of the environment, construction of infrastructure, and spatial development planning.
Investigations for the prediction and modelling of land subsidence in Semarang have been done by various researchers using various methods and approaches, e.g., based on models incorporating geological and hydrological parameters (Tobing et al. 1999), levelling and Global Positioning System (GPS) survey (Abidin et al. 2001; Sutanta 2002; Abidin 2005). This paper will concentrate on monitoring land subsidence in Semarang using digital elevation model (DEM) data in a Geographic Information System (GIS) environment. Future land subsidence prediction based on raster operation will be generated in GIS environment using the Integrated Land and Water Information System (ILWIS) package (ILWIS 2000). A high point map, which was created from photogrammetry high spot is used to generate DEM using a moving average interpolation system. Superimpose benchmark data and DEM data is processed in raster operation to generate the prediction of elevation models, which indicate future land subsidence for the years 2010, 2015, and 2020, respectively. The total area of subsidence in certain levels has been calculated using histogram analysis for each prediction year. This study has benefited from benchmark data issued by the Public Work Department of Semarang (PWD 2000).
Description of Semarang
Description of landuse in Semarang
Types of landuse
Dry land farming
Open pit mining
Industry and tourism
From a geological point of view, Semarang has three main lithologies, namely, volcanic rock, sedimentary rock, which is marine in origin, and alluvial deposits. Volcanic rock consists of volcanic breccias, lava flows, tuff, sandstone, and clay stone. This area is located on the Southern part of Semarang. Sedimentary rock originated from marine consists of clay stone and dominated by sandstone in between. Alluvial sediment consists of beach deposits formed by clay and sand with a thickness of more than 80 m with the age of the Holocene period. It is on these deposits (Marin sediment and alluvial sediment) where the land subsidence is occurring.
Nowadays, the sustainability of groundwater resources in Semarang is at risk from overuse and induces natural hazards such as subsidence and tidal inundation (Sutanta 2002; Marfai 2004). Without adequate recharge, the sediments become increasingly compressed causing the land to settle or subside.
Land subsidence in Semarang
Land subsidence may occur due to building loads (residential houses and industrial buildings) and groundwater pumping, particularly on the clayey sediments layer, by then the ground will be lowered from previous elevation. Therefore, land subsidence is more likely to be a problem in areas underlain by clay-bearing layers such as in the coastal area of Semarang. On this area subsidence occurs gradually and spreads widely. Since the 1980s the land subsidence in several places of Semarang has been measured using several measurement techniques, e.g., benchmark and field surveys, Dutch cone soundings, consolidation tests, standard penetration tests, and groundwater level studies (PWD 2000).
Land subsidence and its rate in Semarang, measured from benchmark data
Elevation in 1996 (m)
Elevation in May 2000 (m)
There are other less visible slow-onset problems that can be expected change of drainage system. Field and channel drainage can submerge because of higher water level in estuaries and river mouths. Submersion reduces then the drainage capacity and may cause problems of water logging and salinisation.
Monitoring the future of land subsidence in Semarang
Monitoring future land subsidence in Semarang has been done using raster data operation of the DEM in GIS environment. A digital representation of relief of Semarang area was generated from a point map which contains elevation data. Several factors play an important role for the quality of DEM-derived products, among them are, terrain roughness, sampling density (elevation data collection method), grid resolution or pixel size, and interpolation method. In this study, the point map has high consistency and variety of the elevation data which indicates a high-quality of terrain roughness both on the hilly and lowland area. The elevation data were obtained from a photogrammetry high spot based on an aerial photo. It has spot heights at 100 m distance in the undulating and hilly area, and 200 m intervals on the lowland and flat area. The last update of the elevation data was made using geodetic technique by Amhar (2001 after Sutanta 2002), using Global Positioning System (GPS) by Sutanta et al. (2003), and through field checking based on the benchmark data from PWD by Marfai (2003). The DEM has been created in the ILWIS software using moving average interpolation method. Point interpolation performs an interpolation on randomly or regularly distributed point values and returns regularly distributed point values which are represented on the raster map. Moving average assigns to pixels weighted averaged point values. Values of points which are close to an output pixel are thus of greater importance to this output pixel value than the values of points which are farther away. Therefore, with regards to the previous processes that have been made, the DEM data on the study area is advantageous and reliable for this research.
Expected area 0–2 m below sea level in Semarang
Elevation (m) below sea level
Number of pixels
Number of pixels
Number of pixels
From Table 3 can be seen that the area of subsidence will increase in the future. With an extending size of the urban area on the coastal plane and a growing population, the impact is predicted to be even more severe. When no action is taken against land subsidence, parts of the coastline will be lost and become permanently inundated by seawater. More inundation areas due to tidal flood overflow will be another negative effect.
The occurrence of land subsidence due to excessive groundwater withdrawal and natural consolidation of alluvium soil is seldom as obvious as it is in the case of mine collapses, limestone collapses, or catastrophic sinkholes. Where groundwater withdrawal and drainage of clay soils are involved, subsidence is typically gradual and widespread, and hard to detect and evaluate. The detection of regional-scale subsidence originates in the discovery that key benchmark have moved. Once unstable benchmarks are discovered, and truly stable benchmark have been established, subsidence can be mapped. Once subsidence is identified and mapped, subsidence-monitoring programs can be implemented and future subsidence rates can also be calculated.
In the case of Semarang City, subsidence is widespread and gradual. This process takes a long time and land-subsidence related problems hardly become evidence immediately; therefore the need for subsidence monitoring and preventive action is difficult to demonstrate. However, land subsidence in Semarang has now become evidence. Field observations demonstrate that there are a lot of damage on settlement, structures and buildings and even tidal inundation occurs due to land subsidence.
Land subsidence in Semarang has been monitored using DEM data accompanied by subsidence data in order to know the subsidence trend. The elevation below sea level is expected to increase in the future, as e.g. the area between 1.5 m up to 2.0 m below sea level will reach 275,000 m2 in the year 2020. The prediction and calculation the area of specific landuse types that fall within a certain elevation class for the year 2010, 2015, and 2020, respectively, will be necessary.
The monitoring of the land subsidence through DEM data analysis using the GIS technology provides valuable data for the prediction of the future subsidence (rate and the spatial distribution) since there are limited data from other techniques, i.e., leveling and GPS survey. However, leveling techniques and GPS surveys are considered as good and reliable for measuring and detecting subsidence in Semarang. Subsidence monitoring and mapping programs are crucial for scientific understanding as well as for the management of land and water resources by government agencies, planners, regulators, and administrators in order to manage and to protect the coastal resources.
This article derives from research in progress by M. A. Marfai on Risk assessment of tidal inundation under the scenarios of sea level rise and land subsidence at the Justus-Liebig-University Giessen, Germany, supported by The German Academic Exchange Service (DAAD). The authors express their gratitude to Mr. Rosyid (Semarang Public Work Department) for providing valuable benchmark data and the reviewers for their helpful advice.