Hello, and welcome to my project report for CE179! My name is Victoria Lim, and I am a current senior at UC Berkeley, studying Civil and Environmental Engineering. In this report, I hope that you will be able to learn more about land subsidence, particularly in Indonesia. My interest piqued for this topic, as my grandparents still currently reside in Jakarta, Indonesia. It was quite enjoyable for me to research into the current happenings, of a land that is special to me. I hope that this report will not only be informational and educational for you, but also be very intriguing. Thank you for your time and interest!
Land subsidence is the settlement and sinking of the ground surface. Although a natural process, it can also be expedited by human activity. Sinking can be both rapid or gradual over time. This sinking has occurred for more than 100 years, and has impacted both the land and inhabitants of Indonesia. In the cities of Jakarta and Semarang, land subsidence has been measured and monitored since the 1970s due to its increasing concern to Indonesian residents and inhabitants. Although there is no explicit reason or understanding of what is causing the increasing rates of land subsidence, there are hypotheses and preventative measures being enacted to monitor and slow the ongoing event. This issue of land subsidence is not a passive phenomenon; Indonesian livelihoods have been and are being affected. For the purpose of this project, there will be a focus upon the land subsidence phenomena within the cities of Jakarta and Semarang, Indonesia.
Below is a short introductory clip, visually depicting the circumstances in Jakarta:
The city of Jakarta, Indonesia is home to about 10 million people, and increasing. Despite its large population, the city is contained within a rough area of 660 km2. Jakarta is characterized by its lowland area---a stretch of land near sea level, and relatively flat with an absence of large hills and valleys. Despite its small area, there are many landforms within the region.
These landforms consist of:
1. Volcanic alluvial landforms (Southern region)
2. Marine-origin landforms (Northern region near the coast)
3. Beach ridge landforms (Northwestern and Northeastern regions)
4. Swamp and mangrove swamp landforms (coastal fringe region)
5. Former channel landforms (region perpendicular to the coastline)
Within these different landforms, the rate of subsidence varies. This is due to not only the different soil composition, but also the different loads that may go on top of each region. Some regions are more prone to high population density than others [Abidin et al., 2011].
The presence of water is great in Jakarta. Not only is Jakarta a coastal city, but it also has 13 natural and artificial rivers flowing through its land. The main rivers contribute as the main drainage system of the city [Abidin et al., 2011].
The issue of land subsidence was first noticed in 1926. Since then, efforts of land subsidence investigation and monitoring have been conducted. Despite this, the occurrence still increases to this present day. Especially with the continuous rise in population and urban development, land subsidence also increases with these trends. Land subsidence is one of the reasons to why there has been discussion of moving the capital of Indonesia from Jakarta to another city in East Kalimantan. Despite all of the research and monitoring that has occurred over the ~100 years of land subsidence in Jakarta, there is still no clear understanding of this occurrence: its clear cause, nor its predicted rates [Abidin et al., 2001].
Located in the province of Central Java, the city of Semarang has also experienced land subsidence from the 1900s. With a population of about nearly 2 million people, and an area of about 370 km2, Semarang has similarly been tracking land subsidence phenomena since the 1990s.
Unlike Jakarta, Semarang consists of both flat regions, and hilly regions. The northern coastal region of Semarang is similar to the land of Jakarta—lowlands with flat coastal areas. The southern region of Semarang is the hillier region, with greater open space, and is more suburban. It is recorded that the settlement rates in the northern flatter regions, are much higher than the southern settlement rates. This is partially due to the different types of soils present within each region.
The northern area consists of volcanic rock, while the southern area is composed of sedimentary rock and alluvial deposits. The sedimentary rock is composed of clay rock and sandstone, and the alluvium deposits are young beach alluvium with high compressibility [Abidin et al., 2013]. It is upon this northern Semarang area of sedimentary rock and alluvial deposits that greater land subsidence occurs. In addition to the soil conditions, the northern region also has a higher population density, and is more urbanized than its southern counterpart. Like Jakarta, Semarang is experiencing exponential population growth and land use (industrial, agricultural, residential) [Marfai & King, 2007].
