**2. Land subsidence and its impacts**

Land subsidence by definition is a lowering of the ground level from a reference height system such as geoids or the level of the sea. The rates of subsidence can commonly vary between 1 and 20 centimeters per year and even more in some places. Over decades a magnitude of 4 to even 10 meters of subsidence can be found in some places in the world [1–9]. Capital or big cities, industrial areas, peatland areas, oil and gas fields, geothermal fields, and underground mining areas are the most common places where significant land subsidence may occur.

> Infrastructure and building cracks, problems with drainage, the wider expansion of flooding areas, and tidal inundation as mentioned earlier are several impacts from subsidence (**Figure 3**). Surprisingly, many capital cities in the world (e.g. Jakarta, Tokyo, Osaka, Venice, Bangkok, and Ho Chi Mien) have suffered from land subsidence. The consumption of huge amounts of groundwater has led each city to subside significantly over many decades [1, 5–8, 10, 11]. Increased stress followed by compaction in an aquifer will result in subsidence of the surface of the ground. Some research has concluded that impact or the consequence of land subsidence is a disaster that takes place over time. However, over time impacts such as cracks on infrastructure or floods are worsening and will result in a real disaster. These infrastuctural cracks will eventually become a real danger. Road or bridge damage can be a danger for transport. The wider expansion of flooded areas is also due to continuing subsidence. Economically, huge

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In many places, short-term measures have been created to mitigate these disasters such as building temporary dykes, fixing or elevating roads, repairing houses and land, including

amounts of money are needed to fix the problems of land subsidence.

**Figure 2.** Field survey GPS observation to derive land subsidence (source: authors).

**Figure 1.** Principle of monitoring land subsidence by geodetic measurements (source: authors).

Geodetic measurements such as GNSS GPS surveys, InSAR, and leveling can reveal the rate and magnitude of land subsidence accurately. Other geotechnical approaches also commonly used are the extensometer and tilting measurement. **Figure 1** shows how to monitor subsidence using GNSS GPS and InSAR survey methods, while **Figure 2** illustrates the use of GPS data acquisition in the field. With GPS, several points, which are placed on the media (e.g. benchmarks) covering the area of investigation, are accurately positioned using GPS survey relative to a certain reference (stable) point. The precise coordinates of the benchmarks are periodically or continuously determined. By simply evaluating the rate of changes of the height coordinates from time to time, subsidence characteristics can be revealed. With InSAR, by using two or more synthetic aperture radar (SAR) images of an area, surface movements over time can be identified. The phase signal of InSAR records information on the distance between the satellite and the Earth's surface. By differential InSAR, where we use two SAR images of the same area acquired at different times, subsidence can be revealed if the distance between the ground and the satellite has changed.

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**Figure 1.** Principle of monitoring land subsidence by geodetic measurements (source: authors).

some decades a magnitude of 3–4 meters of subsidence has taken place in Jakarta and Bandung, and probably in other unsurveyed areas. The impacts or consequences from land subsidence can be seen in several forms such as infrastructure damage, wider expansion of flood water, as well as tidal inundation (flooding by sea water at coastal areas experiencing

In certain regions of Indonesia, the impact of land subsidence in the form of flooding and tidal inundation clearly exists. In Jakarta and Bandung we can see that the areas close to rivers, which suffer significant subsidence, are frequently being flooded. Tidal inundation is also a regular feature in the coastal areas of Jakarta, Blanakan, Semarang, and Demak, which are also experiencing large magnitudes of subsidence. This is inevitable when lowland coastal areas are lower than the level of the sea after experiencing subsidence; this leads to sea water

This chapter will explain in detail the impact of land subsidence on flooding (in other words, the correlation of land subsidence and flooding) in regions of Indonesia. The way this is achieved is mostly descriptive using available data. Nevertheless, descriptive datasets would provide new insights into what is going on in these regions regarding the correlation, and will hopefully give a detailed understanding of these two phenomena so one can better adapt to or mitigate these disastrous situations. These impacts also give rise to the future risk of global

Land subsidence by definition is a lowering of the ground level from a reference height system such as geoids or the level of the sea. The rates of subsidence can commonly vary between 1 and 20 centimeters per year and even more in some places. Over decades a magnitude of 4 to even 10 meters of subsidence can be found in some places in the world [1–9]. Capital or big cities, industrial areas, peatland areas, oil and gas fields, geothermal fields, and underground mining areas are the most common places where significant land subsidence may occur.

