**4. Land subsidence and flooding insight correlation**

As mentioned earlier we will look at the impact of land subsidence in the shape of flooding in some regions of Indonesia. More specifically, we will see how these are significantly correlated with each other and what the consequences of disaster would be. Fortunately, we can see quite clearly the qualitative and quantitative expected correlation between land subsidence and flooding using databases. Below is a detailed explanation of what is happening in Jakarta, Bandung, Semarang, Demak, and Pondok Bali Blanakan, which can best highlight the correlation examples. We will see clearly that the correlation is producing many problems.

Where a lowland area is experiencing land subsidence as a result of a cone of subsidence or there is a subsidence bowl, water will directly flow into it and create a flood zone. If the subsidence continues over time, in this case the cone of subsidence would become larger. As a consequence, wider expansion of flooding will likely occur as well. From all parameters that may create a flood (rain intensity, retention capacity, run-off, infiltration, land subsidence, land use, etc.), the subsidence parameter will likely influence a deeper and wider flood over time.

When we speak of a disaster from subsidence and flooding, we are likely to face economic and other losses. Millions of dollars have to be spent fixing problems from both land subsidence and flooding, and millions more will be spent in the future [17] (**Table 2**). With these kinds of losses, therefore, mitigation and/or adaptation are necessary. One key point regarding better mitigation and/or adaptation is to understand insight correlation between land subsidence and flooding. If flooding is proven to be influenced significantly by land subsidence, in this case reducing or even stopping the subsidence might be the best mitigation.

Geologically speaking, Jakarta is a lowland flood basin area. Thirteen rivers run across the area. Therefore, Jakarta is prone to frequent flooding. When a river is beyond its capacity to retain water from heavy rainfall, then flooding will occur. On the other hand, many places in Jakarta and the surrounding area experience land subsidence. With this situation, Jakarta is even more prone to frequent flooding. Spatially, the correlation between subsidence area and flood-prone area is very clear in Jakarta. Places that are experiencing high rates of subsidence are coincidentally those places most prone to flooding, such as Pluit, Sunter, Kamal Muara, and Joglo (**Figure 9**). In Pluit and Sunter, based on people's experience, floods seem to be

**Figure 8.** Short- and long-term mitigation and adaptation against the consequences of land subsidence (e.g. elevating

**Figure 7.** Pictures of the impact of subsidence (e.g. cracks in buildings and infrastructures and tidal inundation) in other

regions of Indonesia (source: authors).

46 Natural Hazards - Risk Assessment and Vulnerability Reduction

roads and housing, and building dykes, mangrove areas, and giant sea walls) (sources: authors, [12, 16]).


wider and deeper over time. Generally, this is one of the indicators of true spatial correlation

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Jakarta is one of the biggest coastal cities in Indonesia and even in the southeast of Asia. It is quite remarkable how the city has rapidly grown through the decades. Nevertheless, the coastal area of this city is a place where the highest rate of subsidence exists. The lowering of coastal land due to subsidence will result in tidal inundation. Continuing land subsidence in combination with the rise in sea level has made the city prone to significant tidal inundation. We use the LiDAR Digital Elevation Model to look at the subsidence over time and create a projection of tidal inundation in Jakarta. Surprisingly, based on the model we found that around 26.86% of Jakarta would be below sea level in 2025 and potentially suffer from

**Figure 10.** Map of tidal inundation in the area of Jakarta and the future tidal inundation projection as a consequence of continuing land subsidence in the area. Some pictures are given here to describe the existing tidal inundation in the field

between land subsidence and flooding.

(source: authors).

**Table 2.** Cost of fixing problems from land subsidence and floods.

**Figure 9.** Map of land subsidence in Jakarta (modified from [5]) correlated with a model of existing flooding in the area. Some pictures show flooding in the field (sources: authors, [18]). We can see that a significant area suffering from subsidence is also constantly suffering from flooding.

wider and deeper over time. Generally, this is one of the indicators of true spatial correlation between land subsidence and flooding.

Jakarta is one of the biggest coastal cities in Indonesia and even in the southeast of Asia. It is quite remarkable how the city has rapidly grown through the decades. Nevertheless, the coastal area of this city is a place where the highest rate of subsidence exists. The lowering of coastal land due to subsidence will result in tidal inundation. Continuing land subsidence in combination with the rise in sea level has made the city prone to significant tidal inundation. We use the LiDAR Digital Elevation Model to look at the subsidence over time and create a projection of tidal inundation in Jakarta. Surprisingly, based on the model we found that around 26.86% of Jakarta would be below sea level in 2025 and potentially suffer from

**Figure 10.** Map of tidal inundation in the area of Jakarta and the future tidal inundation projection as a consequence of continuing land subsidence in the area. Some pictures are given here to describe the existing tidal inundation in the field (source: authors).