The most obvious indications of land subsidence can be seen by the damage done to existing buildings and infrastructure. This is most notably shown by the cracks through the built environment. These apparent cracks pose a concern for the remaining structural integrity of the buildings. The reduced structural strength of the cracked buildings and infrastructure increases the safety for inhabitants. Beyond cracks, buildings can also tilt due to subsidence. When the cracks and tilt become too great to ignore, there is no other alternative except to abandon the building or infrastructure [Abidin et al., 2013].
In an environmental lens, as land subsidence increases, flooding increases, as well. This occurrence is evident in areas near the coast. The increase in flooding refers not only to the frequency at which floods occur, but also the coverage area of the flood. For example, in Semarang, from 2008 to 2011, the flooded area expanded to 2-3 km from the coast [Abidin et al., 2013]. As flood areas expand, the coastline is threatened, because if the current trends persist, the coastline may eventually be submerged permanently. Increasing inland sea water intrusion is also another result of land subsidence. This means that tides will move further inland, washing over areas that had previously not been affected [Abidin et al., 2011].
As flooding increases, more strain is exerted upon the city’s drainage system. However, land subsidence also affects and changes the drainage system, bringing more malfunctions and difficulty in pumping out sewage and storm water. The drainage system is altered as the land subsidence brings about changes in elevation, as well as changes in slope for streams, canals, drains, etc. As the river canals change, drainage systems then malfunction as they cannot effectively redirect excess water. Additionally, the drainage capacity of the system is reduced, and will result in potential waterlogging or salinization [Marfai & King, 2007].
Beyond structural, infrastructural, and environmental implications, the phenomena of land subsidence also affects the livelihood of building inhabitants. Those living in affected land areas must expect lesser living conditions. This includes lower health and sanitation standards, as well as negatively impacted social and economic aspects. Those living in cracked or tilted buildings are in constant structural danger, and those living in flooded regions do not have access to dry, sanitary homes, along with a disruption to their daily livelihoods.
Land subsidence does not come without a cost (especially economic costs). Great economic loss is brought upon by this phenomenon from the constant maintenance and rehabilitation costs. Due to exacerbated living conditions, and damage to both private and public property, costs are driven up. One reoccurring cost the government must pay for is to have the ground surface frequently raised, in an effort to keep roads and buildings dry during flooding [Abidin et al., 2013].
Overall, the occurrence of land subsidence is not one to be ignored, especially to its inherent consequences for humans, environment, and economy.
Despite land subsidence being a known phenomenon for already over a century, there is still no defined understanding for the direct cause of the prominent land subsidence occurring in Indonesia. There are, however, many factors that are believed to be great contributors to this phenomenon.
Some of the considered factors are listed below:
A. Soil composition
B. Increasing population and industry
C. Tectonic movements
D. Excessive groundwater extraction (digging wells)
Some of these factors may be significant enough to influence land subsidence by itself, but some other factors may just be contributing factors, such that the combination of a number of factors brings and intensifies the land subsidence process.
Depending on the type of soil, the soil itself may cause land subsidence. This refers to the natural consolidation soil (clay) undergoes under the pressure of a vertical load. As discussed earlier, the soil composition in Semarang differs between the north and south---the north having greater subsidence rates than the south. As the northern soil is composed of sedimentary rock and alluvial deposit, these soil materials are much more compressible than the volcanic rock in the south. Especially if no soil preparation nor foundational work had been implemented prior to the building construction in Semarang, then some of the subsidence that is occurring is just the natural phenomenon of clay consolidation under loads.
Based on site investigations, it is found that the soil in the subsiding land of Semarang is a “Holocene clay and sand with a thickness of more than 80 m [Marfai & King, 2007].” This signifies that the soil is of high compressibility, and will thereby be subject to high consolidation.
As both Jakarta and Semarang are some of the largest cities in Indonesia, both population and industrialization have been increasing through the many decades. However, with increasing population and industry, the city must provide for greater loads upon the same ground coverage area. These greater loads are from the increased construction for residential areas, working spaces, and entertainment spaces. Especially if the soil composition is of high compressibility, the increase in load upon the ground surface will only intensify any land subsidence.