Geodetic measurements such as GNSS GPS surveys, InSAR, and leveling can reveal the rate and magnitude of land subsidence accurately. Other geotechnical approaches also commonly used are the extensometer and tilting measurement. **Figure 1** shows how to monitor subsidence using GNSS GPS and InSAR survey methods, while **Figure 2** illustrates the use of GPS data acquisition in the field. With GPS, several points, which are placed on the media (e.g. benchmarks) covering the area of investigation, are accurately positioned using GPS survey relative to a certain reference (stable) point. The precise coordinates of the benchmarks are periodically or continuously determined. By simply evaluating the rate of changes of the height coordinates from time to time, subsidence characteristics can be revealed. With InSAR, by using two or more synthetic aperture radar (SAR) images of an area, surface movements over time can be identified. The phase signal of InSAR records information on the distance between the satellite and the Earth's surface. By differential InSAR, where we use two SAR images of the same area acquired at different times, subsidence can be revealed if the distance

land subsidence).

flooding these areas.

climate change and its consequences.

**2. Land subsidence and its impacts**

40 Natural Hazards - Risk Assessment and Vulnerability Reduction

between the ground and the satellite has changed.

**Figure 2.** Field survey GPS observation to derive land subsidence (source: authors).

Infrastructure and building cracks, problems with drainage, the wider expansion of flooding areas, and tidal inundation as mentioned earlier are several impacts from subsidence (**Figure 3**). Surprisingly, many capital cities in the world (e.g. Jakarta, Tokyo, Osaka, Venice, Bangkok, and Ho Chi Mien) have suffered from land subsidence. The consumption of huge amounts of groundwater has led each city to subside significantly over many decades [1, 5–8, 10, 11]. Increased stress followed by compaction in an aquifer will result in subsidence of the surface of the ground. Some research has concluded that impact or the consequence of land subsidence is a disaster that takes place over time. However, over time impacts such as cracks on infrastructure or floods are worsening and will result in a real disaster. These infrastuctural cracks will eventually become a real danger. Road or bridge damage can be a danger for transport. The wider expansion of flooded areas is also due to continuing subsidence. Economically, huge amounts of money are needed to fix the problems of land subsidence.

In many places, short-term measures have been created to mitigate these disasters such as building temporary dykes, fixing or elevating roads, repairing houses and land, including

**Figure 3.** Example of documentation of the impact of subsidence such as cracks in buildings and infrastructures, and tidal inundation (sources: [12, 13]).

Jakarta city is a well-known place for land subsidence in Indonesia. According to some publications (e.g. [3, 5, 8]) the rates of subsidence in Jakarta generally range from 1 to 10 centimeters per year and may reach 20–26 centimeters in certain places, especially in the northern part of the city (**Figure 5**). Subsidence will continue since mitigation is beyond the priority program.

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Bandung is another well-known city for land subsidence in Indonesia. According to some publications (e.g. [2, 3, 8, 14]) the yearly amount of Bandung's subsidence generally ranges from 1 to 20 centimeters per year. The highest rate existed around Cimahi district in the northwestern part of the city (**Figure 5**). Generally, a linear trend can be seen, which means that

Semarang is also quite well known for land subsidence in Indonesia. According to some reports, subsidence in Semarang has been predicted to continue for more than 100 years. Based on some publications (e.g. [8, 9]) the yearly amount of Semarang's subsidence generally ranges from 1 to 17 centimeters per year, and in certain places, especially in the northeastern

Excessive groundwater extraction in combination with natural compaction of sediments and probably tectonic deformation, land setting/reclamation, loading from the construction of new buildings, oil and gas extraction, underground mining, drainage of peatlands, etc. are considered possible causes of land subsidence in regions of Indonesia, including Jakarta,

The consequences of land subsidence in Jakarta, Bandung, Semarang, and other places in Indonesia can be seen in several forms such as cracking of buildings and infrastructures,

The linear trend of subsidence can be seen as an indicator.

**Figure 4.** Places of land subsidence in Indonesian regions (sources: [3–5, 8, 9, 12, 14, 15]).

part of Semarang, it may reach 20 centimeters (**Figure 5**).

subsidence may continue for quite sometime.

Bandung, and Semarang.

building up mangrove areas. Long-term measures include either building giant sea walls around some coastal areas or stopping land subsidence. Giant sea walls are recognized in places such as New Orleans, Tokyo, and Osaka.