**Figure 9.** Map of land subsidence in Jakarta (modified from [5]) correlated with a model of existing flooding in the area. Some pictures show flooding in the field (sources: authors, [18]). We can see that a significant area suffering from

**Problems to fix Cost Notes** 1. Elevated roads \$1 million for 1 kilometer Spent 2. Elevated bridges \$1 million for one long bridge Spent 3. Fixing drainage \$200,000 for 1 kilometer Spent 4. Social cost \$100 million/year Estimated 5. Temporary dyke \$5 million for 1 kilometer Spent 6. Giant sea wall \$600 million for 40 kilometers Estimated

**Table 2.** Cost of fixing problems from land subsidence and floods.

48 Natural Hazards - Risk Assessment and Vulnerability Reduction

subsidence is also constantly suffering from flooding.

(Source [17]).

significant tidal inundation, and around 35.61% of Jakarta would be below sea level in 2050 and potentially suffer from significant and even permanent tidal inundation (**Figure 10**).

lowering land due to subsidence, this place is prone to either flooding from rainfall or flooding from the sea or tidal inundation (**Figure 12**). When the high tide comes at the same time as heavy rainfall, what happens in northeastern Semarang is a real disaster. The place is drowning in the sea. Indeed, some parts of the area are permanently drowning as endorsed by analysis using the time series of high-resolution satellite image data as depicted in **Figure 12**. It is an undoubted fact of correlation between subsidence and flooding creating a real disaster. Sometime in the future this problem may worsen if we see a significant linear rate of subsidence in this area; on the other hand, attempts to mitigate or adapt to this situation are

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Moving a little further east from Semarang we find a district called Demak. It is a coastal area used mostly for farming and fishing. At the southern part of the district are many industrial areas. It is not surprising that these industrial areas have taken huge amounts of groundwater; what is also surprising is that farming and fishing have also taken huge amounts of groundwater from deep aquifers. As a consequence, the Demak area is suffering from a significant rate of land subsidence. While the low land of the coastal area is sinking, over time more frequent tidal inundation will make things worse and in certain places it has already become permanently inundated. Based on high-resolution satellite image analysis, it is surprising that 3000 hectares of the Demark area are suffering from

**Figure 12.** Map of land subsidence in Semarang (modified from [9]) correlated with existing flooding and tidal inundation in the area and also with satellite images of the land sinking (sources: authors; [18, 19]). We can see that the significant

area suffering from subsidence is also constantly suffering from both flooding and tidal inundation.

still not a priority.

The city of Bandung and its suburbs is located on a high land basin formed from the ancient crater of Gunung Sunda (super volcano). Thick sediment is overlaid below this area. Lying across the basin is the Citarum River. Land subsidence exists in Bandung Basin at a very significant rate per year. In certain places it may reach 20 centimeters per year. If we take a close look, subsidence mostly takes place around the industrial area, which is mainly located near the Citarum River and its tributaries (e.g. Dayeuh Kolot, Gedebage, Majalaya). When industry takes large amounts of groundwater, subsidence occurs in the area and creates a subsidence bowl. When heavy rain comes and the river cannot hold the water, then the surrounding area, including the subsidence bowl, is flooded (**Figure 11**). This clearly shows that the area suffering from land subsidence is constantly suffering from flooding.

Semarang is the biggest coastal city in Indonesia after Jakarta. Half of the city unfortunately is experiencing land subsidence, especially around the northeastern part. As a consequence of

**Figure 11.** Map of land subsidence in Bandung (modified from [14]) correlated with the model of existing flooding in the area. Some pictures are given to describe flooding in the field (sources: authors, [18]). The area suffering from subsidence is constantly suffering from flooding.

lowering land due to subsidence, this place is prone to either flooding from rainfall or flooding from the sea or tidal inundation (**Figure 12**). When the high tide comes at the same time as heavy rainfall, what happens in northeastern Semarang is a real disaster. The place is drowning in the sea. Indeed, some parts of the area are permanently drowning as endorsed by analysis using the time series of high-resolution satellite image data as depicted in **Figure 12**. It is an undoubted fact of correlation between subsidence and flooding creating a real disaster. Sometime in the future this problem may worsen if we see a significant linear rate of subsidence in this area; on the other hand, attempts to mitigate or adapt to this situation are still not a priority.