Beyond the imminent loads that come with increased population and industrialization, the population growth will also trigger the necessity for the increase in and the change in land use. If land use is not regulated, and is left uncontrolled, then the environmental well-being of the land will go for naught. Excessive agricultural land use will deplete the soil of its nutrients, and excessive water drawing to meet the demands of the growing city will sink the water tables [Marfai & King, 2007]. Both of these result in a weaker soil, which will continue the positive trend in land subsidence.
The consideration of the relation between tectonic movements and land subsidence is not a well explored topic for Indonesia, however, this does not mean that this possibility should be ruled out. Although there are earthquakes present in Indonesia, research has not been able to support a strong case for the impact of tectonic movements in the land subsidence of Indonesia. However, in Taiwan, research supported the contribution of tectonic subsidence in total land subsidence. However, this is in the case that the affected land is on a subduction zone [Tran & Wang, 2020]. In Indonesia, there is currently no known active fault lines above Jakarta or Semarang [Abidin et al., 2011]. Although the geographic features of Indonesia do not match the research done in Taiwan, due to the many unknowns and hypotheses of the situation, the possibility that nearby plate tectonic movements could be influencing the land subsidence in Jakarta and Semarang is still valid. At least for the amount of research present currently, it is safe to assume that of all the factors contributing to land subsidence in Indonesia, tectonic movements have the least influence [Abidin et al., 2011].
Another consequence of population increase and industrialization, is that there is in turn a greater demand for both potable and non-potable water. To solve this problem since the 1900s, wells have been dug at an exponential rate across Jakarta and Semarang. From the year 1900 to the year 2000, groundwater extraction from digging wells increased from 0.4 million m3/year to 38 million m3/year [Abidin et al., 2013]. Urban development and economic activity call for more groundwater extraction. The decision to continuously draw from the ground instead of treating used water is due to the current inadequacy of supplies for a treatment procedure, and also because of the greater financial costs that are associated with water treatment. Thus in Jakarta, 64% of the water demand is fulfilled by groundwater extraction [Abidin et al., 2011]. Of all of the factors previously listed above, there is great support that the main cause of Jakarta and Semarang’s land subsidence is from excessive groundwater extraction [Abidin et al., 2001].
When groundwater is extracted excessively, the groundwater level (water table) will decrease. The consequence for this is land subsidence and seawater intrusion [Abidin et al., 2001]. Fluid pressure can also change between soil layers as a result of groundwater extraction. Clay materials under heavy pumping may become permanently compacted. In addition, after excessive pumping occurs, the open pore spaces collapse, decreasing the aquifer’s overall storage capacity [Marfai & King, 2007].
In particular, it is the deep groundwater extraction that is causing the land subsidence in Jakarta. There are two types of wells: shallow and deep. The shallow wells are less than 40 m deep, while the deep wells are greater than 40 m. Before heavy industrialization, shallow wells were more common, as they are dug or driven by the population. However, with industrialization, deep wells became more common; these types of wells are drilled by the industry. It has been found that the excessive groundwater extraction in North Jakarta had caused the water table level to decrease from 12.5 m above sea level to 30-50 m below sea level from around 1910 to 1990 [Abidin et al., 2005]. Deep groundwater extraction is much more impactful than the shallow groundwater extraction and thereby is a lead cause for the land subsidence in Jakarta [Abidin et al., 2001].
Through the 50 years of research and monitoring land subsidence in Jakarta and Semarang, many techniques have been used and developed. These techniques measure the rate at which subsidence occurs, and are used to predict future land subsidence as well.
The following geodetic (land surveying) techniques have been performed:
B. GPS surveys
C. InSAR & DEM in GIS
Leveling surveys are one of the first surveys to be performed to reliably analyze the rate at which land subsidence is occurring. It was first established in 1925 in Jakarta, but the first known Jakarta leveling results are from 1978. In performing the leveling surveys, each leveling line of ~1 km each was measured in double-run. This means that there is both a forward and bakward run being measured; this is to double-check the accuracy of the measurement. There have been six leveling surveys conducted through the 1990s, however only three surveys (1982, 1991, 1997) produced acceptable outcomes per a standard of reliability and quality. Even out of these 3 viable surveys, only 45 leveling points could be considered, as they had more reliable and stable outputs [Abidin et al., 2011]. Stability of the point is determined on how stable and constant the leveling reference benchmark point is.