Moving a little further east from Semarang we find a district called Demak. It is a coastal area used mostly for farming and fishing. At the southern part of the district are many industrial areas. It is not surprising that these industrial areas have taken huge amounts of groundwater; what is also surprising is that farming and fishing have also taken huge amounts of groundwater from deep aquifers. As a consequence, the Demak area is suffering from a significant rate of land subsidence. While the low land of the coastal area is sinking, over time more frequent tidal inundation will make things worse and in certain places it has already become permanently inundated. Based on high-resolution satellite image analysis, it is surprising that 3000 hectares of the Demark area are suffering from

**Figure 12.** Map of land subsidence in Semarang (modified from [9]) correlated with existing flooding and tidal inundation in the area and also with satellite images of the land sinking (sources: authors; [18, 19]). We can see that the significant area suffering from subsidence is also constantly suffering from both flooding and tidal inundation.

**Figure 11.** Map of land subsidence in Bandung (modified from [14]) correlated with the model of existing flooding in the area. Some pictures are given to describe flooding in the field (sources: authors, [18]). The area suffering from subsidence

significant tidal inundation, and around 35.61% of Jakarta would be below sea level in 2050 and potentially suffer from significant and even permanent tidal inundation (**Figure 10**).

The city of Bandung and its suburbs is located on a high land basin formed from the ancient crater of Gunung Sunda (super volcano). Thick sediment is overlaid below this area. Lying across the basin is the Citarum River. Land subsidence exists in Bandung Basin at a very significant rate per year. In certain places it may reach 20 centimeters per year. If we take a close look, subsidence mostly takes place around the industrial area, which is mainly located near the Citarum River and its tributaries (e.g. Dayeuh Kolot, Gedebage, Majalaya). When industry takes large amounts of groundwater, subsidence occurs in the area and creates a subsidence bowl. When heavy rain comes and the river cannot hold the water, then the surrounding area, including the subsidence bowl, is flooded (**Figure 11**). This clearly shows that the area suffer-

Semarang is the biggest coastal city in Indonesia after Jakarta. Half of the city unfortunately is experiencing land subsidence, especially around the northeastern part. As a consequence of

ing from land subsidence is constantly suffering from flooding.

50 Natural Hazards - Risk Assessment and Vulnerability Reduction

is constantly suffering from flooding.

**Figure 13.** Graph of land subsidence in Demak (modified from [8]) correlated with pictures of existing tidal inundation and also with satellite images of land sinking (sources: authors, [18, 19]). We can see the general linear trend of subsidence indicating the sinking of the land.

inundation. **Figure 13** shows a graph of the subsidence in the Demak area along with a picture of tidal inundation in the field, as well as a depiction from a time series of highresolution satellite image.

**5. Discussion**

Based on the correlation between land subsidence and flooding in some areas in Indonesia we can see clearly that the correlation between them is very strong and may result in disastrous situations. We can see that the significant area suffering from subsidence is also constantly suffering from flooding and tidal inundation. When we speak of disaster, we tend to think of economic loss. Indeed, millions of dollars have to be spent fixing problems from both land subsidence and flooding, and it seems more will be spent in the future. So, based on this fact we need better mitigation and/or adaptation. For example, if indeed flooding proves to be influenced significantly by land subsidence, in this case reducing or even stopping the subsidence might be the best mitigation. This fact shows that it is possible to stop land subsidence by

**Figure 14.** Graph of land subsidence in Pondok Bali Blanakan (modified from [8]) correlated with existing tidal inundation and also with the satellite images of land sinking (sources: authors, [18, 19]). We can see the linear trend of

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stopping groundwater extraction and/or recharging artificially [8–10, 14, 20].

subsidence indicating the sinking of land and making it regularly inundated.

Pondok Bali Blanakan is a coastal area a few hundred kilometers east of Jakarta, and is mostly a farming and fishing area. First, it is quite surprising that this natural farming and fishing area is suffering from significant land subsidence. Nevertheless, after a detailed investigation we found that the area is an oil and gas exploitation area. The huge amount of oil and gas exploitation is causing subsidence. While the low land of the coastal area is sinking over time, more frequent tidal inundation is attacking the area and in certain places it has already become permanently inundated. Based on high-resolution satellite image analysis, it was found that a few hundred hectares of the Pondok Bali Blanakan area are suffering from inundation. **Figure 14** shows a graph of the subsidence in the area along with a picture of tidal inundation in the field, as well as a depiction from time series of high-resolution satellite image analysis. Again, we can see clear evidence of disaster.

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**Figure 14.** Graph of land subsidence in Pondok Bali Blanakan (modified from [8]) correlated with existing tidal inundation and also with the satellite images of land sinking (sources: authors, [18, 19]). We can see the linear trend of subsidence indicating the sinking of land and making it regularly inundated.