Through the data, it was shown that the maximum subsidence rate was 8 cm/year between 1982 to 1991. However, this rate increased to 26 cm/year from 1991 to 1997. Leveling surveys have provided a somewhat consistent method in monitoring the land subsidence situation across Jakarta.
Similarly in Semarang, leveling surveys were conducted off of 29 leveling points from 1999 to 2003. The results confirm how the northern coastal Semarang region does in fact have higher subsidence rates in comparison to the South. The rates are above 8 cm/year, and ranged to 17 cm/year [Abidin et al., 2013].
GPS stands for ‘global positioning system.’ This method of GPS surveys utilizes satellites for gathering all of the data, which consists of three-dimensional position and velocity, at a given recorded time. To prepare for this method, several monuments are positioned on the ground relative to stable reference points, elsewhere [Abidin et al., 2013].
The GPS survey methods began monitoring the land subsidence in Jakarta in 1997. The GPS surveys were able to be more frequently conducted---13 surveys were conducted between 1997 to 2010. The stations used to gather data for each survey were not control variables, as some stations could not be observed at each study period. This was due to either damages to the monument, or obstruction from the new surroundings (trees, construction) causing lost signal. Not only were some different stations being used for different surveys, but the number of stations being used increased gradually from 13 to 65 stations from 1997 to 2010 in Jakarta [Abidin et al., 2011].
The duration of each GPS surveying session lasts 9 to 11 hours, with data collections every 30 seconds. This is done using dual-frequency geodetic-type GPS receivers, which will ideally measure to the mm level [Abidin et al., 2011].
Semarang began GPS surveying much later than Jakarta, in 2008. However they has been consistent within the consecutive years from 2008 to 2011 in using the same 44 stations [Abidin et al., 2013].
The southernmost station “SMG1” is the reference point for this network of Semarang GPS stations, as it is most stable.Below, are some examples of GPS-derived outputs for Jakarta and Semarang. It is depicted how the subsidence rates vary per year in different parts of the region, and can have a settlement range of 0 to 16 cm/year:
The InSAR (Interferometric Synthetic Aperture Radar) method began implementation in 2004. The data necessary for this method is gathered through satellite, as well. InSAR data is used to help generate DEM. The advantage of InSAR over the GPS method, is that InSAR "can image the line of sight (LOS) component of deformation over a large area at spatial resolution of tens of meters [Xiao & He, 2013]."
The DEM (digital elevation models) in GIS (geographic information system) assist in the prediction of future land subsidence. First, a high point map is created through photogrammetry high spots. With the help of a moving average interpolation system, the DEM is then generated. As the DEM data is processed, predications of elevation models are then generated, and through this the future land subsidence can be predicted. The quality of the DEM data is determined by the land that is being surveyed, as well. Factors like terrain roughness, sampling density, grid resolution, interpolation method, and etc., will affect the data quality.
Similarly to leveling and GPS, benchmarks are installed in order to have reference points. These benchmarks began installation in 1983 in Semarang. Like the points established for leveling, only 38 of the 70 installed benchmarks provided good data. The ones that did not provide ideal data were damaged from unstable conditions or environmental disturbances [Marfai & King, 2007].
When determining predictions for future year subsidence, analysts employ an equation, under the assumption that the rate of subsidence stays constant (no efforts to subdue subsidence are taken). To ensure as accurate of predictions as possible, three measurements are taken on each benchmark, within the setup to end time. This way, the time-subsidence model can be attained for each point [Marfai & King, 2007].
For more detailed information upon the DEM in GIS process in predicting future land subsidence, please reference Marfai & King (2007).
Land subsidence, although affecting the Indonesian population, is still under close monitoring. Through the efforts of leveling and GPS techniques, the rates at which subsidence is occurring are recorded. Then, through this, the techniques of DEM and InSAR technologies may be utilized to help predict future subsidence, assuming similar conditions. With these predictions, provincial Indonesian government can plan accordingly in how to combat this issue that is affecting the livelihoods of the Indonesian inhabitants. As land subsidence rates decrease, it can be expected that economic, social, and environmental costs will decrease, as well.
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